The standard RSVP periodically re-sends Resv and Path refresh messages to maintain the state of the path, but many trigger messages signaling a new or changed state (such as PathTear and ResvTear messages) are sent only once. The loss of such a message can cause delays in the reservation or release of resources.
RFC 2961 provides extensions to the RSVP to make the transmission of RSVP trigger messages more reliable by creating an ID for each new RSVP message and allowing the sender to request an acknowledgment of the receipt of trigger messages.