9.1 Introduction

Users see the network through the window of distributed applications, which carry out some combination of communication and computation to meet user needs. Familiar examples include web applications for shopping and banking, interactive applications such as video teleconferencing, Voice over Internet Protocol (VoIP) and chat, as well as music sharing and massive multiplayer games. What each of these applications has in common is a model of the interactions between the user and the system, and how the system supports interaction among sets of users.

The variety of applications listed above suggests that these models might differ substantially, but in the abstract these models share the need for a set of communications protocols that deliver network services required by the applications (Figure 9.1). Section 9.2 discusses specific requirements of typical applications, but as the understanding of networks and the variety of services they can provide has improved with experience, the commonality among many application requirements has been exploited to build network protocols that can be used by many applications. The general-purpose nature ("one size fits all") of such protocols is extremely attractive, but may sacrifice the ability to support specific applications as well as they might be supported by a dedicated and optimized protocol.

Figure 9.1: Protocols provide interfaces and services to applications

9.2 9.2 Applications and their Requirements

There is an increasing trend toward mobile applications in the commercial world,...