RSVP (Resource Reservation Protocol) is a set of communication rules that allows channels or paths on the Internet to be reserved for the multicast (one source to many receivers) transmission of video and other high-bandwidth messages. RSVP is part of the Internet Integrated Service (IIS) model, which ensures best-effort service, real-time service, and controlled link-sharing.
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The basic routing philosophy on the Internet is "best effort," which serves most users well enough but isn't adequate for the continuous stream transmission required for video and audio programs over the Internet. With RSVP, people who want to receive a particular Internet "program" (think of a television program broadcast over the Internet) can reserve bandwidth through the Internet in advance of the program and be able to receive it at a higher data rate and in a more dependable data flow than usual. When the program starts, it will be multicast to those specific users who have reserved routing priority in advance. RSVP also supports unicast (one source to one destination) and multi-source to one destination transmissions.
How It Works
Let's assume that a particular video program is to be multicast at a certain time on Monday evening. Expecting to receive it, you send an RSVP request before the broadcast (you'll need a special client program or perhaps your browser includes one) to allocate sufficient bandwidth and priority of packet scheduling for the program. This request will go to your nearest Internet gateway with an RSVP server. It will determine whether you are eligible to have such a reservation set up and, if so, whether sufficient bandwidth remains to be reserved to you without affecting earlier reservations. Assuming you can make the reservation and it is entered, the gateway then forwards your reservation to the next gateway toward the destination (or source of multicast). In this manner, your reservation is ensured all the way to the destination. (If the reservation can't be made all the way to the destination, all reservations are removed.)
When the multicast begins, packets from the source speed through the Internet on a high-priority basis. As packets arrive at a gateway host, they are classified and scheduled out using a set of queues and, in some cases, timers. An RSVP packet is very flexible; it can vary in size and in the number of data types and objects. Where packets need to travel through gateways that don't support RSVP, they can be "tunneled" through as ordinary packets. RSVP works with both Internet Protocol version 4 and IPv6.