Congestion Avoidance and Control - Review by Di Niu

Review: Congestion Avoidance and Control

Reviewer: Di Niu

This paper points out several congestion control problems the old TCP
protocol suffered from and proposed new effective modifications to
some these problems. New modifications include a slow-start
algorithm, a better round trip time estimator and a congestion
avoiding scheme.

The slow-start algorithm was proposed to guarantee that the
connection can indeed reach a steady state. It tackles the congestion
problem when a connection is starting or restarting after a packet
loss. Although the connection could perform with little congestion in
equilibrium given a perfect retransmit timer, yet there is a problem
about how to reach this equilibrium. This is because data
transmission and acks are mutually dependent. The new algorithm tries
to send data at a low rate at first and then increase the
transmission load gradually and carefully.

It then proposes a more accurate way to estimate round trip time. It
is noted that the original TCP protocol did not estimate the
variation of the round trip time, while this variation could be large
in practice and cause problems of spurious retransmissions. Another
problem of the original timer is how should the retransmits be
spaced? The authors propose a cheap method for estimating variation,
and an exponential backoff algorithm to finesse the retransmission.

A congestion avoidance strategy is also proposed. The main components
of this strategy include a network traffic estimation model and a
window size control policy at endpoints.

The paper contributes into the TCP congestion avoidance techniques
significantly. However, though titled as "Congestion Avoidance and
Control", nearly all of its content is about congestion avoidance, be
it slow-start, round trip estimator or adapting to path. It did not
discuss how to recover a network to normal states as quickly as
possible, if it is indeed suffering from severe congestion. The basis
for every algorithm in this paper is that the network starts from
empty or normal states.
Received on Mon Sep 25 2006 - 17:33:32 EDT