| TCP Newreno inefficiency and unfairness in multihop ad hoc wireless networks is well known. The problem originates in part from the nature of the shared wireless medium, from link ARQ and from the interplay between 802.11 MAC and TCP. Several remedies have been proposed, some trying to modify the MAC directly; others operating on cross layer (eg, network layer) feedback; others again proposing only TCP protocol changes. In this paper we follow the third approach. We begin by showing that the use of
packet drop feedback for window adjustment in not a good strategy in ad hoc networks. This is because ad hoc networks implement link ARQ (Automatic Repeat Request) and a packet is retransmitted up to seven times before being dropped. Buffer back up and congestion at intermediate nodes can build up well before a packet is finally dropped by the link layer and thus TCP learns about the loss. In other words, the packet drop feedback &096;&096;comes too late".
In view of packet drop inadequacy as effective congestion indicator, we propose a new feedback mechanism based on the variance of achieved rate. This choice is motivated by the observation that channel contention leads to repeated MAC retransmissions which cause a fluctuation in packet flow and thus increased variance in achieved rate even if no packet is lost. Most important, the change in variance is detected by the source well before the first packet is actually dropped. It makes then sense in ad hoc networks to use variance as feedback to TCP in order to reduce congestion. This argument is valid not only for ad hoc networks, but for all the networks that use link ARQ.
Based on the above considerations we introduce TCP-VAR (TCP with Variance Control), a novel congestion control algorithm for TCP. In TCP-VAR, the source implements rate-based transmission scheduling. It measures the achieved rate variance from returning ACKs and it adjusts its transmission rate based on such variance. Simulation studies confirm that TCP-VAR reacts more promptly to wireless congestion than TCP Newreno and it does substantially improve both efficiency and fairness. TCP – VAR also outperforms TCP AP (Adaptive Pacing), a recent TCP version that uses delay, instead of rate, variance as feedback. An important implementation advantage of TCP-VAR is the |
| Jiwei Chen received the B.S. degree in applied mathematics and M.S. degree in automation from Nanjing University of Science and Technology, China. Currently, he is a Ph.D. candidate in Electrical Engineering of University of California, Los Angeles, where his research topics include protocol design, modeling and performance evaluation of transport, routing and MAC layers in ad hoc and heterogeneous networks. |