&xep0166; can be used to initiate and negotiate a wide range of peer-to-peer sessions. One session type of interest is audio (voice) 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 95), although other encodings are allowed (these "dynamic" types use payload IDs 96 to 127) in accordance with the dynamic assignment rules described in Section 3 of RFC 3551.

The allowable attributes are as follows:

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 XEP-0166. (See Protocol Namespaces regarding issuance of a permanent namespace.)

Each <payload-type/> element MAY contain one or more child elements that specify particular parameters related to the payload. For example, as described in draft-ietf-avt-rtp-speex This Internet-Draft has expired; see <http://www.watersprings.org/pub/id/draft-ietf-avt-rtp-speex-00.txt> for an archived version., the "ebw", "eng", "mode", "sr", and "vbr" parameters may be specified in relation to usage of the Speex See <http://www.speex.org/>. codec. Where such parameters are encoded via the "fmtp" SDP attribute, they shall be represented in Jingle via the following format:

Note: The parameter names are effectively guaranteed to be unique, since &IANA; maintains a registry of SDP parameters (see <http://www.iana.org/assignments/sdp-parameters>).

Upon receiving a Jingle initiate stanza containing a Jingle Audio content description as defined in this document, a receiver iterates through the list of offered payload types, composing an appropriate Jingle Audio response description according to the following rules:

• If the receiver does not support the offered encoding, it MUST NOT include the encoding in its response.
• If the receiver does support the offered encoding, it SHOULD include the encoding in the response, preserving the offered payload type.
• If the receiver is unable to support the offered encoding with the offered payload type, it MAY provide an alternate payload type in its response. This typically will happen only when translating from other signalling protocols.
• The receiver SHOULD preserve the order of the offered encodings, which represents the priority assigned to them by the initator.

If, after applying these rules, the receiver determines it does not support any of the offering encodings, the receiver MUST reject the session by sending a <unsupported-codecs/> error in response to the initiator's "initiate" action. Otherwise, it MUST provisionally accept the session by sending an empty IQ result. If the response content type differs from the one offered, the receiver MUST then propose the change in a "description-modify" action as defined in XEP-0166. If the description is identical, the receiver MUST send a "description-accept" action (either explicitly, or implicitly as part of a "session-accept" or "content-accept" action).

Following is an example of this negotiation:

The receiver now follows the rules provided in this section and determines it can only support PCMU. It provisionally accepts the session:

It then offers the new content description in a 'description-modify' action:

The initiator acknowledges the 'description-modify' with an empty IQ result, and sends a 'description-accept' to accept the new Jingle Audio content description.

Finally, the target acknowledges the 'description-accept'.

If the payload type is static (payload-type IDs 0 through 95 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 96 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:

As noted, if additional parameters are to be specified, they shall be represented as attributes of the <payload-type/> element or of the child <parameter/> element, as in the following example.

If an entity supports the Jingle audio content description format, it MUST advertise that fact by returning a feature of "http://www.xmpp.org/extensions/xep-0167.html#ns" (see Protocol Namespaces) in response to &xep0030; information requests.

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

This document requires no interaction with &IANA;.