# 1.6 Summary

Computer networks, and in particular the Internet, have experienced enormous growth over the past 30 years and are now able to provide a wide range of services, from conducting business to providing access to entertainment to enabling social networks. Much of this growth can be attributed to the general-purpose nature of computer networks, and in particular to the ability to add new functionality to the network by writing software that runs on affordable, high-performance computers. With this in mind, the overriding goal of this book is to describe computer networks in such a way that when you finish reading it you should feel that, if you had an army of programmers at your disposal, you could actually build a fully functional computer network from the ground up. This chapter lays the foundation for realizing this goal.

The first step we have taken toward this goal is to carefully identify exactly what we expect from a network. For example, a network must first provide cost-effective and scalable connectivity among a set of computers. This is accomplished through a nested interconnection of nodes and links and by sharing this hardware base through the use of statistical multiplexing. This results in a packet-switched network, on top of which we then define a collection of process-to-process communication services.

The second step is to define a layered architecture that will serve as a blueprint for our design. The central objects of this architecture are network protocols. Protocols both provide a communication service to higher-level protocols and define the form and meaning of messages exchanged with their peers running on other machines. We have briefly surveyed two of the most widely used architectures: the 7-layer OSI architecture and the Internet architecture. This book most closely follows the Internet architecture, both in its organization and as a source of examples.

The third step is to implement the network's protocols and application programs, usually in software. Both protocols and applications need an interface by which they invoke the services of other protocols in the network subsystem. The socket interface is the most widely used interface between application programs and the network subsystem, but a slightly different interface is typically used within the network subsystem.

Finally, the network as a whole must offer high performance, where the two performance metrics we are most interested in are latency and throughput. As we will see in later chapters, it is the product of these two metrics—the so-called delay $\times$ bandwidth product—that often plays a critical role in protocol design.

Computer networks are not the first communication-oriented technology to have found their way into the everyday fabric of our society. For example, the early part of the last century saw the introduction of the telephone, and then during the 1950s television became widespread. When considering the future of networking—how widely it will spread and how we will use it—it is instructive to study this history. Our first reference is a good starting point for doing this (the entire issue is devoted to the first 100 years of telecommunications).

The second reference is considered one of the seminal papers on the Internet architecture. The final two papers are not specific to networking but present viewpoints that capture the "systems approach" of this book. The Saltzer et al. paper motivates and describes one of the most widely applied rules of network architecture—the end-to-end argument—which continues to be highly cited today. The paper by Mashey describes the thinking behind RISC (Reduced Instruction Set Computer) architectures; as we will soon discover, making good judgments about where to place functionality in a complex system is what system design is all about.

• Pierce, J. Telephony—A personal view. IEEE Communications 22(5):116-120, May 1984.

• Clark, D. The design philosophy of the DARPA Internet protocols. Proceedings of the SIGCOMM '88 Symposium, pages 106-114, August 1988.

• Saltzer, J., D. Reed, and D. Clark. End-to-end arguments in system design. ACM Transactions on Computer Systems 2(4):277-288, November 1984.

• Mashey, J. RISC, MIPS, and the motion of complexity. UniForum 1986 Conference Proceedings, pages 116-124, February 1986.