In-band control

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.

Example
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:  Server: 250 jane.doe@example.net... Sender ok Client: RCPT TO:  Server: 250 john.doe@example.com ... 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.