Session Layer (SL) Retry

In addition to RP User Synchronization, which can be used to try to reestablish an interrupted session, the SWIFTNet FIN protocol offers another protocol element, called Session Layer (SL) Retry, which can be used to attempt to resend session-layer protocol data units (SL PDUs) of types other than OPEN and CLOSE when any of the following occur:
  • No success indicator (that is, no correlated null response) for the SL PDU was received within 90 seconds.
  • An attempt by the session layer to put an SL PDU into the SAG request queue resulted in an IBM MQ error.
  • An SAG or SNL status messages indicated that the attempt to send the SL PDU resulted in an error that was not a fatal error (in SWIFT terminology) or a local authentication (LAU) error.
Note: SL PDUs are not the FIN messages being transferred, but are the InterAct messages containing the FIN messages.

When using SL Retry, the sending LT attempts to retransmit an SL PDU until either the transmission is successful or the retry timeout expires. The retry timeout is 180 seconds. If the retry timeout expires, the session layer sends a USER ABORT message to the SIPN, and the corresponding session is aborted.

The receiving LT must be able to identify duplicate SL PDUs and drop them. A duplicate SL PDU results when an SL PDU is transmitted successfully, but the success indication sent by the receiver does not reach the sender, causing the sender to resend the PDU.

When using the SIPN FIN feature, you can specify for each LT whether it is to employ SL Retry when sending SL PDUs. Each LT always employs SL Retry when receiving SL PDUs.