1. Mobile Adhoc Network: Routing Protocol:AODV
CSE 6806 : Wireless and Mobile Communication Networks
Anika Anwar
Id:

2. What is a Mobile Ad Hoc Network (MANET)?
Collection of mobile nodes
Located in such a manner that the interconnections between nodes are capable of changing on continuous basis
Self-forming, self-maintained and self-healing
Nodes change location and configure itself

4. Routing Protocols Proactive protocols Reactive protocols
Traditional distributed shortest-path protocols
Maintain routes between every host pair at all times
Based on periodic updates; High routing overhead
Example: DSDV (destination sequenced distance vector)
Reactive protocols
Determine route if and when needed
Source initiates route discovery
Example: AODV (Ad hoc On-Demand Distance Vector)
Hybrid protocols
Adaptive; Combination of proactive and reactive
Example : ZRP (zone routing protocol)

5. Ad hoc routing protocols
AD-HOC MOBILE ROUTING PROTOCOLS
ON-DEMAND-DRIVEN REACTIVE
HYBRID
DSDV
OLSR
TABLE DRIVEN/ PROACTIVE
DSR
AODV
ZRP

6. Protocol Trade-offs Proactive protocols Reactive protocols
Always maintain routes
Little or no delay for route determination
Consume bandwidth to keep routes up-to-date
Maintain routes which may never be used
Reactive protocols
Lower overhead since routes are determined on demand
Significant delay in route determination
Employ flooding (global search)
Control traffic may be bursty
Which approach achieves a better trade-off depends on the traffic and mobility patterns

7. MANET Reactive protocol
AODV (Ad hoc On-Demand Distance Vector )
Uses Distance Vector routing protocol

8. Distance Vector routing protocol
What it does:
Computes best paths to all destinations
Fully distributed and Updates are performed periodically
Using as only information the distances from self to all destinations
How it works
Uses Bellman–Ford algorithm, Ford–Fulkerson algorithm, or DUAL FSM
Can faces Count to infinity problem
Do not have knowledge of the entire path to a destination. Instead they use two methods:
Direction in which a packet should be forwarded.
Distance from its destination

9. AODV Protocol
The Ad hoc On-Demand Distance Vector protocol is both an on-demand and a table-driven protocol.
Dynamic, self-starting, multi-hop routing between mobile nodes
A route is maintained only when it is used and hence old and expired routes are never used.
AODV maintains only one route between a source-destination pair.
AODV avoids the “counting to infinity” problem by using destination sequence numbers. This makes AODV loop free.
The packet size in AODV is uniform and supports multicasting and unicasting within a uniform framework.
Each route has a lifetime after which the route expires if it is not used.

10. AODV Protocol AODV defines 3 message types: Route Requests (RREQs)
Route Replies (RREPs)
Route Errors (RERRs)
RREQ messages are used to initiate the route finding process.
RREP messages are used to finalize the routes
RERR messages are used to notify the network of a link breakage in an active route.
Route table information must be kept for all routes even short-lived routes.

11. AODV Routing Table

12. Route Request Type 1 J Join flag : reserved for multicast.
R Repair flag : reserved for multicast.
G Gratuitous RREP flag
D Destination only flag
U Unknown sequence number
Reserved Sent as 0
Hop Count: The number of hops from the Originator IP Address to the node handling the request.
R Repair flag; reserved for multicast.
G Gratuitous RREP flag; indicates whether a gratuitous RREP should be unicast to the node specified in the Destination IP Address field
D Destination only flag; indicates only the destination may respond to this RREQ
U Unknown sequence number; indicates the destination sequence number is unknown

13. Route Reply Type 2 R Repair flag; used for multicast.
A Acknowledgment required
Reserved Sent as 0
Prefix Size: If nonzero, the 5-bit Prefix Size specifies that the indicated next hop may be used for any nodes with the same routing prefix
Hop Count : The number of hops from the Originator IP Address to the Destination IP Address.
R Repair flag; reserved for multicast.
G Gratuitous RREP flag; indicates whether a gratuitous RREP should be unicast to the node specified in the Destination IP Address field
D Destination only flag; indicates only the destination may respond to this RREQ
U Unknown sequence number; indicates the destination sequence number is unknown

14. Route Request and Route Reply
Route Request (RREQ) includes the last known sequence number for the destination
An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender
Intermediate nodes that forward the RREP, also record the next hop to destination
A routing table entry maintaining a reverse path is purged after a timeout interval
A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval

15. Maintaining Sequence Numbers
Keep AODV loop-free and avoiding the “counting to infinity” problem.
Nodes originating RREQ messages must increment their own sequence number before transmitting the RREQ.
Before a destination node originates a RREP in response to a RREQ, it MUST update its own sequence number
Forwarding nodes update their stored sequence number when forwarding RREP when:
The sequence number in the routing table is invalid
The sequence number in the RREP message is greater than the stored number
The sequence numbers are identical, but the route is marked as inactive
The sequence numbers are the same, but the hop count is smaller for the RREP message.

16. AODV Operation
Route Requests (RREQ) are broadcasted it to its neighbors with initial TTL of 1.
When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source
AODV assumes symmetric (bi-directional) links
When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP)
Route Reply travels along the reverse path set-up when Route Request is forwarded

17. Route Requests in AODV Y Z S E F B C M L J A G H D K I N
Represents a node that has received RREQ for D from S

18. Broadcast transmission
Route Requests in AODV Y
Broadcast transmission Z S E F B C M L J A G H D K I N
Represents transmission of RREQ

19. Route Requests in AODV Y Z S E F B C M L J A G H D K I N
Represents links on Reverse Path

20. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N
Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once

21. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N

22. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N
Node D does not forward RREQ, because node D
is the intended target of the RREQ

23. Forward Path Setup in AODVY Z S E F B C M L J A G H D K I N
Forward links are setup when RREP travels along
the reverse path
Represents a link on the forward path

24. Link Failure
A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry
Neighboring nodes periodically exchange hello message
When the next hop link in a routing table entry breaks, all active neighbors are informed
Link failures are propagated by means of Route Error (RERR) messages, which also update destination sequence numbers

25. Route Error
When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message
Node X increments the destination sequence number for D cached at node X
The incremented sequence number N is included in the RERR
When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N
When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

26. Performance of AODV
AODV does not retransmit data packets that are lost and hence does not guarantee packet delivery.
However, the packet delivery percentage is close to 100 with relatively small number of nodes.
The packet delivery percentage drops with increased mobility.

27. Control Overheads
The overhead packets in AODV are due to RREQ, RREP and RERR messages.
AODV needs much less number of overhead packets compared to DSDV.
The number of overhead packets increases with increased mobility, since this gives rise to frequent link breaks and route discovery.

28. Latency in Route Discovery
The route discovery latency in AODV is low compared to DSR and DSDV.
The latency is almost constant even with increased mobility if the concentration of the nodes remain similar.
The average path length for discovered routes is also quite low.

29. AODV: Summary Routes need not be included in packet headers
Nodes maintain routing tables containing entries only for routes that are in active use
At most one next-hop per destination maintained at each node
DSR may maintain several routes for a single destination
Sequence numbers are used to avoid old/broken routes
Sequence numbers prevent formation of routing loops
Unused routes expire even if topology does not change