Data flow control protocol reference

The following topics describe the byte-level protocols for data flow control (DFC). The protocols are presented here.

BID protocol
IMS does not send the SNA BID command, but receives the command when acting as a secondary half session.
Bracket and half-duplex protocol
IMS uses SNA bracket protocol to resolve contention, and uses the change-direction indicator of the half-duplex protocol to control normal flow send/receive mode while within bracket state.
CANCEL protocol
IMS sends and receives the SNA CANCEL command, which allows the sender to terminate an output SNA chain without having to send all of the chain following a sender- or receiver-detected error.
Chaining protocol
Whether operating as the primary or secondary half session, IMS sends both single and multiple RU chains. Half sessions can operate with either single or multiple RU chains as specified in the bind parameters.
CHASE protocol
IMS sends and receives SNA CHASE.
ERP purging
After sending an exception response and before continuing to send or receive, the exception response sender might need to enter error recovery process (ERP) PURGE mode until the DFC state managers of both half sessions are synchronized.
LUSTATUS protocol
IMS sends and receives LUSTATUS as summarized in the following table. The DFC bracket, send/receive, and response requirements illustrated in this figure are subject to the considerations detailed in the balance of this topic.
Paged messages ERP
For sender-detected errors (/DEQ command) detected after transmission of an IMS multichain demand or autopaged output message has been initiated, IMS must send the CANCEL command (as necessary) in conjunction with LUSTATUS - abort.
Ready-to-receive protocol
IMS does not send the SNA ready-to-receive (RTR) command, but receives the command when acting as a primary half session.
RSHUT protocol
IMS does not send, but does receive, the RSHUT command.
Selective receiver ERP
Selective receiver error recovery procedure (ERP) is initiated using the X'08460000' sender ERP sense code.
Sender ERP
The SNA CANCEL request can be used during sender error recovery processing as necessary to terminate the chain in error and to provide synchronization between half sessions. CANCEL can be either solicited (by the sender of the exception response) or unsolicited (because of a receiver-detected error, which results in an exception response).
Sense codes that IMS receives
A sense code is a 2-byte field containing the category and modifier defined for the particular exception condition that has occurred. The 2 bytes following the sense code can contain optional user data, and are not supported for all sense codes.
Sense codes that IMS sends
The following table shows the output sense codes that IMS sends.
SIGNAL protocol
IMS sends the SIGNAL command to request change-direction (SIGNAL RCD - X'00010000') at the end of an input message chain that does not indicate change-direction and that produces output that must be sent prior to subsequent input.
Symmetrical session shutdown for LU 6.1 (SBI and BIS)
Two data flow control commands allow a symmetrical and orderly termination for peer level LU 6.1 half sessions: stop bracket initiation (SBI) and bracket initiation stopped (BIS).

Parent topic: ISC protocols for VTAM connections