The Set and Test Sequence Numbers (STSN) and Request Recovery (RQR) Requests

About this task

Another session-control request sent with the SESSIONC macroinstruction is Set and Test Sequence Numbers (STSN). This request allows the PLU to reset the normal-flow sequence numbers and to communicate with the SLU to establish the proper sequence numbers. For example, an attempt to resynchronize sequence numbers can begin when one of the LUs recognizes that the sequence number of a request it has received on the session is not 1 greater than the sequence number of the previous request it received. When the SLU recognizes the sequence number error, it sends the Request Recovery (RQR) session-control request to ask the PLU to take recovery action. When the VTAM® application program PLU receives the request, its SCIP exit routine is scheduled.

Another use of STSN is for restarting request flow, where the PLU that periodically checks normal-flow requests that it sends to an SLU on a session wants to inform the SLU of the sequence numbers at which it is restarting after a system, session, application program, or LU failure.

Procedure

A VTAM application program acting as the PLU in a session normally uses the following procedure to resynchronize sequence numbers with the SLU:

The PLU issues SESSIONC CONTROL=CLEAR to stop the request/response flow and to remove all requests not delivered and all responses pertaining to its session.
The PLU then issues SESSIONC CONTROL=STSN to question the LU about normal-flow sequence numbers on the session. With this macroinstruction, the PLU can do one of the following tasks:
- Send sequence number values to the SLU and, from the response, determine whether the SLU "agrees" with those numbers
- Request that the SLU return whatever values it considers to be the correct sequence numbers.
- Tell the SLU to set its sequence numbers to particular values.
To reach agreement with the SLU, the PLU might have to send several STSN requests, with the SLU responding to each request. When agreement is finally reached, either session partner or both, might have to return to a previous point in their operations and resend one or more normal-flow requests.
After agreement on sequence numbers is reached, the PLU issues SESSIONC CONTROL=SDT to restart the flow of requests and responses.

Results

For examples of the use of SESSIONC CONTROL=STSN, see Figure 3 and Figure 13. For details about the options available with STSN, refer to SESSIONC—Send a session-control request or response.