RSVP "teardown" messages remove path or reservation state immediately. Although it is not necessary to explicitly tear down an old reservation, we recommend that all end hosts send a teardown request as soon as an application finishes.
There are two types of RSVP teardown message, PathTear and ResvTear. A PathTear message travels towards all receivers downstream from its point of initiation and deletes path state, as well as all dependent reservation state, along the way. An ResvTear message deletes reservation state and travels towards all senders upstream from its point of initiation. A PathTear (ResvTear) message may be conceptualized as a reversed-sense Path message (Resv message, respectively).
A teardown request may be initiated either by an application in an end system (sender or receiver), or by a router as the result of state timeout or service preemption. Once initiated, a teardown request must be forwarded hop-by-hop without delay. A teardown message deletes the specified state in the node where it is received. As always, this state change will be propagated immediately to the next node, but only if there will be a net change after merging. As a result, a ResvTear message will prune the reservation state back (only) as far as possible.
Like all other RSVP messages, teardown requests are not delivered reliably. The loss of a teardown request message will not cause a protocol failure because the unused state will eventually time out even though it is not explicitly deleted. If a teardown message is lost, the router that failed to receive that message will time out its state and initiate a new teardown message beyond the loss point. Assuming that RSVP message loss probability is small, the longest time to delete state will seldom exceed one refresh timeout period.
It should be possible to tear down any subset of the established state. For path state, the granularity for teardown is a single sender. For reservation state, the granularity is an individual filter spec. For example, refer to Figure 7. Receiver R1 could send a ResvTear message for sender S2 only (or for any subset of the filter spec list), leaving S1 in place.
A ResvTear message specifies the style and filters; any flowspec is ignored. Whatever flowspec is in place will be removed if all its filter specs are torn down.