Function management headers
In SNA, function management (FM) headers are an optional part of the request unit sent over a link. This topic describes the FM headers supported by IMS on ISC sessions.
The headers addressed in this topic include:
- The ATTACH function management header. This type 5 header is used to attach a process so it can receive session input. It carries the name of the process to be attached synchronously, as well as other parameters to be used by the attached process. All messages have an explicit or implicit ATTACH FM header. The header is implicit when not actually sent with the message. An implicit ATTACH FM header assumes reset parameter values or values of a previous ATTACH as defined later in this topic.
- The error recovery procedure (ERP) function management header. This type 7 header is sent after an error condition requiring an error message to be sent is detected.
- The reset attached process (RAP) function management header. This type 5 header is input to IMS to cause the attached process and the associated data flow control states to be reset.
The following header types are subordinate to the ATTACH
FM header and cannot be used unless the ATTACH FM header has previously
been sent within that bracket to attach the associated process:
- The SCHEDULER function management header. This type 6 header is sent after the SCHEDULER process has been attached. It carries the name of the process to be asynchronously scheduled within IMS, as well as other parameters required by the SCHEDULER process.
- The SYSMSG function management headers. The type 6 system message headers precede system messages sent by or received by IMS.
- The data descriptor function management header. This type 4 header is used in conjunction with MFS to send a data structure name, version ID, or a current output field tab separator.
- The QMODEL function management headers. These type 6 headers are used for demand-paged messages sent by IMS Message Format Service.
All function management headers must be on a first-in-chain or only-in-chain. The following table summarizes the header types that IMS sends and receives (Y=Supported). These header types are described in the topics that follow.
| Function management header type | Sent by IMS | Received by IMS |
|---|---|---|
| FMH4 - Data Descriptor | Y | Y |
| FMH5 | ||
| ATTACH | Y | Y |
| Reset Attach Process (RAP) | Y | |
| FMH6 - QMODEL | ||
| QGET | Y | |
| QGETN | Y | |
| QPURGE | Y | |
| QSTAT | Y | |
| QXFR | Y | |
| FMH6 - SCHEDULER | Y | Y |
| FMH6 - SYSMSG | ||
| SYSSTAT (default) | Y | |
| SYSSTAT | Y | Y |
| SYSERROR | Y | Y |
| FMH7 - ERP | Y | Y |
When receiving a header, IMS ignores any coded parameters beyond the last parameter to which it is sensitive (that is, that IMS supports). The other LU 6.1 subsystem should do the same when receiving headers that IMS sends.