Routing Algorithms in Telecommunications: Distance Vector and Link-State, Slides of Computer Science

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Routing

An Engineering Approach to Computer Networking An Engineering Approach to Computer Networking

What is it? nn

Process of finding a path from a source to every destination inProcess of finding a path from a source to every destination inthe networkthe network

n n

Suppose you want to connect to Antarctica from your desktopSuppose you want to connect to Antarctica from your desktop

u u

what route should you take? what route should you take?

uu

does a shorter route exist? does a shorter route exist?

uu

what if a link along the route goes down? what if a link along the route goes down?

u u

what if you’re on a mobile wireless link? what if you’re on a mobile wireless link?

n n

Routing deals with these types of issuesRouting deals with these types of issues

Key problem nn

How to make correct local decisions?How to make correct local decisions?

u u

each router must know each router must know

something something about global state

n n

Global stateGlobal state

uu

inherently large inherently large

uu

dynamic dynamic

u u

hard to collect hard to collect

n n

A routing protocol must intelligently summarize relevant A routing protocol must intelligently summarize relevantinformationinformation

Requirements nn

Minimize routing table spaceMinimize routing table space

u u

fast to look up fast to look up

u u

less to exchange less to exchange

nn

Minimize number and frequency of control messagesMinimize number and frequency of control messages

nn

Robustness: avoidRobustness: avoid

u u

black holes black holes

u u

loops loops

uu

oscillations oscillations

nn

Use optimal pathUse optimal path

Outline nn

Routing in telephone networksRouting in telephone networks

n n

Distance-vector routingDistance-vector routing

n n

Link-state routingLink-state routing

nn

Choosing link costsChoosing link costs

nn

Hierarchical routingHierarchical routing

n n

Internet routing protocolsInternet routing protocols

n n

Routing within a broadcast LANRouting within a broadcast LAN

nn

Multicast routingMulticast routing

nn

Routing with policy constraintsRouting with policy constraints

n n

Routing for mobile hostsRouting for mobile hosts

Telephone network topology nn

3-level hierarchy, with a fully-connected core3-level hierarchy, with a fully-connected core

nn

AT&T: 135 core switches with nearly 5 million circuitsAT&T: 135 core switches with nearly 5 million circuits

n n

LECs may connect to multiple coresLECs may connect to multiple cores

Features of telephone network routing nn

Stable loadStable load

u u

can predict pairwise load throughout the day can predict pairwise load throughout the day

u u

can choose optimal routes in advance can choose optimal routes in advance

nn

Extremely reliable switchesExtremely reliable switches

uu

downtime is less than a few minutes per year downtime is less than a few minutes per year

u u

can assume that a chosen route is available can assume that a chosen route is available

u u

can’t do this in the Internet can’t do this in the Internet

nn

Single organization controls entire coreSingle organization controls entire core

uu

can collect global statistics and implement global changes can collect global statistics and implement global changes

n n

Very highly connected networkVery highly connected network

n n

Connections require resources (but all need the same)Connections require resources (but all need the same)

The cost of simplicity nn

Simplicity of routing a historical necessitySimplicity of routing a historical necessity

n n

But requiresBut requires

u u

reliability in every component reliability in every component

uu

logically fully-connected core logically fully-connected core

nn

Can we build an alternative that has same features as theCan we build an alternative that has same features as thetelephone network, but is cheaper because it uses moretelephone network, but is cheaper because it uses moresophisticated routing?sophisticated routing?

u u

Yes: that is one of the motivations for ATM Yes: that is one of the motivations for ATM

u u

But 80% of the cost is in the local loop But 80% of the cost is in the local loop

FF

not affected by changes in core routingnot affected by changes in core routing

uu

Moreover, many of the software systems assume topology Moreover, many of the software systems assume topology

F F

too expensive to change themtoo expensive to change them

Metastability n n

Burst of activity can cause network to enter metastable stateBurst of activity can cause network to enter metastable state

u u

high blocking probability even with a low load high blocking probability even with a low load

nn

Removed by trunk reservationRemoved by trunk reservation

uu

prevents spilled traffic from taking over direct path prevents spilled traffic from taking over direct path

Trunk status map routing (TSMR) nn

DNHR measures traffic once a weekDNHR measures traffic once a week

n n

TSMR updates measurements once an hour or soTSMR updates measurements once an hour or so

u u

only if it changes “significantly” only if it changes “significantly”

nn

List of alternative paths is more up to dateList of alternative paths is more up to date

Outline nn

Routing in telephone networksRouting in telephone networks

n n

Distance-vector routingDistance-vector routing

n n

Link-state routingLink-state routing

nn

Choosing link costsChoosing link costs

nn

Hierarchical routingHierarchical routing

n n

Internet routing protocolsInternet routing protocols

n n

Routing within a broadcast LANRouting within a broadcast LAN

nn

Multicast routingMulticast routing

nn

Routing with policy constraintsRouting with policy constraints

n n

Routing for mobile hostsRouting for mobile hosts

Distance vector routing nn

EnvironmentEnvironment

u u

links and routers unreliable links and routers unreliable

u u

alternative paths scarce alternative paths scarce

uu

traffic patterns can change rapidly traffic patterns can change rapidly

nn

Two key algorithmsTwo key algorithms

u u

distance vector distance vector

u u

link-state link-state

nn

Both assume router knowsBoth assume router knows

uu

address of each neighbor address of each neighbor

u u

cost of reaching each neighbor cost of reaching each neighbor

n n

Both allow a router to determine global routing information byBoth allow a router to determine global routing information bytalking to its neighborstalking to its neighbors

Example

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‘

2

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Why does it work nn

Each node knows its true cost to its neighborsEach node knows its true cost to its neighbors

n n

This information is spread to its neighbors the first time it sendsThis information is spread to its neighbors the first time it sendsout its distance vectorout its distance vector

n n

Each subsequent dissemination spreads the truth one hopEach subsequent dissemination spreads the truth one hop

nn

Eventually, it is incorporated into routing table everywhere in theEventually, it is incorporated into routing table everywhere in thenetworknetwork

nn

Proof: Bellman and Ford, 1957Proof: Bellman and Ford, 1957