RSVP (Resource Reservation Protocol) is an industry standard protocol used to implement IntServ QoS architecture. RSVP is used by the sender to request a certain QoS level from the routers along the path to the receiver. Devices along the way should be RSVP aware and they will let the sender know if they can implement its requirements.
The sender first sends an RSVP request message (PATH). This message will identify the flows and what QoS level they require. The first router in the path towards destination will receive this message and if it can offer the desired QoS level it will forward the traffic to the next router towards the destination. If all routers along the path can deliver the required QoS level, the PATH message will arrive at the destination router. If it can also deliver the QoS level, it will start to send RESV messages towards the sender, letting it and all the routers along the path that the network can offer the required QoS level.
By sending RESV messages, the receiver is the one that actually requests the QoS level, now that we know that the network can implement it. In order for the routers to maintain the reservation, senders must send PATH messages every 30 seconds.