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Network Congestion Control, M. Welzl, Wiley 2005. (A Book Review)

Posted by David Mayer on August 31, 2007

Network Congestion Control: Managing Internet Traffic — Michael Welzl (Wiley Series on Communications Networking & Distributed Systems 2005).

The control of Internet congestion is a field entangling numerous research, engineering and economic topics, each of which, on itself, offers only a limited perspective on the problem. Some books on this topic treat it as a part of a broader field of network communications, others provide a narrow formal mathematical exposition. Michael Welzl’s book is an exception. It exposes congestion control in several facets, always succinctly explaining the principles and anticipating reader’s questions. Aimed mainly at starting PhD students and interested networking people outside the research community, it avoids formal mathematics and builds up the material through common sense. But it is far from trivial: the author provides a profound overview of the principles, problems and solutions and manages to cover a vast number of diverse topics, focusing on underlying principles, practicality, drawbacks. The author co-chairs the Internet Congestion Control Research Group and works at the University of Innsbruck in Austria.

Essential introduction into the problematics is developed in chapter 2, in which the less informed reader becomes familiar with the basic concepts such as control feedback, stability, queue management, scalability, incentives, fairness. Present technology is the topic of chapter 3, 70% of which takes a fairly detailed description of TCP. The exposition is incremental and problem-motivated. The rest of this chapter briefly describes SCTP, RED and ATM’s Available Bit Service.

The main value of the book I see in its latter part. Chapter 4 is an excellent exposition of very recent experimental enhancements to congestion control. Most of this chapter is dedicated to TCP enhancements and active queue management enhancements. About 10 recent queue management techniques are presented, including such specialities as RED with Preferential Dropping and Stochastic Fair BLUE. A view from the Internet provider’s side is briefly treated in chapter 5. The topics here share one property: they operate on a much larger timescale than those of other chapters. They include Traffic Engineering, MPLS and QoS architectures. Although the level of detail here is much lower than in other chapters, the chapter puts the others into a perspective. The last chapter makes for an exciting read: it presents a mixture of open problems waiting for the hungry PhD mind. It also contains the author’s personal view on the subject. It could be characterised as common-sense reflections of a well-informed pragmatic.

Two appendices conclude the book. Appendix A shows some practical teaching techniques, Appendix B introduces the workings of the IETF.

As great as the book seems, I spotted a few over-simplifications:
Section 2.17.3 describes the concept of proportional fairness. The author states here that the maximization of total utility maximises financial gain. I think this is quite misleading because the term financial gain remains undefined and it is not clear from the text where the revenue comes from. Even if the reader knew it is here meant to come from congestion prices, this would be still problematic. It is true that revenue is maximised if prices charged equal to the congestion prices corresponding to the utility maximisation, but congestion prices are typically not meant to generate revenue as they serve as a control mechanism.
Further the author states that the maximisation of utility functions can be seen as an “optimisation (linear programming)”. But only the constraints are linear and the objective function is non-linear, concave, hence the optimization problem is nonlinear.
In section 5.1 the author explains long-range dependence of Internet traffic. It is here stated that Poisson process’s distribution flattens out as the timescale grows. Surely it is meant that the autocorrelation function flattens out. (As opposed to that of self-similar Internet traffic.) [Update: See comments below by the book’s author. ]

A word of warning for PhD students
This book is a very good one but I think one should be cautious about using some of the open problems as a basis of one’s PhD. The problems border with engineering rather than research and in some colleges solving these problems cannot satisfy PhD requirements by definition. Ideally, solutions to these problems will come about as a byproduct of a more fundamental framework called the thesis. Perhaps these problems can motivate and trigger a PhD topic, but they should not constitute it.

Any alternative books out there?
A book treating congestion control in detail is The Mathematics of Internet Congestion Control by R. Srikant, 2004, Birkhäuser, but be aware: the exposition is quite straightforward and very formal. Another adept is High Performance TCP/IP Networking by M. Hassan and R. Jain, 2003, Prentice Hall, which is limited to TCP and focuses on performance evaluation.

In summary, the material in this unique book is excellently exposed, a good balance between depth and clarity is kept throughout and the book should be keenly received by its audience.

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