A feedback–based scheme A feedback–based scheme for improving TCP for improving TCP

performance in performance in Ad Hoc Wireless NetworksAd Hoc Wireless Networks

Group :Group : Manish MehtaManish MehtaAditya BarveAditya Barve

BackgroundBackground

• 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 ??

•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 What is an Ad Hoc network ?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.

AcronymsAcronyms

• 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 networksTCP 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 ??Why is this undesirable ??

• When route is lost, retransmission of When route is lost, retransmission of packets will waste scarce bandwidth.packets will waste scarce bandwidth.

• Retransmissions will waste precious Retransmissions will waste precious MH battery power.MH battery power.

• After restoration of the route, the After restoration of the route, the throughput will be unnecessarily low, throughput will be unnecessarily low, as a result of slow start recovery as a result of slow start recovery mechanism.mechanism.

TCP in other networksTCP in other networks

• WiredWired TCP already optimized for it. TCP already optimized for it.

• Wireless Wireless There are many proposed There are many proposed Cellular solutions for these networks.Cellular solutions for these networks.

Use the solution for wireless ad hoc?Use the solution for wireless ad hoc?All schemes proposed for wireless cellular All schemes proposed for wireless cellular heavily depend on the presence of the heavily depend on the presence of the wired base station network, and hence wired base station network, and hence cannot be directly applied.cannot be directly applied.

Proposed Solution (TCP-F)Proposed Solution (TCP-F)

• In this scheme, the source is informed of In this scheme, the source is informed of the route failure so that it does not the route failure so that it does not misinterpret the situation as congestion.misinterpret the situation as congestion.

• This will prevent the sender from pumping This will prevent the sender from pumping data packets into the network until route is data packets into the network until route is restored.restored.

• Sender will not invoke congestion controlSender will not invoke congestion control..

So How does it work ?So How does it work ?

• Consider data transfer between MH Consider data transfer between MH AA and and MH MH B. B.

• As soon as an intermediate MH detects a As soon as an intermediate MH detects a route failure,route failure,

– It sends a RFN packet to the source. It sends a RFN packet to the source. – Each intermediate node that receives RFN Each intermediate node that receives RFN

packet, invalidates the particular route. If that packet, invalidates the particular route. If that node knows alternate route to the destination, node knows alternate route to the destination, RFN is discarded and new route is used.RFN is discarded and new route is used.

– On receiving RFN, the source goes into snooze On receiving RFN, the source goes into snooze state.state.

State Diagram of TCP-FState Diagram of TCP-F

What happens in snooze What happens in snooze state?state?

• It marks all the existing timers as invalid.It marks all the existing timers as invalid.• Freezes the Freezes the window sizewindow size..• Freezes other state variables Freezes other state variables

(for e.g. retransmit timer)(for e.g. retransmit timer)• Starts route failure timer (Worst-case Starts route failure timer (Worst-case

route reestablishment time)route reestablishment time)

Sender remains in this snooze state until it Sender remains in this snooze state until it receives RRN.receives RRN.

When route is restored …When route is restored …

• The intermediate node learning about a The intermediate node learning about a possible route will send RRN to the source.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.It switches back to active state.-- Flushes out all unacknowledged Flushes out all unacknowledged packets.packets.-- Starts communication at the same Starts communication at the same rate rate as before the failure.as before the failure.

Observations from Observations from simulationssimulations

Observations from Observations from simulationssimulations

Observations from Observations from simulationssimulations

Future work & ExtensionsFuture work & Extensions

• Success of proposed scheme is Success of proposed scheme is crucially dependent on the ability of crucially dependent on the ability of routers to detect route failures. routers to detect route failures. Therefore we need something more Therefore we need something more than Best Effort Routing.than Best Effort Routing.

• Caching of RFN and RRN entries can Caching of RFN and RRN entries can be integrated into routing table be integrated into routing table maintenance modules.maintenance modules.

Three concepts to Three concepts to rememberremember

• Practically, the performance of higher Practically, the performance of higher layer protocols may depend on underlying layer protocols may depend on underlying protocols/technology. protocols/technology. (Ad hoc wireless on TCP in our case)(Ad hoc wireless on TCP in our case)

• TCP-F attempts to distinguish packet loss TCP-F attempts to distinguish packet loss due to congestion from that due to route due to congestion from that due to route failure without interfering with TCP’s failure without interfering with TCP’s congestion control mechanism.congestion control mechanism.

• Effect of TCP-F on multiple TCP Effect of TCP-F on multiple TCP connections is yet to be determined.connections is yet to be determined.