Congestion Avoidance and Control

This paper presented techniques to avoid congestion in the network, which stemmed from the data throughput rate from LBL to Berkeley being dropped from 32Kbps to 40 bps. It states that congestion is not caused by the protocol itself, but the implementation of the protocol. I think although this might be true in the TCP case, but not all protocols are designed to avoid congestion, and a poorly designed protocol would still run into the problem even with good implementation. Thus, the congestion avoidance mechanism much also be built into the protocol. The author also generalizes all congestion avoidance algorithms to contain a conversation of packet principle.

It then lists the several techniques (which are mostly pretty simple but effective) to be able to avoid congestion. First is a slow start, which restricts communications to start slower, and increase if there is bandwidth. The problem with starting too slow is that it's overhead and wasting bandwidth that could be used to transfer. But starting too fast between two different linked speeds would cause congestion (it's interesting that some of these methods are enforced to maintain better compatibility with older connection links, which brings us back to the previous week's paper that internet needs to support a wide range of networks).

The second technique is round trip timing, which is used to calculate when a packet is dropped from the network. This is important because if the calculation is not conservative enough, we would start inserting the packet retries earlier that the packet is dropped, wasting bandwidth. The third technique is exponential back off, where the sender adjusts to the link. More proof on why exponential back off is required in the next paper. The future work is interesting in that it is also related to the end to end argument, that not just the end points should implement avoidance, but maybe the gateways as well, to improve the congestion control.

Congestion control is not just limited to networks, traffic jam is the perfect example. They employ techniques like slowing down incoming traffic by using stop lights at freeway entrances to slow the incoming rate (back off). Without congestion control, then we are very vulnerable to congestion collapses as mentioned in the paper, which will easily turn a high bandwidth link to a low one, just like with the LBL and UCB link.