1. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) How a Node Deals with a Route Request Message S B D  If B has a route to D in its route cache, B sends back that route to S. D S B  If B does not have a route to D, B broadcasts the route request message.

2. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) DSR Route Request  When a Route Request message reaches a node, the accumulated route indicates the nodes through which it has passed.  This accumulated route is used by a node to send a Route Reply message back to the initiator.  The Route Reply message can be sent either by the destination node or by an intermediate node that finds a route to the destination in its route cache.

3. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) How Often Should a Node Initiate a Route Discovery?  Suppose S is the initiator of a route discovery message to a node D.  If S does not receive a route reply message, S would like to initiate another route request message.  However, S should not send route request messages frequently, because D may be unreachable at the moment.

4. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) How Often Should a Node Initiate a Route Discovery?  S may have a better chance of finding a route to D later. S should not flood the network with route requests. S P D P

5. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) How Fast Should a Node Send a Route Reply?  A node can send a route reply if it finds a route to the target in its route cache.  However, if many nodes try to send route reply for the same destination, it may result in a route reply storm.  Too many replies for the same route request may flood the network.

6. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Route Reply Storm A E D B C F G  Each of the nodes B, C, D, E and F knows a route to G.

7. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Waiting Before Replying  Simultaneous replies from all these nodes will result in network congestion and packet collisions.  Each node should wait for a random amount of time and listen to the traffic.  If there is a better (shorter) route reply, or the initiator (node A in this example) starts using another route, there is no need to reply.

8. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Caching Overheard Routing Information  An important aspect of DSR is to maintain an up-to-date route cache.  A good and current route cache helps a node to find (i) routes for itself faster, and (ii) reply fast to route request messages.  If the nodes overhear other messages, they should analyse these messages and update their own route cache if necessary.

9. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Caching Overheard Routing Information A B C D E P  P overhears the route A-B-C-D-E when B is sending a packet to C. This packet was originally sent by A.  P stores this route in its route cache for future use. If P receives a route request for any of the destinations C,D or E, P can use this information.

10. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Route Request Hop Limit  Sometime it is not good to propagate a route request message throughout the network.  Suppose S is the source of a route request message for a destination D.  In case D is in the neighbourhood of S, the route request message from S should not propagate too far away.

11. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Route Request Hop Limit  If D is near S, propagating the route request message too far will result in too many unnecessary route reply messages in future. S D

12. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Restricted Propagation of Route Request  A better strategy is to propagate route request messages with increasing hop count.  Initially, send the route request to a distance of 2 hops. If no route reply is received after sometime, send the route request to a distance of 4 hops and so on.  This reduces network congestion by reducing the number of route reply messages.

13. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) DSR Route Maintenance  Route maintenance is important for correct delivery of messages.  When forwarding a packet, each node should ensure that the packet reaches the next hop.  A packet reaches its correct destination if each node ensures this correct delivery along the path.

14. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Ensuring Correct Delivery E A B C D  Node A is sending a packet to E, using the route A-B-C- D-E.  A is responsible for the correct delivery of the packet to B  B is responsible for the correct delivery of the packet to C and so on.

15. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Active or Passive Acknowledgment  The correct delivery can be ensured through an active acknowledgment.  An active acknowledgment may be part of the MAC protocol in use. IEEE 802.11 standards provide such link level active acknowledgment.  In a passive acknowledgment, node A may overhear the forwarding of the packet by node B and knows that the packet has been received by B.

16. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Route Error Message E A B C D  When A initiates a message to E, there are two parts in the message. The route from A to E and the actual message.  Each intermediate node tries to forward the mesage by looking up the next hop from the route.

17. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) When a message cannot be forwarded A B C D E  A node like C tries to forward the message and waits for acknowledgment. C will retransmit the message a fixed number of times if no acknowledgment arrives.  After that, C will initiate a route error message.

18. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Route Error back to the Initiator E A B C D  In this example, C will initiate a route error message back to A indicating that the link to D is currently broken.  A will remove this route from its route cache and try another route to E, if it has one. Or, A may start a new route discovery.

19. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Packet Salvaging  After sending a route error message, a node may try to send the packet that caused the route error.  In the previous example, C may try to find a route to E from its own route cache.  If C can find a route to E, it will replace the previous route by the new route and send the packet to E.  C should also indicate that this packet has been salvaged, so that other nodes do not try to salvage it.

20. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Automatic Route Shortening  Some routes may become unnecessarily long when nodes move around. A BF G C E  C overhears the transmission when A sends the packet to B. Recall that A sends the complete route to B.  C informs A that B,F,G can be removed from the route.

21. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Spreading of Route Error Message A B C D E  When A receives a route error message from C, A knows that the link C-D is broken.  A removes this route from its route cache. In future, A may try a new route or try to discover a new route.  A piggybacks the route error message so that other nodes know that the link C-D is broken.

22. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Performance of DSR  DSR has been simulated by various authors using the ns-2 network simulator.  Here, we use simulation results by Johnson, Maltz and Brock (the original authors of DSR).  The simulation is done within a flat rectangular area of 1500 m X 300 m  There are 50 mobile nodes in the simulation environment.

23. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Performance of DSR  All simulations were run for 900 secs of simulation time.  Data taffic is generated using Constant Bit Rate (CBR) sources. There are 20 source nodes.  Each node starts at a random location within the simulation area.

24. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Performance of DSR  Each mobile node moves independently during the simulation.  A node remains stationary for a specified period (pause time) and then moves in a random direction with a random speed.  If the pause time is 0, the nodes do not stop at all. If the pause time is 900, there is no movement.

25. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Packet Delivery Ratio in DSR (1m / sec)

26. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Routing Overhead Packets (1m / sec)

27. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Packet Delivery Ratio in DSR (20m / sec)

28. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Routing Overhead Packets (20m/ sec)

29. Mobile and Wireless Computing Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC) Advantages and Disadvantages  DSR is a simple and efficient routing protocol with low overhead of control messages.  However, DSR has relatively high latency in finding routes.  DSR is not very scalable since packet size increases with increasing hop numbers in a route.