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8/9/2019 Adhocslide for Blog

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Ad-hoc On-Demand

Distance Vector(AODV)

Routing

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100110001010101010101010100101111000100100101010101010000011111100110101010


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What Does AODV Mean?

An ad hoc network is a network with noexisting infrastructure

No routers or access points

Instead each node acts as a router

On Demand refers to the nature thenetwork routes information.

it only creates and maintains routes b/wnodes as and when they are necessary andactive


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What Does AODV Mean?(cont.)

Distance Vector refers to the way inwhich routes are implemented

Routers using distance vector protocoldo not have knowledge of the full pathto the destination

They have a routing table which

dictates the next node to forward theinformation to and the distance to thedestination


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What is AODV?

Its a routing protocol for wireless ad hocnetworks

Provides a way for nodes to communicate w/oneanother

Can be directly from one node to another

Needed when nodes need to talk to nodes theyare not in range with

Node 1 Node

2

Node 3


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How Does it Work?

Each node has a routing table

The table indicates routes to knowndestinations

It stores: destination address, nexthop address,destination sequence number, and life time.

Life time is updated each time the route is used

If the route isnt used in that time the info is

deleted from the table


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How Does it work?continued

When a node wants to send a packet to another node it checks itsrouting table to see if it has a route

If it does, it forwards it to the next node

If not, it sends out a Route Request (RREQ) packet

RREQ contains: source node IP address, and current sequencenumber, destination IP address and sequence number, a time to livenumber, and a broadcast ID

Broadcast ID and source IP address is unique to each RREQ

The RREQ is sent to all nodes it can reach within a time limit

Limits network congestion


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How does it work?continued

When an intermediate node receives a RREQ itlogs a reverse route entry in its table Includes the destination, the next hop (the next node in

line), the hop count (incremented 0 at the source to

i=number of hops since then) If a neighbor of the source doesnt know a route to

the destination, it rebroadcasts the RREQ.

If a neighbor does know a route to the destination,

it sends a route reply (RREP) back to the source.Below, Node 1 is trying to talk to node 5.


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How does it work?continued

As seen in the last slide, node 4 hada route to node 5

Node 5 sends node 1 a RREP alongthe route the RREQ came on

Once node 1 receives the RREP, itnotes the route to node 5

and sends the packet on thatroute.


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Lets get more specific!

Sequence Number: Each message contains asequence number, which is essentially the ageof the message. This allows nodes to knowhow recent a message was sent, and it mayallow nodes to find new, quicker routes.

Life Span: Each message only last the timethat is specified by its life span. If the

message dies before it reaches thedestination, the source will resend themessage with a longer life span.


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And more specific

Hello Messages: These are simplemessages that nodes send at certaintime intervals to all its neighbors to

let them know that it is still there. Ifa node stops receiving hellomessages from one of its neighbors,

it knows that any routes through thatnode no longer exist.


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It gets complicated!

Things get much more complicatedwith many nodes. This is becausenodes have many neighbors so RREQ

get rebroadcasted a lot! Thats whysequence numbers and life spans

are so key.


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Error Messages RERR

RERR are used mainly when nodes get movedaround and connections are lost. If a nodereceives a RERR, it deletes all routes

associated with the new error. Errormessages are sent either when a message(not RREQ or RREP) is sent to a node that hasno route to the destination, or when a route

becomes invalid, or if it cannot communicatewith one of its neighbors.

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