A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

Summary

This paper explores different mechanisms to deal with loss in wireless networks. Specifically, it compares the performance of three classes of solutions:

1. End-to-end protocols
2. Link-layer protocols
3. Split-connection protocols

End-to-end protocols include standard TCP Reno and modifications. The basic problem is that standard TCP uses packet loss as a signal of congestion, however, in wireless networks packet losses can occur due to noise or interference, treating these losses as congestion degrades performance. The authors find that basic TCP Reno achieves a throughput of 50% of the maximum with a 2.3% packet loss rate on the wireless link. Other variations of TCP Reno include partial acknowledgments (new Reno) which performs better because it can fill large holes, resending one packet for each additional ACK (after 3 duplicates). A different approach is to use explicit loss notification, this allows the sender to maintain the congestion window size while retransmitting lost packets. Both of these modifications perform similarly.

The second class of solutions is link-layer protocols. These solutions perform re-transmissions at the link layer and attempt to hide the lossy link from TCP. The problem with this approach is that re-transmitting at the link-layer can lead to out of order packets received at the destination. This will cause duplicate ACKs which will activate TCP fast retransmit unnecessarily. Several solutions are explored, all involve "TCP awareness" or suppressing duplicate ACKs for packets that have already been retransmitted. The authors find that this solution is best, out performing the end-to-end solutions.

The final class of solutions is split-connection protocols. These isolate TCP from the wireless link by using a different connection to make the wireless hop. If the protocol used for the wireless hop is optimized to handle losses then the overall throughput is quite good. However, the authors argue that this approach breaks the end-to-end principle and is not necessary for good performance.

Thoughts

I am curious to know what is actually done in 802.11b/g/n networks. The link-layer recovery seems like a fairly clear solution but admittedly requires duplicating recovery functionality.