CSE 123ACSE 123AComputer NetworksComputer Networks

Winter 2005Winter 2005

Lecture 9Lecture 9Internet Routing: Internet Routing:

Overview and Distance Vector AlgorithmsOverview and Distance Vector Algorithms

Some portions courtesy David Wetherall

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 2

Last classLast classHow to build Big/Huge Fast Routers

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 3

This classThis classNew topic: routing

How do I get there from here?

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 4

OverviewOverviewIntro & Design choices

Intra-domain routingDistance vectorLink state

Inter-domain routingPolicy

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IntraIntra--domain routingdomain routingRouting within a network/organizationA single administrative domain

Overall goalsProvide intra-network connectivityAdapt quickly to failures or topology changesOptimize use of network resources

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IntraIntra--domain routing:domain routing:Problem statementProblem statement

Routing is essentially a problem in graph theory

Network is a directed graph G=(V,E)Routers are vertices, links are edges labeled with some metric

For simplicity ignore hosts, they are part of each VFor each V, find the shortest path to every other V

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Find “best” path between every pair of vertices

D

G

A

F

E

B

C=router

=link

Network as a GraphNetwork as a Graph

X

11

1

11

1

1

11 =cost

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 8

Routing QuestionsRouting QuestionsHow to choose best path?

Defining “best” can be slippery

How to scale to millions of users?Minimize control messages and routing table size

How to adapt to failures or changes?Node and link failures, plus message loss

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 9

Recap: What does a router do?Recap: What does a router do?

ForwardingMove packet from input link to the appropriate output linkPurely local computation Must go be very fast (executed for ever packet)

RoutingDoing work so you’re sure that the “next hop” actually leads to the destinationGlobal decisions; distributed computation and communicationCan go slower (only important when topology changes)

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WhatWhat’’s in a Routing Table?s in a Routing Table?The routing table at A, lists – at a minimum –the next hops for the different destinations

D

G

A

F

E

B

C

FFFG

EECDCCBB

Next HopDest

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Kinds of routing/forwardingKinds of routing/forwardingSource routing

Complete path in packetVirtual circuits

Set up path out-of-band and store path identifier in routersLocal path identifier in packet

Destination-based forwardingRouter looks up address in forwarding tableForwarding table contains (address, next-hop) tuples

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Source routingSource routingRouting

Host computes path Must know global topology and detect failuresPacket contains complete ordered path information

» I.e. node A then D then X then J…Requires variable length path header

ForwardingRouter looks up next hop in packet header, strips it off and forwards remaining packetVery quick forwarding, no lookup requiredVery flexible

In practice ad hoc networks (DSR), SANs (Myrinet), and for debugging on the Internet (LSR,SSR)

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Virtual circuitsVirtual circuitsRouting

Hosts sets up path out-of-band, requires connection setupWrite (input id, output id, next hop) into each router on pathFlexible (one path per flow)

ForwardingSend packet with path idRouter looks up input, swaps for output, forwards on next hopRepeat until reach destinationTable lookup for forwarding (why faster than IP lookup?)

In practiceATM: fixed VC identifiers and separate signaling codeMPLS: ATM meets the IP world (why? traffic engineering)

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DestinationDestination--based routingbased routingRouting

All addresses are globally known» No connection setup

Host sends packet with destination address in header» No path state; only routers need to worry about failure

Distributed routing protocol used to routing tablesForwarding

Router looks up destination in table» Must keep state proportional to destinations rather than

connections(Address, next-hop) tupleLookup address, send packet to next-hop link

» All packets follow same path to destinationIn Practice: IP routing

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Three approaches to IP routingThree approaches to IP routing

StaticType in the right answers and hope they are always true

Distance vectorTell your neighbors when you know about everyone

Link stateTell everyone what you know about your neighbors

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Distance Vector routingDistance Vector routingAssume

Each router knows own address & cost to reach neighbors

GoalCalculate routing table containing next-hop information for every destination at each router

Distributed Bellman-Ford algorithmEach router maintains a vector of costs to all destinations

» Initialize neighbors with known cost, others with infinityPeriodically send copy of distance vector to neighborsOn reception of a vector

» If neighbor’s path to a destination is shorter, switch to it

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Initial conditionsInitial conditions

1

7

8

2

2

1

A

E

B C

D

02∞81E202∞∞D∞201∞C8∞107B1∞∞70AEDCBA

Distance to NodeInfo at node

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E receives DE receives D’’s vectors vector

1

7

8

2

2

1

A

E

B C

D

02481E202∞∞D∞201∞C8∞107B1∞∞70AEDCBA

Distance to NodeInfo at node

I’m 2 from C, 0 from D and 2 from E

D is 2 away, 2+2< ∞, so best path to C is 4

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A receives BA receives B’’s vectors vector

1

7

8

2

2

1

A

E

B C

D

02481E202∞∞D∞201∞C8∞107B1∞870AEDCBA

Distance to NodeInfo at node

I’m 7 from A, 0 from B, 1 from X &

8 from D

B is 7 away, 1+7< ∞so best path to C is 8

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A receives EA receives E’’s vectors vector

1

7

8

2

2

1

A

E

B C

D

02481E202∞∞D∞201∞C8∞107B13570AEDCBA

Distance to NodeInfo at node

I’m 1 from A, 8 from B, 4 from C, 2 from D & 0 from E

E is 1 away, 4+1<8 so C is 5 away, 1+2< ∞

so D is 3 away

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 21

Final stateFinal state

1

7

8

2

2

1

A

E

B C

D

02451E20233D42015C53106B13560AEDCBA

Distance to NodeInfo at node

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View from a node (B)View from a node (B)

1

7

8

2

2

1

A

E

B C

D

588E31010D11212C697ACEA

Next hopDest

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Link failureLink failure

1

7

8

2

2

1

A

E

B C

D

0104512E20233D42018C53107B

1210870AEDCBA

Distance to NodeInfo at

node

• A marks distance to E as ∞ , and tells B• E marks distance to A as ∞ , and tells B and D• B and D recompute routes and tell C, E and E• etc… until converge

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Problems: Problems: Count to InfinityCount to Infinity

1A CB

23 2

1A CB3 4

Update 3

1A CB

Update4

5 4

Etc…

Distance to C

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Why?Why?Updates don’t contain enough information

Can’t totally order bad news above good news

B’s accepts A’s path to C that is implicitlythrough B!

Aside: this also causes delays in convergence

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SolutionsSolutionsHold downs

As metric increases, delay propagating informationLimitation: Delays convergence

Split horizonNever advertise a destination through its next hop

» A doesn’t advertise C to BPoison reverse: Send negative information when advertising a destination through its next hop

» A advertises C to B with a metric of ∞Limitation: Only works for “loop”s of size 2

Loop avoidanceFull path information in route advertisementExplicit queries for loops (e.g. DUAL)

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How split horizon/How split horizon/pvpv failsfails

1

A C

B

D

1

1

• A tells B & C that D is unreachable

• B tells C that D is unreachable

• B tells A that D is reachable with cost=3 (since route is through C, split horizon doesn’t apply)

• A tells C that D is reachable through A (cost=4)

• Etc…

1

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Other issuesOther issuesWhen to send route updates?Periodically

Limits granularity of failure recoveryGlobal synchronization can cause packet loss

JitteredRandom offset from periodic deals with synchronization problem

TriggeredSend updates immediately when metric changesConverges more quickly, but causes flood of packets

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Routing Information Protocol Routing Information Protocol (RIP)(RIP)

DV protocol with hop count as metricInfinity value is 16 hops; limits network sizeIncludes split horizon with poison reverse

Routers send vectors every 30 secondsWith triggered updates for link failuresTime-out in 180 seconds to detect failures

RIPv1 specified in RFC1058www.ietf.org/rfc/rfc1058.txt

RIPv2 (adds authentication etc.) in RFC1388www.ietf.org/rfc/rfc1388.txt

February 2, 2005 CSE 123A -- Lecture 8 – Internet Routing: Overview and Distance Vector Algorithms 30

Key ConceptsKey ConceptsRouting is a global process, forwarding is local oneThe Distance Vector algorithm and RIP

Simple and distributed exchange of shortest paths.Weak at adapting to changes (loops, count to infinity)

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For next timeFor next time……No new reading… although review the section about link state routing since that’s what we’ll be talking about