In the z/OS® UNIX Common INET (CINET) environment, the application is associated with multiple TCP/IP stacks unless the application specifically associates itself with a particular stack using the socket call setibmopt(). Figure 1 shows an example of a z/OS UNIX system that uses multiple stacks.

Figure 1. A z/OS UNIX system using multiple stacks.

CINET is defined by the presence of a FILESYSTYPE statement with an ENTRYPOINT of BPXTCINT and the presence of one or more SUBFILESYSTYPE statements that define the actual TCP/IP stacks to be used. By convention, and in this topic, TYPE(CINET) parameter for CINET configurations is used. The entrypoints for the SUBFILESYSTYPE statements are typically EZBPFINI for several instances of the daemon, which is used to route messages, is shipped with (TCP/IP Services) or the entry point for a conforming TCP/IP stack that is provided by a vendor.

For the example in Figure 1, you would have the two SUBFILESYSTYPE statements to define the two stacks shown in the illustration.