Queuing a request for a session with an SLU

When a session is requested with a particular SLU, that LU might not be available to act as the SLU in the requested session. This might be because the LU is either at its session limit, or is not currently enabled to act as an SLU.

Table 1 shows how an SLU application program can change from the disabled state to the enabled state by using SETLOGON OPTCD=START. Some terminals also have the ability to switch back and forth between enabled and disabled states. For example, for certain IBM® 3274 control units, if a printer or display is powered off, the control unit notifies the SSCP that the terminal is disabled for sessions. If the terminal is then powered on, the control unit notifies the SSCP that the terminal is enabled for sessions. Thus, the SSCP is able to keep track of the session capability of these LUs.

If an application program requests a session with such an LU by using SIMLOGON OPTCD=Q and the LU is available, the session is established at that time. However, if the LU is currently disabled for sessions as an SLU, or if the LU is at its session limit, then SIMLOGON is posted complete, but the session is not established until sometime after the LU becomes available (for example, by powering on).

The LU can change from an enabled to a disabled state (for example, by powering off) just before the BIND reaches the LU. In this case, VTAM® rejects the BIND request. The SNA sense code in the response can be used to determine the appropriate recovery procedure. This sense code is made available in the SSENSEI and SSENSMI fields of the RPL that issued OPNDST OPTCD=ACCEPT.

Sense code X'08450000' indicates that the LU notifies the SSCP when it is again enabled. For this sense code, SIMLOGON OPTCD=Q can be reissued immediately and the SSCP queues the request for the session until the LU becomes available.

Sense code X'080A0000' indicates that the LU does not notify the SSCP when it becomes available for a session. This code is used if either the LU or its SSCP does not support the required notification facility. In this case, the application program should not immediately reissue SIMLOGON OPTCD=Q because another BIND rejection results. Manual intervention is required. For example, when the terminal operator powers on the terminal, the operator can notify the host operator, who in turn can send a message to the program to issue SIMLOGON at that time.

Table 1. Session outage notification UNBIND type codes and reason codes
Session outage notification signals SCIP UNBIND RU with type code NSEXIT CLEANUP RU (CU) LOSTERM reason code (Type and reason codes shown in decimal) “ns” means “exit routine not scheduled” Session outage notification reasons	If LU is PLU			If LU is SLU
	SCIP	NSEXIT	LOSTERM	SCIP	NSEXIT	LOSTERM
UNBIND type=01 received by PLU UNBIND 01 sent by SLU.	01	CU	24, then 16	ns	ns	ns
UNBIND type=01 received by SLU CLSDST OPTCD=RELEASE by PLU See "CTERM Forced received by PLU" in the following rows CLOSE by PLU.	ns	ns	ns	ns	01	ns
UNBIND type=02 received by SLU CLSDST OPTCD=PASS by PLU.	ns	ns	ns	ns	02	ns
UNBIND type=12 received by this LU Unrecoverable failure of other LU.	12	CU	12	ns	12	ns
UNBIND type=TC received by this LU for any UNBIND type not listed above, including (but not restricted to): TC=07 Virtual route inoperative TC=08 Route extension inoperative TC=09 Hierarchical reset because of SSCP session activation TC=10 SSCP session deactivated or failed TC=11 Virtual route deactivated TC=14 Recoverable failure of other LU TC=15 Cleanup done by other LU TC=254 session protocol is not valid or user-supplied sense code.	TC	CU	24, then 16	ns	TC	ns
CTERM Forced received by PLU (PLU sends UNBIND type=01 to SLU. No action is taken if PLU is an independent LU.) VARY INACT,I of either LUs, CDRSC, CDRM, or associated component HALT NET,QUICK REQDISCONT Immediate from PU Cross-Domain Takedown (Forced).	ns	ns	12	ns	ns	ns
CTERM Forced received by PLU (PLU sends UNBIND type=01 to SLU. No action is taken if PLU is an independent LU.) Terminate Forced by SLU LOGOFF UNCOND by SLU TERMSESS UNCOND by SLU VARY TERM,UNCOND of session.	ns	ns	20	ns	ns	ns
CLEANUP received by this LU (UNBIND type=0C sent to other LU) VARY INACT,F or R of LU, CDRSC, CDRM, or associated component VARY TERM,FORCE of session Cross-Domain Takedown Cleanup CLOSE or ABEND by other LU RU received larger than maximum size specified in BIND Route extension inoperative	ns	CU	24 then 16 or 48	ns	CU	ns
CLEANUP received by this LU (UNBIND type=01 sent to other LU) Terminates Cleanup by this or other LU (includes Terminate Forced changed to Terminate Cleanup by the SSCP).	ns	CU	12	ns	CU	ns
Global failure of this LU (UNBIND type=01 sent to other LU) (TPEND exit routine with reason code 8) HALT NET,CANCEL ABEND of application program VTAM failure	ns	ns	ns	ns	ns	ns
Failure of this LU for this session (UNBIND type=14 sent to other LU)	14	CU	24, then 16	14	ns	ns
Buffer overflow at this LU (Does not cause session termination)	ns	ns	36	ns	ns	36
Test Request message received by PLU (UNBIND type=01 sent to the SLU) From BSC 3270 attached to NCP	ns	ns	12	ns	ns	ns
CTERM Orderly received by PLU (No action taken if PLU is an independent LU) (Does not cause session termination) VARY TERM,COND of session Terminate Orderly by SLU TERMSESS COND by SLU LOGOFF COND by SLU Cross-Domain Takedown Orderly.	ns	ns	32	ns	ns	ns