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ASCII Request/Reply

ASCII Request/Reply protocols include FTP, HTTP, SMTP, POP, IMAP and others. They all run over reliable stream connections (TCP), and are designed to be usable by either a program or a human (usually for testing purposes). They all feature the following common attributes:

Protocols need not strictly confirm to this outline. For example, the IMAP protocol introduces the idea of tagged commands and replies. Each client command is prefixed with a tag, which the server echos back in the reply, allowing replies to be matched to commands.

ASCII request/reply protocols use the underlying TCP protocol to provide flow control and windowing. A state machine, specified as part of the protocol, typically sequences the messages. Timers on both the client and server place an upper limit on how long each will wait for a message from the other side, disconnecting the session if it becomes hung.

RFC 821 includes several sample SMTP sessions which illustrate how this kind of protocol operates.

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