What does "An out-of-band data transmission mechanism may be supported" mean, in socket man page

Question

The man pages for socket contains the below: SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported.

What does this mean " An out-of-band data transmission mechanism may be supported. "...?

Answer 1

Out-of-band (OOB) means "communication that you do that is not the actual data that you are communicating".

For example, if you are in a "conversation" with an end point - it is sending you requests and you are sending it data - and it wants to tell you to slow down midway through a bunch of data, it would need an "out of band" transmission mechanism to say "whoa, stop for a moment".

http://en.wikipedia.org/wiki/Out-of-band_data

The way it works is that each piece of information received can be flagged as OOB - if this is the case, the receiver looks at it to see what the control message is, instead of passing it as part of data transmission.

What it means by "may be supported" is that sockets can be transported over a range of protocols, as determined by the domain of the socket. In order for SOCK_STREAM (in particular send(2)) to support OOB transmission, the underlying protocol also has to support it.

If you try to send OOB messages and it's not supported, you get an EOPNOTSUPP error from send(2) .