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A protocol is a controlled sequence of
messages that is exchanged between two or more systems to accomplish a
given task. Protocol specifications define this sequence together with
the format or layout of the messages that are exchanged. In
coordinating the work between systems, protocols use control
structures in each system that operate like a set of interlocking
gears. This is so computers can precisely track their points as they
move through the sequence of exchanges. Timing is crucial to network
operation. Protocols require messages to arrive within certain time
intervals, so systems maintain one or more timers during protocol
execution. They also take alternative actions if the network does not
meet the timing rules. To do their work, many protocols depend on the
operation of other protocols in the group or suite of protocols.
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