Internet Core Protocols: The Definitive Guide

Presentation Services

Whenever application protocols wish to communicate with each other, they must do so using a predefined set of rules that define the types of data that will be exchanged. For example, if an application protocol is to use textual data, then those characters must have the same byte-order and binary values on both systems. For example, one system cannot use US-ASCII while the other system uses EBCDIC characters. Nor can one system pass data in “big-endian” form to a processor that only understands “little-endian” data, since the bits will be interpreted backwards.

For these reasons, the application protocols must agree to use certain types of data, and must also agree on how to present that data so that it is interpreted consistently. Typically, this falls under the heading of “presentation layer services,” with some network architectures providing detailed presentation-layer specifications that cover everything from character sets to numeric formatting rules. However, TCP/IP does not have a formally defined presentation layer. Instead, it has many informal mechanisms that act as presentation layers, with each of them providing specific kinds of presentation services to different kinds of applications.

Most of the application protocols used on the Internet today use the Network Virtual Terminal (NVT) specification for presentation services. NVTs are a subset of the Telnet specification, and provide a basic terminal-to-terminal session that applications use to exchange ...