8. Transaction Model

DLEP defines a simple Message transaction model: only one request per destination may be in progress at a time per session. A Message transaction is considered complete when a response matching a previously issued request is received. If a DLEP participant receives a request for a destination for which there is already an outstanding request, the implementation MUST terminate the session by issuing a Session Termination Message (Section 12.9) containing a Status Data Item (Section 13.1) with status code set to 129 'Unexpected Message' (see Table 2) and transition to the Session Termination state. There is no restriction on the total number of Message transactions in progress at a time, as long as each transaction refers to a different destination.

It should be noted that some requests may take a considerable amount of time for some DLEP participants to complete; for example, a modem handling a multicast Destination Up request may have to perform a complex network reconfiguration. A sending implementation MUST be able to handle such long-running transactions gracefully.

Additionally, only one session request, e.g., a Session Initialization Message (Section 12.5), may be in progress at a time per session. As noted above for Message transactions, a session transaction is considered complete when a response matching a previously issued request is received. If a DLEP participant receives a session request while there is already a session request in progress, it MUST terminate the session by issuing a Session Termination Message containing a Status Data Item with status code set to 129 'Unexpected Message' and transition to the Session Termination state. Only the Session Termination Message may be issued when a session transaction is in progress. Heartbeat Messages (Section 12.20) MUST NOT be considered part of a session transaction.

DLEP transactions do not time out and are not cancellable, except for transactions in flight when the DLEP session is reset. If the session is terminated, canceling transactions in progress MUST be performed as part of resetting the state machine. An implementation can detect if its peer has failed in some way by use of the session heartbeat mechanism during the In-Session state; see Section 7.3.