Adhoc 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
Route Request
3
4
2
5
7
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. gt 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 Black
  List.

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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
Unspecified RREP-ACK
A
B
C
D
How node D knows which RREP arrives and which not
does not?
16
Initial unidirectional links are useless!
Node A adds B to its routing table
No entry for node B in As routing table
Node B sends Hello msg to A
B also cannot send data 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.