VIPA, Sysplex Distributor and the TN3270 server

Why a virtual IP address

A normal IP address belongs to one interface. If that OSA card or its cable fails, the address disappears with it. A VIPA (Virtual IP Address) belongs to the stack itself, not to any adapter. As long as at least one interface still works and routing knows how to reach the VIPA, clients keep connecting. This is why important z/OS services are usually published on a VIPA.

Dynamic VIPA

A static VIPA protects against adapter failure but stays on one system. A dynamic VIPA (DVIPA) can move between stacks in a sysplex. Each DVIPA has an owning stack and one or more backup stacks; if the owner fails, a backup takes over the address. Some DVIPAs are tied to an application: the address becomes active on whichever system the application starts on. All of this is defined in the VIPADYNAMIC section of the profile.

Sysplex Distributor

Sysplex Distributor goes a step further. One stack owns a distributed DVIPA and receives new connections, then spreads them across target stacks where the same application is running. It can use WLM information to favour systems with spare capacity or servers meeting their goals. Clients see a single address; the sysplex behaves like one server.

The TN3270 server

Users no longer have real 3270 terminals. They run a 3270 emulator on a PC, which speaks the TN3270E protocol over TCP/IP to the TN3270 server in Communications Server. The server converts each connection into an SNA session with VTAM, using a VTAM LU, so applications such as TSO, CICS and IMS see an ordinary 3270 terminal.

• The server is configured with TELNETPARMS (port, security, timers) and BEGINVTAM sections (which LUs to use, default application, mapping rules).
• It usually runs in its own address space, so it can be restarted without touching the TCP/IP stack.
• LUs used by the server are defined to VTAM as application definitions, typically with names drawn from a pattern.
• Secure connections use TLS, configured either in the server or through AT-TLS policy. Port 992 is the usual secure port, but sites choose their own.
• The server can show a logon screen (often an Unformatted System Services table, which has nothing to do with Unix System Services) or send users straight to one application.

TELNETPARMS
  PORT 23
  INACTIVE 3600
ENDTELNETPARMS
BEGINVTAM
  PORT 23
  DEFAULTLUS TCPM0001..TCPM0500 ENDDEFAULTLUS
  ALLOWAPPL *
ENDVTAM

Exact statement names and options depend on release and site style, so read your own site's configuration before relying on any example.