&jep0166; can be used to initiate and negotiate a wide range of peer-to-peer sessions. The first session type of interest is audio chat. This document specifies a format for describing Jingle audio sessions.

The Jingle content description format defined herein is designed to meet the following requirements:

1. Enable negotiation of parameters necessary for audio chat over Realtime Transport Protocol (RTP; see &rfc3550;).
2. Map these parameters to Session Description Protocol (SDP; see &rfc4566;) to enable interoperability.
3. Define informational messages related to audio chat (e.g., busy and ringing).

A Jingle audio session is described by one or more encodings contained within a wrapper <description/> element. In the language of RFC 4566 these encodings are payload-types; therefore, each <payload-type/> element specifies an encoding that can be used for the audio stream. In Jingle Audio, these encodings are used in the context of RTP. The most common encodings for the Audio/Video Profile (AVP) of RTP are listed in &rfc3551; (these "static" types are reserved from payload ID 0 through payload ID 96), although other encodings are allowed (these "dynamic" types use payload IDs 97 to 127) in accordance with the dynamic assignment rules described in Section 3 of RFC 3551. The 'id' attribute is REQUIRED. The 'name' attribute is RECOMMENDED for static payload types, and REQUIRED for dynamic payload types. The 'clockrate' attribute is RECOMMENDED and should specify the sampling frequency in hertz. The 'channels' attribute is RECOMMENDED and should specify the number of channels; if omitted, it MUST be assumed to contain one channel.

The encodings SHOULD be provided in order of preference.

The <description/> element is intended to be a child of a &JINGLE; element as specified in JEP-0166.

When the session is provisionally accepted, as indicated by the target entity sending an empty IQ result in response to an 'initiate' message, both receiving and sending entities SHOULD start listening for audio as defined by the negotiated transport method. For interoperability with telephony systems, each entity SHOULD play any audio received at this time, before the target sends an 'accept' message.

If the payload type is static (payload-type IDs 0 through 96 inclusive), it MUST be mapped to a media field defined in RFC 4566: Session Description Protocol (SDP). The generic format for the media field is as follows:

In the context of Jingle audio sessions, the <content> is "audio", the <port> is the preferred port for such communications (which may be determined dynamically), the <transport> is whatever transport method is negotiated via the Jingle negotiation (e.g., "RTP/AVT"), and the <fmt list> is the payload-type ID.

For example, consider the following static payload-type:

If the payload type is dynamic (payload-type IDs 97 through 127 inclusive), it SHOULD be mapped to an SDP media field plus an SDP attribute field named "rtpmap".

For example, consider a payload of 16-bit linear-encoded stereo audio sampled at 16KHz associated with dynamic payload-type 98:

If an entity supports the Jingle audio content description format, it MUST advertise that fact by returning a feature of "http://jabber.org/protocol/jingle/description/audio" in response to &jep0030; information requests.

The description of a format for audio sessions introduces no known security vulnerabilities.

This JEP requires no interaction with &IANA;.