XEP-0294: Jingle RTP Header Extensions Negotiation

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

Table of Contents

1. Introduction

This documents specifies how to negotiate the use of the RTP Header Extensions as defined by RFC 5285 [1] with Jingle RTP sessions.

2. Requirements

The Jingle extension defined herein is designed to meet the following requirements:

1. Enable negotiations of the RTP Header extensions as defined in RFC 5285.
2. Map these parameters to Session Description Protocol (SDP; see RFC 4566 [2]) to enable interoperability.

3. New element

This specification defines a new element, <rtp-hdrext/>, that can be inserted in the <description/> element of a XEP-0167 RTP session.

The attributes of the <rtp-hdrext/> element are:

Any type of RTP Header Extension that requires extra parameters in the a=b form can embed <parameter/> elements to describe it. Any other form of parameter can be stored as the 'key' attribute in a parameter element with an empty value.

4. Negotiation

RTP header extensions are negotiated along the codecs. They follow the same Offer/Answer mechanism based on SDP Offer/Answer. The initiator signals which RTP header extensions it wants to send or receive in the the <session-initiate/> iq stanza. If the responder does not understand the type of header extensions, it MUST remove the element from the reply. If the responder does not wish to provide or receive some kind of RTP header extension, it MUST remove the relevant element from the reply. It MUST then send the remaining elements it wants to keep as-is without modifying them in the <session-accept/> iq stanza.

It MUST NOT add any <rtp-hdrext/> element that was not offered by the initiator. The responder MAY downgrade the senders field from "both" to "initator" or "responder", but MUST NOT modify it if it is "initator" or "responder".

Example negotiation where the initiator offers to use the timestamp offset header extension as defined in RFC 5450 [3] and also the requests synchronisation metadata header extension (RFC 6051 [4]) with either the 56-bit or the 64-bit format.

Example 1. Initiator sends description inside session-initiate

    
<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:toffset'
	      id='1'/>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:ntp-64'
	      id='4907'/>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:ntp-56'
	      id='4907'/>
  <payload-type id='96' name='THEORA' clockrate='90000'/>
</description>
  

Example reply where the responder accepts the timestamp offset and the 56-bit synchronisation metadata header extensions.

Example 2. Reponder sends description inside session-accept

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:toffset'
	      id='1'/>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:ntp-56'
	      id='2'/>
  <payload-type id='96' name='THEORA' clockrate='90000'/>
</description>

Another reply to the same request where the responder accepts only the synchronisation data header extension with the 64-bit format.

Example 3. Responder sends description inside session-accept with only the synchronisation data accepted

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      uri='urn:ietf:params:rtp-hdrext:ntp-64'
	      id='2'/>
  <payload-type id='96' name='THEORA' clockrate='90000'/>
</description>

5. Mapping to Session Description Protocol

The <rtp-hdrext/> element maps to the "a:extmap=" SDP line defined in RFC 5285. The ID is mapped to the 'id' attribute, the direction to the 'senders' attribute and the URI to the 'uri' attribute.

Example conversion of a incomplete sample fragment of a SDP taken from RFC 5285 section 6 into equivalent XMPP:

Example 4. SDP fragment

m=video
a=sendrecv
a=extmap:1 URI-toffset
a=extmap:2/recvonly URI-gps-string
a=extmap:3 URI-frametype

Example 5. The same description in XMPP format

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      id='1'
	      uri='URI-toffset'/>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      id='2'
	      uri='URI-gps-string'
	      senders='initiator'/>
  <rtp-hdrext xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
	      id='3'
	      uri='URI-frametype'/>
</description>

6. Determining support

To advertise its support for Generic Header extensions in Jingle RTP Sessions, when replying to Service Discovery (XEP-0030) [5] information requests an entity MUST return the following features:

1. URNs for any version of this protocol that the entity supports -- e.g., "urn:xmpp:jingle:apps:rtp:rtp-hdrext:0" for the current version

An example follows:

Example 6. Service discovery information request

    <iq from='romeo@montague.lit/orchard'
	id='bh3vd715'
	to='juliet@capulet.lit/balcony'
	type='get'>
      <query xmlns='http://jabber.org/protocol/disco#info'/>
    </iq>
  

Example 7. Service discovery information response

    <iq from='juliet@capulet.lit/balcony'
	id='bh3vd715'
	to='romeo@montague.lit/orchard'
	type='result'>
      <query xmlns='http://jabber.org/protocol/disco#info'>
	<feature var='urn:xmpp:jingle:1'/>
	<feature var='urn:xmpp:jingle:apps:rtp:1'/>
	<feature var='urn:xmpp:jingle:apps:rtp:video'/>
	<feature var='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'/>
      </query>
    </iq>
  

7. IANA Considerations

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

8. XMPP Registrar Considerations

9. XML Schemas

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

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
    xmlns='urn:xmpp:jingle:apps:rtp:rtp-hdrext:0'
    elementFormDefault='qualified'>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0294: http://www.xmpp.org/extensions/xep-0294.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='rtp-hdrext'>
    <xs:complexType>
      <xs:sequence>  
        <xs:element name='parameter'
            type='parameterElementType'
            minOccurs='0' 
            maxOccurs='unbounded'/>
      </xs:sequence>
      <xs:attribute name='id' type='xs:unsignedInt' use='required'/>
      <xs:attribute name='uri' type='xs:string' use='required'/>
      <xs:attribute name='senders'
                  use='optional'
                  default='both'>
        <xs:simpleType>
          <xs:restriction base='xs:NCName'>
            <xs:enumeration value='both'/>
            <xs:enumeration value='initiator'/>
            <xs:enumeration value='none'/>
            <xs:enumeration value='responder'/>
          </xs:restriction>
        </xs:simpleType>
      </xs:attribute>
    </xs:complexType>
  </xs:element>

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

</xs:schema>
  

10. Acknowledgements

Thanks to Youness Alaoui for his feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0294
Publisher: XMPP Standards Foundation
Status: Draft
Type: Standards Track
Version: 1.0
Last Updated: 2015-08-11
Approving Body: XMPP Council
Dependencies: XEP-0167, RFC 5285
Supersedes: None
Superseded By: None
Short Name:
Schema: <http://xmpp.org/schemas/jingle-apps-rtp-rtp-hdrext.xsd>
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards 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.

Appendix E: Discussion Venue

There exists a special venue for discussion related to the technology described in this document: the <jingle@xmpp.org> mailing list.

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements 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".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

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