A Comparision of Mechanisms for Improving TCP Performance over Wireless Links

This paper compares the performance of TCP over a wireless link using various schemes believed to improve TCP performance, primarily by dealing better with losses over the wireless link. The used a real wireless link between one TCP endpoint and a gateway and the gateway was connected over a wired link(s) (Ethernet or the wide-area US Internet) to the other TCP endpoint. Though the authors used a real wireless link, they simulated single-bit losses (by corrupting TCP checksums) according to an exponential distribution. (The authors also performed one simulation where introduced errors caused more than one packet to be lost, finding that selective acknowledgment helped more than usual.)

The authors compare several types of schemes for improving TCP performance: gateway solutions where the gateway resends missing segments and (in TCP-aware schemes) possibly suppresses spurious duplicate ACKs also; gateway solutions that split the TCP connection into two TCP connnections; and purely end-to-end solutions. The authors find that several proposed improvements do help with caveats:

• link-layer retransmission schemes help by themselves but work a lot better when they also suppress spurious duplicate acknowledgments, which lead to spurious retransmits and congestion window size reductions (TCP-aware link-layer schemes provided the most improvement of all the schemes tested in the paper);
  
• end-to-end selective acknowledgments make good use of much of the bandwidth of the link without gateway modifications, but do not perform as well as gateway modifications especially with a wide-area connection; and
  
• splitting the TCP stream at the gateway performs not as well as link-layer snooping because of limited buffer sizes at the gateway
  

Though the authors mentioned “very high interactive delays” as a motivating problem observed in TCP over wireless, they disappointingly did not attempt to measure it in this paper.