IUCV/VMCF considerations

The IUCV/VMCF inter-address space communication API enables applications running in the same MVS™ image to communicate with each other without requiring the services of the TCP/IP protocol stack. The VMCF/TNF subsystems provide these services, which are still available in z/OS® Communications Server. Several components of TCP/IP in z/OS Communications Server continue to make some use of these services for the purpose of inter-address space communications. These include:

• The AF_IUCV domain sockets for the TCP/IP C socket interface. The AF_IUCV domain enables applications executing in the same z/OS image and using the TCP/IP C socket interface to communicate with each other using a socket API, but without requiring the services of the TCP/IP protocol stack, as no network flows result in these communications. This is quite different from the more common AF_INET domain that enables socket communication over an IP network. AF_IUCV sockets continue to be supported in z/OS Communications Server.

  An example of a TCP/IP-provided application that exploits AF_IUCV sockets is the SNMP Query Engine component (SQESERVE). The z/OS UNIX socket library provides a similar functionality to the AF_IUCV domain sockets with its AF_UNIX domain. Users creating new applications should consider using AF_UNIX domain sockets.

• The Pascal socket interface also makes use of the IUCV/VMCF services for a limited set of inter-address space communication flows. As a result, any applications (provided by IBM® or others) that use the Pascal socket API also still have a requirement for the VMCF/TNF subsystems. TCP/IP provides several applications and commands that use these interfaces, such as the SMTP and LPD servers, and the TSO TELNET, HOMETEST, TESTSITE, and LPR commands. IUCV/VMCF services require the usage of an address space name of SYSTEM. This means a TSO user cannot have the user ID name of SYSTEM.