XEP-0293: Jingle RTP Feedback 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 Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF) with Jingle RTP sessions.

2. Requirements

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

1. Enable negotiations of the required parameters for the transmissions of RTP Feedback messages as defined in RFC 4585 [1].
2. Map these parameters to Session Description Protocol (SDP; see RFC 4566 [2]) to enable interoperability.

3. New elements

This specification defines two new elements, <rtcp-fb/> and <rtcp-fb-trr-int/>, that can be inserted in the <description/> or the <payload-type/> elements of Jingle RTP Sessions (XEP-0167) [3]. The presence of any of these elements in a content's description means that the RTP/AVPF profile should be used for the whole content. If any of these elements are inside the <payload-type/> element, the parameters specified apply only to that payload type, if they are directly inside the <description/> tag, then the specified parameters apply to the whole content.

The attributes of the <rtcp-fb/> element are:

Any type or subtype of feedback message that requires extra parameters in the a=b form can use the <parameter/> element to describe it. Any other form of parameter can be stored as the 'key' attribute in a parameter element with an empty value.

Note: this overlaps with the subtype attribute. If there is only one parameter, use the subtype. The only known example where this is required is ccm.

The element <rtcp-fb-trr-int/> is used to specify the minimum interval between two Regular (full compound) RTCP packets in milliseconds for this media session. It corresponds to the "a=rtcp-fb:* trr-int" line in SDP. The attributes of the <rtcp-fb-trr-int/> element are:

4. Negotiation

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

To conform with the negotiation rules outlined in RFC 4585 Section 4, the responder MUST send any <rtcp-fb/> element as-is if it accepts it. It MUST NOT change any parameter. It MUST NOT add any <rtcp-fb/> element that was not offered by the initiator. It MUST NOT modify the 'value' of any <rtcp-fb-trr-int/> element. It can only remove the <rtcp-fb-trr-int/> element or reject the content. If all the feedback messages are removed but the responder wants to stay in the RTP/AVPF profile, it MUST put a <rtcp-fb-trr-int/> element with the same 'value' that it received from the intiator, if the initiator did not provide a <rtcp-fb-trr-int/> element, then this value is "0".

Example negotiation where the initiator requests Packet Loss Indications (pli) as defined in RFC 4585 on both H.263 and H.264, but also requests Slice Loss Indications for H.264 with a minimum interval between regular full compound RTCP packets of 100 milliseconds.

Example 1. Initiator sends description inside session-initiate

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtcp-fb xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' type='nack' subtype='pli'/>
  <payload-type id='96' name='H264' clockrate='90000'>
    <rtcp-fb-trr-int xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' value='100'/>
    <rtcp-fb xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' type='nack' subtype='sli'/>
  </payload-type>
  <payload-type id='34' name='H263' clockrate='90000'/>
</description>
  

Example reply where the responder rejects the "sli" but accepts the "pli".

Example 2. Responder sends description inside session-accept

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtcp-fb xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' type='nack' subtype='pli'/>
  <payload-type id='96' name='H264' clockrate='90000'>
    <rtcp-fb-trr-int xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' value='100'/>
  </payload-type>
  <payload-type id='34' name='H263' clockrate='90000'/>
</description>
  

Another reply to the same request where the responder wishes to stay in the AVPF profile but rejects all specific feedback messages by using the <rtcp-fb-trr-int/> with the default value.

Example 3. Responder sends description inside session-accept without any feedback message

<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
  <rtcp-fb-trr-int xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' value='0'/>
  <payload-type id='96' name='H264' clockrate='90000'/>
  <payload-type id='34' name='H263' clockrate='90000'/>
</description>

5. Mapping to Session Description Protocol

The <rtcp-fb/> element maps to the a:rtcp-fb= SDP line with the exception of the 'trr-int' parameter which is mapped into it's own element (<rtcp-fb-trr-int/>) in XMPP. The payload types are also not explicitly written in the <rtcp-fb/> and <rtcp-fb-trr-int/> elements. Instead, each payload type has its own set of <rtcp-fb/> and <rtcp-fb-trr-int/> elements if they do not apply to the whole content.

Example conversion of a sample fragment of a SDP containing an audio session using the RTP/AVP profile for audio and the RTP/AVPF profile for video:

Example 4. SDP fragment

 v=0
 o=remeo 3203093520 3203093520 IN IP4 host.example.com
 s=Video call with feedback
 t=3203130148 3203137348
 m=audio 49170 RTP/AVP 0
 c=IN IP4 10.0.1.1
 a=rtpmap:0 PCMU/8000
 m=video 59172 RTP/AVPF 98 99
 c=IN IP4 10.0.1.1
 a=rtpmap:98 H263-1998/90000
 a=rtpmap:99 H261/90000
 a=rtcp-fb:* nack
 a=rtcp-fb:98 nack rpsi
 a=rtcp-fb:98 trr-int 100

Example 5. The same description in XMPP format

<iq from='romeo@montague.lit/orchard'
    id='ph37a419'
    to='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'>
    action='session-initiate'
    initiator='romeo@montague.lit/orchard'
    sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='voice'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='audio'>
        <payload-type id='0' name='PCMU' />
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:raw-udp:1'>
        <candidate component='1'
	     generation='0'
	     id='a9j3mnbtu1'
	     ip='10.0.1.1'
	     port='49170'/>
        <candidate component='2'
 	     generation='0'
	     id='a9j3mnbtu1'
	     ip='10.0.1.1'
	     port='49171'/>
      </transport>
    </content>
    <content creator='initiator' name='face'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
        <rtcp-fb xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' type='nack'/>
        <payload-type id='98' name='H263-1998'>
          <rtcp-fb xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' type='nack'
                   subtype='rpsi'/>
          <rtcp-fb-trr-int xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0' value='100'/>
        </payload-type>
        <payload-type id='99' name='H264' />
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:raw-udp:1'>
        <candidate component='1'
                   generation='0'
                   id='a9j3mnbtu1'
                   ip='10.0.1.1'
                   port='49172'/>
        <candidate component='2'
                   generation='0'
                   id='a9j3mnbtu1'
                   ip='10.0.1.1'
                   port='49173'/>
      </transport>
    </content>
  </jingle>
</iq>
  

6. Determining support

To advertise its support for Extended RTCP Feedback in Jingle RTP Sessions and a minimum interval between regular RTCP packets, when replying to Service Discovery (XEP-0030) [4] 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:rtcp-fb: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:rtcp-fb: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:rtcp-fb:0'
    xmlns='urn:xmpp:jingle:apps:rtp:rtcp-fb:0'
    elementFormDefault='qualified'>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0293: http://www.xmpp.org/extensions/xep-0293.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='rtcp-fb'>
    <xs:complexType>
      <xs:sequence>  
        <xs:element name='parameter'
            type='parameterElementType'
            minOccurs='0' 
            maxOccurs='unbounded'/>
      </xs:sequence>
      <xs:attribute name='type' type='xs:string' use='required'/>
      <xs:attribute name='subtype' type='xs:string' use='optional'/>
    </xs:complexType>
  </xs:element>

  <xs:element name='rtcp-fb-trr-int'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='value' type='xs:positiveInteger' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </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: 0293
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 4585
Supersedes: None
Superseded By: None
Short Name:
Schema: <http://xmpp.org/schemas/jingle-apps-rtp-rtcp-fb.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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