The speeds of communications facilities at the physical and link levels have improved rapidly in recent times as represented by gigabit rate point-to-point fiber-optic links and high-speed local area networks such as the fiber distributed data interface (FDDI). It has been apparent for some time, however, that the intricate procedures needed to achieve end-to-end reliable transfer in complex networks can become bottlenecks in fully exploiting the potential high link-level rates for communications among hosts. This limitation led to the search for “lightweight” higher-layer protocols for more efficient implementation of these services. This book describes the Xpress Transfer Protocol (XTP), one of the more recent of the class of lightweight protocols, whose development also involved considerations of its implementation in hardware. The book is thus a timely contribution to the literature on network protocols.
The book is a research and design monograph on XTP, with some discussion of other transport protocols in general. It is intended to serve as a professional reference for scientists, engineers and engineering managers, and researchers and advanced students of communications protocols. It serves this purpose well. The book has nine chapters. A good selection of pertinent references appears at the end of each chapter. A judicious use of well-designed figures and tables guides the reader through the material.
Chapter 1 explains the rationale and motivation for developing XTP, from the points of view of both expanded functionality compared to the existing protocols and hardware implementation. Chapter 2 briefly describes the OSI model and its layers (with emphasis on the network and transport layers) and provides a setting to explain XTP functions. Next is a brief overview of XTP’s data communication model (single-node and end-to-end views) and service requirements. Chapter 3 summarizes other transport protocol designs that influenced XTP development, such as transmission control protocol (TCP), ISO transport protocol (TP), Delta-t, network block transfer (NETBLT), GAM-T-103, versatile message transaction protocol (VMTP), Datakit, and universal receiver protocol. Descriptions of each of these protocols are interlaced with comparisons and relationships of their functionality to that of XTP. Table 3.1 gives an informative summary of comparisons of the mechanisms and services of these protocols.
Chapters 4 through 6 give detailed descriptions of XTP protocol procedures, packet structures, and packet formats, respectively. Chapter 7 discusses addressing and encapsulation information needed for carrying a packet through intermediate XTP nodes. Chapter 8 discusses reliable group communications needs and the XTP multicast procedures, which are particularly important in the context of emerging multimedia group communications. Chapter 9 gives a brief description of XTP implementation using VLSI circuits in the Protocol Engine Project.
One missing element in the book is a discussion of formal validation of the protocol. As the authors stress the richer functionality and streamlining of XTP compared to other transport protocols, it is reasonable for a reader to be concerned about possible inconsistencies in the protocol. Discussion on any investigations of verification of the protocol would have been informative.
Overall, this book is a valuable addition to the body of literature on high-speed, lightweight protocols.
