Paper Review: General AIMD Congrestion Control

Reviewer: Mark Meras (mm446)

Main Contribution

This paper by Yang and Lam discusses a new analysis of the additive increase/multiplicative decrease congestion control algorithm present in TCP. Their analysis generalizes the AIMD algorithm into the GAIMD algorithm (General AIMD) and shows that the parameters of the GAIMD can be set so that the congestion control is TCP friendly.

Key Ideas

We can generalize the AIMD scheme by adding two variables that control the two aspects of AIMD:
- alpha - the increase in the window size if there is no packet loss in round-trip time
- beta - the fraction of the window size that it is decreased to when packet loss is detected
A simple formula can be derived for the average sending rate of a flow controlled by GAIMD. This formula is a function of alpha, beta, loss rate, average RTT, average timeout value, and the number of packets acknowledged per ACK. This formula is validated up to 20% loss rate.
A relationship between alpha and beta is determined so that the resulting congestion control is TCP-friendly. This formula is: alpha = (1 - (beta * beta)) / 3

Critique of Contribution

This contribution is very important, as it provides and experimentally verifies two new formulae for calculation the mean sending flow rate as well as the relationship between alpha and beta
More importantly, the paper introduces and validates a general AIMD congestion control mechanism and shows it to be effective and TCP-friendly

Lesson

We can get better perfomance by generalizing the AIMD and tuning its parameters.

Open Question

What are the tradeoffs among rate fluctuations, responsiveness, and speed of convergence?