Mobile Adhoc Network: Routing Protocol:AODV CSE 6806 : Wireless and Mobile Communication Networks Anika Anwar Id: 1014052024

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

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)

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

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

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

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

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.

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.

AODV Routing Table

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

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

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

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.

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

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

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

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

Reverse Path Setup in AODV Y 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

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

Reverse Path Setup in AODV Y 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

Forward Path Setup in AODV Y 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

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

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

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.

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.

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.

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