Connected: An Internet Encyclopedia
1. Introduction

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1. Introduction

1. Introduction

This document defines RSVP, a resource reservation setup protocol designed for an integrated services Internet [RSVP93, RFC 1633]. The RSVP protocol is used by a host to request specific qualities of service from the network for particular application data streams or flows. RSVP is also used by routers to deliver quality-of-service (QoS) requests to all nodes along the path(s) of the flows and to establish and maintain state to provide the requested service. RSVP requests will generally result in resources being reserved in each node along the data path.

RSVP requests resources for simplex flows, i.e., it requests resources in only one direction. Therefore, RSVP treats a sender as logically distinct from a receiver, although the same application process may act as both a sender and a receiver at the same time. RSVP operates on top of IPv4 or IPv6, occupying the place of a transport protocol in the protocol stack. However, RSVP does not transport application data but is rather an Internet control protocol, like ICMP, IGMP, or routing protocols. Like the implementations of routing and management protocols, an implementation of RSVP will typically execute in the background, not in the data forwarding path, as shown in Figure 1.

RSVP is not itself a routing protocol; RSVP is designed to operate with current and future unicast and multicast routing protocols. An RSVP process consults the local routing database(s) to obtain routes. In the multicast case, for example, a host sends IGMP messages to join a multicast group and then sends RSVP messages to reserve resources along the delivery path(s) of that group. Routing protocols determine where packets get forwarded; RSVP is only concerned with the QoS of those packets that are forwarded in accordance with routing.

In order to efficiently accommodate large groups, dynamic group membership, and heterogeneous receiver requirements, RSVP makes receivers responsible for requesting a specific QoS [RSVP93]. A QoS request from a receiver host application is passed to the local RSVP process. The RSVP protocol then carries the request to all the nodes (routers and hosts) along the reverse data path(s) to the data source(s), but only as far as the router where the receiver's data path joins the multicast distribution tree. As a result, RSVP's reservation overhead is in general logarithmic rather than linear in the number of receivers.

              HOST                              ROUTER

 _____________________________       ____________________________
|  _______                    |     |                            |
| |       |   _______         |     |            _______         |
| |Appli- |  |       |        |RSVP |           |       |        |
| | cation|  | RSVP <---------------------------> RSVP  <---------->
| |       <-->       |        |     | _______   |       |        |
| |       |  |process|  _____ |     ||Routing|  |process|  _____ |
| |_._____|  |       -->Polcy||     ||       <-->       -->Polcy||
|   |        |__.__._| |Cntrl||     ||process|  |__.__._| |Cntrl||
|   |data       |  |   |_____||     ||__.____|     |  |   |_____||
|===|===========|==|==========|     |===|==========|==|==========|
|   |   --------|  |    _____ |     |   |  --------|  |    _____ |
|   |  |        |  ---->Admis||     |   |  |       |  ---->Admis||
|  _V__V_    ___V____  |Cntrl||     |  _V__V_    __V_____ |Cntrl||
| |      |  |        | |_____||     | |      |  |        ||_____||
| |Class-|  | Packet |        |     | |Class-|  | Packet |       |
| | ifier|==>Schedulr|================> ifier|==>Schedulr|===========>
| |______|  |________|        |data | |______|  |________|       |data
|                             |     |                            |
|_____________________________|     |____________________________|

                  Figure 1: RSVP in Hosts and Routers

Quality of service is implemented for a particular data flow by mechanisms collectively called "traffic control". These mechanisms include (1) a packet classifier, (2) admission control, and (3) a "packet scheduler" or some other link-layer-dependent mechanism to determine when particular packets are forwarded. The "packet classifier" determines the QoS class (and perhaps the route) for each packet. For each outgoing interface, the "packet scheduler" or other link-layer-dependent mechanism achieves the promised QoS. Traffic control implements QoS service models defined by the Integrated Services Working Group.

During reservation setup, an RSVP QoS request is passed to two local decision modules, "admission control" and "policy control". Admission control determines whether the node has sufficient available resources to supply the requested QoS. Policy control

determines whether the user has administrative permission to make the reservation. If both checks succeed, parameters are set in the packet classifier and in the link layer interface (e.g., in the packet scheduler) to obtain the desired QoS. If either check fails, the RSVP program returns an error notification to the application process that originated the request.

RSVP protocol mechanisms provide a general facility for creating and maintaining distributed reservation state across a mesh of multicast or unicast delivery paths. RSVP itself transfers and manipulates QoS and policy control parameters as opaque data, passing them to the appropriate traffic control and policy control modules for interpretation. The structure and contents of the QoS parameters are documented in specifications developed by the Integrated Services Working Group; see [RFC 2210]. The structure and contents of the policy parameters are under development.

Since the membership of a large multicast group and the resulting multicast tree topology are likely to change with time, the RSVP design assumes that state for RSVP and traffic control state is to be built and destroyed incrementally in routers and hosts. For this purpose, RSVP establishes "soft" state; that is, RSVP sends periodic refresh messages to maintain the state along the reserved path(s). In the absence of refresh messages, the state automatically times out and is deleted.

In summary, RSVP has the following attributes:

Further discussion on the objectives and general justification for RSVP design are presented in [RSVP93] and [RFC 1633].

The remainder of this section describes the RSVP reservation services. Section 2 presents an overview of the RSVP protocol mechanisms. Section 3 contains the functional specification of RSVP, while Section 4 presents explicit message processing rules. Appendix A defines the variable-length typed data objects used in the RSVP protocol. Appendix B defines error codes and values. Appendix C defines a UDP encapsulation of RSVP messages, for hosts whose operating systems provide inadequate raw network I/O support.

Next: 1.1 Data Flows

Connected: An Internet Encyclopedia
1. Introduction