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.