Explain the DSR routing protocol used in ad-hoc network

DSR Routing Protocol in Ad-hoc Network

• The Dynamic Source Routing protocol (DSR) is a routing protocol specially used in multi-hop wireless ad hoc networks of mobile nodes.
• DSR routing protocol self-organizes, self-configure the network without any help from other existing network infrastructure or administration.
• DSR routing uses source routing and is completely based on demand.
• It creates the route on-demand when the request arrives from the transmitting computer.
• DSR follows source routing instead of maintaining a routing table at each intermediate node device.
• DSR routing consists of two mechanisms such as "Route Discovery" and "Route Maintenance", which discover and maintain routes to arbitrary destinations in the ad hoc network.

Route Discovery

• It finds out the route from a source node to a destination node.
• When a source node wants to send a message to some destination node, then it first searches for its route cache to find whether there is a route to the destination is already exists or not.
• If there is no route to the destination, then the source node will initiate a Route Discovery and send out Route Request Message which is broadcasted to all the nodes within its transmission range.
• The Route Request Message contains the destination address, the source address, and a unique identification number.
• Each node that receives the Route Request Message checks whether it has a route to the destination or not. If it does not, it adds its address to the route record of the message and then rebroadcasts the Route Request Message on its outgoing nodes.
• When a message reaches the destination node, it will send a Route Reply Message towards the source node and this message contains the source route record list which is collected when the Route Request message is forwarded along its way to the destination.
• When the destination sends the Route Reply it uses MAC protocols such as IEEE 802.11 that require a bidirectional link.
• When the source node receives the Route Reply message, it stores returned route into its route cache. From then onwards all the messages destined to the same destination will use this route.

Route maintenance

• The ad hoc network is dynamic in nature and the topology of the network changes frequently therefore, existing routes in route cache are broken frequently. Hence, route maintenance is very important.
• After forwarding a message, a node must need to confirm the reachability of the next-hop node.
• If the node does not receive any confirmation from the next hop during a certain period, it will retransmit the packet. If after the number of retransmission still does not receive any confirmation, it will think the link to the next hop is broken and will send a Route Error message to the source node.
• To check this DRS uses three acknowledgment methodologies such as
  • Link-layer acknowledgment - Provided by MAC layer protocol such as IEEE 802.11.
  • Passive acknowledgment - Node hears the next-hop node forwarding the message and thus confirms the reachability of the route.
  • Network-layer acknowledgment - Node sends an explicit acknowledgment request to its next-hop node.

Advantages of DSR

• Reduces overhead on route maintenance because routes maintain only between nodes involved in communication.
• Route cache also reduces overhead during route discovery time.
• Single route discovery generates numerous routes because intermediate nodes provide replies from their local caches.

Disadvantages of DSR

• Packet header size grows with route length due to source routing.
• A Flood of route requests may reach all nodes in the network.
• Potential collisions between route requests broadcasted by neighboring nodes
• Insertion of random delays before forwarding RREQ.
• Increased contention because too many route replies come back due to nodes replying using their local cache.
• Route Reply may create a Storm problem sometimes.
• Stale caches wilL cause increased overhead