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Jingle Audio Content Description Format This document defines a content description format for Jingle audio sessions. &LEGALNOTICE; 0167 Proposed Standards Track Standards JIG Council XMPP Core XEP-0166 jingle-audio &scottlu; &stpeter; &seanegan; 0.6 2006-10-30 psa/se

Specified how to include SDP parameters and codec-specific parameters; added Speex examples.

 0.5 2006-08-23 psa

Modified namespace to track XEP-0166.

 0.4 2006-07-12 se/psa

Specified when to play received audio (early media); specified that DTMF must use in-band signalling (XEP-0181).

 0.3 2006-03-20 psa

Defined info messages for hold and mute.

 0.2 2006-02-13 psa

Defined info message for busy; added info message examples; recommended use of Speex; updated schema and XMPP Registrar considerations.

 0.1 2005-12-15 psa

Initial version.

 0.0.3 2005-12-05 psa

Described service discovery usage; defined initial informational messages.

 0.0.2 2005-10-27 psa

Added SDP mapping, security considerations, IANA considerations, XMPP Registrar considerations, and XML schema.

 0.0.1 2005-10-21 psa/sl

First draft.

&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:

Attribute Description Inclusion
channels The number of channels; if omitted, it MUST be assumed to contain one channel RECOMMENDED
clockrate The sampling frequency in Hert RECOMMENDED
id The payload identifier REQUIRED
maxptime Maximum packet time as specified in RFC 4566 OPTIONAL
name The appropriate subtype of the audio MIME type RECOMMENDED for static payload types, REQUIRED for dynamic payload types
ptime Packet time as specified in RFC 4566 OPTIONAL

The encodings SHOULD be provided in order of preference.

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

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:

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

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:

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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:

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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:

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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.

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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 &xep0030; information requests.

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Informational messages may be sent by either party within the context of Jingle to communicate the status of a Jingle audio session, device, or principal. The informational message MUST be an IQ-set containing a &JINGLE; element of type "description-info", where the informational message is a payload element qualified by the 'http://jabber.org/protocol/jingle/info/audio' namespace; the following payload elements are defined: A <trying/> element (equivalent to the SIP 100 Trying response code) is not necessary, since each session-level action is acknowledged via XMPP IQ semantics.

Element Meaning
<busy/> The principal or device is currently unavailable for a session because busy with another (audio or other) session.
<hold/> The principal is temporarily pausing the chat (i.e., putting the other party on hold).
<mute/> The principal is temporarily stopping audio input but continues to accept audio output.
<queued/> The audio session request is queued for pickup by the principal.
<ringing/> The device is ringing but the principal has not yet interacted with it to answer.

Note: Because the informational message is sent in an IQ-set, the receiving party MUST return either an IQ-result or an IQ-error (normally only an IQ-result to acknowledge receipt; no error flows are defined or envisioned at this time).

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Support for the Speex codec is RECOMMENDED.

If it is necessary to send Dual Tone Multi-Frequency (DTMF) tones, it is REQUIRED to use the XML format specified &xep0181;.

When the session is provisionally accepted, as indicated by the receiver 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.

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

This document requires no interaction with &IANA;.

The ®ISTRAR; shall include 'http://jabber.org/protocol/jingle/description/audio' and 'http://jabber.org/protocol/jingle/info/audio' in its registry of protocol namespaces.

The XMPP Registrar shall include the name "audio" in its registry of Jingle content description formats. The registration is as follows:

audio Jingle sessions that support audio exchanges XEP-0167 ]]> ]]> ]]>