Improving performance of TCP over wireless networks

Abstract

Transmission Control Protocol (TCP) assumes a relatively reliable underlying network where most packet losses are due to congestion. In a wireless network, however, packet losses will occur more often due to unreliable wireless links than due to congestion. When using TCP over wireless links, each packet loss on the wireless link results in congestion control measures being invoked at the source. This causes severe performance degradation. In this paper, we study the effect of: burst errors on wireless links; packet size variation on the wired network; local error recovery by the base station; and explicit feedback by the base station, on the performance of TCP over wireless networks. It is shown that the performance of TCP is sensitive to the packet size, and that significant performance improvements are obtained if a good packet size is used. While local recovery by the base station using link-level retransmissions is found to improve performance, timeouts can still occur at the source, causing redundant packet retransmissions. We propose an explicit feedback mechanism, to prevent these timeouts during local recovery. Results indicate significant performance improvements when explicit feedback from the base station is used. A major advantage of our approaches over existing proposals is that no state maintenance is required at any intermediate host. Experiments are performed using the Network Simulator (NS) from Lawrence Berkeley Labs. The simulator has been extended to incorporate wireless link characteristics.