The previous chapter gave an overview of the ISO seven-layer model, describing in some detail the lower layers. In this chapter, we will discuss a class of layer 7— application protocols known as directory services. NIS is an example of such a directory service.
The purpose of a directory service is to map names of one form to names of another form. Often the names of the first form are alphanumeric strings, and the second form are numbers. Or the names of the first form are simple, whereas the names of second form are complex. In the days before computing, we used directories, but they were published on paper. The most obvious one, and perhaps the inspiration for network directory services, is the telephone book. A typical telephone book for a city in the United States consists of three directories:
The white pages of residence listings
The white pages of business listings
The yellow pages of business listings
The residence white page listings contains a list of names, last name first, and for each entry, the telephone number. In many cases, each entry contains the street address of the residence. Thus you can think of the residence white pages in a telephone book, as a way to direct you from a person's name to his telephone number and address. Hence, a telephone book's formal name is a telephone directory.
The white pages of residence listings can be thought of as a "structured set of data." If this data were ...