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University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS

An overview on

TCP

in Wireless Ad hoc Networks

Ruy de OliveiraMarch 27, 2002

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Topics addressed

TCP in wireless Ad hoc networks Lower layers effects on TCPPath asymmetry effectsSome existent proposed solutionsOpen issuesPreliminary ideas for our intended proposalSummary

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TCP in wireless Ad hoc environment

Std TCP handles any pck loss as indication of network congestion.

A robust TCP needs to distinguish losses due to:• Network congestion• Lossy medium• Mobility Partition

Error-detection and error-recovery strategies must be improved

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An example of a partition condition

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TCP response under a long partition

Exponential backoff mechanism in action

RTO

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Lower layers impacts on TCP

MAC layer– TCP and MAC retransmission mechanisms can interfere with each

other• Hidden node, exposed node and capture problems • Smaller values for pck size and congestion window help to

mitigate the effects of such problems• …

Network layer– Some on-demand routing protocols, such as DSR, are highly prone

to stale route– Others, like TORA, provides multiple paths (multi-path) between

sender and receiver– …

Needed: Cooperation among the layers.

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Hidden and exposed problem

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Other factors

Path asymmetry – Presumably, three sort of asymmetries can occur here:

• Bandwidth asymmetry• Loss rate asymmetry• Latency asymmetry

– Proposed solutions attempt either to minimize the lack of ACK or decouple CWND growing from the number of ACKs

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Existing solution proposals

There is a lack of appealing proposed solutionsMain idea followed by the existent ones:

• Lead the sender into “persist mode” or similar one whenever a timeout due to mobility is imminent

• Use routing cooperation to detect link failure• Rely on feedback information such as ECN and ICMP

messages to distinguish the nature of the error• Fix the RTO under route failure

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Open issues

AsymmetryProper CWND upon resumingSender mobilityPower management InteroperationScalabilitySecurityQoS

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The future•finer gran.•robust error detection •fast recov.• …

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Preliminary ideas

Distinguished features to mobile-mobile and fixed-mobile connections

Probe network continuously to detect congestion, like TCP VEGAS does

Use ICMP to detect intermediate route failure Use MAC layer to detect sender disconnection Under congestion to invoke congestion control normally Under route failure to stop transmitting Under medium losses to retransmit ASAP Main Advantages

• Interoperation• No security problems• Energy saving

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Summary

Existent proposals are too limited as well as the evaluated scenarios

Representative scenarios are really tough to be simulated/emulated

Interoperation is do necessary Probe schemes appear to be the most appropriate

way for detecting congestionComplete solution can be too complexThis is a work in progress !