we'll discuss the advantages of breaking complicated applications up
into cooperating processes speaking an application-specific command
set or protocol with each other. All the good reasons for data file
formats to be textual apply to these application-specific protocols as
When your application protocol is textual and easily parsed
by eyeball, many good things become easier. Transaction dumps become
much easier to interpret. Test loads become easier to write.
Server processes are often invoked by harness programs such as
in such a way that the server sees commands on standard input and
ships responses to standard output. We describe this “CLI
server” pattern in more detail in Chapter11.
A CLI server with a command set that is designed for simplicity has
the valuable property that a human tester will be able to type
commands direct to the server process to probe the software's
Another issue to bear in mind is the end-to-end design
principle. Every protocol designer should read the classic
End-to-End Arguments in System Design [Saltzer]. There are often serious questions about which
level of the protocol stack should handle features like security and
authentication; this paper provides some good conceptual tools for
thinking about them. Yet a third issue is designing application
protocols for good performance. We'll cover that issue in more detail
The traditions of Internet application protocol design evolved
separately from Unix before 1980.
But since the 1980s these traditions have become thoroughly
naturalized into Unix practice.
We'll illustrate the Internet style by looking at three
application protocols that are both among the most heavily used, and
are widely regarded among Internet hackers as paradigmatic: SMTP,
POP3, and IMAP. All three address different aspects of mail transport
(one of the net's two most important applications, along with the
World Wide Web), but the problems they address (passing messages,
setting remote state, indicating error conditions) are generic to
non-email application protocols as well and are normally addressed
using similar techniques.
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