RFC 3264

RFC 3264
Title	RFC 3264
Othernames	"An Offer/Answer Model with SDP"
Authors	"R. Mahy, C. Jennings, D. M. Israel"
Published	"June 2002"
Status	"Standards Track"
Pages	"23"
Doi	""

RFC 3264

RFC 3264 is a Standards Track document published in June 2002 that specifies the Offer/Answer Model using the Session Description Protocol for negotiating multimedia sessions. It formalizes how initiating and responding endpoints exchange session parameters for media streams and integrates with signaling protocols used in real-time communications.

Introduction

RFC 3264 defines an Offer/Answer Model that uses the Session Description Protocol to negotiate session parameters between endpoints. It builds on prior work by defining precise procedures for generating offers, answering offers, and modifying sessions, addressing interoperability among implementations of the Session Initiation Protocol, H.323, and other signaling systems. The specification aims to reduce ambiguity in establishing multimedia sessions and to provide a clear normative reference for implementers.

Background and Purpose

The purpose of RFC 3264 is to provide a stable, interoperable mechanism for session negotiation in environments that use the Session Description Protocol as a payload format. It was motivated by the need for consistent behavior across implementations of the Session Initiation Protocol and PSTN/VoIP gateways, and by earlier standards work such as the original Session Description Protocol and signaling practices used in telephony and multimedia conferencing. The document situates itself amid standards activity by bodies and projects working on interoperable real-time communications and gateway interconnection.

Offer/Answer Model Specification

RFC 3264 formalizes how an endpoint generates an "offer" that contains a session description listing media streams, codecs, transport addresses, and attributes, and how a remote endpoint constructs an "answer" that selects compatible parameters. The model prescribes rules for SDP fields when negotiating symmetric and asymmetric media directions, for handling formats and payload types, and for re-offers and mid-session modifications. It also specifies behavior for handling ambiguous or incompatible offers, default attribute values, and the lifecycle of media streams across session changes.

Protocol Details and Message Flow

The protocol details cover how offers and answers are created, transmitted, and applied by endpoints. RFC 3264 describes how to interpret m-lines, connection information, and attribute semantics within SDP payloads during initial session setup and subsequent modifications. It enumerates state transitions for local and remote session descriptions, the relationship between provisional and final signaling responses, and the expected behavior when media formats are rejected or when multiple transport addresses are present. The message flow considerations map to signaling interactions performed by protocols that carry SDP payloads, emphasizing deterministic outcomes to aid gateways, proxies, and user agents.

Security Considerations

RFC 3264 discusses security implications related to negotiation of media characteristics, such as potential for denial-of-service via resource-intensive offers, media redirection through manipulated connection fields, and privacy concerns when exposing IP addresses and ports in session descriptions. The document recommends using secure transport mechanisms provided by signaling protocols, authentication of endpoints, and careful resource management policies in media handling components. It highlights the need for implementers to consider interaction with encryption and integrity mechanisms for both signaling and media.

Implementation and Interoperability

Implementers rely on the normative rules in RFC 3264 to achieve interoperability among diverse stacks, including user agents, softswitches, and gateways connecting packet networks and circuit-switched networks. The specification’s deterministic handling of payload negotiation, media direction attributes, and re-offer semantics reduces ambiguities that previously caused interop failures among vendors and projects. Conformance testing, interoperability events, and profile documents often reference the RFC for clarifying edge cases in codec selection, bundling, and mid-call modifications.

Reception and Impact on SIP/VoIP Standards

RFC 3264 has been widely cited and adopted by standards-track work and product implementations in the VoIP and conferencing ecosystem. Its clear offer/answer mechanics influenced subsequent extensions and best current practices for Session Initiation Protocol usage, codec negotiation, media security, and NAT traversal techniques. The model underpins many later efforts to standardize call features, media switching, and gateway behavior in operational deployments and in standards maintained by relevant organizations.

Category:Internet Standards