Paper Review: TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms

Reviewer: Mark Meras (mm446)

Main Contribution

This paper by Stevens documents four algorithms used by TCP to deal with packet congestion. Two of them, Slow Start and Congestion Avoidance, are required for a TCP, while the other two, Fast Retransmit and Fast Recovery Algorithm, are new improvements over the TCP requirement.

Key Ideas

Slow Start deals with congestion by avoiding the usual behavior of sending as many segments as possible (i.e. the advertised window size). Instead, it increases the congestion window, cwnd, by the number of ACKs it receives. Until congestion occurs, Slow Start will therefore create exponential growth in the congestion window size.
Congestion Avoidance deals with avoiding congestion once the congestion window gets to an "average" size. Once cwnd gets to half the size of its previous value (i.e. when congestion occured), the growth in cwnd is linear, not exponential. The half of the previous value is stored in ssthresh, the slow start threshold size.
Fast Retransmit and Fast Recovery are recent algorithms that adjust the cwnd and ssthresh to allow congestion control to be used without slow start. This is to allow congestion control to take place without immediately decreasing the number of segments sent back to 1.

Critique of Contribution

The RFC does a good job explaining slow start and congestion control
However, the details on Fast Retransmit and Fast Recovery are very skimpy. It would seem those sections would be of the most value, as they are not in the TCP requirements.
The contribution is not a new design or analysis, but only a review. In that respect, it is a successful one.