12.1 Introduction -- The AF_X25 Domain

This chapter assumes some familiarity with SunOS sockets and address domains (families). Briefly, the socket layer of the network system deals with the interprocess communications provided by the system. A socket is a descriptor that acts as a bidirectional endpoint for communications and is "typed" by the semantics of the communications it supports. The type of the socket is defined at socket creation time and used in selecting those services which are appropriate to support it. The socket type SOCK_STREAM provides sequenced, reliable, two-way, connection-based byte streams with an out-of-band data transmission mechanism. An address domain specifies an address format which is used to interpret addresses specified in later operations using the socket.

Solstice X.25 defines an address domain, AF_X25. Within this domain only the socket type SOCK_STREAM is supported. Like other SOCK_STREAM sockets, an AF_X25 domain socket is composed of two byte streams: an in-band stream and an out-of-band stream. However, unlike other sockets, there are two different kinds of out-of-band messages: X.25 status and interrupt data.