RSVP ResvError Message

RSVP uses various message types to reserve bandwidth across a topology, including Path messages and Resv messages
  RSVP also uses what is known as a ResvError message. ResvErr messages are sent downstream as a response to Resv messages that may be incorrect/corrupt/incomprehensible or just unexpected.   The sending of a ResvError message doesn't necessarily mean that the reservation failed. It does mean that the requested reservation was not fulfilled, but a reservation at a lower bandwidth may have been successful.

Links

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