Paper review: A Comparison of Mechanism for Improving TCP Performance over Wireless Links

Reviewer: Mike Liu

1. State the problem the paper is trying to solve.
The main problem the paper is trying to solve is how to classify, compare, and evaluate different mechanisms for improving TCP performance over wireless links.
2. State the main contribution of the paper: solving a new problem, proposing a new algorithm, or presenting a new evaluation (analysis). If a new problem, why was the problem important? Is the problem still important today? Will the problem be important tomorrow?  If a new algorithm or new evaluation (analysis), what are the improvements over previous algorithms or evaluations? How do they come up with the new algorithm or evaluation? 
The main contribution of the paper is that it provides a framework for evaluating new mechanisms for improving TCP performance over wireless links and shows how improvements to each of the improvements to each scheme can maximize performance. Prior to this, different schemes were proposed but no one had determined a way to classify them or measure their performance. In addition, improvement under each class of mechanisms were made to seek out the most optimal performance in each class. This framework is still rather relevant today because wireless technologies for the internet are still developing and it is yet to be decided which will be dominant scheme before it is widely implemented amongst the general publi.
3. Summarize the (at most) 3 key main ideas (each in 1 sentence.) 
The three 3 key main ideas are: (1) The three basic groups of schemes for improve TCP performance over Wireless Link are: end-to-end proposals, split-connection protocols, and link-layer proposals, with the last of the three having the best performance overall. (2) Other discoveries amongst each of the three groups are: it is important for link-layer schemes to be aware of TCP algorithms in order to achieve high end-to-end throughput; using a SMART-based selective acknowledgement scheme for the wireless hops yields higher throughput in a split-connection approach, though the throughput is still slightly less than a well-tuned link-layer scheme, indicating that splitting the end-to-end connections is a requirement of good performance; end-to-end schemes, while not as effective as local techniques in handling wireless losses, can use explicit loss notification to acheive an improvement in throughput and are promising since significant performance gains can be achieved without any extensive support from intermediate nodes in the network. (3) The SMART-based selective acknowledgement scheme is quite efective in dealing with a high packet loss rate, when employed over the wireless hop or by a sender in a LAN environment while a SACK scheme based on the IETF Draft resulted in significantly improved end-to-end performance in a WAN environment.
4. Critique the main contribution
   • Rate the significance of the paper on a scale of 5 (breakthrough), 4 (significant contribution), 3 (modest contribution), 2 (incremental contribution), 1 (no contribution or negative contribution). Explain your rating in a sentence or two.
   I give this paper a rating of 4 because it sets up a framework for understanding Wireless technologies involved with the Internet and shows how to evaluate them. However, it was not very clear in its description of the different algortihms and strategies and may have been better if it showed actual code or used clear diagrams or graphs.
   • Rate how convincing the methodology is: how do the authors justify the solution approach or evaluation? Do the authors use arguments, analyses, experiments, simulations, or a combination of them? Do the claims and conclusions follow from the arguments, analyses or experiments? Are the assumptions realistic (at the time of the research)? Are the assumptions still valid today? Are the experiments well designed? Are there different experiments that would be more convincing? Are there other alternatives the authors should have considered? (And, of course, is the paper free of methodological errors.)
   Their methodology was somewhat convincing. They used a number of different protocols and tested them against one another in actual experiments. These experiments, however, could have been more clearly explained along with the protocols themselves. Because the experimental methods were a bit vague and only listed the results, there is the possiblity that there may have been errors in their methods but which are undetectable since they only posted the data rates acheived from each of their tests. It would have been more convincing to have graphs of the data rates as they changed with time and then demonstrated the effects of introducing several pathological conditions to show which had the best data rate given a harsh environment as one encounters in the real world.
   • What is the most important limitation of the approach?
   The most important limitation of their experimental approach is that their experiments were so specialized that they do not generate the wireless conditions one would encounter in using such devices on the road or in a normal everyday setting.
5. What lessons should researchers and builders take away from this work. What (if any) questions does this work leave open?
Researchers and builders should take away from this work the fact that well-tuned link-layer schemes that are TCP aware and use a SMART-based selective acknowledgement scheme are currently the most optimal scheme to be used for maximizing performance in wireless environments. The questions that the work leaves open are that are there perhaps even better or more implementations of the link-layer scheme or also, are there other schemes in the other two classes, end-to-end and split-connection, that may acheive better performance but were not considered in this paper. Finally, further work can also be done to evaluate the performance of different protocols under the influence of wireless losses in different network topologies.