Paper review: TCP Vegas: End to End Congestion Avoidance on a Global Internet

Reviewer: Mike Liu

  1. State the problem the paper is trying to solve.
  2. The main problem the paper is dealing with is explaining the TCP Vegas protocol, analyzing its performance, and arguing for its strengths and adoption.
  3. 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? 
  4. The main contribution of this paper is that it outlines in detail and analyzes the performance of the TCP Vegas protocol. The three key techniques employed by TCP Vegas that are an improvement over the current protocol, TCP Reno, are a new timeout mechanism, a novel approach to congestion avoidance, and a modified slow-start mechanism. After detailing the improvements, the paper also presents extensive analysis on the performance improvements of TCP Vegas over TCP Reno. To perform this analysis, they built their own network simulator based on the x-kernel. The x-kernel-based simulator provided a realistic setting for evaluating protocols by modeling the actual C code rather than a more abstract version of it. The simulator used a protocol called TRAFFIC that implemented TCP Internet traffic based on tcplib.
  5. Summarize the (at most) 3 key main ideas (each in 1 sentence.) 
  6. The three 3 key main ideas are: (1) TCP Vegas uses three techniques to improve its performance over TCP Reno: A more timely decision to retransmit a dropped segment; The ability to anticipate congestion an adjust its transmission rate accordingly; Modifying the TCP slow-start mechanism to avoid packet losses while trying to find the available bandwidthduring the initial use of slow-start. (2) TCP Vegas achieves 37 to 71% better throughput and one-fifth to one-half as many as losses as TCP Reno. (3) TCP Vegas is just as fair as TCP Reno, does not suffer from stability problems, and does not adversely affect latency.
  7. Critique the main contribution
  8. What lessons should researchers and builders take away from this work. What (if any) questions does this work leave open?
  9. The lessons that researchers and builders should take away from this work are that it is possible to build a relatively accurate and elaborate real-world simulator to test the performance of new protocols and to evaluate their performance with current ones and that TCP Vegas presents itself as a very convincing improvement over TCP Reno that is more efficient and safe and easy to implement. The questions that this paper leaves open are the performance of TCP Vegas vs. TCP Reno in terms of Queue Behavior when better analytical tools become available to study their real-world performance to determine if latency is in fact affected by TCP Vegas. Another question would be to study TCP Vegas under different BSD variations such as having an ACK every segment, an ACK every segment, increasing the window during linear growth by one segment per RTT or half a segment per RTT per 1/8th of the maximum segment sizer per ACK received during that RTT, or using a time-stamp option. Finally, more work could be done to analyze TCP Vegas' effect on fairly and effectively allocating resources on the Internet, in terms of guaranteeing bandwidth to real-time connections, or using select ACKs to decrease the number of unnecessarily retransmitted packets. Also, the situations in which the addition of Vegas* would be more beneficial and its actual performance in such situations have yet to be studied. Finally, the most important work that may follow this paper is actual empirical tests of the TCP Vegas protocol on the real Internet.