Resource Reservation Protocol

Resource Reservation Protocol. It is a network control protocol used by IP networks to reserve a specific quality of service (QoS) for data flows. Developed by the Internet Engineering Task Force and first standardized in RFC 2205, it enables applications to request dedicated bandwidth and low latency for their traffic streams. The protocol operates by establishing and maintaining state information in routers along a path, ensuring network resources are allocated before data transmission begins.

Overview

The protocol was designed to address the need for guaranteed service levels in packet-switched networks, which traditionally offered only best-effort delivery. Its architecture is influenced by earlier telephony signaling systems like Signaling System No. 7 and borrows concepts from connection-oriented communication models. Key components include the PATH message, which traces a route, and the RESV message, which installs reservation state in network elements. It integrates with various network layer protocols, primarily IPv4 and IPv6, and functions at the transport layer of the OSI model, though it does not carry application data itself. The specification has been updated through several subsequent Request for Comments documents to enhance scalability and security.

Protocol operation

Operation begins when a sender host transmits a PATH message toward the receiver, traversing routers such as those from Cisco Systems or Juniper Networks. Each router along the path records the previous hop, creating a reverse route for reservation messages. Upon receiving the PATH message, the receiver can send a RESV message back along the recorded path to request resources. Intermediate routers, like those running Cisco IOS or Junos OS, process the RESV message and commit resources like buffer space and scheduler priority if available, otherwise returning an error. The protocol maintains soft state, meaning reservations must be periodically refreshed with messages like PATH refresh and RESV refresh to prevent timeout. Key operational models include the Fixed Filter and Shared Explicit styles, which define how reservations are shared among multiple senders.

Message types

The protocol defines several fundamental message types to manage the reservation lifecycle. PATH and RESV messages are the core signaling commands for establishing a flow. Error conditions are reported via PATH Error and RESV Error messages, which are sent to the originator of the faulty message. To tear down a reservation explicitly, PATH Tear and RESV Tear messages are used, removing state from routers immediately. For diagnostic purposes, PATH Probe and RESV Confirm messages provide feedback without altering reservation state. The format of these messages includes a common header followed by objects specifying details like the Session Object, which identifies the flow's destination IP address and transport protocol.

Applications and use cases

A primary application is in Voice over IP and video conferencing systems, such as those from Microsoft Teams or Zoom Video Communications, to ensure clear audio and video by minimizing jitter and packet loss. It is critical in Internet2 and other research and education networks for high-performance data transfers between institutions like CERN and Stanford University. Within ISP backbones, it can be used to provision VPN services with guaranteed SLAs. The protocol also supports multicast applications, such as IP television distribution, allowing efficient resource reservation for one-to-many data streams. Furthermore, it finds use in software-defined networking architectures and emerging fields like telesurgery, which demand ultra-reliable low-latency communication.

Related protocols and standards

It is closely associated with the Integrated Services architectural model, which defines service classes like Guaranteed Service and Controlled Load Service. For policy control and authorization, it interacts with the Common Open Policy Service protocol. In Multiprotocol Label Switching networks, extensions like RSVP-TE are used for traffic engineering and label-switched path setup. The Next Steps in Signaling working group developed Generalized Multiprotocol Label Switching signaling, which extends its concepts. Competing and complementary QoS frameworks include Differentiated services, defined in RFC 2475, and the Stream Control Transmission Protocol. Other related signaling efforts include the Session Initiation Protocol for multimedia sessions and the ITU-T's H.323 standard for audiovisual communications.

Category:Network protocols Category:Internet standards