13.  Out of band data

      Out of band data is a facility peculiar to the stream socket abstraction defined. Little agreement appears to exist as to what its semantics should be. TCP defines the notion of ``urgent data'' as in-line, while the NBS protocols [Burruss81] and numerous others provide a fully independent logical transmission channel along which out of band data is to be sent. In addition, the amount of the data which may be sent as an out of band message varies from protocol to protocol; everything from 1 bit to 16 bytes or more.

      A stream socket's notion of out of band data has been defined as the lowest reasonable common denominator (at least reasonable in our minds); clearly this is subject to debate. Out of band data is expected to be transmitted out of the normal sequencing and flow control constraints of the data stream. A minimum of 1 byte of out of band data and one outstanding out of band message are expected to be supported by the protocol supporting a stream socket. It is a protocol's prerogative to support larger-sized messages, or more than one outstanding out of band message at a time.

      Out of band data is maintained by the protocol and is usually not stored in the socket's receive queue. A socket-level option, SO_OOBINLINE, is provided to force out-of-band data to be placed in the normal receive queue when urgent data is received; this sometimes amelioriates problems due to loss of data when multiple out-of-band segments are received before the first has been passed to the user. The PRU_SENDOOB and PRU_RCVOOB requests to the pr_usrreq routine are used in sending and receiving data.