A feedback–based scheme for improving TCP performance in Ad Hoc Wireless Networks Group : Manish Mehta Aditya Barve

Background For data communication, we have 7 layer hierarchy defined in Open Systems Interconnection (OSI) reference model. For inter-operation of heterogeneous systems, OSI requires all layers to be independent of each other. In theory, protocol used or changes made at any layer should not affect any other layer.

TCP ?? TCP (Transmission Control Protocol) is a transport layer protocol used for data communication. TCP uses the concept of window size to keep up with slow receivers and congested networks. Most TCP implementations are optimized for wired networks.

What is an Ad Hoc network ? A set of mobile hosts communicating among themselves without using any other communication support facilities. (e.g. Base stations) Every mobile host acts as a router, allowing packets destined to other mobile hosts to pass through it. Topology of the network rapidly changes due to movement of mobile hosts.

Acronyms MH Mobile Host TCP Transmission Control Protocol TCP-F TCP-Feedback ACK Acknowledgement RFN Route Failure Notification RRN Route reestablishment notification RRD Route reestablishment delay

TCP in Ad HOC networks As a MH moves away from other MHs, routes through it are invalidated. It takes finite amount of time to reestablish the routes. Due to this, all the data packets/ACKs in this interval are lost. Sender TCP module misinterprets this loss as congestion in the network. Implications Retransmission upon timeout. Invoking congestion control mechanism. Entering slow start recovery state.

Why is this undesirable ?? When route is lost, retransmission of packets will waste scarce bandwidth. When route is lost, retransmission of packets will waste scarce bandwidth. Retransmissions will waste precious MH battery power. Retransmissions will waste precious MH battery power. After restoration of the route, the throughput will be unnecessarily low, as a result of slow start recovery mechanism. After restoration of the route, the throughput will be unnecessarily low, as a result of slow start recovery mechanism.

TCP in other networks Wired TCP already optimized for it. Wired TCP already optimized for it. Wireless There are many proposed Cellular solutions for these networks. Wireless There are many proposed Cellular solutions for these networks. Use the solution for wireless ad hoc? All schemes proposed for wireless cellular heavily depend on the presence of the wired base station network, and hence cannot be directly applied.

Proposed Solution (TCP-F) In this scheme, the source is informed of the route failure so that it does not misinterpret the situation as congestion. In this scheme, the source is informed of the route failure so that it does not misinterpret the situation as congestion. This will prevent the sender from pumping data packets into the network until route is restored. This will prevent the sender from pumping data packets into the network until route is restored. Sender will not invoke congestion control. Sender will not invoke congestion control.

So How does it work ? Consider data transfer between MH A and MH B. Consider data transfer between MH A and MH B. As soon as an intermediate MH detects a route failure, As soon as an intermediate MH detects a route failure, – It sends a RFN packet to the source. – Each intermediate node that receives RFN packet, invalidates the particular route. If that node knows alternate route to the destination, RFN is discarded and new route is used. – On receiving RFN, the source goes into snooze state.

State Diagram of TCP-F

What happens in snooze state? It marks all the existing timers as invalid. It marks all the existing timers as invalid. Freezes the window size. Freezes the window size. Freezes other state variables (for e.g. retransmit timer) Freezes other state variables (for e.g. retransmit timer) Starts route failure timer (Worst-case route reestablishment time) Starts route failure timer (Worst-case route reestablishment time) Sender remains in this snooze state until it receives RRN. Sender remains in this snooze state until it receives RRN.

When route is restored … The intermediate node learning about a possible route will send RRN to the source. The intermediate node learning about a possible route will send RRN to the source. As soon as the source receives RRN, As soon as the source receives RRN, -It switches back to active state. -Flushes out all unacknowledged packets. -Starts communication at the same rate as before the failure.

Observations from simulations

Future work & Extensions Success of proposed scheme is crucially dependent on the ability of routers to detect route failures. Therefore we need something more than Best Effort Routing. Success of proposed scheme is crucially dependent on the ability of routers to detect route failures. Therefore we need something more than Best Effort Routing. Caching of RFN and RRN entries can be integrated into routing table maintenance modules. Caching of RFN and RRN entries can be integrated into routing table maintenance modules.

Three concepts to remember Practically, the performance of higher layer protocols may depend on underlying protocols/technology. (Ad hoc wireless on TCP in our case) Practically, the performance of higher layer protocols may depend on underlying protocols/technology. (Ad hoc wireless on TCP in our case) TCP-F attempts to distinguish packet loss due to congestion from that due to route failure without interfering with TCPs congestion control mechanism. TCP-F attempts to distinguish packet loss due to congestion from that due to route failure without interfering with TCPs congestion control mechanism. Effect of TCP-F on multiple TCP connections is yet to be determined. Effect of TCP-F on multiple TCP connections is yet to be determined.