Irrecoverable messages

Irrecoverable, non-MFS input and output messages basically have the same requirements as those for recoverable messages, except that DR1 and exception DR1 can optionally be requested instead of DR2 and exception DR2, respectively. Irrecoverable MFS-paged input and output messages have the same requirements as recoverable ones.

When messages are being sent, only one message can be outstanding at a time. This means that the sender can send one message and must wait for the response or reply before sending another.

The required response and sync-point protocol allows message integrity to be maintained by allowing the half sessions' sync-point managers to mutually understand when messages are accepted (committed) or rejected (backed out). This also allows change-direction to be solicited using SIGNAL RCD as required. RQE1 or RQE2 is recommended where change-direction is indicated, because these capabilities are automatic. An RQD1 or RQD2 are valid, but reduce performance because of the unnecessary response. A response or sync point is implied when a reply is received to a sent message indicating change-direction and RQE1 or RQE2. That is, the reply is an implied DR1 or DR2 response. Requesting either a definite response (RQD1 or RQD2) or an exception response (RQE1 or RQE2) with change-direction is valid for the session bind option DEFINITE RESPONSE CHAINS.

A failure can occur between sending a recoverable message and receiving the sync-point response (or reply) and receiving the sync-point response (or reply message). During the session restart procedure, the STSN command is used to inform both half sessions of the sequence number of the last sent or received sync-point message. If either half session has not completely received a given message, that message can then be retransmitted.