Each chapter begins by describing one central problem that motivates it, and ends with a discussion of an "open question" — the relationship between IP and ATM, for example, or the balance between handling congestion control inside and outside networks. The effect is to convey a feel for networking as a dynamic and evolving field. Side boxes contain digressions, a device which works quite well here.
It's hard to put my finger on exactly what sets Peterson and Davie's offering apart from the many other introductions to networking available. The depth of their coverage, certainly, and also their good coverage of implementation and architecture issues. But the key to their success is what they call a "systems approach", a concentration on first principles and basic building blocks and concepts. So there are no lengthy descriptions of protocols for their own sake — they are used instead to illustrate fundamental ideas. (Protocols are, after all, merely solutions to problems.) If the focus is very much on the Internet and internet protocols, that is because they are the only systems which address some problems.
A Systems Approach really does manage to highlight the most important ideas. It is also an attractive and clearly laid out volume and, despite the price, one which is good value for money. It is easily the best general networking textbook I have seen. Though it may daunt weaker students, it would be suitable as a text for higher undergraduate courses; it is the first book I would recommend to computing professionals approaching networking for the first time or seeking to refresh their knowledge of the field.
Note: a third edition of Computer Networks: A Systems Approach was published in 2003.