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.
Server: 220 example.com Client: HELO example.net Server: 250 Hello example.net, pleased to meet you Client: MAIL FROM: <email@example.com> Server: 250 firstname.lastname@example.org... Sender ok Client: RCPT TO: <email@example.com> Server: 250 firstname.lastname@example.org ... 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.