Congestion Avoidance and Control

Summary

This paper provides some simple algorithms to deal with congestion avoidance and control. It also gives some examples of what happens with and without those algorithms. Specifically, the authors put forth 7 new algorithms: RTT variance estimation, exponential retransmit timer backoff, slow-start, more aggressive receiver ack policy and dynamic window sizing on congestion. They determined that the main cause for congestion control is a violation of conversation of packets, which can be caused by (i) the connection failing to get to equilibrium, (ii) sender injecting a new packet before old one has exited and (iii) by being unable to reach equilibrium because of resource limits along the path.

To solve the first problem, they propose the slow-start algorithm. This algorithm increases the congestion window by 1 each time it gets an ack for new data. One the first problem is solved, the second problem is caused by a incorrect implementation of the retransmit timer. The authors say that not estimating the variation in RTT is usually the cause of an incorrect implementation and they provide some suggestions on how to improve it. They also state that exponential backoff is the best scheme for backoff after a retransmit. Finally, for the third problem, they suggest using a retransmit to indicate that the network is congested. Once the sender knows this, they should use the additive increase/multiplicative decrease algorithm to adjust their congestion window size.

Finally, the authors give information about their future work. It involves using the gateways to ensure fair sharing of the network capacity. They argue that the gateway should 'self-protect' against misbehaving hosts. They finish off by mentioning work on being able to detect congestion early because it can be fixed faster the earlier it is detected.

Criticisms & Questions

I liked that they talked about how to deal with uncooperative hosts and suggested gateways as a possible place to automatically moderate them. I would like to know more about this mechanism and if it's been implemented, whether or not it works in the real world.

I also liked that they point out that congestion avoidance and slow-start are 2 different algorithms with 2 different objectives. I was a little confused about that myself and was glad to have that cleared up.