3rd Edition: Chapter 3

Introduction

Lecture 19

Network Layer

(Routing Protocols)

slides are modified from J. Kurose & K. Ross

University of Nevada – Reno

Computer Science & Engineering Department

Fall 2011

CPE 400 / 600Computer Communication Networks

Network Layer

4-2

Comparison of LS and DV algorithms

Message complexity

rLS: with n nodes, E links,O(nE) msgs sent

rDV: exchange betweenneighbors only

mconvergence time varies

Speed of Convergence

rLS: O(n2) algorithm requiresO(nE) msgs

mmay have oscillations

rDV: convergence time varies

mmay be routing loops

mcount-to-infinity problem

Robustness: what happensif router malfunctions?

LS:

mnode can advertiseincorrect link cost

meach node computes onlyits own table

DV:

mDV node can advertiseincorrect path cost

meach node’s table used byothers

•error propagate thrunetwork

Network Layer

4-3

Chapter 4: Network Layer

r4. 1 Introduction

r4.2 Virtual circuit anddatagram networks

r4.3 What’s inside arouter

r4.4 IP: InternetProtocol

mDatagram format

mIPv4 addressing

mICMP

mIPv6

r4.5 Routing algorithms

mLink state

mDistance Vector

mHierarchical routing

r4.6 Routing in theInternet

mRIP

mOSPF

mBGP

r4.7 Broadcast andmulticast routing

Network Layer

4-4

Hierarchical Routing

scale: with 200 milliondestinations:

rcan’t store all dest’s inrouting tables!

rrouting table exchangewould swamp links!

administrative autonomy

rInternet = network of networks

reach network admin may wantto control routing in its ownnetwork

Our routing study thus far - idealization

rall routers identical

rnetwork “flat”

… not true in practice

Network Layer

4-5

Hierarchical Routing

raggregate routers intoregions, “AutonomousSystems” (AS)

rrouters in same AS runsame routing protocol

m“intra-AS” routing protocol

mrouters in different AS canrun different intra-ASrouting protocol

Gateway router

rDirect link to router inanother AS

Network Layer

4-6

AS3

AS1

AS2

Intra-AS

Routing

algorithm

Inter-AS

Routing

algorithm

Forwarding

table

Interconnected ASes

rforwarding table configuredby both intra- and inter-ASrouting algorithm

mintra-AS sets entries forinternal dests

minter-AS & intra-As setsentries for external dests

Network Layer

4-7

AS3

AS1

AS2

Inter-AS tasks

rsuppose router in AS1receives datagramdestined outside of AS1:

mrouter should forwardpacket to gatewayrouter, but which one?

AS1 must:

1.learn which dests arereachable through AS2,which through AS3

2.propagate thisreachability info to allrouters in AS1

Job of inter-AS routing!

Network Layer

4-8

Example: Setting forwarding table in router 1d

rsuppose AS1 learns (via inter-AS protocol) that subnetx reachable via AS3 (gateway 1c) but not via AS2.

rinter-AS protocol propagates reachability info to allinternal routers.

rrouter 1d determines from intra-AS routing info thatits interface I is on the least cost path to 1c.

minstalls forwarding table entry (x,I)

AS3

AS1

AS2

…

Network Layer

4-9

Example: Choosing among multiple ASes

rnow suppose AS1 learns from inter-AS protocol thatsubnet x is reachable from AS3 and from AS2.

rto configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

mthis is also job of inter-AS routing protocol!

AS3

AS1

AS2

…

Network Layer

4-10

Learn from inter-AS

protocol that subnet

x is reachable via

multiple gateways

Use routing info

from intra-AS

protocol to determine

costs of least-cost

paths to each

of the gateways

Hot potato routing:

Choose the gateway

that has the

smallest least cost

Determine from

forwarding table the

interface I that leads

to least-cost gateway.

Enter (x,I) in

forwarding table

Example: Choosing among multiple ASes

rnow suppose AS1 learns from inter-AS protocol thatsubnet x is reachable from AS3 and from AS2.

rto configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

mthis is also job of inter-AS routing protocol!

rhot potato routing: send packet towards closest oftwo routers.

Network Layer

4-11

Chapter 4: Network Layer

r4. 1 Introduction

r4.2 Virtual circuit anddatagram networks

r4.3 What’s inside arouter

r4.4 IP: InternetProtocol

mDatagram format

mIPv4 addressing

mICMP

mIPv6

r4.5 Routing algorithms

mLink state

mDistance Vector

mHierarchical routing

r4.6 Routing in theInternet

mRIP

mOSPF

mBGP

r4.7 Broadcast andmulticast routing

Network Layer

4-12

RIP (Routing Information Protocol)

rdistance vector algorithm

rincluded in BSD-UNIX Distribution in 1982

rdistance metric: # of hops (max = 15 hops)

destination hops

u 1

v 2

w 2

x 3

y 3

z 2

From router A to subnets:

Network Layer

4-13

RIP advertisements

rdistance vectors: exchanged amongneighbors every 30 sec via ResponseMessage

malso called advertisement

reach advertisement: list of up to 25destination subnets within AS

Network Layer

4-14

RIP: Example

Destination Network Next Router Num. of hops to dest.

wA2

yB2

zB7

x--1

….….....

Routing/Forwarding table in D

Network Layer

4-15

RIP: Example

Destination Network Next Router Num. of hops to dest.

wA2

yB2

zB A7 5

x--1

….….....

Routing/Forwarding table in D

Dest Next hops

w - 1

x - 1

z C 4

…. … ...

from A to D

Network Layer

4-16

RIP: Link Failure and Recovery

If no advertisement heard after 180 sec -->neighbor/link declared dead

mroutes via neighbor invalidated

mnew advertisements sent to neighbors

mneighbors in turn send out new advertisements iftables changed

mlink failure info propagates to entire net

mpoison reverse used to prevent ping-pong loops

•infinite distance = 16 hops

Network Layer

4-17

RIP Table processing

rRIP routing tables managed by application-levelprocess called route-d (daemon)

radvertisements sent in UDP packets, periodicallyrepeated

physical

link

network forwarding

(IP) table

Transprt

(UDP)

routed

physical

link

network

(IP)

Transprt

(UDP)

routed

forwarding

table

Network Layer

4-18

Chapter 4: Network Layer

r4. 1 Introduction

r4.2 Virtual circuit anddatagram networks

r4.3 What’s inside arouter

r4.4 IP: InternetProtocol

mDatagram format

mIPv4 addressing

mICMP

mIPv6

r4.5 Routing algorithms

mLink state

mDistance Vector

mHierarchical routing

r4.6 Routing in theInternet

mRIP

mOSPF

mBGP

r4.7 Broadcast andmulticast routing

Network Layer

4-19

OSPF (Open Shortest Path First)

r“open”: publicly available

ruses Link State algorithm

mLS packet dissemination

mtopology map at each node

mroute computation using Dijkstra’s algorithm

rOSPF advertisement carries one entry per neighborrouter

radvertisements disseminated to entire AS (viaflooding)

mOSPF messages directly over IP (rather than TCP or UDP)

Network Layer

4-20

OSPF “advanced” features (not in RIP)

rsecurity: all OSPF messages authenticated

mto prevent malicious intrusion

rmultiple same-cost paths allowed

monly one path in RIP

rFor each link, multiple cost metrics for different TOS

me.g., satellite link cost set “low” for best effort; high for realtime

rintegrated uni- and multicast support:

mMulticast OSPF (MOSPF) uses same topology database as OSPF

rhierarchical OSPF in large domains

Network Layer

4-21

Hierarchical OSPF

Network Layer

4-22

Hierarchical OSPF

rtwo-level hierarchy: local area, backbone

mLink-state advertisements only in area

meach nodes has detailed area topology

•only know direction (shortest path) to nets in other areas

rarea border routers: “summarize” distances to netsin own area, advertise to other Area Border routers

rbackbone routers: run OSPF routing limited tobackbone

rboundary routers: connect to other AS’s

Network Layer

4-23

Chapter 4: Network Layer

r4. 1 Introduction

r4.2 Virtual circuit anddatagram networks

r4.3 What’s inside arouter

r4.4 IP: InternetProtocol

mDatagram format

mIPv4 addressing

mICMP

mIPv6

r4.5 Routing algorithms

mLink state

mDistance Vector

mHierarchical routing

r4.6 Routing in theInternet

mRIP

mOSPF

mBGP

r4.7 Broadcast andmulticast routing

Network Layer

4-24

Internet inter-AS routing: BGP

rBGP (Border Gateway Protocol):

mthe de facto standard

rallows subnet to advertise its existence torest of Internet: “I am here”

rBGP provides each AS a means to:

1.Obtain subnet reachability information fromneighboring ASs

2.Propagate reachability information to all AS-internal routers

3.Determine “good” routes to subnets based onreachability information and policy

Network Layer

4-25

BGP basics

rpairs of routers (BGP peers) exchange routing infoover semi-permanent TCP connections: BGP sessions

mBGP sessions need not correspond to physical links

rwhen AS2 advertises a prefix to AS1:

mAS2 promises it will forward datagrams towardsthat prefix

mAS2 can aggregate prefixes in its advertisement

AS3

AS1

AS2

eBGP session

iBGP session

Network Layer

4-26

Distributing reachability info

rusing eBGP session between 3a and 1c, AS3 sendsprefix reachability info to AS1

m1c can then use iBGP do distribute new prefix infoto all routers in AS1

m1b can then re-advertise new reachability info toAS2 over 1b-to-2a eBGP session

rwhen router learns of new prefix, it creates entry forprefix in its forwarding table

AS3

AS1

AS2

eBGP session

iBGP session

Network Layer

4-27

Path attributes & BGP routes

radvertised prefix includes BGP attributes

mprefix + attributes = “route”

rtwo important attributes:

mAS-PATH: contains ASs through which prefixadvertisement has passed: e.g, AS 67, AS 17

mNEXT-HOP: indicates specific internal-AS routerto next-hop AS

•may be multiple links from current AS to next-hop-AS

rwhen gateway router receives route advert.,uses import policy to accept/decline

Network Layer

4-28

BGP route selection

rrouter may learn about more than 1 routeto some prefix

mRouter must select route

relimination rules:

1.local preference value attribute: policy decision

2.shortest AS-PATH

3.closest NEXT-HOP router: hot potato routing

4.additional criteria

Network Layer

4-29

BGP messages

rBGP messages exchanged using TCP

rBGP messages:

mOPEN: opens TCP connection to peer andauthenticates sender

mUPDATE: advertises new path (or withdraws old)

mKEEPALIVE keeps connection alive in absence ofUPDATES; also ACKs OPEN request

mNOTIFICATION: reports errors in previous msg;also used to close connection

Network Layer

4-30

BGP routing policy

rA,B,C are provider networks

rX,W,Y are customer (of provider networks)

rX is dual-homed: attached to two networks

mX does not want to route from B via X to C

m.. so X will not advertise to B a route to C

legend:

customer

network

provider

network

Network Layer

4-31

BGP routing policy (2)

rA advertises path AW to B

rB advertises path BAW to X

rShould B advertise path BAW to C?

mNo way! B gets no “revenue” for routing CBAWsince neither W nor C are B’s customers

mB wants to force C to route to w via A

mB wants to route only to/from its customers!

legend:

customer

network

provider

network

Network Layer

4-32

Why different Intra- and Inter-AS routing ?

Policy:

rInter-AS: admin wants control over how its trafficrouted, who routes through its net

rIntra-AS: single admin, so no policy decisions needed

Scale:

rhierarchical routing saves table size and reducesupdate traffic

Performance:

rInter-AS: policy may dominate over performance

rIntra-AS: can focus on performance