Socket protocols

Socket protocols provide the network transportation of application data from one machine to another (or from one process to another within the same machine).

The application specifies the transport provider on the protocol parameter of the socket() API.

The AF_INET and AF_INET6 sockets over TCP/IP can also specify a SOCK_RAW type, which means that the socket communicates directly with the network layer known as Internet Protocol (IP). The TCP or UDP transport providers normally communicate with this layer. When you use SOCK_RAW sockets, the application program specifies any protocol between 0 and 255 (except the TCP and UDP protocols). This protocol number then flows in the IP headers when machines are communicating on the network. In fact, the application program is the transport provider, because it must provide for all the transport services that UDP or TCP transports normally provide.

For the AF_UNIX and AF_UNIX_CCSID address families, a protocol specification is not really meaningful because there are no protocol standards involved. The communications mechanism between two processes on the same machine is specific to the machine.