On TCP Performance in Multihop Wireless Networks

This paper studies TCP performance over static, ad hoc networks that use IEEE 802.11 protocol as the access method. Our study reveals some interesting results. First, there exists an optimal value for TCP congestion window size, at which the TCP throughput is maximized. However, TCP does not operate around this optimal point, and typically grows its window much larger; this leads to decreased throughput and increased packet loss. To better understand this behavior, we further study the characteristics of TCP packet loss. Our results show that, network overload is mainly signified by wireless link contention. As long as the buffer size at each node is reasonable (larger than 10 packets), buffer overflow-induced packet loss is rare and packet drops due to link-layer contention dominate. Link-layer drops offer the first sign for network overload. We further observe that multihop wireless links collectively demonstrate Random Early Detection (RED) like graceful drop behavior, but the current TCP protocol does not adapt well to this built-in grace drop characteristic. We further propose two techniques of link RED and adaptive spacing at the link layer, and simulations show that they can improve TCP throughput by 5% to 30%.