Proper handling of delivery settlement for Multicast distribution

Deployers and system architects who would (or did) avoid the use of multicast distribution due to its lack of reliability shall be able to consider its use.

Previously, any unsettled (reliable) deliveries sent to a multicast address were treated as pre-settled and delivered on a best-effort basis, without any reliable indication to the sender. In other words, it is possible that a sender could be told that a delivery was accepted when it was not actually delivered to any consumer.

With this enhancement, delivery settlement shall be given to the sender only after the delivery has been settled by every attached consumer. If the message was not delivered to any consumer, then the disposition given to the sender shall be RELEASED or MODIFIED. RELEASED disposition indicates that the message was definitely not delivered to any consumer. MODIFIED disposition indicates that the message may have been delivered but no acknowledgement has been received.