Consider this example situation. Message number 49 arrives at the listener out of sequence—it has not yet received messages number
46,
47 and
48. Rendezvous software automatically requests that the sender retransmit messages
46–
48. However, the time-out has expired for those messages, and they are no longer available in the sender’s ledger. The listening transport first presents a
DELIVERY.UNAVAILABLE advisory, indicating that messages
46–48 are lost; then it queues message
49.
In this example a range of messages are unavailable. The advisory indicates this range by noting the sequence numbers of the first unavailable message (seqno_begin is
46) and the last unavailable message (
seqno_end is
48). In a situation where only one message is unavailable, the fields
seqno_begin and
seqno_end both contain the same value.
The subject element is the subject name of the unavailable certified messages. The subject often consists of several elements, so the wildcard character
">" (rather than
"*") is the correct way to match all subjects in this position.
DELIVERY.UNAVAILABLE advisories can indicate either of these difficulties:
