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5 DataLink Layer 5-1
Chapter 5Link Layer and LANs
A note on the use of these ppt slidesWersquore making these slides freely available to all (faculty students readers) Theyrsquore in PowerPoint form so you can add modify and delete slides (including this one) and slide content to suit your needs They obviously represent a lot of work on our part In return for use we only ask the following
If you use these slides (eg in a class) in substantially unaltered form that you mention their source (after all wersquod like people to use our book)
If you post any slides in substantially unaltered form on a www site that you note that they are adapted from (or perhaps identical to) our slides and note our copyright of this material
Thanks and enjoy JFKKWR
All material copyright 1996-2009JF Kurose and KW Ross All Rights Reserved
Computer Networking A Top Down Approach 5th edition Jim Kurose Keith RossAddison-Wesley April 2009
5 DataLink Layer 5-2
Chapter 5 The Data Link LayerOur goals
understand principles behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressingreliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextdatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-2
Chapter 5 The Data Link LayerOur goals
understand principles behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressingreliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextdatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-3
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextdatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-4
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-5
Link layer contextdatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-5
Link layer contextdatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-6
Link Layer Servicesframing link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressreliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit-error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-7
Link Layer Services (more)
flow controlpacing between adjacent sending and receiving nodes
error detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-8
Where is the link layer implemented
in each and every hostlink layer implemented in ldquoadaptorrdquo (aka network interface card NIC)
Ethernet card PCMCI card 80211 cardimplements link physical layer
attaches into hostrsquos system busescombination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-9
Adaptors Communicating
sending sideencapsulates datagram in frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-10
Link Layer
51 Introduction and services52 Error detection and correction53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-11
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-12
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-13
Internet checksum (review)
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into UDP checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-14
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-15
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-16
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-17
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)old-fashioned Ethernetupstream HFC80211 wireless LAN
humans at acocktail party
(shared air acoustical)shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-18
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send at
rate R2 when M nodes want to transmit each can send at
average rate RM3 fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-20
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquonodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes result in ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-24
Slotted ALOHA
Assumptionsall frames same sizetime divided into equal size slots (time to transmit 1 frame)nodes start to transmit only slot beginning nodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-25
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability pprob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-27
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
worse than slotted AlohaPrice for no synchronization
Can you improve upon this protocolKey reduce collision
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Q Can collisions still occur
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-30
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turntypically used with ldquodumbrdquo slave devicesconcerns
polling overhead latencysingle point of failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-35
ldquoTaking Turnsrdquo MAC protocolsToken passing
control token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
T
data
(nothingto send)
T
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-36
Summary of MAC protocols
channel partitioning by time frequency or codeTime Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACDcarrier sensing easy in some technologies (wire) hard in others (wireless)CSMACD used in EthernetCSMACA used in 80211
taking turnspolling from central site token passingBluetooth FDDI IBM Token Ring
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-37
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-38
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address
function get frame from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settablebull How many addresses are there
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-39
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-40
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Each block is 2^24 analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressCompare IP address and MAC address
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-41
LAN Address (more)
ComparisonMAC flat address portability
can move LAN card from one LAN to anotherIP hierarchical address NOT portable
address depends on IP subnet to which node is attached
Q What happens if two nodes have the same IP (MAC) address
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-42
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP tableARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-43
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-44
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-45
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-46
What happens if you put cnncom in your browser
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-47
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-48
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for NICfirst widely used LAN technologysimpler cheaper than token LANs and ATMkept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
PARC and 3com
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-49
Star topologybus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-50
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-51
Ethernet Frame Structure (more)Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocolotherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)CRC checked at receiver if error is detected frame is dropped
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-52
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICsunreliable receiving NIC doesnrsquot send acks or nacksto sending NIC
What happens to the upper layer stream of datagrams passed to network layer can have gaps (missing datagrams)gaps will be filled if app is using TCPotherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-53
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mthcollision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-54
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-55
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LANttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-56
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standardscommon MAC protocol and frame formatdifferent speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bpsdifferent physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-57
Manchester encoding
used in 10BaseTeach bit has a transitionallows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized global clock among nodesHey this is physical-layer stuff
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-58
Link Layer
51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet
56 Link-layer switches LANs VLANs57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-59
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rateall nodes connected to hub can collide with one anotherno frame bufferingno CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-60
Switchlink-layer device smarter than hubs take active role
store forward Ethernet framesexamine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-61
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions
not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-62
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entry
(MAC address of host interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
C
Crsquo
Brsquo
switch with six interfaces(123456)
1 2 345
6
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-63
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-64
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood forward on all but the interface
on which the frame arrived
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-65
Self-learning forwarding example
A
Arsquo
B
C
Crsquo
Brsquo
1 2 345
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquoframe destination unknown flood
Arsquo A
destination A location knownselective send
Arsquo 4 60
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-66
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
HI
G
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-67
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
HI
G
12 S3
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-68
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-69
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-70
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-71
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-72
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-73
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-74
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-75
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-76
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-77
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-78
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-79
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-80
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-81
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-82
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-83
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-84
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-85
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineeringcomputing examples virtual memory virtual devices
Virtual machines eg javaIBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-86
The Internet virtualizing networks
1974 multiple unconnected nets
ARPAnetdata-over-cable networkspacket satellite network (Aloha)packet radio network
hellip differing inaddressing conventionspacket formatserror recoveryrouting
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on CommunicationsMay 1974 pp 637-648
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-87
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork appears as single uniform entity despite underlying local network heterogeneitynetwork of networks
Gateway ldquoembed internetwork packets in local packet format or extract themrdquoroute (at internetwork level) to next gateway
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-88
Cerf amp Kahnrsquos Internetwork Architecture
What is virtualizedtwo layers of addressing internetwork and local networknew layer (IP) makes everything homogeneous at internetwork layerunderlying local network technology
cablesatellite56K telephone modemtoday ATM MPLS
hellip ldquoinvisiblerdquo at internetwork layer Looks like a link layer technology to IP
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-89
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-90
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-91
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-92
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-93
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-94
Link Layer
51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-Layer Addressing55 Ethernet
56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-95
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-96
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-97
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-98
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-99
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-100
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-101
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-102
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-103
Chapter 5 Summaryprinciples behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressing
instantiation and implementation of various link layer technologies
Ethernetswitched LANS VLANsPPPvirtualized networks as a link layer MPLS
synthesis a day in the life of a web request
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
5 DataLink Layer 5-104
Chapter 5 letrsquos take a breathjourney down protocol stack complete(except PHY)solid understanding of networking principles practicehellip could stop here hellip but lots of interesting topics
wirelessmultimediasecurity network management
