3.8. Communications software

A communications protocol is a set of rules for controlling communication. It may define a particular, ordered sequence of messages to be used in a communication. The order is agreed by convention between the communicating entities to satisfy their requirements. A simple example, taken from a different context, is:

message:'Hi beta, this is alpha, are you receiving me? OVER'
reply:'Yes alpha, this is beta, I'm receiving you. OVER'

The above two messages implement a connection establishment protocol.

An example of an application protocol (see Figure 3.16) is a client's interaction with a file server, for example:

file-id =open (filename, write-mode)
data-bytes =read (file-id, byte-range)
 close (file-id)
Figure 3.16. ...

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