Session-initiation transmission sequence

The following list shows the sequence of transmissions that occurs when a workstation requests session initiation.

The numbers relate to major events in the sequence.

  1. The controller sends the INITIATE-SELF command. VTAM® checks the validity of the command, looking for syntax errors that might have been introduced by either the controller or NCP.
  2. If the command is not valid or if VTAM does not find a match when verifying the initiation request, VTAM returns an EXC/DR1 response. EXC/DR1 terminates the initiation request. The SLU P system can retry the initiation request or notify the operator to begin corrective action.

    If the command is valid, VTAM returns a DR1 response. VTAM then verifies the content of the resource field in the INITIATE-SELF command. This field must contain the name of the VTAM application program with which the workstation wants to enter a session. VTAM compares the resource field content to its list of active application programs (those programs with an open VTAM ACB).

  3. If a match is found, VTAM passes the logon request to the specified application program (IMS).

    In an XRF environment, VTAM routes the request to the currently active system.

    When IMS receives the request, it compares the workstation node name supplied by VTAM to the node names defined during IMS system definition.

  4. If IMS does not recognize the workstation, it issues a VTAM CLSDST command, which causes VTAM to send a procedure error command. A procedure error terminates the session initiation request. The system notifies the workstation operator that the session is denied.

    If IMS recognizes the workstation, it issues a VTAM OPNDST command, which causes VTAM to send a BIND command. The BIND command contains the name of the application program (IMS) that issued the OPNDST command.

    In an XRF complex that uses USERVAR instead of MNPS, the BIND command from VTAM contains the USERVAR segment in the user data field and the active APPLID in the PLUNAME field.

    IMS supplies a set of BIND parameters, which define the communication rules and protocol that must be followed. IMS ignores the parameters of the mode table entry supplied by the workstation or network operator.

    The workstation must respond to the BIND command.

    Related reading: For more information on the contents of the BIND data, see Bind Parameters for SLU P and LU 6.1 in IMS Version 15.3 System Programming APIs.

  5. If the workstation cannot begin a session, it returns an EXC/DR1. This can occur when either VTAM or IMS is requesting session initiation and the workstation is currently unable to communicate (for example, because it is involved in offline processing).

    If the workstation can begin a session, it returns a DR1. When IMS receives the DR1, it performs message resynchronization, if necessary.

  6. If message resynchronization is not necessary, or when it is complete, IMS sends a VTAM start-data-traffic (SDT) command.

    The workstation is in session and can transmit data to IMS.