Paper review: < TCP Vegas: End to End Congestion Avoidance on a Global Internet [BP95] >

Reviewer: <Ryan Gehl>

1. State the problem the paper is trying to solve.

The purpose of this paper is to describe another implementation of TCP (Vegas) that has 37 to 71% higher throughput on the Internet than the current distribution (Reno) of BSD Unix.

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 this paper is to concretely describe a new implementation of TCP.

3. Summarize the (at most) 3 key main ideas (each in 1 sentence.) 

1) The Vegas implementation does not involve any changes to the TCP specification, as all of the changes are confined to the sending side.
2) The improvement in throughput is not achieved by an aggressive retransmission strategy that effectively steals bandwidth away from TCP connections that use the Reno implementation. Rather, it is achieved by a more efficient use of the available bandwidth.
3) The three new techniques implemented in Vegas are (1) a new retransmission mechanism, (2) a congestion avoidance mechanism, and (3) a modified slow-start mechanism.

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 would rate this paper as a 5 because it significantly improves the throughput on the Internet by providing a better implementation of TCP without requiring any modifications to the protocol itself.

   • 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.)

   By presenting both simulated and actual performance results, they make an effective argument to upgrade to the Vegas implementation. I thought it was particularly interesting that "Reno connections do slightly better when the background traffic is running on top of Vegas as compared to when the traffic is running on top of Reno." (In fact, the authors seem to emphasize this point; a choice that I believe strengthens their argument).

   • What is the most important limitation of the approach?

   Given the multitude of tests (both actual and simulated) run both the authors, as well as the extended discussion at the end of the paper, I found very few limitations in the authors approach. The only potential limitation I saw was that the authors presented a weak defense for Vegas as being a fair algorithm: "we conclude that Vegas is no less fair than Reno."

5. What lessons should researchers and builders take away from this work. What (if any) questions does this work leave open?

One lesson researchers should take away from this work is that it is often possible to improve upon existing implementations of a given protocol without having to modify the protocol itself.