In-band control

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In-band control is a characteristic of network protocols with which data control is regulated. In-band control passes control data on the same connection as main data. Protocols that use in-band control include HTTP and SMTP. This is as opposed to Out-of-band control used by protocols such as FTP.

Here is an example of an SMTP client-server interaction:

Server: 220 example.com   Client: HELO example.net   Server: 250  Hello example.net, pleased to meet you   Client: MAIL FROM: <[email protected]>   Server: 250 [email protected]... Sender ok   Client: RCPT TO: <[email protected]>   Server: 250 [email protected] ... Recipient ok   Client: DATA   Server: 354 Enter mail, end with "." on a line by itself   Client: Do you like ketchup?   Client: How about pickles?   Client: .   Server: 250 Message accepted for delivery   Client: QUIT   Server: 221 example.com closing connection 

SMTP is in-band because the control messages, such as "HELO" and "MAIL FROM", are sent in the same stream as the actual message content.

Out-of-band control

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