XEP-0167: Jingle Audio Content Description Format

This document defines a content description format for Jingle audio sessions.

NOTICE: This document is currently within Last Call or under consideration by the XMPP Council for advancement to the next stage in the JSF standards process.

XEP Information

Status: Proposed
Type: Standards Track
Number: 0167
Version: 0.6
Last Updated: 2006-10-31
Publishing Organization: Jabber Software Foundation
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0166
Supersedes: None
Superseded By: None
Short Name: jingle-audio
Wiki Page: <http://wiki.jabber.org/index.php/Jingle Audio Content Description Format (XEP-0167)>

Author Information

Legal Notice

This XMPP Extension Protocol is copyright 1999 - 2006 by the Jabber Software Foundation (JSF) and is in full conformance with the JSF's Intellectual Property Rights Policy <http://www.xmpp.org/extensions/ipr-policy.shtml>. This material may be distributed only subject to the terms and conditions set forth in the Creative Commons Attribution License (<http://creativecommons.org/licenses/by/2.5/>).

Discussion Venue

The preferred venue for discussion of this document is the Standards-JIG discussion list: <http://mail.jabber.org/mailman/listinfo/standards-jig>.

Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the Jabber Software Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Conformance Terms

The following keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Table of Contents

1. Introduction

Jingle [1] 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.

2. Requirements

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 RFC 3550 [2]).
2. Map these parameters to Session Description Protocol (SDP; see RFC 4566 [3]) to enable interoperability.
3. Define informational messages related to audio chat (e.g., busy and ringing).

3. Content Description Format

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 RFC 3551 [4] (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:

Table 1:

The encodings SHOULD be provided in order of preference.

Example 1. Audio Description Format

    <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
      <payload-type id='96' name='speex' clockrate='16000'/>
      <payload-type id='97' name='speex' clockrate='8000'/>
      <payload-type id='18' name='G729'/>
      <payload-type id='103' name='L16' clockrate='16000' channels='2'/>
      <payload-type id='98' name='x-ISAC' clockrate='8000'/>
      <payload-type id='102' name='iLBC'/>
      <payload-type id='4' name='G723'/>
      <payload-type id='0' name='PCMU' clockrate='16000'/>
      <payload-type id='8' name='PCMA'/>
      <payload-type id='13' name='CN'/>
    </description>
  

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 [5], the "ebw", "eng", "mode", "sr", and "vbr" parameters may be specified in relation to usage of the Speex [6] codec. Where such parameters are encoded via the "fmtp" SDP attribute, they shall be represented in Jingle via the following format:

<parameter name='foo' value='bar'/>
  

Note: The parameter names are effectively guaranteed to be unique, since the Internet Assigned Numbers Authority (IANA) [7] maintains a registry of SDP parameters (see <http://www.iana.org/assignments/sdp-parameters>).

4. Negotiating a Jingle-Audio Session

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:

Example 2. Initiation Example

<iq to='juliet@capulet.com/balcony' from='romeo@montague.net/orchard' id='jingleaudio1' type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='session-initiate'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <content name='audio'>
      <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
        <payload-type id='96' name='speex' clockrate='16000'/>
        <payload-type id='0' name='PCMU'/>
      </description>
      <transport xmlns='http://jabber.org/protocol/jingle/transport/ice'>
        ...
      </transport>
    </content>
  </jingle>
</iq>
  

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

Example 3. Receiver Provisionally Accepts Session

  <iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio1' type='result'/>
  

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

Example 4. Initiation Example

<iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio2' type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='description-modify'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <content name='audio'>
      <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
        <payload-type id='0' name='PCMU'/>
      </description>
    </content>
  </jingle>
</iq>
  

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

Example 5. Initiator Accepts New Content Description

<iq to='juliet@capulet.com/balcony' from='romeo@montegue.net/orchard' id='jingleaudio2' type='result'/>

<iq to='juliet@capulet.com/balcony' from='romeo@montegue.net/orchard' id='jingleaudio3' type='set'/>
  <jingle xmlns='http://jabber.org/protocol/jingle'
   action='description-accept' initiator='romeo@montague.net/orchard' sid='a73sjjvkla37jfea'>
     <content name='audio'>
       <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
         <payload-type id='0' name='PCMU'/>
       </description>
     </content>
  </jingle>
</iq>
  

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

Example 6. Receiver Provisionally Accepts Session

<iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio3' type='result'/>
  

5. Negotiating a Jingle-Audio Session

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

• If the target entity does not support the offered encoding, it MUST NOT include the encoding in its response.
• If the target entity does support the offered encoding, it SHOULD include encoding in the response, preseriving the offered payload type.
• If the target entity 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 target entity SHOULD preserve the order of the offered encodings, which represents the priority assigned to them by the initator.

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

Following is an example of this negotiation:

Example 7. Initiation Example

  <iq to='juliet@capulet.com/balcony' from='romeo@montague.net/orchard' id='jingleaudio1' type='set'>
    <jingle xmlns='http://jabber.org/protocol/jingle'
            action='session-initiate'
            initiator='romeo@montague.net/orchard'
            sid='a73sjjvkla37jfea'>
      <content name='audio'>
        <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
          <payload-type id='96' name='speex' clockrate='16000'/>
          <payload-type id='0' name='PCMU' />
        </description>
        <transport xmlns='http://jabber.org/protocol/jingle/transport/ice'>
          ...
        </transport>
      </content>
    </jingle>
  </iq>
    

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

Example 8. Target Provisionally Accepts Session

  <iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio1' type='result' />
  

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

Example 9. Initiation Example

  <iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio2' type='set'>
    <jingle xmlns='http://jabber.org/protocol/jingle'
            action='description-modify'             initiator='romeo@montague.net/orchard'
            sid='a73sjjvkla37jfea'>
      <content name='audio'>
        <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
          <payload-type id='0' name='PCMU' />
        </description> 
      </content>
    </jingle>
  </iq>
    

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

Example 10. Initiator Accepts New Content Description

  <iq to='juliet@capulet.com/balcony' from='romeo@montegue.net/orchard' id='jingleaudio2' type='result' />

  <iq to='juliet@capulet.com/balcony' from='romeo@montegue.net/orchard' id='jingleaudio3' type='set' />
    <jingle xmlns='http://jabber.org/protocol/jingle'
     action='description-accept' initiator='romeo@montague.net/orchard' sid='a73sjjvkla37jfea'>
       <content name='audio'>
         <description xmlns='http://jabber.org/protocol/jingle/description/audio'>
           <payload-type id='0' name='PCMU' />
         </description>
       </content>
    </jingle>
  </iq>
  

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

Example 11. Target Provisionally Accepts Session

  <iq to='romeo@montague.net/orchard' from='juliet@capulet.com/balcony' id='jingleaudio3' type='result' />
  

6. Mapping to Session Description Protocol

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:

m=<media> <port> <transport> <fmt list>
  

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:

Example 12. Jingle Format for Static Payload-Type

<payload-type id="13" name="CN"/>
  

Example 13. SDP Mapping of Static Payload-Type

m=audio 9999 RTP/AVP 13
  

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:

Example 14. Jingle Format for Dynamic Payload-Type

<payload-type id='96' name='speex' clockrate='16000'/>
  

Example 15. SDP Mapping of Dynamic Payload-Type

m=audio 9999 RTP/AVP 96
a=rtpmap:96 speex/16000
  

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.

Example 16. Jingle Format for Dynamic Payload-Type With Parameters

<payload-type id='96' name='speex' clockrate='16000' ptime='40'>
  <parameter name='vbr' value='on'/>
  <parameter name='cng' value='on'/>
</payload-type>
  

Example 17. SDP Mapping of Dynamic Payload-Type With Parameters

m=audio 9999 RTP/AVP 96
a=rtpmap:96 speex/16000
a=ptime:40
a=fmtp:96 vbr=on;cng=on
  

7. Service Discovery

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 Service Discovery [8] information requests.

Example 18. Service Discovery Information Request

<iq from='romeo@montague.net/orchard'
    id='disco1'
    to='juliet@capulet.com/balcony'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  

Example 19. Service Discovery Information Response

<iq from='juliet@capulet.com/balcony'
    id='disco1'
    to='romeo@montague.net/orchard'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
    <feature var='http://jabber.org/protocol/jingle'/>
    <feature var='http://jabber.org/protocol/jingle/description/audio'/>
    ...
  </query>
</iq>
  

8. Informational Messages

<iq from='juliet@capulet.com/balcony'
    to='romeo@montague.net/orchard'
    id='busy1'
    type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='description-info'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <busy xmlns='http://jabber.org/protocol/jingle/info/audio'/>
  </jingle>
</iq>
    

<iq from='juliet@capulet.com/balcony'
    to='romeo@montague.net/orchard'
    id='hold1'
    type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='description-info'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <hold xmlns='http://jabber.org/protocol/jingle/info/audio'/>
  </jingle>
</iq>
    

<iq from='juliet@capulet.com/balcony'
    to='romeo@montague.net/orchard'
    id='mute1'
    type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='description-info'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <mute xmlns='http://jabber.org/protocol/jingle/info/audio'/>
  </jingle>
</iq>
    

<iq from='juliet@capulet.com/balcony'
    to='romeo@montague.net/orchard'
    id='ringing1'
    type='set'>
  <jingle xmlns='http://jabber.org/protocol/jingle'
          action='description-info'
          initiator='romeo@montague.net/orchard'
          sid='a73sjjvkla37jfea'>
    <ringing xmlns='http://jabber.org/protocol/jingle/info/audio'/>
  </jingle>
</iq>
    

9. Error Handling

Table 3: Other Error Conditions

10. Implementation Notes

11. Security Considerations

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

12. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [11].

13. XMPP Registrar Considerations

<content>
  <name>audio</name>
  <desc>Jingle sessions that support audio exchanges</desc>
  <doc>XEP-0167</doc>
</content>
    

14. XML Schemas

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://jabber.org/protocol/jingle/description/audio'
    xmlns='http://jabber.org/protocol/jingle/description/audio'
    elementFormDefault='qualified'>

  <xs:element name='description'>
    <xs:complexType>
      <xs:sequence minOccurs='0' maxOccurs='unbounded'/>
        <xs:element ref='payload-type'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='payload-type'>
    <xs:complexType>
      <xs:sequence minOccurs='0' maxOccurs='unbounded'>
        <xs:element ref='parameter'/>
      </xs:choice>
      <xs:attribute name='channels' type='xs:byte' use='optional' default='1'/>
      <xs:attribute name='clockrate' type='xs:short' use='optional'/>
      <xs:attribute name='id' type='xs:unsignedByte' use='required'/>
      <xs:attribute name='maxptime' type='xs:short' use='optional'/>
      <xs:attribute name='name' type='xs:string' use='optional'/>
      <xs:attribute name='ptime' type='xs:short' use='optional'/>
    </xs:complexType>
  </xs:element>

  <xs:element name='parameter'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='name' type='xs:string' use='required'/>
          <xs:attribute name='value' type='xs:string' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>
    

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://jabber.org/protocol/jingle/info/audio'
    xmlns='http://jabber.org/protocol/jingle/info/audio'
    elementFormDefault='qualified'>

  <xs:element name='busy' type='empty'/>
  <xs:element name='hold' type='empty'/>
  <xs:element name='mute' type='empty'/>
  <xs:element name='ringing' type='empty'/>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>
    

Notes

Revision History

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