Using SON/COS support in IMS

Session outage notification (SON) and class of service (COS) are facilities of VTAM® and SNA that allow IMS to recognize a session outage.

IMS attempts to restart the session for recoverable session failures. Both SON and COS must be specified in VTAM to be available for IMS use. In addition, the ASR option must be specified for the IMS node using the system definition process or the /CHANGE command.

The IMS SON and COS support are available for all the VTAM SNA terminals and MSC VTAM links. When multiple virtual routes exist between IMS and the other LU, use an alternate route to avoid a network outage.

The SON facility enables VTAM to inform IMS that a session failure has occurred. The SON facility gives installations the option of allowing IMS to automatically restart sessions (without end-user intervention) if a recoverable session failure occurs in the network.

You need to code SONSCIP=YES on the VTAM APPL definition statement for the IMS application in order to activate automatic session restart after a session outage. If you decide not to include the SONSCIP definition for IMS, VTAM and IMS proceed with session termination without automatic session restart.

The COS facility allows VTAM to control selection of actual routes by designating a set of virtual communication routes based on speed of traffic, data type, and security considerations. These sets of routes are defined in the VTAM COS table. The mode table entry that is used for session establishment can specify a COS entry name.

When the set of session BIND parameters associated with the terminal contains a COS name, these specified virtual routes are scanned to determine which one can be used for the session. If the selected virtual route fails, restart of the session causes the COS set to be scanned again for use of a different virtual route for session reestablishment.

For program LUs (as well as Finance, LU P, MSC, and ISC), in-flight messages can be recovered and retransmitted as necessary, thereby preserving the integrity of the message exchange across the restart.

For device LUs, in-flight messages might be lost or duplicated.

SON and COS support preserves the session setup options (BIND parameters), as well as the class of service associated with the failing session by using the same mode table entry name in order to reestablish a new session.

Restrictions: Because the boundary node NCP cannot distinguish an upstream route failure from a failure of a host CPC, SON and COS support cannot be used for terminals in XRF configurations that have backup sessions.