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How Session Establishment Works

Regardless of the source of the session initiation request, and whether your product region is to be the primary or secondary session partner, session establishment proceeds through a number of phases:

• Phase 1. Session parameter validation ensures that the BIND to be sent (when processing a logon request), or BIND received (when processing an unsolicited BIND), is a valid LU6.2 BIND within the parameters supported by the product region.

  If necessary, the BIND parameters are modified before being sent to the secondary LU, or negotiated with the primary LU, so that they are within the bounds supported by the product region.

• Phase 2. The unqualified network LU name provided by VTAM is extracted. If other LU6.2 sessions already exist with this network node, or are in the process of being established, then Step 3 is performed; otherwise Step 4 is performed next.
• Phase 3. Where other LU6.2 sessions exist, or are activating, to the destination involved, additional checks are made to determine whether the new session is acceptable within the session limits determined by the MCT entries (see Step 5). If so, the session is accepted as a parallel session without further processing; otherwise it is rejected.
• Phase 4. Where no other LU6.2 sessions exist, and none are being activated, to the destination involved, the unqualified network LU name is used to locate a matching entry in the DLT. If an entry cannot be located the session initiation is rejected. Otherwise the DLT details are extracted and used to initialize the APPC link connection. If an OSCT entry was nominated in the DLT, it is located and the processing options are extracted.
• Phase 5. The OSCT entry determines whether this link supports parallel sessions or not. Any mode names listed for the OSCT entry are used to locate the corresponding MCT entries, and the details of each MCT is extracted to form a valid MODE name list for this destination. For each mode name, this list contains the session limits for that mode, as well as other session control information.

Note: If a session initiation request comes from an &APPC ALLOCATE verb, Phase 4 and 5 are performed first, before the session setup request is passed to VTAM. Following this, and for all other session initiation types, processing begins at Phase 1 when VTAM notifies the region of the arrival of a logon request or BIND request.