Session limits for single-session-capable partners

CNOS negotiation does not occur when session limits are set with a single-session-capable partner. A single-session-capable LU is an LU that can have only one active session at a time with a partner LU. Therefore, the only nonzero session limits it can use are (1,1,0), (1,0,1), or (1,0,0). They can have only one mode with nonzero session limits.

VTAM® determines single-session or parallel-session capability in one of the following ways:

The application program can specify the session capability of the partner with the SLDSCAP bit field in the CNOS session limits control block. Setting the bit on indicates that the partner does not support parallel sessions. (This is applicable only for the first CNOS request involving the partner LU.)
For an initial CNOS request with a partner LU, where the application program does not set the session capability indicator bit (SLDSCAP), VTAM assumes the partner LU is parallel-session capable and attempts to establish a SNASVCMG mode session to negotiate the session limits. VTAM receives the information regarding the partner's session capability, and the application program issues the APPCCMD CONTROL=OPRCNTL, QUALIFY=DISPLAY macroinstruction to determine the partner's session capability. (This is true regardless of the mode to which the CNOS request applies.)
For SLU session initiation, VTAM will use the parallel/single session indicated in the CINIT to force a nonparallel session starting BINPSS=0 on PSERVIC.

The default session limits used for single-session LUs are shown in Table 1.

Table 1. Default session limits for single-session partners
How Capability is Detected	Default Session Limits
LU-mode table indicates partner LU is single-session capable.	(1,1,0)
LU-mode table indicates partner LU is pending parallel capable but partner LU actually is single-session capable.	(1,0,0)
BIND negotiation received from single-session-capable LU and no LU entry exists in the LU-mode table.	(1,0,0)

Some types of the APPCCMD CONTROL=ALLOC macroinstruction are not appropriate for single-session LUs. For example, the APPCCMD CONTROL=ALLOC, QUALIFY=CONWIN macroinstruction and the APPCCMD CONTROL=ALLOC, QUALIFY=IMMED macroinstruction are possible for one of the LUs, the contention winner, and the contention-winner role can be reversed at BIND (not at CNOS negotiation). The APPCCMD CONTROL=ALLOC, QUALIFY=ALLOCD macroinstruction is recommended for single-session LUs.

Note: When VTAM receives a BIND request for a single-session-capable LU and no LU-mode entry exists for the LU, VTAM creates an LU-mode entry with session limits of (1,0,0) for the LU. Because of these limits, the application program receiving the BIND cannot successfully issue an APPCCMD CONTROL=ALLOC, QUALIFY=CONWIN macroinstruction once the session is available.