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Chapter 10 - Resource Reservation Protocol

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Chapter 10: Resource Reservation Protocol
Overview
The Resource ReSerVation Protocol (RSVP) is an Internet control protocol that is used by unicast and multicast receivers to request a specific quality of service (QoS) for the data flow from a unicast or multicast source. RSVP would typically be used to establish a bandwidth reservation for real-time traffic, such as voice or video, as opposed to data traffic, such as a file transfer. RSVP can prevent a data application from depleting the bandwidth available for real-time traffic. Without a guaranteed bandwidth along the path from sender to receiver, real-time traffic can suffer from jitter or delay inconsistencies.
RSVP is also used by routers to forward QoS requests on the path from the receiver to the source. RSVP is not a routing protocol but is a transport layer control protocol used to establish a QoS along a routed path. RSVP interoperates with unicast and multicast routing protocols to determine the path along which QoS reservations need to be made. If available, resources are reserved in each router along the selected path from the receiver to the source. QoS Reservations are unidirectional, typically from the source to the receiver (see Figure 10-1).
Figure 10-1: RSVP request flows along the shared or source-based multicast tree
The RSVP request will flow along the source-based or shared multicast tree depending on which multicast routing protocol has been enabled. The RSVP requests are forwarded towards the source by examining the routing table and determining the next hop toward the source. The functional components of RSVP run as a background process in parallel with the data path as shown in Figure 10-2.
Figure 10-2: RSVP functional modules for host and router implementations
When a resource reservation request is initiated, the request is sent to the policy and admission control modules. The admission control module will check to see if the node can satisfy the request. The policy control module determines if the entity requesting the reservation has the required privileges to do so. If either of these checks is unsuccessful, the application will be notified of the failure. If no failures occur, the classifier and the packet scheduler establish the requested reservation. Multicast membership is usually dynamic. Hosts can join or leave a multicast group at any time. To accommodate the dynamic nature of multicast data flows, RSVP will periodically send refresh messages along the data flow path in order to maintain the established reservation. When refresh messages stop being sent, the reservation will timeout, releasing the resources back to the system.

 


 
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