RSVP (RFC 2205) is a protocol used by a host (or switch) to request a specific quality of service (QoS) by reserving resources in the IP network to support a particular application's data flow. RSVP is also used by routing services to deliver the QoS requests to nodes along the path of the flow and to establish and maintain the requested service. RSVP is logically located on top of IP and supports unicast and multicast traffic flows.

Reservations

RSVP is used by sending stations to advertise the particular QoS requirements to support an application. This advertisement, called a path message, contains traffic specifications (or Tspecs) that describe the requirements of the flow. End stations (receivers) use this information to make requests, called reservations, for resources from switches along the path between themselves and the senders. Switches guarantee the delivery of a flow by accepting the reservation request (a switch may also deny the request if so configured or if unable to support the request).

Using RSVP, a sender (such as a video workstation) initially sends RSVP path messages through the network to advertise the requirements of the flow. When a receiver receives the path message, it sends a reservation request for network resources it needs to receive the flow. The reservation request is passed to each node in the network, which then validates or rejects the request. If the reservation is validated, the desired resource reservation is made on that part of the link, and the request is sent to the next node. The reservation request is dynamic. It may be modified as it travels upstream because of traffic control mechanisms, or it may be merged with another similar reservation in the multicast tree. Once the reservation request confirmation reaches the sender (the video workstation), it starts sending data. A confirmation is also sent to the receiver informing it that its reservation has been accepted.

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