Ad-hoc On-Demand Distance Vector (AODV)

Routing

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What Does AODV Mean?

• An ad hoc network is a network with no existing infrastructure– No routers or access points– Instead each node acts as a router

• On Demand refers to the nature the network routes information.– it only creates and maintains routes b/w nodes as

and when they are necessary and active

What Does AODV Mean? (cont.)

• Distance Vector refers to the way in which routes are implemented

• Routers using distance vector protocol do not have knowledge of the full path to the destination

• They have a routing table which dictates the next node to forward the information to and the distance to the destination

What is AODV?• It’s a routing protocol for wireless ad hoc networks• Provides a way for nodes to communicate w/one

another • Can be directly from one node to another• Needed when nodes need to talk to nodes they are

not in range with

Node 1 Node 2 Node 3

How Does it Work?

• Each node has a routing table• The table indicates routes to known

destinations– 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 isn’t used in that time the info is

deleted from the table

• When a node wants to send a packet to another node it checks its routing 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 sequence number, destination IP address and sequence number, a time to live number, 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

How does it work?

• When an intermediate node receives a RREQ it logs 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 doesn’t 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.

Node 3 Node 2 Node 1 Node 4 Node 5

RREQRREQ RREQ

RREP

How does it work? continued

• As seen in the last slide, node 4 had a route to node 5

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

• Once node 1 receives the RREP, it notes the route to node 5 and sends the packet on that route.

Node 1 Node 4 Node 5

RREQ

RREP

Packet Packet

Node 1

Node 2

Node 3

Node 4

Node 5

RREQ

RREP

Now we can see it all in action!

Lets get more specific!

• Sequence Number: Each message contains a sequence number, which is essentially the age of the message. This allows nodes to know how recent a message was sent, and it may allow nodes to find new, quicker routes.

• Life Span: Each message only last the time that is specified by its life span. If the message dies before it reaches the destination, the source will resend the message with a longer life span.

And more specific…

• Hello Messages: These are simple messages that nodes send at certain time intervals to all its neighbors to let them know that it is still there. If a node stops receiving hello messages from one of its neighbors, it knows that any routes through that node no longer exist.

It gets complicated!

• Things get much more complicated with many nodes. This is because nodes have many neighbors so RREQ get rebroadcasted a lot! That’s why sequence numbers and life spans

are so key.

Error Messages RERR

• RERR are used mainly when nodes get moved around and connections are lost. If a node receives a RERR, it deletes all routes associated with the new error. Error messages are sent either when a message (not RREQ or RREP) is sent to a node that has no route to the destination, or when a route becomes invalid, or if it cannot communicate with one of its neighbors.