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A common way to establish a session with an LU is to have the LU send an Initiate request, which asks for a session with a particular application program. A VTAM® application program might have a LOGON exit routine that is automatically entered when a CINIT request resulting from the Initiate request is received. The LOGON exit routine establishes the session with the LU by issuing OPNDST OPTCD=ACCEPT. The OPNDST points to a node initialization block (NIB). The NIB contains information that VTAM associates with the session. When the OPNDST processing is completed, the application program and the LU can exchange requests and responses.

When an application program establishes a session with another LU, a 32-bit communication identifier (CID) is returned in two control blocks used in establishing the session (the request parameter list (RPL) and the node initialization block (NIB)). The CID identifies the session. Whenever the application program sends a request or response unit (RU) to the LU on a particular session, the CID of that session must be in the RPL used to send the RU.