24.5 Summary

TCP does not have true out-of-band data. It provides an urgent pointer that is sent in the TCP header to the peer as soon as the sender goes into urgent mode. The receipt of this pointer by the other end of the connection tells that process that the sender has gone into urgent mode, and the pointer points to the final byte of urgent data. But all the data is still subject to TCP's normal flow control.

The sockets API maps TCP's urgent mode into what it calls out-of-band data. The sender goes into urgent mode by specifying the MSG_OOB flag in a call to send. The final byte of data in this call is considered the out-of-band byte. The receiver is notified when its TCP receives a new urgent pointer by either the SIGURG signal, or by an indication from select that the socket has an exception condition pending. By default, TCP takes the out-of-band byte out of the normal stream of data and places it into its own one-byte out-of-band buffer that the process reads by calling recv with the MSG_OOB flag. Alternately, the receiver can set the SO_OOBINLINE socket option, in which case, the out-of-band byte is left in the normal stream of data. Regardless of which method is used by the receiver, the socket layer maintains an out-of-band mark in the data stream and will not read through the mark with a single input operation. The receiver determines if it has reached the mark by calling the sockatmark function.

Out-of-band data is not heavily used. telnet and rlogin use it, as does FTP; they all use it to notify the remote end of an exceptional condition (e.g., client interrupt), and the servers discard all input received before the out-of-band mark.