Increase and Decrease

It was the spring of hope, it was the winter of despair, and so it is with protocols; the trick is knowing when you're in spring and when you're in winter. This paper is by far the more interesting of the two, since I think it manages to take the control problem of endpoints communicating at rates subject to interference by other senders and collapse it to a linear model where they demonstrate stability under conditions not depending on the number of participants. One key idea is to reduce the feedback from the network to a single congestion signal; of course, in TCP, there isn't actually a congestion bit but rather it infers congestion from loss; in any case, the idea is the same. The authors demonstrate how multiplicative increase and additive decrease lead to streams which are "fair" and "efficient" for all participants. The work was very closely contemporaneous with Congestion Avoidance and Control, and so represents an effort underway at the time to more thoroughly understand the conditions for stable transport protocols.

Without much of a background in control theory, the authors provide enough information on linear controls to assert that the major benefit these controls will have over nonlinear (say, quadratic) control is that they will be less brittle because they will require less information about the state of the system to function; in particular, not knowing the number of participants in the control scheme seems like a key requirement for internet protocols where this number ("n") is going to vary by many orders of magnitude.

An interesting observation is that the authors seem to have been more concerned with the OSI suite then the internet protocols. The OSI protocols, although eventually finished and implemented, were generally regarded as failing because they were too complicated and were never available in partly-finished yet function forms like TCP was. By the time solid implementations were available, it was too late; this papers seems to be a symptom of the impulse to research and design every facet of the protocols; while this can probably be both a virtue and a vice, it certainly led to commercial failure...

The paper doesn't include any measurement studies of actual network connections, although it can serve to more fully explain figures 8 & 9 in Congestion Avoidance. I think this work is mostly valuable as an examination of the assumptions underlying the internet congestion control algorithms, and grounding them in a series of justified assumptions. They do note that they don't consider the asynchronous case where streams come and go rather then all start at once; this is actually an important divergence from the real world, although apparently the control scheme is stable in this regime as well.