Send a Switch request initiating the switch from backup to primary session status

About this task

CONTROL=SWITCH,STYPE=REQ

The application program sends a Switch request that causes the backup XRF session to become the primary XRF session. The former primary XRF session, if still "active", is terminated with an UNBIND(CLEANUP).

Either end of a session has the ability to check its inbound and outbound sequence numbers and, if necessary, to suspend traffic flow and ask that correct numbers be reestablished. When the primary logical unit requests to reestablish correct sequence numbers, it issues a Clear (if allowed in the TS profile for the session) followed by an STSN request. If the logical unit acting as the secondary end of the session requests to reestablish correct sequence numbers, it issues an RQR request. The PLU application program then issues the Clear and STSN requests.

The PLU application program can send four STSN options (CONTROL=STSN,STYPE=REQ) to the logical unit: SET, TESTSET, INVALID, and IGNORE. The logical unit acting as the secondary end of the session responds with an STSN response (CONTROL=STSN, STYPE=RESP) as shown in Table 1. A SESSIONC macroinstruction can be used to send STSN requests that apply to either the inbound or the outbound sequence numbers, or that apply to both independently.

Table 1. Types of STSN requests and their possible responses
PLU STSN request	Meaning	SLU STSN response(s)	Meaning
Set (01) (set)	CPMGR sequence number at PLU and SLU is to be set to value in IBSQVAL or OBSQVAL field.	TESTPOS (01) (ignore)	Secondary logical unit accepts value. No value is returned.
TESTSET (11) (set and test)	CPMGR sequence number at PLU and SLU is to be set to value in IBSQVAL or OBSQVAL field. The SLU NAU sequence number is to be tested against the value in the IBSQVAL or OBSQVAL field; the SLU NAU sequence number is returned in the response.	TESTPOS (01)	Secondary logical unit agrees with value. Value (=SLU NAU sequence number) is returned in IBSQVAL or OBSQVAL field.
		TESTNEG (11)	Secondary logical unit disagrees with value. SLU NAU sequence number is returned in IBSQVAL or OBSQVAL field.
		INVALID (10)	Secondary logical unit does not maintain or has lost its NAU sequence number. No value returned.
		RESET (00)	Meaning depends on LU type.
INVALID (10) (sense)	Primary logical unit is not setting or testing the sequence number for the associated flow. The CPMGR sequence number is not changed. The SLU sequence number is to be returned in the response.	TESTNEG (11)	Secondary logical unit returns NAU sequence number in IBSQVAL or OBSQVAL field.
		INVALID (10)	Secondary logical unit does not maintain or has lost its NAU sequence number. No value returned.
		RESET (00)	Meaning depends on LU type.
IGNORE (00)	Primary logical unit is not setting or testing the sequence number for the associated flow. The CPMGR sequence number is not changed.	TESTPOS (01)	Secondary logical unit ignores this part of the STSN request. No value is returned.
Notes: 1. The numbers in parentheses are the corresponding bit settings for the action and result codes in the STSN RU. The names in parentheses are the associated SNA names for the action and result codes. Refer to SNA Formats for the format of the STSN RU. If there is no name in parentheses, SNA does not currently have an abbreviated name for the result code. 2. Responses to PLU requests are returned in the RPL fields (IBSQAC and OBSQAC) originally used to contain the request. 3. The term CPMGR sequence number, as used in this table, refers to the sequence number assigned by VTAM® when a normal-flow request is sent or received. This is a number returned to the application program in the SEQNO field of the SEND or RECEIVE RPL. The term NAU sequence number refers to a related sequence number kept by the logical unit. For example, for a NAU outbound sequence number, the logical unit might keep a log on a disk file of the CPMGR outbound sequence number assigned to the last request that the logical unit sent for which it received a positive response. Similarly, the NAU inbound sequence number might be the inbound CPMGR sequence number of the last normal-flow request received by the logical unit for which it sent a positive response. The NAU sequence numbers are of interest during recovery operations, because they can be used to let each logical unit in the session understand which RUs have been successfully processed by its session partner. The inbound and outbound sequence numbers are handled independently by SESSIONC. Either one or both can be set by a single STSN request.

When an STSN, SDT, or Clear request is sent to the logical unit, a response type 1 is returned as part of the SESSIONC operation. That is, the request is sent as though POST=RESP and RESPOND=(NEX,FME,NRRN) had been specified on a SEND macroinstruction. If a negative response is returned, the SSENSEI, SSENSMI, and USENSEI fields are set as they would be for SEND POST=RESP.