RFC 5576 [1] provides a mechanism to describe attributes of individual media sources (identified by their synchronization source) within a media stream. A mapping to Jingle as an extension to Jingle RTP Sessions (XEP-0167) [2] is defined in this document.
The SDP format defined in RFC 5576 is shown below.
a=ssrc:<ssrc-id> <attribute>
a=ssrc:<ssrc-id> <attribute>:<value>
This maps to Jingle as a <source/> element qualified by the 'urn:xmpp:jingle:apps:rtp:ssma:0' namespace. Since 'ssrc' is a media attribute in SDP, the <source/> element is included as child of the Jingle <description/> element.
<source ssrc='ssrc-id' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='attribute'/>
<parameter name='attribute' value='value'/>
</source>
Each ssrc-id maps to a <source/> element whose 'ssrc' attribute is set to the ssrc-id. The associated attributes map to <parameter/> children with 'name' and 'value' attributes. If there is no value in the SDP, the value parameter shall be omitted.
An example follows:
a=ssrc:1656081975 cname:Yv/wvbCdsDW2Prgd a=ssrc:1656081975 msid:MLTJKIHilGn71fNQoszkQ4jlPTuS5vJyKVIv MLTJKIHilGn71fNQoszkQ4jlPTuS5vJyKVIva0
<source ssrc='1656081975' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='cname' value='Yv/wvbCdsDW2Prgd'/>
<parameter name='msid' value='MLTJKIHilGn71fNQoszkQ4jlPTuS5vJyKVIv MLTJKIHilGn71fNQoszkQ4jlPTuS5vJyKVIva0'/>
</source>The SDP format defined in RFC 5576 is shown below.
a=ssrc-group:<semantics> <ssrc-id> ...
This maps to Jingle as a <ssrc-group/> element qualified by the 'urn:xmpp:jingle:apps:rtp:ssma:0' namespace. Like the <source/> element, this is included as child of the Jingle <description/> element. The SDP 'semantics' parameter is mapped to the semantics attribute (for consistency with Jingle Grouping Framework (XEP-0338) [3]) and the list of ssrc-ids is mapped to <source/> elements whole 'ssrc' attribute is set to the ssrc-id.
<ssrc-group xmlns='urn:xmpp:jingle:apps:rtp:ssma:0' semantics='semantics'> <source ssrc='ssrc-id'/> [...] </ssrc-group>
A minimal example follows:
m=video 1 RTP/SAVPF 100 116 117
a=rtpmap:100 VP8/90000
a=ssrc-group:FID 2301230316 386328120
a=ssrc:2301230316 cname:T5qvrIZj42v//eYQ
a=ssrc:386328120 cname:uEYgNtStZyTF74sM
a=ssrc-group:FID 3139499595 2613715171
a=ssrc:3139499595 cname:re8jhxkly9bxzuxr
a=ssrc:2613715171 cname:f83avsiw6n1m7vi
This is mapped to Jingle as follows:
<content creator='initiator' name='webcam'>
<description xmlns='urn:xmpp:jingle:apps:rtp:1' media='video'>
<payload-type id='100' name='VP8' clockrate='90000'/>
<ssrc-group xmlns='urn:xmpp:jingle:apps:rtp:ssma:0' semantics='FID'>
<source ssrc='2301230316'/>
<source ssrc='386328120'/>
</ssrc-group>
<ssrc-group xmlns='urn:xmpp:jingle:apps:rtp:ssma:0' semantics='FID'>
<source ssrc='3139499595'/>
<source ssrc='2613715171'/>
</ssrc-group>
<source ssrc='2301230316' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='cname' value='T5qvrIZj42v//eYQ'/>
</source>
<source ssrc='386328120' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='cname' value='uEYgNtStZyTF74sM'/>
</source>
<source ssrc='3139499595' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='cname' value='re8jhxkly9bxzuxr'/>
</source>
<source ssrc='2613715171' xmlns='urn:xmpp:jingle:apps:rtp:ssma:0'>
<parameter name='cname' value='f83avsiw6n1m7vi'/>
</source>
</description>
</content>
If an entity supports source specific media attributes as described in RFC 5576, it MUST advertise that fact in its responses to Service Discovery (XEP-0030) [4] information ("disco#info") requests by returning a feature of 'urn:ietf:rfc:5576':
<iq type='get'
from='calvin@usrobots.lit/lab'
to='herbie@usrobots.lit/home'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='herbie@usrobots.lit/home'
to='calvin@usrobots.lit/lab'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<feature var='urn:xmpp:jingle:1'/>
<feature var='urn:ietf:rfc:5576'/>
</query>
</iq>
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) [5]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
This document introduces no additional security considerations above and beyond those defined in the documents on which it depends.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
This specification defines the following XML namespace:
The XMPP Registrar [7] includes the foregoing namespace to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) [8].
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.
TODO
This document in other formats: XML PDF
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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".
1. RFC 5576: Source-Specific Media Attributes in the Session Description Protocol (SDP) <http://tools.ietf.org/html/rfc5576>.
2. XEP-0167: Jingle RTP Sessions <https://xmpp.org/extensions/xep-0167.html>.
3. XEP-0338: Jingle Grouping Framework <https://xmpp.org/extensions/xep-0338.html>.
4. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
5. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.
6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
7. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
8. XEP-0053: XMPP Registrar Function <https://xmpp.org/extensions/xep-0053.html>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Move to Draft as per Council vote from 2020-05-20.
remove obsolete mslabel and label lines.
Initial published version approved by the XMPP Council.
First draft.
END