Fundamentals of Computer NetworksECE 478/578

Lecture #17: Routing 2Instructor: Loukas Lazos

Dept of Electrical and Computer EngineeringUniversity of Arizona

Broadcast RoutingBroadcast Communication: All nodes of the network must receive the same

messageTV, radio are by default broadcast operations

Unicasting to each nodeSource needs transmit message N timesAddresses of all recipients must be knownA total of N messages must traverse the network

FloodingInefficient use of bandwidth – Nodes receive the same message multiple timesDoes not scale with network size N

Spanning Tree RoutingBuild a (minimum) spanning tree from source to all nodesSource need send the message only onceMessage is relayed a total of (N - 1) times

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Building a Spanning TreeKruskal’s Algorithm: Greedy approach

Add the edge with the smallest weight that adds connects one or two new nodes togetherRepeat until all nodes are added

BroadcastingAll edges have the same weightAny spanning tree is a minimum spanning treeSpanning tree is the same regardless of the source of the message

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Reverse Path ForwardingIf packet arrives from a “preferred router” forward, otherwise discard

Preferred router: router on the reverse shortest path to the source

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Source

Broadcasting in Wireless Multi-hop NetsMust take into account energyThe Broadcast advantage

SBn > SAn + ABn, broadcast to A and A will forward to BElse use one transmission to B

BIP: Broadcast Incremental Power

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S

B

AS

A

B

C

DE

• T={S}• Add node j, which minimally increases the

power required to reach new node• Repeat until all nodes are added to T• Greedy algorithm, that leads to a non-

optimal solution

Routing for Mobile HostsHost moves from one network to another

If IP is changed, certain services may not work

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Optimizing Routing for Mobile Hosts

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Routing in Ad Hoc NetworksAd Hoc Distance Vector Routing (AODV)

Source node floods a RREQ message Every receiving hosts keeps a reverse path to where the packet came fromOnce the message reaches the destination, it replies with a RREPRREP is forwarded on a unicast path back to the source

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Message FormatRoute Request Message

Destination Sequence Number: Freshness of route to dest.Source Sequence Number: Freshness of RREQ

Route Reply Message

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Route MaintenanceActive neighbors: Neighbors that maintain active paths

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Dynamic Source Routing (DSR)The source determines the sequence of nodes that the packet must follow based

onCashed InformationA route discovery phase

Advantages of DSRNo periodic route updates as in DV, saves energy resourcesNo need for bidirectional links as in AODVCan quickly adapt to changes

Disadvantages of DSRRoute is specified on the packet header that can grow fairly longUses flooding for routing discovery as well

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Route Discovery in DSRSender broadcasts a route request (RREQ) packet

Similar format to AODV packet

Each Intermediate node<Source address, request id>

If same, discard

The address of the intermediate node is already in the route recordThis is a loop - discard

This node is the targetSend a route reply (RREP)

Else Append node’s address to the route record, and re-broadcast

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Example: Routing with DSR

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A

BC

E

D

F

H

G

I

A

A

A, B

A, BA, D

A, D, F

A, D

A, D, F, H

A, D, GA, B, D, G

A, B, C

A, B, C, E

Geographical RoutingRoute based on geographical location

Send packet to the neighbor closest to the destination, based on geographic coordinates

No need for route discovery, or routing tables

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A

BC

E

D

F

H

G

I

Geographical Routing May Get StuckA node is closest to the destination, yet there is no transition to the

destination

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Avoid local MinimaCreate a planar graph of the network

Route in the perimeter until you hit the straight line

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