1

Link Layer

11/18/2009

Admin

Next exam?

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Recap: Internet Routing

Intradomain routing and interdomain routing

Intradomain routes are aggregated and announced to interdomain routing CIDR to allow flexibility in aggregation of

destination addresses

Longest prefix matching to determine the next hop to a destination

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Example 1 (same network): A->B

Look up dest address find dest is on same net link layer will send the

datagram directly inside a link-layer frame

miscfields223.1.1.1223.1.1.3data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

Dest. Net. next router Nhops

223.1.1/24 1223.1.2/24 223.1.1.4 2223.1.3/24 223.1.1.4 2

forwarding table in A

0.0.0.0/0 223.1.1.4 -

223.1.4.1

To Internet

src dst

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Example 2 (Different Networks): A-> E

look up dest address in forwarding table

routing table: next hop router to dest is 223.1.1.4

link layer sends datagram to router 223.1.1.4 inside a link-layer frame the dest. of the link layer

frame is 223.1.1.4

miscfields223.1.1.1223.1.2.3 data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.3

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Dest. Net. next router Nhops

223.1.1/24 1223.1.2/24 223.1.1.4 2223.1.3/24 223.1.1.4 2

forwarding table in A

0.0.0.0/0 223.1.1.4 -

223.1.4.1

To Internet

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Example 2 (Different Networks): A-> E

Arriving at 223.1.1.4, destined for 223.1.2.2

look up dest address in router’s forwarding table

E on same network as router’s interface 223.1.2.9 router, E directly

attached link layer sends datagram

to 223.1.2.2 inside link-layer frame via interface 223.1.2.9

datagram arrives at 223.1.2.2!! (hooray!)

miscfields223.1.1.1223.1.2.3 data Dest. Net router Nhops interface

223.1.1/24 - 1 223.1.1.4 223.1.2/24 - 1 223.1.2.9

223.1.3/24 - 1 223.1.3.27

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.3

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in router

0.0.0.0/0 - - 223.1.4.1

223.1.4.1

To Internet

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What A Router Looks Like: Outside

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Look Inside a Router

Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) switching datagrams from incoming to outgoing ports

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Input Port Functions

physical layer:bit-level reception

data link layer:e.g., Ethernet network layer:

lookup output port using forwarding

table

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Switching: Low End

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Overcome bus bandwidth limitations fragmenting datagram into fixed length

cells, switch cells through the fabric. Crossbar, Banyan networks, and others

Cisco 12416: switches 320 Gbps (upgradeable to 1.28 Tbps) with 16 slots (each 10G full-duplex) through the crossbar interconnection network

Switching Via An Interconnection Network

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New Potential Bottleneck: Output Ports

Due to output port contention and head-of-the-Line (HOL) blocking (i.e., queued datagram at front of queue prevents others in queue from moving forward)

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Head-of-Line Blocking Limits Thrput

Due to output-port contention and HOL blocking, the stable throughput is only around 2 - sqrt(2) = 0.586 of line speed !

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Avoiding Port Contention and HOB

Virtual output queueing

Input/output ports matching algorithm Switch fabric speedup, e.g., two cells to

one output port

For more details: http://www.cisco.com/warp/public/63/arch12000-swfabric.html

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Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rate

Queueing (delay) and loss due to output port buffer overflow !

Scheduling and queue/buffer management choose among queued datagrams for transmission

Summary

We have covered the basics of the network layer routing and forwarding

There are multiple other topics that we did not cover Multicast/anycast routing QoS slides linked on the schedule page just in case

you want to take a quick look

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Recap: The Hourglass Architecture of the Internet

IP

Ethernet FDDIWireless

TCP UDP

Telnet Email FTP WWW

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Link Layer: Introduction

Some terminology: hosts and routers are nodes (bridges and switches too)

communication channels that connect adjacent nodes along a communication path are links wired, wireless dedicated, shared

2-PDU is a frame, encapsulates datagram

“link”

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Link layer: Context

Data-link layer has responsibility of transferring datagram from one node to another node over a link

Datagram transferred by different link protocols over different links, e.g., Ethernet on first link, frame relay on

intermediate links 802.11 on last link

transportation analogy

trip from New Haven to San Francisco taxi: home to union

station train: union station

to JFK plane: JFK to San

Francisco airport shuttle: airport to

hotel

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Link Layer Services Framing

o encapsulate datagram into frame, adding header, trailer and error detection/correction

Multiplexing/demultiplexingo frame headers to identify src, dest

• different from IP address ! Flow control Link access (interference and quality of service

control) Reliable delivery between adjacent nodes

o we learned how to do this already !o seldom used on low bit error link (fiber, some twisted

pair)o common for wireless links: high error rates

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Adaptors Communicating

link layer implemented in “adaptor” (aka NIC) Ethernet card,

modem, 802.11 card

adapter is semi-autonomous, implementing link & physical layers

sending side: encapsulates datagram

in a frame adds error checking bits,

rdt, flow control, etc.

receiving side looks for errors, rdt, flow

control, etc extracts datagram,

passes to receiving node

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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LAN/MAC/Physical Address

Each adapter has a unique link layer address (also called MAC address)

• used as address in datalink frames to identify the interface

• 48 bit MAC address (for most types of LANs) burned in the adapter ROM

• MAC address allocation administered by IEEE;manufacturer buys portion of MAC address space (to assure uniqueness)

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Recall Earlier Routing Discussion

Starting at A, given IP datagram addressed to E:

look up net. address of E, find C

link layer sends datagram to C inside link-layer frame; the dest. address should be C’s MAC address

C’s MACaddr

A’s MACaddr

A’s IPaddr

E’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

C

Question: how to determine MAC address of C knowing C’s IP address?

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ARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL> TTL (Time To Live): time

after which address mapping will be forgotten (typically 20 min)

[yry3@cicada yry3]$ /sbin/arpAddress HWtype HWaddress Flags Mask Ifacezoo-gatew.cs.yale.edu ether AA:00:04:00:20:D4 C eth0artemis.zoo.cs.yale.edu ether 00:06:5B:3F:6E:21 C eth0lab.zoo.cs.yale.edu ether 00:B0:D0:F3:C7:A5 C eth0

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ARP Protocol

ARP is “plug-and-play”: nodes create their ARP tables without

intervention from net administrator

A broadcast protocol: Source broadcasts query frame, containing

queried IP address • all machines on LAN receive ARP query

destination D receives ARP frame, replies• frame sent to A’s MAC address (unicast)

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Comparison of IP address and MAC Address

IP address is hierarchical for routing scalability

IP address needs to be globally unique (if no NAT)

IP address depends on IP network to which an interface is attached NOT portable

MAC address is flat

MAC address does not need to be globally unique, but the current assignment ensures uniqueness

MAC address is assigned to a device portable

Outline

Admin Link layer overview Error detection

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Error Detection

D = Data protected by error checking, may include header fieldsED = Error Detection bits (redundancy)

• Error detection not 100% reliable!• a good error detector may miss some errors, but rarely• larger ED field generally yields better detection

‘

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Cyclic Redundancy Check: Background Widely used in practice, e.g.,

Ethernet, DOCSIS (Cable Modem), FDDI, PKZIP, WinZip, PNG

For a given data D, consider it as a polynomial D(x) consider the string of 0 and 1 as the

coefficients of a polynomial• e.g. consider string 10011 as x4+x+1

addition and subtraction are modular 2, thus the same as xor

Choose generator polynomial G(x) with r+1 bits, where r is called the degree of G(x)

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Cyclic Redundancy Check: Encode Given data G(x) and D(x), choose R(x)

with r bits, such that D(x)xr+R(x) is exactly divisible by G(x)

The bits correspond to D(x)xr+R(x) are sent to the receiver

+x

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Cyclic Redundancy Check: Decode

Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x) if non-zero remainder: error detected! if zero remainder, assumes no error

Encode:CRC(G)

DT = D(x)xr+R(x) T’

check

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CRC: Steps and an Example

Suppose the degree of G(x) is r

Append r zero to D(x), i.e. consider D(x)xr

Divide D(x)xr by G(x). Let R(x) denote the reminder

Send <D, R> to the receiver

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The Power of CRC Let T(x) denote D(x)xr+R(x), and E(x) the polynomial of the

error bits the received signal is T’(x) = T(x)+E(x)

Since T(x) is divisible by G(x), we only need to consider if E(x) is divisible by G(x)

Encode:CRC(G)

DT = D(x)xr+R(x) T’

check

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The Power of CRC

Detect a single-bit error: E(x) = xi

if G(x) contains two or more terms, E(x) is not divisible by G(x)

Detect an odd number of errors: E(x) has an odd number of terms: lemma: if E(x) has an odd number of terms, E(x) cannot

be divisible by (x+1)• suppose E(x) = (x+1)F(x), let x=1, the left hand will be 1, while

the right hand will be 0 thus if G(x) contains x+1 as a factor, E(x) will not be

divided by G(x)

Many more errors can be detected by designing the right G(x)

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Example G(x)

16 bits CRC: CRC-16: x16+x15+x2+1,

CRC-CCITT: x16+x12+x5+1 both can catch

• all single or double bit errors• all odd number of bit errors• all burst errors of length 16

or less• >99.99% of the 17 or 18 bits

burst errors

CRC-16 hardware implementationUsing shift and XOR registers

http://en.wikipedia.org/wiki/CRC-32#Implementation

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Example G(x) 32 bits CRC:

CRC32: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

used by Ethernet, FDDI, PKZIP, WinZip, and PNG GSM phones

For more details see the link below and further links it contains: http://en.wikipedia.org/wiki/Cyclic_redundancy_check

                      .

Outline

Admin Link layer overview Error detection Link access

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Multiple Access Links and Protocols

Two types of “links”: point-to-point

e.g., a leased dedicated line, PPP for dial-up access

broadcast (shared wire or medium) traditional Ethernet 802.11 wireless LAN satellite

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Multiple Access Protocols Single shared broadcast channel

thus, if two or more simultaneous transmissions by nodes, due to interference, only one node can send successfully at a time (see CDMA later for an exception)

multiple access protocol Protocol that determines how nodes share

channel, i.e., determines when nodes can transmit Communication about channel sharing must use

channel itself !

Discussion: properties of an ideal multiple access protocol.

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Ideal Mulitple Access Protocol

Broadcast channel of rate R bps- Efficiency: when one node wants to transmit, it

can send at rate R

- Fairness: when N nodes want to transmit, each can send at average rate R/N

- Decentralized: no special node to coordinate transmissions no synchronization of clocks

- Simple

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MAC Protocols: a Taxonomy

Goals efficient, fair, decentralized, simple

Three broad classes: channel partitioning

divide channel into smaller “pieces” (time slot, frequency, code)

Non-partitioning random access

• allow collisions “taking-turns”

• a token coordinates shared access to avoid collisions

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Outline

Admin. and recap Link layer overview Error detection and correction Media access control (MAC) protocols

channel partitioning

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Channel Partitioning: TDMA

TDMA: time division multiple access Access 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, 1,3,4 have pkt, slots

2,5,6 idle

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Channel Partitioning: 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 bands 2,5,6 idle

frequ

ency

bands time

5

1

4

3

2

6

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1 2 3 4 5 6 7 8

935-960 MHz124 channels (200 kHz)downlink

890-915 MHz124 channels (200 kHz)uplink

frequ

ency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA

S: indicates data or control

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Channel Partitioning: CDMA

CDMA (Code Division Multiple Access) Used mostly in wireless broadcast channels

(cellular, satellite, etc) A spread-spectrum technique

Example: Sprint , Verizon 3G802.11

History: http://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/hedy/lemarr.htm

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CDMA: Encoding

All users share same frequency, but each user m has its own unique “chipping” sequence (i.e., code) cm to encode data, i.e., code set partitioning e.g. cm = 1 1 1 -1 1 -1 -1 -1

Assume original data are represented by 1 and -1

Encoded signal = (original data) modulated by (chipping sequence) assume cm = 1 1 1 -1 1 -1 -1 -1

if data is d, send d cm, • if data d is 1, send cm

• if data d is -1 send -cm

CDMA: Encoding

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user data d(t)

chipping sequence c(t)

resultingsignal

1 -1

-1 1 1 -1 1 -1 1 -11 -1 -1 1 11

X

=

tb

tc

tb: bit periodtc: chip period

-1 1 1 -1 -1 1 -1 11 -1 1 -1 -11

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CDMA: Decoding

Inner-product (summation of bit-by-bit product) of encoded signal and chipping sequence if inner-product > 0, the data is 1; else -1

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CDMA Encode/Decode

Code of user m cm: 1 1 1 -1 1 -1 -1 -1

- The number of bitsof each chipping sequence is M

Encode

Decode

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CDMA: Deal with Multiple-User Interference

Two codes Ci and Cj are orthogonal, if , where we use “.” to denote inner

product, e.g.

If codes are orthogonal, multiple users can “coexist” and transmit simultaneously with minimal interference:

iiij

jj cdccd )(

0 ij cc

C1: 1 1 1 -1 1 -1 -1 -1 C2: 1 -1 1 1 1 -1 1 1-----------------------------------------C1 . C2 = 1 +(-1) + 1 + (-1) +1 + 1+ (-1)+(-1)=0

Analogy: Speak in different languages!

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CDMA: Two-Sender Interference

Code 1: 1 1 1 -1 1 -1 -1 -1Code 2: 1 -1 1 1 1 -1 1 1

Discussions

Advantages of channel partitioning

Problems of channel partitioning

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Backup Slides

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Backup: IP Multicast

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IP Fragmentation & Reassembly Network links have MTU

(max.transfer size) - largest possible link-level frame. different link types,

different MTUs, e.g. Ethernet MTU is 1500 bytes

Large IP datagram divided (“fragmented”) one datagram

becomes several datagrams

“reassembled” only at final destination

IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

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IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=1480

fragflag=1

length=1500

ID=x

offset=2960

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte

datagram MTU = 1500

bytes

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IP Multicast: Service Model

Multicast group concept: use of indirection A group is identified by a location-independent

logical address (class D IP address: prefix 1110) Open group model

Anyone can send packets to the “logical” group address Anyone can join a group and receive packets

Normal, best-effort delivery semantics of IP

128.119.40.186

128.59.16.12

128.34.108.63

128.34.108.60

multicast group

226.17.30.197

Needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group

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Multicast Across LANs

shared tree source-based trees

Goal: find a tree (or trees) connecting routers having local mcast group members source-based: different tree from sender to each receiver

– Distance-vector multicast routing protocol (DVMRP)– Protocol-independent multicast-dense mode (PIM-DM)

shared-tree: same tree used by all group members– Core-Based Tree (CBT)– Protocol-independent multicast-sparse mode (PIM-SM)

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Source Tree: Reverse Path Flooding (RPF)

A router x forwards a packet from source (S) iff it arrives via neighbor y, and y is on the shortest path from x back to S

A packet is replicated to all but the incoming interface

xxyy

tt

SS

a

zz

1

1

1

1

1

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Reverse Path Forwarding: Improvement Basic idea: forward a packet from S only

on child links for S A child link of router x for source S

a link that has x as parent on the shortest path from thelink to S

a child x notifies its parent y(through the routing protocol)that it has selected y as itsparent

xxyy

tt

SS

a

zz

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Reverse Path Forwarding: Pruning No need to forward datagrams down

subtree with no mcast group members

“prune” msgs sent upstream by router with no downstream group members

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

prune message

LEGENDS: source

links with multicastforwarding

P

P

P

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Pruning

Prune (Source, Group) at a leaf router if no members send No-Membership Report (NMR) up tree

If all children of router R prune (S,G) propagate prune for (S,G) to its parent

What do you do when a member of a group (re)joins? send a Graft message to upstream parent

How to deal with failures? prune dropped flow is reinstated down stream routers re-prune

Note: again a soft-state approach

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Implementation of Source Trees in the Internet

Multicast OSFP (MOSFP) Membership is part of the link state distribution;

calculate source specific, pre-pruned trees

Reverse Path Forwarding Distance Vector Multicast Routing Protocol (DVMRP) Protocol Independent Multicast – Dense Mode (PIM-DM)

• very similar to DVMRP

Difference: PIM uses any unicast routing algorithm to determine the path from a router to the source; DVMRP uses distance vector

Question: the state requirement of Reverse Path Forwarding

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Building a Shared Tree

Steiner Tree: minimum cost tree connecting all routerswith attached group members

A Steiner tree is not a spanning tree because you do not need to connect all nodes in the network

Problem is NP-hard Excellent heuristics exists Not used in practice:

computational complexity information about entire network needed monolithic: rerun whenever a router needs to join/leave

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Center (Core) based Shared Tree

Single delivery tree shared by all One router identified as “center” of tree Tree construction is receiver-based

edge router sends unicast join-msg addressed to center router

join-msg “processed” by intermediate routers and forwarded towards center

join-msg either hits existing tree branch for this center, or arrives at center

path taken by join-msg becomes new branch of tree for this router

A sender unicasts a packet to center The packet is distributed on the tree when it hits the

tree

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Example: M3 Joins

Group members: M1, M2

core

M1

M2 M3

shared tree

S1join message

Discussion: what is property of the constructed tree?

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Example: M1 Sends Data Group members: M1, M2, M3 M1 sends data

core

M1

M2 M3

control (join) messagesdata S1

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Shared Tree Protocols in the Internet

Core Based Tree Protocol Independent Multicast (PIM)

Sparse mode The catch: how do you know the center?

session announcement

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Mbone: Tunneling

Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?

mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram

normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router

receiving mcast router unencapsulates to get mcast datagram

physical topology logical topology