A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

H. Balakrishnan, V. Padmanabhan, S. Seshan, R. H. Katz, "A Comparison of Mechanisms for Improving TCP Performance over Wireless Links," IEEE/ACM Transactions on Networking, (December 1997).

One line summary: This paper compares several end-to-end, link layer, and split connection schemes for improving the performance of TCP over wireless links, and concludes that link layer schemes incorporating TCP awareness and selective acknowledgments, as well as end-to-end schemes using Explicit Loss Notification and selective acknowledgments, were the most promising.

Summary

This paper compares a number of schemes for improving TCP performance in wireless networks using an experimental testbed consisting of a wired and a wireless link. The approaches compared fall under three categories: end-to-end, link layer, and split connection. In the end-to-end category, six specific protocols were tested. These included the de facto TCP implementation, TCP Reno (labeled E2E), TCP New Reno (labeled E2E-NEWRENO), which improves upon the performance of TCP Reno by remaining in fast recovery after a partial acknowledgement to recover from losses at the rate of one packet per RTT, two TCP schemes using selective acknowledgments (SACKs), labeled E2E-SMART and E2E-IETF-SACK, and two schemes using Explicit Loss Notification (ELN), labeled E2E-ELN and E2E-ELN-RXMT. There were four link layer schemes tested. These included a base link layer algorithm, LL, a link layer algorithm that uses SACKs, labeled LL-SMART, and two additional schemes that add to the first two TCP awareness, labeled LL-TCP-AWARE and LL-SMART-TCP-AWARE. Two split connection schemes were tested, one labeled SPLIT modeled after I-TCP, and another that adds SACKs to the first, called SPLIT-SMART.

The authors perform simple initial experiments on the mechanisms in each category. Of the link layer schemes, the authors conclude that the ones that use knowledge of TCP semantics perform the best. Of the end-to-end protocols, the results suggest that TCP New Reno is better than TCP Reno and that a scheme that makes use of both SACKs and ELN would probably perform the best. Of the split-connection schemes, SPLIT-SMART is better, but the performance of these schemes is overall not as good as those in the other categories. Using the most promising schemes from their initial experiments, the authors then test these schemes’ reaction to burst errors and performance at different error rates. Overall, the authors drew four main conclusions: (1) of all the schemes investigated, a link layer protocol with TCP awareness and selective acknowledgements performs the best, (2) splitting the end-to-end is not necessary for good performance and violates the end-to-end semantics, (3) selective acknowledgements are especially useful over lossy links when losses occur in bursts, and (4) end-to-end schemes are not as good as link layer schemes but they are promising because they do not require any support from the link layer or intermediate nodes.

Critique

The comparisons and overviews of the different techniques for improving TCP over wireless were informative. One thing that is a bit questionable is that the authors themselves implemented each of the protocols. The reason I mention this is because in class I remember Prof. Katz telling us about a company that claimed to be able to distinguish between different implementations of TCP because they each had unique signatures of sorts in the way they operated. This suggests that two different implementations of the same protocol aren’t necessarily the same. It may have been more of a fair comparison to use the “standard implementation” of each protocol but if this was not possible in all cases then implementing them all is probably better anyway. On second thought, this method still works for the sake of comparing techniques, if not implementations, and you probably wouldn’t want the results to be implementation dependent anyway. This paper was the first time I had heard about split-connection protocols, and I have to say, they don’t seem like a very good idea at all, but that could have been because of the way it was presented and tested. It confused me that they still used TCP over the wireless link using the split-connection schemes. That didn’t seem very useful. One other thing that confused me is why at one point in a footnote, the paper states that they only measure “receiver throughput”, when at an earlier point in the paper they state they measure “throughput at the receiver”. I think that in this case they intended the same meaning by these two phrases but generally I would think that these two phrases don’t mean the same thing. I also found the charts not so great to read. Lastly, I think an interesting thing to study next would be a comparison of these protocols over more complex wireless topologies, as they mention in the section on future work.