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7/24/2019 UDLAP Computer Networks Spring 2015 pages 191-271
1/41
Dr. Vicente Alarcn Aqui no
Copyright 1996-2015 J.F Kurose
and K.W. Ross
1
23
0111
value in arrivingpackets header
routing algorithm
local forwarding table
header value output link
0100010101111001
3221
4.8 Routing algorithms
Interplay between
routing and forwarding
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Dr. Vicente Alarcn Aqui no
Copyright 1996-2015 J.F Kuroseand K.W. Ross
u
yx
wv
z2
2
13
1
1
2
53
5
Graph: G = (N,E)
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (u,w), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Graph abstraction
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Dr. Vicente Alarcn Aqui no
Copyright 1996-2015 J.F Kurose
and K.W. Ross
Graph abstraction: costs
u
yx
wv
z2 21
3
1
1
2
5
3
5 c(x,x) = cost of link (x,x)
- e.g., c(w,z) = 5
cost could always be 1, orinversely related to bandwidth,or inversely related tocongestion
Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)
Question: Whats the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
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Routing Algorithm classification
Global or decentralizedinformation?
Global: all routers have complete
topology, link cost info
link state algorithms
Decentralized: router knows physically-
connected neighbors, linkcosts to neighbors
iterative process ofcomputation, exchange ofinfo with neighbors
distance vector algorithms
Static or dynamic?
Static:
routes change slowlyover time
Dynamic:
routes change more
quickly periodic update in response to link cost
changes
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A Link-State Routing Algorithm
Dijkstras algorithm
net topology, link costsknown to all nodes
accomplished via link
state broadcast all nodes have same info computes least cost paths
from one node (source)to all other nodes
gives forwarding table forthat node
iterative: after k iterations,
know least cost path to kdest.s
Notation: c(x,y): link cost from node x
to y; = if not direct
neighbors D(v): current value of cost of
path from source to dest. v
p(v): predecessor nodealong path from source to v
N': set of nodes whose leastcost path definitively known
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Dijsktras Algorithm
1 Initialization:2 N' = {u}3 for all nodes v4 if v adjacent to u5 then D(v) = c(u,v)6 else D(v) = 78 Loop
9 find w not in N' such that D(w) is a minimum10 add w to N'11 update D(v) for all v adjacent to w and not in N' :12 D(v) = min( D(v), D(w) + c(w,v) )13 /* new cost to v is either old cost to v or known14 shortest path cost to w plus cost from w to v */15 unti l all nodes in N'
u
yx
wv
z2
2
13
1
1
2
53
5
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Dr. Vicente Alarcn Aqui no
Copyright 1996-2015 J.F Kurose
and K.W. Ross
Dijkstras algorithm: example
Step01
2345
N'u
ux
uxyuxyv
uxyvwuxyvwz
D(v),p(v)2,u2,u
2,u
D(w),p(w)5,u4,x
3,y3,y
D(x),p(x)1,u
D(y),p(y)
2,x
D(z),p(z)
4,y4,y4,y
u
yx
wv
z2
2
1 3
1
1
2
53
5
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Dijkstras Algorithm Example
The first 5 steps used in computing the shortest path from A toD. The arrows indicate the working node.
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Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Define dx(y) := cost of least-cost path from x to y
dx(y) = min {c(x,v) + dv(y) } for each node yN
where min is taken over all neighbors of x
Basic idea:
Each node periodically sends its own distance vectorestimate to neighbors
When node a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
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Bellman-Ford example
u
yx
wv
z2
2
13
1
1
2
53
5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
du(z) = min { c(u,v) + dv(z),c(u,x) + d
x(z),
c(u,w) + dw(z) }= min {2 + 5,
1 + 3,5 + 3} = 4
Node that achieves minimum is nexthop in shortest path forwarding table
B-F equation says:
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x y z
xy
z
0 2 7
from
cost to
from
from
x y z
xy
z
0 2 3
from
cost tox y z
xy
z
0 2 3
from
cost to
x y z
xyz
cost tox y z
xyz
0 2 7
from
cost to
x y z
xyz
0 2 3
from
cost to
x y z
xyz
0 2 3
from
cost tox y z
xyz
0 2 7
from
cost to
x y z
xyz
7 1 0
cost to
2 0 1
2 0 1
7 1 0
2 0 17 1 0
2 0 13 1 0
2 0 1
3 1 0
2 0 1
3 1 0
2 0 1
3 1 0
time
x z12
7
y
node x table
node y table
node z table
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
Dx(z)= min{c(x,y) + Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
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Comparison
Distance vector algorithm
Very simple to implement May have convergence problems Used in RIP and EIGRP
Link-state algorithm
Much more complex Switches perform independent computations Used in OSPF (Open Shortest Path First Protocol)
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4.9 Hierarchical Routing
scale: with 200 milliondestinations:
cant store all dests inrouting tables!
routing table exchangewould swamp links!
administrative autonomy
Internet = network ofnetworks
each network admin maywant to control routing in itsown network
Our routing study thus far - idealization
all routers identical
network flat
not true in practice
aggregate routers into regions,autonomous systems (AS)
routers in same AS run samerouting protocol
intra-AS routing protocol
routers in different AS canrun different intra-ASrouting protocol
Gateway router: Direct link torouter in another AS
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Classification Of Internet Routing Protocols
Two broad classes Interior Gateway Protocols (IGPs): aka Intra-AS routing protocols.
Used among routers within autonomous system Destinations lie within IGP RIP: Routing Information Protocol. RIPng for IPv6 OSPF: Open Shortest Path First. OSPFv3 for IPv6 IGRP/EIGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Exterior Gateway Protocols (EGPs) Used among autonomous systems (BGP) Destinations lie throughout Internet
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3b
1d
3a
1c
2a
AS3
AS1
AS21a
2c
2b1b
Intra-ASRoutingalgorithm
Inter-ASRoutingalgorithm
Forwardingtable
3c
Interconnected ASes
Forwarding table isconfigured by both intra-and inter-AS routingalgorithm
Intra-AS sets entries forinternal dests
Inter-AS & Intra-As setsentries for external dests
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RIP ( Routing Information Protocol)
Distance vector algorithm
Distance metric: # of hops (max = 15 hops)
DC
BA
u v
w
x
yz
destination hopsu 1v 2
w 2x 3y 3z 2
Distance vectors:exchanged amongneighbors every 30 secvia Response Message(also calledadvertisement)
Each advertisement: listof up to 25 destinationnets within AS
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Destination Network Next Router Num. of hops to dest.
w A 2y B 2
z B A 7 5x -- 1. . ....
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RIP: Example
w x y
z
A
C
D B
Routing table in D207
Dest Next hopsw - -x - -z C 4. ...
Advertisementfrom A to D
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An OSPF AS Consists of Multiple Areas Linked by
Routers
(a) an interconnect of routers andnetworks, and
(b) an equivalent OSPF graph Router corresponds to a node in the
graph
Illustration Of OSPF Graph
Carried in OSPF messages directly over IP(rather than TCP or UDP): protocol number89 for the IP Protocol field
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BGP basics
Pairs of routers (BGP peers) exchange routing info oversemi-permanent TCP conctns: BGP sessions on port 179.
Note that BGP sessions do not correspond to physicallinks.
When AS2 advertises a prefix to AS1, AS2 is promising it
will forward any datagrams destined to that prefix towardsthe prefix. AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3c
eBGP session
iBGP session
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5. Link Layer and LANs
Some terminology: hosts and routers are nodes
communication channels thatconnect adjacent nodes alongcommunication path are links
wired links wireless links
LANs layer-2 packet is a frame,
encapsulates datagram
link
data-link layer has responsibility oftransferring datagram from one nodeto adjacent node over a link
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Two types of networks at the data link layer
Broadcast Networks: All stations share a singlecommunication channel
Point-to-Point Networks: Pairs of hosts (or routers) aredirectly connected
Typically, local area networks (LANs) are broadcast andwide area networks (WANs) are point-to-point
Broadcast Network Point-to-Point Network
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Local Area Networks
Local area networks (LANs) connect computers within a building or a enterprisenetwork
Almost all LANs are broadcast networks
Typical topologies of LANs are bus or star orring. There are also mesh, hybrid,hierarchical star, star-wireless.
We will work with Ethernet LANs. Ethernet has a bus or star topology.
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IEEE 802 Standards
Number Description
802.1 Internetworking
802.2 Logical Link Control
802.3 Ethernet (CSMA/CD)
802.3u Fast Ethernet
802.3z Gigabit Ethernet
802.3ae 10 Gigabit Ethernet
802.4 Token Bus
802.5 Token Ring
802.6 Distributed Queue Dual Bus (MAN)
802.7 Broadband Technology
802.8 Fiber Optic Technology
802.10 LAN Security
802.11a/b/g/n Wireless LAN
802.15 Wireless Personal Area Network (WPAN)
802.15.3a UWB (Ultra-WideBand)
802.16 Wireless MAN WiMAX802.20 Wireless WAN (Mobile Broadband Wireless Access MBWA).
802.22 Wireless RAN (Regional Area Network)
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5.1 Link layer: context and services
Datagram transferred by different link protocolsover different links:
e.g., Ethernet on first link, frame relay onintermediate links, 802.11 on last link
Framing, link access:
encapsulate datagram into frame, adding header, trailer
channel access if shared medium MAC addresses used in frame headers to identify source,
dest
different from IP address! Reliable delivery between adjacent nodes
seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates
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Link Layer Services (more)
Flow Control: pacing between adjacent sending and receiving nodes
Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors:
signals sender for retransmission or drops frame
Error Correction: receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex
with half duplex, nodes at both ends of link cantransmit, but not at same time
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Adaptors Communicating
link layer implemented inadaptor (aka NIC)
Ethernet card, 802.11 card sending side:
encapsulates datagram in aframe
adds error checking bits, rdt(reliable data transfer), flowcontrol, etc.
receiving side
looks for errors, rdt, flowcontrol, etc
extracts datagram, passes torcving node
adapter is semi-autonomous
link & physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
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5.2 Error Detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields
Error detection not 100% reliable! protocol may miss some errors, but rarely
larger EDC field yields better detection and correction
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Parity Checking
Single Bit Parity:Detect single bit errors
Two Dimensional Bit Parity:Detect and correct single bit errors
0 0
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Checksumming: Cyclic Redundancy Check
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
exactly divisible by G (modulo 2) receiver knows G, divides by G. If non-zero
remainder: error detected!
can detect all burst errors less than r+1 bits widely used in practice
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CRC Example
[D.2rXOR R]/G = Q
[D.2r]/G = Q XOR R/G
if we divide D.2rbyG, want remainder R
R = remainder[ ]D.2r
G
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Polynomials
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5.3 Multiple Access Links and Protocols
Two types of links:
point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)
traditional Ethernet
upstream HFC 802.11 wireless LAN
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Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes:interference collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes sharechannel, i.e., determine when node can transmit
communication about channel sharing must usechannel itself!
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Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can sendat rate R.
2. When M nodes want to transmit, each cansend at average rate R/M
3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots
4. Simple
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MAC Protocols: a taxonomy
Three broad classes:
Channel Partitioning divide channel into smaller pieces (time slots,
frequency, code) allocate piece to node for exclusive use
Random Access channel not divided, allow collisions recover from collisions
Taking turns
Nodes take turns, but nodes with more to send cantake longer turns
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Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt transtime) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, oneper user; inefficient with low duty cycle users and at light load.FDM (Frequency Division Multiplexing): frequency subdivided.
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Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands2,5,6 idle
frequencybands
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Channel Partitioning MAC protocols: CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular, satellite, etc)standards
unique code assigned to each user; i.e., code set partitioning
all users share same frequency, but each user has own chippingsequence (i.e., code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chipping sequence
allows multiple users to coexist and transmit simultaneously with
minimal interference (if codes are orthogonal)
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Hadamard Matrix (Walsh Codes)
A Hadamard matrix of ordern is a matrix with elements 1 or1 such that
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CDMA Encode/Decode Example
slot 1 slot 0
d1 = -1
1 1 1 11- 1- 1- 1-
Zi,m= di.cm
d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zi,m
sender
code
databits
slot 1 slot 0
d1 = -1
d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0
channeloutput
slot 1
channeloutputreceiver
code
receivedinput
Di = Zi,m.cm
m=1
M
M
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CDMA: two-sender interference
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Random Access Protocols
When node has packet to send
transmit at full channel data rate R. no a priori coordination among nodes
two or more transmitting nodes
collision, random access MAC protocol specifies:
how to detect collisions how to recover from collisions (e.g., via delayed
retransmissions)
Examples of random access MAC protocols: slotted ALOHA ALOHA
CSMA, CSMA/CD (IEEE 802.3), CSMA/CA (IEEE 802.11)
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The ALOHA Protocol: Pure (unslotted) ALOHA
Multiple users share a single broadcast channel Users do not test the channel before they transmit. They
transmit fixed-size frames at arbitrary times
If a collision occurs (i.e., by overlapping transmissions), thecolliding frames get garbed and thus discarded Retransmission by upper-layer protocols
Maximum throughput is ~18.4%
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Slotted ALOHA
Pure Aloha performance can be substantially improved if werequire frame transmissions to occur on time slot boundaries
Time slots are the same as the frame size Assume that all stations are globally synchronized The frame will be scheduled for transmission at the start of the
next time slot User sends the frame without testing the state of the channel If a collision occurs, the colliding frames discarded
Retransmission by upper-layer protocols Maximum throughput is ~37%
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CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission
Multiple Access (MA) Multiple computers attach to shared media Each uses same access algorithm
Carrier Sense (CS) Wait until medium idle Begin to transmit frame
Simultaneous transmission possible
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CSMA collisions
collisions can still occur:propagation delay meanstwo nodes may not heareach others transmission
collision:entire packet transmissiontime wasted
spatial layout of nodes
note:role of distance & propagationdelay in determining collision
probability
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CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing,deferral as in CSMA collisions detected within short
time
colliding transmissionsaborted, reducing channelwastage
collision detection: easy in wired LANs: measuresignal strengths, comparetransmitted, received signals
difficult in wireless LANs:receiver shut off whiletransmitting
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The CSMA Protocols 1-persistent CSMA:
listens to the channel prior to its transmission. If it is idle then it sends its frame. If a collision occurs the station waits for a random time and tries again. If it is busy it waits for the ongoing transmission to finish.
Non-persistent CSMA Waits for a random period before it sends the frame if the channel is in use
P - Persistent:
1. If medium idle, transmit with probability p, and delay one time unit with probability (1 p) Time unit typically maximum propagation delay2. If medium busy, listen until idle and repeat step 13. If transmission is delayed one time unit, repeat step
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237
Taking Turns MAC protocolsPolling: master node invites slave
nodes to transmit in turn
concerns:
polling overhead latency single point of failure
(master)
Token passing: control token passed from one
node to next sequentially; tokenmessage; concerns (token overhead,latency, single point of failure (token)).
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master
slaves
poll
data
data
T
data
(nothingto send)
T
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5.4 MAC Addresses and ARP
32-bit IP address: network-layeraddress used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address:
used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs)
burned in the adapter ROM
RARP
Ethernet MAC
address
(48 bit)
ARPIP address(32 bit)
IGMP Internet Group Management ProtocolICMP Internet Control Message ProtocolNDP Neighbor Discovery Protocol for IPv6 and ICMPv6
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LAN Addresses and ARP
Each 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)
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ARP: Address Resolution Protocol
Each IP node (Host,Router) on LAN hasARP table
ARP Table: IP/MACaddress mappings forsome LAN nodes< IP address; MAC address; TTL>
TTL (Time To Live): timeafter which addressmapping will be forgotten(typically 10 min)
Question: how to determineMAC address of Bknowing Bs 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
237.196.7.23
237.196.7.78
237.196.7.14
237.196.7.88
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Address Translation with ARP
ARP Request:Argon broadcasts an ARP request to all stations on thenetwork: What is the hardware address of Router137?
Argon128.143.137.144
00:a0:24:71:e4:44
Router137128.143.137.1
00:e0:f9:23:a8:20
ARP Request:
What is the MAC address
of 128.143.137.1?
ARP Request from Argon:
Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: FF:FF:FF:FF:FF:FFTarget protocol address: 128.143.137.1
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Address Translation with ARP
ARP Reply:Router 137 responds with an ARP Reply which containsthe hardware address
Argon128.143.137.144
00:a0:24:71:e4:44
Router137128.143.137.1
00:e0:f9:23:a8:20
ARP Reply:
The MAC address of 128.143.137.1
is 00:e0:f9:23:a8:20
ARP Reply from Router137:
Source hardware address: 00:e0:f9:23:a8:20Source protocol address: 128.143.137.1
Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144
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Internet Architecture & Internet Application
Example
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5.5 High-Speed LANs: Ethernet, Token Ring
dominant wired LAN technology:
cheap $20 for 100Mbs!
first widely used LAN technology
Simpler, cheaper than token LANs and ATM Kept up with speed race: 10 Mbps 10 Gbps
Robert Metcalfes (1946)(MIT, 1973) EthernetSketch.
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Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layerprotocol packet) in Ethernet frame
Preamble: 7 bytes with pattern 10101010 followed by one byte withpattern 10101011. Used to synchronize receiver, sender clock rates
Addresses: 6 bytes
if adapter receives frame with matching destination address, or withbroadcast address (e.g., ARP packet), it passes data in frame to net-layer protocol; otherwise, adapter discards frame
Type: indicates the higher layer protocol (mostly IP)
CRC: checked at receiver, if error is detected, the frame is simply dropped
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Ethernet CSMA/CD algorithm
1. Adaptor receives datagramfrom net layer & createsframe
2. If adapter senses channelidle, it starts to transmitframe. If it senses channelbusy, waits until channel idleand then transmits
3. If adapter transmits entireframe without detectinganother transmission, theadapter is done with frame !
4. If adapter detects anothertransmission whiletransmitting, aborts andsends jam signal
5. After aborting, adapterenters exponentialbackoff: after the mthcollision, adapter choosesa K at random from{0,1,2,,2m-1}. Adapterwaits K512 bit times andreturns to Step 2
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Ethernets CSMA/CD (more)
Jam Signal: make sure all othertransmitters are aware ofcollision; 48 bits or 32bits.
Bit time: 0.1 microsec for 10Mbps Ethernet ;for K=1023, wait time is
about 50 msec
Exponential Backoff: Goal: adapt retransmission
attempts to estimated currentload
heavy load: random wait willbe longer
first collision: choose K from
{0,1}; delay is K
512 bittransmission times
after second collision: choose Kfrom {0,1,2,3}
after ten collisions, choose Kfrom {0,1,2,3,4,,1023}
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Ethernet Technologies: LAN Wiring
Ethernet Technologies Three schemes correspond to three generations
(a) 10Base5, (b) 10Base2, (c) 10Base-T
All run same link speed of 10Mbps
All use same frame format
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Ethernet Wiring In An Office
10Base5
10Base2
10BaseT
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10BaseT and 100BaseT
10/100 Mbps rate; latter called fast ethernet
T stands for Twisted Pair
Nodes connect to a hub: star topology; 100m
max distance between nodes and hub
twisted pair
hub
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Repeater
Hardware device
Connects two LAN segments
One repeater can effectively double the length of an LANsegment
Copies signal from one segment to the other
Connection can be extended with Fiber Optic Intra-Repeater Link
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Limits On Repeaters
Can't extend Ethernet with repeaters indefinitely CSMA/CD requires low delay; if medium is too long, CSMA/CD
won't work Ethernet standard includes limit of 4 repeaters between any two
Ethernet stations Very easy to use - just plug in
Repeaters simply re-transmit analog signals Collisions affect entire network Transient problems - noise propagates throughout network
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Fast (802.3u) and Gbit Ethernet (802.3z)
uses standard Ethernet frame format allows for point-to-point links and shared broadcast channels
in shared mode, CSMA/CD is used; short distances between nodesrequired for efficiency
uses hubs, called here Buffered Distributors or Full Duplex Repeater orBuffered Repeater
Full-Duplex at 1 Gbps for point-to-point links
10 Gbps now !
T4: 1 pair is used to receive while3 pairs are used to transmit.
FX, TX: it uses 1 pair to receiveand 1 pair to transmit
In 1000Base-T, each of four wirepairs transmits 250 Mbps inboth directions.
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CSMA/CD efficiency
Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
N
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity
Much better than ALOHA, but still decentralized, simple, andcheap
transprop tt /3.441
1efficiency
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IEEE 802.5 Token Ring
Second most popular LAN topology
Bits flow in single direction
Several technologies exist
Used with ring topology
Guarantees fair access: IEEE 802.5 standards
Token
Special (reserved) message
Small (a few bits) Frame formats:
token:SD AC FC
packet: SD AC FC dest addr src addr data checksum ED FS
8 8 8 48 48 variable 32 8 8 bits
SD, ED: mark start, end of frame
FC: Frame Control: used for monitoringand maintenance, distinguish controlframes and information frames.
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Frame Formats
AC: Access Control Byte:
T - token bit: value 1 means token can be seized, value 0 means data
packet to follow access control byte.
PPP - priority bits: priority level of this packet.
RRR - reservation bits : station can write these bits to prevent stations withlower priority packet from seizing the token next time it becomes free.
M - is the monitor bit which is set by the Active Monitor (AM) station whenit sees this frame.
Example: Priority-Based Ring
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Frame Formats (contd)
Source and destination address: as in Ethernetdata: packet from network layer
checksum: error detection
FS: Frame Status: set by destination, read by receiver
set to indicate that destination is up, packet copied OK from ring
Destination station nonexistent or not active (A=0, C=0)Destination station exists but frame not copied (A=1, C=0)Frame received (A=1, C=1)
A C 0 0 A C 0 0
1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit
Example:At a propagation speed of 200m/s, what is the effective length Ladded to a ring by a bit delay at each repeater? Now suppose a token holdingtime of 10ms, what is the frame length for each case?.a) At 5Mbpsb) At 40Mbpsc) At 1Mbps
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5.6 Interconnecting with hubs and switches
Backbone hub interconnectsLAN segments
Extends max distance betweennodes
But individual segment collisiondomains become one largecollision domain
Cant interconnect 10BaseT &100BaseT
hub hub hub
hub
Data link layer technologies (switched networks): Switched Ethernet ATM (Asynchronous Transfer Mode) Frame Relay Multi-Protocol Label Switching (MPLS)
Some switched networks are intended for enterprise networks (SwitchedEthernet), wide area networks (MPLS, Frame Relay), or both (ATM)
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Hubs
Hubs are essentially physical-layer repeaters:
bits coming from one link go out all other links at the same rate no frame buffering no CSMA/CD at hub: adapters detect collisions
twisted pair
hub
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Bridge
Hardware device
Connects two LAN segments
handles complete frames Uses NIC like any other station Performs some processing on frame
Does not forward noise or collisions
Learns addresses and filters
Allows independent transmission
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Filtering Example
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Break Cycles--Spanning Tree Algorithm
Bridges must cooperate to broadcast frames exactly onceon each segment
Used by all bridges to
Discover one another
Break cycle(s)
Known as Distributed Spanning Tree (DST)-- resulting in aunique path from each source to each destination
As each bridge joins the network, it communicates withother bridges on special hardware (typically multicast)
address
Learns network topology Performs spanning tree computation Determines if bridge will form a cycle
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Spanning Tree Algorithm
Step-1: Take the least weighing edge of the graph
and add it to the minimum Spanning Tree.
Step-2: Find the adjacent edges for the vertices in the
spanning tree.
Step-3: Find the least among all those edges.
Step-4: Add that edge to the spanning tree if its notforming a loop in the tree and that its not already
there in the tree.
Step-5: Stop if all vertices in the Graph have been
covered by the Tree else go to step-2.
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Spanning Tree Example
AB - 2 Least weighing
AD - 1 Already in t he tree
DB - 2
Least weighing
DE - 3
> value
DA - 1
Already in t he tree
BA - 2 Alr eady i n the tree
BD - 2 > value
BC - 1
Least weighing
BE - 3 > value
DB - 2 > value
DE - 3 > value
CB - 1 Alr eady in the tr ee
CE - 4 > value
CF - 2 Least weighing
BD - 2 Forming a loop
BE - 3 > value
DB - 2 Forming a loop
DE - 3 > value
FC - 2
Alr eady in the t ree
FE - 3 least weighing
CE - 4
> value
BE - 3
least weighing
DE - 3
least weighing
EB - 3 Alr eady i n the tree
EC - 4 Forming a loop
ED - 3
Forming a loop
EF - 3 Forming a loop
FE - 3
Forming a loop
CE - 4
Forming a loop
DE - 3 Forming a loop
The resultant Spanning Tree with no loops giving you the shortest distancebetween any two vertices.
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Switch
Link layer device
stores and forwards Ethernet frames examines frame header and selectively
forwards frame based on MAC dest address
when frame is to be forwarded on segment,uses CSMA/CD to access segment
transparent
hosts are unaware of presence of switches plug-and-play, self-learning
switches do not need to be configured
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Forwarding
How do determine onto which LAN segment to
forward frame? Looks like a routing problem...
hub hubhub
switch1
2 3
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Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table, switch forwards frame into
interfaces 2 and 3
frame received by D
hub hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
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Switch example
Suppose D replies back with frame to C.
Switch receives frame from from D notes in bridge table that D is on interface 2
because C is in table, switch forwards frame only to interface 1 frame received by C
hub hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
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Switches vs. Routers
both store-and-forward devices
routers: network layer devices (examine network layerheaders)
switches are link layer devices routers maintain routing tables, implement routing
algorithms
switches maintain switch tables, implement filtering,learning algorithms
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Summary comparison
hubs routers switches
traffic
isolation
no yes yes
plug & play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
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