Congestion Avoidance and Control

Jacobson, V. 1988. Congestion avoidance and control. In Symposium Proceedings on Communications Architectures and Protocols (Stanford, California, United States, August 16 - 18, 1988) 

The underlying idea highlighted by this paper is that most of the problems arise from incorrect implementations of transport layer protocols rather than the protocols themselves. In view of this, the paper highlighted plenty of instances where things could go wrong and described some algorithms to set them right. The whole paper is set in a very practical approach, and is a result of the work done in solving the problem of a sudden massive throughput decrease between LBL and Berkeley due to network congestion in October 1986.

 The following contributions were proposed by the paper:

• Slow start algorithm: This was an algorithm to establish the initial flow. It started off with a one packed window and then kept on doubling the size of the window whenever all the current packets sent by the window were acknowledged (limited by the max size of the window wmax).
  
• Round-trip-time variance estimation: This talks about estimating the mean round trip time R = aR + (1 - a)M where R is the average RTT estimate and M is the recently calcuated RTT time from tthe network.
• Exponential retransmit timer backoff: This deals with how should the transmits be spaced if the packet has to be re-transmitted. The author claims (with a little bit of intuitive explanation) that exponential retransmit works best in this case.
• Dynamic window resizing on congestion: This is more of a caveat taken fromt he last paper, and talks about additive increase (by 1/(window size)) and multiplicative decrease (by 0.5).

Further, the authors claim that if fair sharing has to be taken into account, the congestion detection must be implemented at gateways instead of end points (End-to-end Principle violation?).  In the end, the paper has a very informative appendix which gives a good insight into the implementation of the above proposed algorithms.

Overall, I felt the paper to be perfectly complimentary to the previous paper.  It carefully highlighted the practical aspects of the congestion avoidance problem highlighted in the previous paper. I felt that if I had taken a course in queuing theory, I would have been in a much better position to appreciate the subtleties in this paper. It would be great if we could have a short optional reading on queuing theory along with this paper. Moreover, I am not very convinced by the author's argument of using b = 0.5 instead of 7/8 in the original paper. Though the reason highlighted is that it is due to the nature of  slow start algorithm, I somehow find it difficult to appreciate it. Doesn't slow start run at the beginning just for establishing the flow? Once the flow is established, subsequent throughput is increased/decreased by the additive increase /multiplicative decrease. In this case, halving the window size seems to be pretty harsh!