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RSVP provides quality of service
Resource Reservation Protocol has been designed to provide end-to-end quality of service to Internet data flows.
Typically all IP traffic on the Internet is delivered on a best-effort basis. This delivery method does not address the requirements of multimedia applications such as videoconferencing, real-time IP multicasting and Internet telephony. Resource Reservation Protocol (RSVP) is an effort to address the performance needs of such applications.
RSVP is a signaling and control protocol that doesn't carry application data. It operates on top of IP in the transport layer of the Open Systems Interconnection (OSI) protocol stack.
Host applications use RSVP to request the necessary QoS (such as guaranteed bandwidth) from the network for specific data flows. The QoS request is sent through all the routers along the path of the data flow on a hop-by-hop basis, and at each device the RSVP process attempts to establish and maintain a reservation state to provide the requested service.
Refresh messages are sent periodically by hosts and routers to maintain this state during the duration of the data transfer. The established state ends when the end host sends an explicit "teardown" message after the application has finished sending the data. RSVP also adapts to routing topology and multicast group membership changes.
While RSVP provided QoS for data flows, it does not change existing routing protocols that determine where the packets get forwarded. To obtain routing-related information, RSVP consults the existing IP routing database.
For handling bidirectional data flows, it treats each flow independently and makes unidirectional reservations. The receiver host is responsible for initiating and maintaining the reservation for the flow. This receiver-focused approach accommodates the requirements of dynamic group memberships in IP multicasting environments and addresses diverse receiver requirements.
How it works
Host A wants to send multimedia traffic at a constant rate to Host B using the Internet. To facilitate this process, Host A requests QoS (example: bandwidth on demand) from the network using RSVP. The following steps are involved in establishing end-to-end QoS, which is set up before any data is transferred.
1.The RSVP program in Host A receives the desired QoS request from the multimedia application and sends an RSVP path message to Host B.
2.This message follows the exact path that the multimedia traffic would take and creates a "path state" across all nodes in the direction of the traffic flow. The path message contains the sender's IP address, format of data packets and details on the traffic characteristics.
How it works
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3.On receiving the path message, Host B originates an RSVP reservation request message. The next hop for this message is obtained from the previously established path state and follows the exact reverse direction of the path message. It contains the desired QoS information (in this case the requested bandwidth) and a filter condition that identifies the subset of data packets that should receive the QoS.
4.The RSVP program in the router, on receiving the reservation request message, passes it to two local decision modules: an admission control module, which determines if the node has sufficient available resources to supply the requested QoS; and a policy-control module, which verifies whether the requester has the administrative privileges to make the reservation. If either check fails, it returns an error notification to the application that made the request. If both checks succeed, the RSVP program configures the packet classifier in the node to determine the data packets that receive the QoS. The RSVP program also configures the packet scheduler to provide the requested QoS on the outgoing link. This creates a reservation state in the node.
5.After having made the reservation locally, the router sends the reservation request to the next node in the direction of the sender. The process continues on a hop-by-hop basis, and at each node the RSVP program attempts to establish and maintain a reservation state to provide the requested QoS. Finally, the reservation request reaches the sender host and creates a reservation locally. At this point, the multimedia data stream originating from the sender will receive the requested bandwidth from the network.
6.After having successfully reserved the bandwidth end to end, Host A sends multimedia traffic to Host B.
Joy is senior network architect at Fujitsu America. She can be reached at email@example.com.
Resource Reservation Setup Protocol
The official IETF site on the topic.
RSVP Protocol Overview
A quick overview of the protocol from the USC Information Sciences Institute.
RSVP backgrounder (PDF)
Cisco gives background on the Resource Reservation Setup Protocol.