1 University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS An overview on TCP in Wireless Ad hoc Networks Ruy de Oliveira March 27, 2002

2 Topics addressed TCP in wireless Ad hoc networks Lower layers effects on TCP Path asymmetry effects Some existent proposed solutions Open issues Preliminary ideas for our intended proposal Summary

3 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 MobilityPartition Error-detection and error-recovery strategies must be improved

4 An example of a partition condition

5 TCP response under a long partition Exponential backoff mechanism in action RTO

6 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.

7 Hidden and exposed problem

8 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

9 Existing solution proposals There is a lack of appealing proposed solutions Main 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

10 Open issues Asymmetry Proper CWND upon resuming Sender mobility Power management Interoperation Scalability Security QoS

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

12 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

13 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 congestion Complete solution can be too complex This is a work in progress !