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.
The Jingle extension defined herein is designed to meet the following requirements:
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) . 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:
|type||The type of feedback||REQUIRED||ack, nack, ccm, app, etc..|
|subtype||The subtype (depends on the type)||OPTIONAL (possibly REQUIRED depending on the type)||ack: rpsi, app; nack: sli, pli, rpsi, app, rai; ccm: fir, tmmbr, tstr, vbcm; app: depends on the application;|
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:
|value||Number of milliseconds between regular RTCP reports||REQUIRED||0 to MAXUINT (default to 0)|
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 reply where the responder rejects the "sli" but accepts the "pli".
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.
The <rtcp-fb/> element maps to the a:rtcp-fb= SDP line with the exception of the 'trr-int' parameter which is mapped into its 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:
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)  information requests an entity MUST return the following features:
An example follows:
This document requires no interaction with the Internet Assigned Numbers Authority (IANA).
This specification defines the following XML namespaces:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
Thanks to Youness Alaoui for his feedback.
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