4 NetworkLayer PartI

8/12/2019 4 NetworkLayer PartI

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Part 4: Network LayerPart A: Algorithms


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Summary

1) The Problem

2) The Dijkstras Shortest Path Algorithm

3) Distance Vector Routing

4) Link State Routing5) Hierarchical Routing

6) Routing in Ad Hoc Networks


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1. The Problem (1)

Store-and-Forward Packet SwitchingServices Provided to the Transport Layer

Connectionless Service

Connection-Oriented Service


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1. The Problem (2): Packet Switching

The environment of the network layer protocols.

fig 5-1


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1. The Problem (3): Connectionless Service

Routing within a diagram subnet.


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1. The Problem (4): Connection-Oriented

Service

Routing within a virtual-circuit subnet.


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1. The Problem (5): Connectionless VS.

Connection-Oriented


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2 Shortest Path Algorithm

The first 5 steps used in computing the shortest path from A to D.

The arrows indicate the working node.


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3 Distance Vector Routing (1)

(a)A subnet. (b)Input from A, I, H, K, and the new

routing table for J.


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3 Distance Vector Routing (2)

The count-to-infinity problem.


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3 Distance Vector Routing (3): Loop-BreakingHeuristics

Set infinity to a limited number, e.g. 16.

Split horizon

Split horizon with poison reverse


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Initialization

3 Example:


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Direct

Neighbours


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Neighboursof neighbours


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Neighbours

of neighbours

of neighbours


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Stableconvergence


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Good news:

A new link!


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Directendpoints

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Neighboursknow


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Neighbours

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A happy andstable network


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Bad news:

Link crash!!


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Direct

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Get help

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Routing loop

(due toinconsistent

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Countingto infinity


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Stabilityagain


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4 Link State Routing (1)

Each router must do the following:

Discover its neighbors, learn their network address.

Measure the delay or cost to each of its neighbors.

Construct a packet telling all it has just learned.

Send this packet to all other routers.

Compute the shortest path to every other router.

4 Li k St t R ti (2) L i b t


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4 Link State Routing (2): Learning about

the Neighbors

(a) Nine routers and a LAN.

(b) A graph model of (a).

4 Li k St t R ti (3) M i Li


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4 Link State Routing (3): Measuring Line

Cost

A subnet in which the East and West parts are connected by two

lines.

4 Li k St t R ti (4) B ildi Li k


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4 Link State Routing (4): Building Link

State Packets

(a) A subnet. (b) The link state packets for this subnet.

4 Li k St t R ti (5) Di t ib ti th


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4 Link State Routing (5): Distributing the

Link State Packets

The packet buffer for router B in the previous slide (Fig. 5-13).


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5 Hierarchical Routing

Hierarchical routing.


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6. Routing in Ad Hoc Networks (1)

Possibilities when the routers are mobile:

Military vehicles on battlefield.

No infrastructure.

A fleet of ships at sea. All moving all the time

Emergency works at earthquake .

The infrastructure destroyed.

A gathering of people with notebook computers. In an area lacking 802.11.

6 Ro ting in Ad Hoc Net orks (2): Ro te


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6. Routing in Ad Hoc Networks (2): Route

Discovery

(a)Range of A's broadcast.

(b)After B and D have received A's broadcast.

(c)After C, F, and G have received A's broadcast.

(d)After E, H, and I have received A's broadcast.

Shaded nodes are new recipients. Arrows show possible reverse routes.

6 Routing in Ad Hoc Networks (3): Route


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Discovery

Format of a ROUTE REQUEST packet.



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Discovery

Format of a ROUTE REPLY packet.



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Maintenance

(a)D's routing table before G goes down.

(b)The graph after G has gone down.

Documents

4 NetworkLayer PartI