Ad-hoc On Demand Distance Vector Protocol Hassan Gobjuka

Agenda Introduction Message types. AODV concepts. Problems. Conclusions.

Introduction Only nodes that lie on active paths participate in route detection and maintain routing information. Broadcast discovery packets are sent only when necessary. Distinguishing between local connectivity management and general topology maintenance. Disseminating information about changes with nodes that may require these information.

Path Discovery Initiated when a node needs to communicate with another node which has no information in the local route table. Type J R G Reserved Hop count Flooding ID Destination IP Address Destination sequence number Source IP address Source sequence number

Maintaining Sequence Numbers Sequence number increased only in the following cases: Immediately before generating NEW RREQ. Immediately before generating RREP. If link failure has been detected.

Route Request

RREQ, Cont. 142 RREQ Since node 3 is not a neighboring node, node 1 has to discover a route by regenerating and broadcasting the RREQ. RREQ Packet: Dest: 3, Src: 1, Dest Seq: 0 (unknown), Src Seq: 2, Flooding ID: 2, Hopcount: 0

Route Request Forwarding Compare Sequence numbers: Local Seq. No. > Source Seq. No. Sequence numbers are equal but less Hop Count. Pointer to reversed node has been created. Increment Hop Count.

Route Reply When a node has a route to the destination, or the destination itself, it sends RREP. Type R A Reserved Prefix Sz Hop count Destination IP Address Destination sequence number Source IP address Source sequence number

RREP Node 2 has a route to node 3, it sends a RREP to node 1. It also sends RREP to node 3 so it will know how to contact node 1. Node 3 receives RREP from node 2 and adds it to its route table. Each intermediate node sends the RREP back to the previous node (after incrementing Hop Count).

Concept of Precursor list All active nodes that maintain a path through the local node are added in the precursor table list. Precursor node members are informed by any link failure by sending RERR message. RERR message is transitive (each node multicast it to all nodes in local precursor table list).

Route Error Message (RERR) A node initiates RERR if: It detects a link break. It receives a RREQ to a node, and it has no route to that node. It receives an RERR from another node, it resends the RERR to nodes in its precursor list.

Concept of unidirectional link Unidirectional link can be used to send messages in one direction only. Initial unidirectional links cannot be detected by all nodes! Nodes that are not accessible due to non-initial unidirectional link will be added to the Black List.

Problems Selecting non-optimal path. Unspecified RREP-ACK. Unidirectional links are useless! What happen when a non-accessible node (due to unidirectional link) moves and become out-of-range (Link failure) ?

Unspecified RREP-ACK ABCD RREQ1 RREQ2 RREQ1 RREP1 RREP2 RREP-ACK How node D knows which RREP arrives and which not does not?

Initial unidirectional links are useless! A B Node A is able to send and receive Node B is only able to receive Hello msg. Node A adds B to its routing table Node B sends Hello msg to A ? No entry for node B in A’s routing table A cannot send data to B B also cannot send data to A! Physical connection

Conclusions AODV is an efficient protocol for ad-hoc networks. –Loop free. –Efficient resource utilization. There are still unsolved problems.