RSVP is the Resource reSerVation Protocol.

A soft state QoS signaling protocol that traverses each aware hop on path that given flow of traffic will go across. It contains at the least 5-tuple and servicetype information, and normally flowspec information. These messages can reserve resources on the router, or get vetoed. For a reservation to get installed both a path and resv message must go through un-vetoed. These messages are marked as IP Protocol 46 and are defined by RFC 2205. RSVP is generally considered an intserv QoS implementation, however more recencently have been refocused to addmision control to a diffserv QoS network.