Principles of Protocol Design

This book introduces the reader to the principles used in the construction of a large range of modern data communication protocols. The approach we take is rather a formal one, primarily based on descriptions of protocols in the notation of CSP. This not only enables us to describe protocols in a concise manner, but also to reason about many of their interesting properties and formally to prove certain aspects of their correctness with respect to appropriate specifications. Only after considering the main principles do we go on to consider actual protocols where these principles are exploited. This is a completely new edition of a book which was first published in 1994, where the main focus of many international efforts to develop data communication systems was on OSI – Open Systems Interconnection – the standardised architecture for communication systems developed within the International Organisation for Standardization, ISO. In the intervening 13 years, many of the specific protocols developed as part of the OSI initiative have fallen into disuse. However, the terms and concepts introduced in the OSI Reference Model are still essential for a systematic and consistent analysis of data communication systems, and OSI terms are therefore used throughout. There are three significant changes in this second edition of the book which particularly reflect recent developments in computer networks and distributed systems. Firstly, the rise of the Internet has led to the development of large numbers of new protocols, particularly in the Application Layer, and a new chapter has been introduced focussing on some of the most important of these. These new protocols typically use new styles of encoding, particularly ASCII encodings such as MIME and XML, and rely on middleware solutions based on a variety of support protocols. Chapters 8 and 10 have therefore been extended to deal with these topics. Finally, there is today much more focus on security issues in networks. Chapter 6, which deals with the general concepts of security, has been heavily revised and brought up to date, while Chapters 8, 9, 10 and 11 now include sections discussing specific technologies such as IPsec, SSL/TLS, and secure protocols for e-mail and webbased applications. The book has arisen from material used for teaching a course on Distributed Systems at the Technical University of Denmark. The exercises at the ends of Chapters 2 to 8 also originate in this course, either as weekly assignments or, in some cases, as examination questions. I hope you find them as interesting—and maybe even as challenging—as my students have done. This text was originally written for third or fourth year students in Computer Science or Electrical Engineering, and is intended to be suitable for most final-year undergraduate or postgraduate courses on advanced data communications or computer networks. The reader is expected to have a software background, in particular including a basic knowledge of functional programming and parallel programming, combined with some knowledge of computer systems architecture and data transmission. Knowledge of formal methods, for example based on languages such as VDM or Z, is not essential, but to get the most out of the book you should know about the sort of discrete mathematics which is used in computer science, and be aware of the basic concepts of mathematical proof. Many people deserve thanks for helping me in the task of preparing this text, and especially for reading and commenting on the various drafts which have seen the light of day. I would particularly like to thank my colleagues Hans-Henrik Løvengreen of this department, Arne Skou of Aalborg University, Klaus Hansen of the University of Copenhagen and Henrik Reif Andersen of the IT University in Copenhagen for their comments on important parts of the text. I also owe a great debt of thanks to my many students, who have been exposed to several provisional versions of this book, a fate which they have borne stoically. Their help in finding errors and pointing out the shakier parts of the presentation, where the arguments were weakest or the explanations most difficult to follow, has been invaluable. Finally, I would like to thank my wife Lisbeth and daughter Melissa for continuing to be so tolerant about having an author in the house. The competition for the PC at home has at times been intense – and they have been really sweet about letting me win. . . . Technical University of Denmark, Robin Sharp October 2007.