Each intermediate RSVP-capable router along the distribution tree intercepts Path messages and checks them for validity. If an error is detected then the router will drop the Path message and send a PathErr message upstream to inform the sender who can then take appropriate action.
Assuming the Path message is valid the router does the following: Update the path state entry for the sender identified by the Sender Template. If no path state exists then create it. Path state includes the Sender Tspec, the address, Phop of the previous hop upstream router and optionally an Adspec. The Phop address needs to be stored in order to route Resv messages in the reverse direction up the tree. The Sender Tspec provides a ceiling to clip any inadvertently over- specified Tspecs subsequently received in Resv messages Set cleanup timer equal to cleanup timeout interval and restart timer. Associated with each path state entry is a cleanup timer, the expiration of which triggers deletion of the path state. Expiration of the timer will be prevented if a Path message for the entry is received at least once every cleanup timeout interval. This is the so-called RSVP “soft state” mechanism and ensures that state automatically times out if routing changes while subsequent Path messages install state along the new routing path. In this way the use of soft-state rather than hard- state helps to maintain much of the robustness of the initial Internet design concepts whereby all flow-related state was restricted to the end systems[#!clark88!#]
The router is also responsible for generating Path messages based on the stored path state and forwarding them down the routing tree making sure that for each outgoing interface the Adspec(see section 2.9.4) and Phop objects are updated accordingly. Path messages will be generated and forwarded whenever RSVP detects any changes to stored path state or is informed by the underlying routing protocol of a change in the set of outgoing interfaces in the data forwarding path. Otherwise, a Path message for each specific path state entry is created and forwarded every refresh period timeout interval in order to refresh downstream path state. The refresh period timeout interval is several times smaller than the cleanup timeout interval so that occasional lost Path messages can be tolerated without triggering unnecessary deletion of path state.
However it is still a good idea that a minimum network bandwidth and router processing resources be configured for RSVP messages to protect them from congestion losses.
Although all path state would eventually timeout in the absence of any refreshes via Path messages, RSVP includes an additional message, PathTear to expedite the process. PathTear messages travel across the same path as Path messages and are used to explicitly tear down path state. PathTear messages are generated whenever a path state entry is deleted and so a PathTear message generated by a sender will result in deletion of all downstream path state for that sender. Typically, senders do this as soon as they leave the communications session. Also, deletion of any path state entry triggers deletion of any dependent reservation state(see section 2.9).