SNA indicators and records

SNA indicators and records can be generated either explicitly as a result of a CICS® command, or automatically when CICS detects that they are needed. The most common SNA indicators and records are described below:

Begin_bracket and conditional_end_bracket
The begin_bracket (BB) and condition_end_bracket (CEB) indicators in the request header (RH) denote respectively the beginning and end of a conversation between two transactions. Because the BB is generated automatically at the start of a conversation, you need only consider the CEB. The CEB is generated by a SEND with the LAST option, an ISSUE ABEND, a FREE command, or task termination before the conversation is ended.
Function management headers
Function management headers (FMHs) are records sent on a conversation which contain SNA control data. Several types of FMH are defined under SNA; but only two (FMH5 and FMH7) are relevant to APPC DTP.

The FMH5, also known as the attach FMH, is sent with BB and contains the information required to initiate the back-end transaction.

The FMH7 is issued by the ISSUE ERROR, ISSUE ABEND, and SYNCPOINT ROLLBACK commands. In addition, if the back-end system rejects the FMH5, an FMH7 is sent to the front-end transaction. The FMH7 contains a 4-byte code, called the sense code, which describes the error. This code is set in EIBERRCD (or CDBERRCD for basic conversations). The FMH7 may be followed by log data. This log data is included in message DFHZN2701 on the sending system and DFHZC3433 on the receiving system.

Change direction
The change direction (CD) indicator, found in the RH, switches the issuing transaction from send state (state 2) to receive state (state 5). CD is generated explicitly by either of the following:
PS header (type 10)
PS headers (type 10) are records sent on a conversation which contain syncpoint requests. These headers contain a 2-byte syncpoint request code (for example, prepare, request commit, committed, and forget). In addition, the initial record sent contains a 2-byte modifier specifying the conversation state after a successful syncpoint exchange.

Request mode and responses

When data is sent, a response confirming receipt of the data is not normally expected. This is because data is normally sent in RQE (request exception response) mode, meaning that a response is required only if an error condition needs to be transmitted. This response is called -RSP (negative response) and might precede an FMH7. However, if data is sent with the CONFIRM option, the data is sent in RQD (request definite response) mode. This means that the sending transaction will suspend until a DR (definite response) or -RSP is received. The partner transaction generates a DR with the ISSUE CONFIRMATION command.

When SNA indicators are transmitted

To optimize the use of ISC sessions, CICS defers output processing for SEND commands. Deferred output often enables CICS to add SNA indicators to waiting data before transmitting it. The number of transmissions on the session is thereby reduced.

For APPC sessions, this reduction is achieved by accumulating as much data as possible in a CICS buffer before actually transmitting it across the link. Thus the data from a series of SEND commands is transmitted only when the buffer becomes full or when transmission must be forced (for example, if SEND WAIT is encountered).

Optimization of ISC transmission does not affect the number of data flows that the application programming interface sees.

For more information on the APPC protocol, see the SNA LU6.2 Reference: Peer Protocols book, SC31-6808.

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