TCP supports urgent data.  Urgent data is used to signal the receiver that some important message is part of the data stream and that it should be processed as soon as possible.

To send urgent data specify the MSG_OOB option to send().  When urgent data is received, the kernel sends a SIGURG signal to the process or process group that has been  set  as

the  socket  "owner" using the SIOCSPGRP or FIOSETOWN ioctls (or the POSIX.-specified fcntl() F_SETOWN operation).  When the SO_OOBINLINE socket option is enabled, urgent data

is put into the normal data stream (a program can test for its location using the SIOCATMARK ioctl described below), otherwise it can be received only when the MSG_OOB  flag  is

set for recv() or recvmsg().