Although most application programs that use LU 6.2 protocols are capable of parallel sessions, some are not. (Application can refer to code that implements LU 6.2 in devices in the network.) A number of limitations apply to single-session partners.

Session limits designating local and remote contention-winner sessions cannot exceed 1 for any mode name group for a single-session partner. Only one mode name group at any given time can have nonzero session limits because only one session can be active at a time. Before it issues an ALLOC or PREALLOC using a second mode name, the application program must issue a CNOS request setting the session limits for the first mode name to 0, which causes the session to be deactivated when the conversation ends. It then issues a CNOS request for the second mode name.

When an application program issues a CNOS request, session limits are negotiated if the partner is capable of parallel sessions. No negotiation takes place for single-session partners. Instead, all processing is done by VTAM® in the local host system. For single-session devices, the session limits are always 0 or 1.

If the application program knows that the partner LU is a single-session LU, it can specify this information in the CNOS request. This prevents the initiation of a SNASVCMG session for initial CNOS requests involving the partner LU. A session partner that does not support parallel sessions can initiate an LU 6.2 session as an SLU. The mode entry used must specify that parallel sessions and CNOS are not supported. Reference the byte 'BINFLG2' at hex offset 17 in the BIND DSECT. (Refer to z/OS Communications Server: SNA Programmer's LU 6.2 Reference for this byte.)