Restarting and Resynchronizing a Session

If the PLU or the SLU encounters an unrecoverable error, such as a line failure, you might need to resynchronize the LU-LU session after restarting it. Resynchronizing the LU-LU session includes reprocessing recoverable messages and (optionally) resetting the message sequence numbers. The application programs can include routines to retransmit lost messages.

When a session is restarted and resynchronized, the PLU sends the BIND, the STSN, and the SDT commands. When the STSN command is sent, a dialog can occur to establish the sequence numbers that are acceptable to both the PLU and the SLU. This dialog consists of a series of STSN messages and positive responses.

If the SLU determines that resynchronization is required, the SLU can send a Request Recovery (RQR) command, a negative response, or an LU-Status command (LUSTAT) with a description of the failure in the user sense bytes. If the PLU discovers the failure or receives an RQR command from the SLU, the PLU sends a CLEAR command to purge all LU-LU messages from the network, an STSN command to establish new sequence numbers, and then an SDT command.