An application program can attempt to initialize session limits with APPCCMD CONTROL=OPRCNTL, QUALIFY=CNOS before VTAM® has determined the partner LU's session capability. If the partner LU is single-session capable, the session request sent by VTAM returns that information. (For information on how a partner LU can return this indication, see Session limits for single-session-capable partners.) VTAM determines the session-capability of the partner from the response, and the CNOS request completes with an RCPRI, RCSEC combination of X'0000', X'0004', indicating that the partner LU is single-session capable. The session limits are, however, initialized. If session limits greater than 1 are specified on the macroinstruction or if no session limits are specified, the default values of (1,0,0) are used.

An installation should always include the IBM-supplied entry for the SNASVCMG mode in its logon mode tables. Even if no parallel sessions are planned for an application program, VTAM can still use this mode as part of its processing for session limits with partner LUs whose session capability is unknown.

As with CNOS processing for a single-session partner, no CNOS negotiation is done for the SNASVCMG mode. The local LU handles all CNOS processing. The SNASVCMG session limits cannot be reset to 0 until all other mode name groups with the partner LU have session limits of 0. VTAM deactivates the SNASVCMG session only after all other sessions to the partner LU have been deactivated. No draining of pending conversation allocation requests is to take place on any of these SNASVCMG sessions for the local LU.