Ad-hoc On-Demand Distance Vector (AODV)

Routing

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When/Why do we need AODV?

• Basically when there is one node that wants to communicate with another node that is not in range, it finds a route through other nodes. In the example below node 1 is not in range with node 3, so it simply talks to node 3 through node 2.

Node 1 Node 2 Node 3

How does it work?• AODV Routing works by using Route Request Messages

(RREQ) and Route Reply Messages (RREP). If a node is not in range with a node that it wants to talk to, it sends a RREQ to its neighbors. The RREQ contains source IP address and sequence number, and destination IP address and sequence number, as well as the life span of the RREQ. 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 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, so it sent node 1 a RREP. Once node 1 receives the RREP, it notes the route to node 5 and sends the packet on that route.

• SUCCESS!!Route 5 receivedthe packet.

Node 1 Node 4 Node 5

RREQ

RREP

Packet Packet

Node 1

Node 2

Node 3

Node 4

Node 5

RREQ

RREP

AODV Protocol

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.