The M_IOCPROTO message is the same as the M_PROTO message type, except for the priority and the following additional attributes. When an M_PCPROTO message is placed on a queue, its service procedure is always enabled. The stream head allows only one M_PCPROTO message to be placed in its read queue at a time. If an M_PCPROTO message is already in the queue when another arrives, the second message is discarded and its message blocks freed.
This message is intended to allow data and control information to be sent outside the normal flow control constraints.