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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Internet Message Access Protocol (IMAP/IMAP4)

IMAP General Operation, Client/Server Communication and Session States 1 2 3 4 IMAP Not Authenticated State: User Authentication Process and Commands

Once an IMAP session is established, all communication between the client and server takes place in the form of commands sent by the client and responses returned by the server. Like POP3, commands and responses are sent as strings of ASCII text and terminated with a “CRLF” sequence, making them compatible with the way data is sent using the Telnet Protocol. However, IMAP does a few things quite differently than POP and many other TCP/IP application protocols.

The first interesting thing about IMAP commands is that most are not abbreviated into codes of three or four letters—they are spelled out in full. So, where POP3 has a STAT command, the one in IMAP is called STATUS. Commands are normally shown in upper case, as I do in this Guide, but are in fact case-insensitive.

IMAP also uses an interesting system of command tagging to explicitly match client commands with certain server responses. Each time a client sends a command, it prefixes it with a tag that is unique for the particular session. The tags are usually short strings with a monotonically increasing number in them; the examples in the IMAP standards have the first command tagged “a0001”, then the second “a0002” and so on. That said, as long as each command is uniquely labelled, it doesn't matter what tagging scheme is used. When the server needs to send a response that is specific to a command, it tags the reply with the appropriate command tag. Not all replies are tagged, however.

The standard doesn't state explicitly why this tagging scheme is needed, but I believe it is probably related to IMAP's multiple command feature. IMAP clients are allowed to send a sequence of commands to the server to be processed, rather than only sending commands one at a time. This can improve performance when certain commands would take a long time to complete. The only restriction is that the commands must be independent enough that the result of executing them all would be the same regardless of the order in which they were processed. For example, sending a command to read a particular entity in combination with a command to store a value into the same entity is not allowed.

IMAP General Operation, Client/Server Communication and Session States 1 2 3 4 IMAP Not Authenticated State: User Authentication Process and Commands