Ad-hoc On Demand Distance Vector Protocol

1. Ad-hoc On Demand Distance Vector Protocol Hassan Gobjuka

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

3. 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.

4. Path Discovery • Initiated when a node needs to communicate with another node which has no information in the local route table. 0 7 0 1 --- 9 0 1 --- 3 4 1 Type J R G Reserved Hop count Flooding ID Destination IP Address Destination sequence number Source IP address Source sequence number

5. 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.

6. 1 Route Request 3 4 2 5

7. 1 4 2 RREQ, Cont. • 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 RREQ RREQ

8. 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.

9. Route Reply • When a node has a route to the destination, or the destination itself, it sends RREP. 0 7 0 8 9 3 4 1 Type R A Reserved Prefix Sz Hop count Destination IP Address Destination sequence number Source IP address Source sequence number

10. 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).

11. 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).

12. 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.

13. 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 BlackList.

14. 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) ?

15. RREP1 RREQ1 RREQ1 RREQ1 RREQ2 RREQ2 RREP-ACK RREP2 Unspecified RREP-ACK A B C D How node D knows which RREP arrives and which not does not?

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

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