The Common INET layer (CINET) is inserted between the LFS and a sockets PFS to allow multiple AF_INET transports to be used by a single application socket. A sockets PFS can be attached directly to the LFS when it is the only AF_INET transport on the system, or attached through the CINET layer when it is one of several. To be attached to CINET, the PFS must implement the "master socket" and support several additional ioctl command types, as described in this topic. The interface to the PFS is the same in both cases. Once the additional support for CINET is written, the PFS does not have to distinguish between the two cases.

When Common INET is used, the sockets file system is initialized by the SUBFILESYSTYPE statement in the parmlib member, instead of by the FILESYSTYPE statement, which initializes the Common INET support. The operands of the SUBFILESYSTYPE statement are similar to those for the FILESYSTYPE statement.

The general model is that of a sockets PFS that is split into two pieces: a PFS layer that runs in the kernel address space, and additional programming that runs in a separate address space and that actually controls the transport interface to the network. For the purposes of this discussion, the PFS layer piece will be called the transport driver (or TD) and the separate address space piece will be called the transport provider (or TP).

The transport driver is started by z/OS UNIX, as a PFS, and communicates with the transport provider through its own internal mechanisms, usually by a space switching program call (PC).

The transport provider (for instance, a TCP/IP stack) is started independently, and communicates with the transport driver through the master socket.

Figure 1. Common INET sockets PFS structure

A TD/TP that is structured entirely within the PFS in the kernel address space still has to establish the master socket and pass the minimum ioctl commands to run under the CINET layer.