RSVP Bandwidth Allocation Request and Response

RSVP operates by sending messages from devices requesting bandwidth for particular flows. Routers in the path process and forward these requests. If a requested bandwidth exceeds the allowed parameters or available resources of a particular router in the path, a router may respond providing a successful reservation at a lower bandwidth.

If this occurs, a ResvErr message may be sent, signaling policy failures. Policy failures do not necessary mean that no reservation was made, but that the requested policy was not fulfilled. It may have been provided with something less than what was asked for.

https://networklessons.com/quality-of-service/introduction-to-rsvp