This documents specifies how to negotiate the use of the RTP Header Extensions as defined by RFC 5285  with Jingle RTP sessions.
The Jingle extension defined herein is designed to meet the following requirements:
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:
|id||The ID of the extensions||REQUIRED||1-256, 4096-4351|
|uri||The URI that defines the extension||REQUIRED||Any valid URI|
|senders||Which party is allowed to send the negotiated RTP Header Extensions||OPTIONAL (defaults to "both")||"initiator", "responder", and "both"|
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.
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  and also the requests synchronisation metadata header extension (RFC 6051 ) with either the 56-bit or the 64-bit format.
Example reply where the responder accepts the timestamp offset and the 56-bit synchronisation metadata header extensions.
Another reply to the same request where the responder accepts only the synchronisation data header extension with the 64-bit format.
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:
To advertise its support for Generic Header extensions in Jingle RTP Sessions, 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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Advanced to Draft per a vote of the XMPP Council.
Added XML Schema; Updated based on last call feedback.
Initial published version.