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LINK STATE PROTOCOLS(contents)

Disadvantages of the distance vector protocolsLink state protocolsWhy is a link state protocol better?

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Disadvantages of the Distance Vector Protocols

Slow convergenceCounting to infinity problemLack of variety of metricsNo possibility of hierarchical routingBad performance in large networks

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Link State Protocols

developed to overcome the disadvantages of the distance vector protocolscentralized database describes the topology of the whole networkcalculation and routing are still distributed

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Link State Routing Algorithm

1. Each router is responsible for meeting its neighbors and learning their names.

2. Each router constructs a link state packet (LSP) which consists of a list of names and cost for each of its neighbors.

3. The LSP is transmitted to all other routers. Each router stores the most recently generated LSP from each other router.

4. Each router uses complete information on the network topology to compute the shortest path route to each destination node.

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Characteristics

information about adjacencies sent to all routers only when there is a changeeach router maintains an identical databasea “shortest path” algorithm is used to find the best routeconverge as quickly as databases can be updated

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Link State Database

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From To Link CostA B 1 1 2 A D 3 1 2B A 1 1 2B C 2 1 1B E 4 1 2C B 2 1 1C E 5 1 1D A 3 1 2D E 6 1 1E B 4 1 2E C 5 1 1E D 6 1 1

From A to B, link 1, distance = 1 number = 2

Link State Announcement (LSA)

LSseq.num.

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The Flooding Protocol

every node sends the message on every link except the one from where it received the message

very fast and very reliable, but wastes bandwidth

used for ordinary traffic in military networksmessages are sent only when there is a change or every 45 minutes

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Securing the Map Updatesthe flooding procedure includes hop-by-hop acknowledgmentsthe database description packets are transmitted in a secure fashioneach link state record is protected by a timer and is removed from the database if a refreshing packet does not arrive in due timeall records are protected by checksum

the messages can be authenticated, e. g. by passwords

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Dijkstra’s Algorithm

• Actual implementation in routers called “Forward Search”• Builds up a list of “Tentative” and “Confirmed” paths• Automatically prunes out longer paths

Average time needed to compute the routing table is about 200ms fora 200 node network on a typical router.

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Why Is a Link State protocol Better?

fast loopless convergencesupport of precise metrics and, if needed multiple metricssupport of a multiple paths to a destinationsplitting very large networks in areas

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Link State ProtocolsDisadvantages

more memory requiredthe link state database is needed in addition to the routing tables

much more complex procedurehigher probability for a bug in the program

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Comparison of LS and DV algorithms

Message complexityLS: with n nodes, E links, O(nE) msgs sent each DV: exchange between neighbors only

convergence time varies

Speed of ConvergenceLS: O(n2) algorithm requires O(nE) msgs

may have oscillationsDV: convergence time varies

may be routing loopscount-to-infinity problem

Robustness: what happens if router malfunctions?

LS: node can advertise incorrect link costeach node computes only its own table

DV:DV node can advertise incorrect path costeach node’s table used by others

error propagate thru network

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Path-vector paradigmA variant of distance-vector Each router maintains its path to a destinationMethod of choosing the path

based on available paths through neighbors, choose the one of highest “preference” (not necessarily the shortest)

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