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  
Client: HELO  
Server: 250  Hello, pleased to meet you  
Client: MAIL FROM: <>  
Server: 250 Sender ok  
Client: RCPT TO: <>  
Server: 250 ... 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 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.

See also[edit]

Out-of-band control