In addition to Sandip's answer, I see two reasons for RSVP being a soft state protocol.
First, the periodic signalling allows for easy adaptation when the network topology changes and the so does the path between the source and destination. Imagine that you have a reservation along a path, and during the session, the path changes considerably because some link in that path fails and routers converge on a new topology. It would be generally difficult to have a smart mechanism with hard state that would act upon reconvergence and have the reservations re-done across the changed portion of the path. Simply re-sending the PATH and RESV messages causes them to be always forwarded through the then-current path between the source and the destination, so if the path changes, these messages will also be forwarded over the changed path, performing the reservation as necessary.
Second, the PATH and RESV messages actually originate on end hosts. Routers forward the PATH and RESV messages between the source and destination of the flow along with creating a reservation, but both these messages are created by the end hosts that want to reserve bandwidth or other network resources. When the session is closed, end hosts should originate a PathTear or a ResvTear message to free the resources. However, because end hosts can not be generally trusted to behave well (for example, they can get disconnected before they tear down a session, they can crash and restart at any moment, their IP address can change, etc.), having a hard state would be dangerous. It would cause the reservations in the network to become stale if the hosts do not tear down the session properly. Therefore, periodic signalling and soft state is a better approach. If a host goes away just like that, without doing any housekeeping before disconnection, the reservations will expire in soft state, so no big harm is done.
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