Information Formats

The data and control information that is transmitted through internetworks takes a variety of forms. The terms used to refer to these information formats are not used consistently in the internetworking industry but sometimes are used interchangeably. Common information formats include frames, p acketS| datagrams, segments, messages, cells, and data units.

A frame is an information unit wOose sou rce and destination are data link layer entities. A frame is composed of the data link layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper-layer entities is encapsulated in the data link layer header and trailer. Figure 1-9 illustrates the basic components of a data link layer frame.

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