Protocols used by CICS function shipping

Requests and replies exchanged between systems for CICS® interval control, CICS transient data, CICS temporary storage, and DL/I functions are shipped using the standard protocol as defined for SNA logical unit type 6.1. Requests and replies for CICS file control functions are shipped using a private protocol (with function management headers of type 43).

Symmetrical bracket protocol

Logical unit type 6.1 (LU6.1) sessions between two CICS systems require most protocols to be symmetrical; therefore, CICS receives (as well as sends) end bracket.

Shutdown protocol

The LU6.1 shutdown protocol does not use the SHUTDOWN command; it uses the data flow control commands SBI (stop bracket initiation) and BIS (bracket initial stopped). Shutdown is executed as part of session termination (by DFHZCLS) and ensures that, when a session is terminated normally (as a result of a main terminal release command or a normal CICS shutdown), there are no unfinished syncpoint requests on the session. This means that when the session is initiated, no resynchronization sequence is required.

Sender error recovery protocol (ERP)

CICS support for LU6.1 uses a symmetrical SNA protocol called Sender ERP. In addition, when CICS wants to send a negative response to a remote system, it sends a special negative response (0846), which indicates that an ERP message is to follow. This ERP message contains the real system and user sense values, together with a text message. The negative response and ERP message are built by DFHZEMW, and are received and processed by DFHZRAC, DFHZRVX, and DFHZNAC.

Resynchronization protocol

CICS support for LU6.1 sessions that use the syncpoint protocol has associated resynchronization logic, which is used during the initiation of a session after a previous session has terminated abnormally. This logic is used to generate messages concerning the outcome of any logical units of work that were in doubt when the previous session failed. The modules involved are DFHZRSY, DFHZSCX, and DFHZNAC.