This document specifies a method for establishing a direct connection using SCTP channels over DTLS. As WebRTC supports the SCTP partial reliability extension (see RFC 3758 [1]), the connection established can either be reliable or partial-reliable. However due to the wider use (e.g. Jingle File Transfer (XEP-0234) [2]) and easier maintenance, this transport method defines the mandatory use of reliable and in-order-delivery datachannels (thus a streaming transport). If the need for partial-reliability is there, a second specification for datagram type datachannels might be created. The establishment of this connection is managed by Jingle ICE-UDP Transport Method (XEP-0176) [3]. The establishment of security provided by DTLS is described in Use of DTLS-SRTP in Jingle Sessions (XEP-0320) [4].
There are several use cases. Given below are a few examples:
In accordance with Section 10 of XEP-0166, this document specifies the following information related to the Jingle DTLS/SCTP transport method:
The transport negotiation process is defined in the Protocol Description section of this document.
The semantics of the <sctpmap/> element are defined in the Syntax section of this document.
This protocol requires the Stream Control Transmission Protocol (SCTP) to run within the security context of the Datagram Transport Layer Security (DTLS) protocol. As determined by RTCWeb Data Channels running SCTP on top of DTLS is preferred, as in this order the control messages are encrypted as well and the DTLS channel can be shared with several applications
In order for the initiator in a Jingle exchange to start the negotiation, it sends a Jingle "session-initiate" stanza that includes at least one content type, as described in XEP-0166. If the initiator wishes to negotiate the SCTP transport method for an application format, it MUST include a <sctpmap/> child element qualified by the 'urn:xmpp:jingle:transports:dtls-sctp:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number). The <transport/> element SHOULD in turn contain one <candidate/> element for each of the initiator's higher-priority transport candidates as determined in accordance with the ICE methodology, but MAY instead be empty (with each candidate to be sent as the payload of a transport-info message).
<iq from='romeo@montague.lit/orchard'
id='ixt174g9'
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='file-552da749930852c69ae5d2141d3766b1'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:3'>
<offer>
<file>
<date>1969-07-21T02:56:15Z</date>
<desc>This is a test. If this were a real file...</desc>
<name>test.txt</name>
<range/>
<size>1022</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</offer>
</description>
<transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'
pwd='asd88fgpdd777uzjYhagZg'
ufrag='8hhy'>
<sctpmap xmlns='urn:xmpp:jingle:transports:dtls-sctp:1' number='5000' protocol='webrtc-datachannel' streams='1024'/>
<fingerprint xmlns='urn:xmpp:jingle:apps:dtls:0' hash='sha-256' setup='actpass'>
02:1A:CC:54:27:AB:EB:9C:53:3F:3E:4B:65:2E:7D:46:3F:54:42:CD:54:F1:7A:03:A2:7D:F9:B0:7F:46:19:B2
</fingerprint>
<candidate component='1'
foundation='1'
generation='0'
id='el0747fg11'
ip='10.0.1.1'
network='1'
port='8998'
priority='2130706431'
protocol='udp'
type='host'/>
<candidate component='1'
foundation='2'
generation='0'
id='y3s2b30v3r'
ip='192.0.2.3'
network='1'
port='45664'
priority='1694498815'
protocol='udp'
rel-addr='10.0.1.1'
rel-port='8998'
type='srflx'/>
</transport>
</content>
</jingle>
</iq>
As seen in the example, there is no notion about DataChannel internals. The only things negotiated here are the port on which the SCTP Stream runs and the name of the DataChannel. Further negotiation is happening through the WebRTC Data Channel Protocol [5], thus the DataChannel is identified through the content name.
For some uses like negotiated datachannels, it might be required to explicitly signal the channel properties. This is done by including a <channel/> element qualified by the 'urn:xmpp:jingle:transports:webrtc-datachannel:0' namespace (see Namespace Versioning regarding the possibility of incrementing the version number) as child of the <transport/> for each channel that has to be signaled. An abbreviated example follows:
[...]
<transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'
pwd='asd88fgpdd777uzjYhagZg'
ufrag='8hhy'>
<sctpmap xmlns='urn:xmpp:jingle:transports:dtls-sctp:1' number='5000' protocol='webrtc-datachannel' streams='1024'/>
<channel xmlns='urn:xmpp:jingle:transports:webrtc-datachannel:0' id="1" negotiated="true"/>
<channel xmlns='urn:xmpp:jingle:transports:webrtc-datachannel:0' id="3" negotiated="true"/>
<fingerprint mlns='urn:xmpp:jingle:apps:dtls:0' hash='sha-256' setup='actpass'>
[...]
The values and names for the <sctpmap/> element are taken from the corresponding SDP. The attributes of the <sctpmap/> element are as follows.
| Name | Description | SDP Syntax | Example |
|---|---|---|---|
| port | A port number where the channel is to be contacted | Port value in a=sctpmap line | 5000 |
| protocol | Media format | Protocol value in a=sctpmap line | webrtc-datachannel |
| streams | Number of supported incoming streams | Stream value in a=sctpmap line | 16 |
The attributes of the <channel/> element map directly to the attributes of the RTCDataChannelInit dictionary as defined in WebRTC 1.0: Real-time Communication Between Browsers [6].
As described in XEP-0166, to acknowledge receipt of the session initiation request, the responder immediately returns an IQ-result.
<iq from='juliet@capulet.lit/balcony'
id='ixt174g9'
to='romeo@montague.lit/orchard'
type='result'/>
Depending on the application type, a user agent controlled by a human user might need to wait for the user to affirm a desire to proceed with the session before continuing. When the user agent has received such affirmation (or if the user agent can automatically proceed for any reason, e.g. because no human intervention is expected or because a human user has configured the user agent to automatically accept sessions with a given entity), it returns a Jingle session-accept message. This message MUST contain a <transport/> element qualified by the 'urn:xmpp:jingle:transports:ice-udp:1' namespace, which SHOULD in turn contain one <sctpmap/> element qualified by the 'urn:xmpp:jingle:transports:dtls-sctp:1' namespace for the channel used by the responder.
Note: See the Security Considerations section of this document regarding the exposure of IP addresses by the responder's client.
<iq from='juliet@capulet.lit/balcony'
id='rw782g55'
to='romeo@montague.lit/orchard'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-accept'
initiator='romeo@montague.lit/orchard'
responder='juliet@capulet.lit/balcony'
sid='a73sjjvkla37jfea'>
<content creator='initiator' name='file-552da749930852c69ae5d2141d3766b1'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:3'>
<offer>
<file>
<date>1969-07-21T02:56:15Z</date>
<desc>This is a test. If this were a real file...</desc>
<name>test.txt</name>
<range/>
<size>1022</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</offer>
</description>
<transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'
pwd='YH75Fviy6338Vbrhrlp8Yh'
ufrag='9uB6'>
<fingerprint xmlns='urn:xmpp:jingle:apps:dtls:0' hash='sha-256' setup='actpass'>
02:1A:CC:54:27:AB:EB:9C:53:3F:3E:4B:65:2E:7D:46:3F:54:42:CD:54:F1:7A:03:A2:7D:F9:B0:7F:46:19:B2
</fingerprint>
<sctpmap xmlns='urn:xmpp:jingle:transports:dtls-sctp:1' number='5000' protocol='webrtc-datachannel' streams='1024'/>
<candidate component='1'
foundation='1'
generation='0'
id='or2ii2syr1'
ip='192.0.2.1'
network='0'
port='3478'
priority='2130706431'
protocol='udp'
type='host'/>
</transport>
</content>
</jingle>
</iq>
The initiator acknowledges the Jingle session-accept.
<iq from='juliet@capulet.lit/balcony'
id='ixt174g9'
to='romeo@montague.lit/orchard'
type='result'/>
Even after media has begun to flow, either party MAY continue to send additional channels to the other party (e.g., because the user wants to transmit a new file). Such events are shared by sending a content-add message.
<iq from='romeo@montague.lit/orchard'
id='uh3g1f48'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='content-add'
initiator='romeo@montague.lit/orchard'
sid='a73sjjvkla37jfea'>
<content creator='initiator' name='this-is-the-new-content'>
<description xmlns='urn:xmpp:jingle:apps:some-file' media='file'/>
<transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'/>
</content>
</jingle>
</iq>
The receiving party MUST acknowledge receipt.
<iq from='juliet@capulet.lit/balcony'
id='uh3g1f48'
to='romeo@montague.lit/orchard'
type='result'/>
By design is the sent media already encrypted by the DTLS layer
Special thanks to István Koren at i5 and their XMPP Experience
Thanks to Philipp Hancke and Tobias Markmann for their help and support and all the XEP-Authors for their examples.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].
This specification defines the following XML namespace:
The XMPP Registrar [8] includes the foregoing namespace in its registry at <https://xmpp.org/registrar/namespaces.html>, as governed by XMPP Registrar Function (XEP-0053) [9].
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.
The registry submission is as follows.
<var>
<name>IETF Draft MMUSIC</name>
<desc>
Signals support for SCTP in the SDP as described in draft-ietf-mmusic-sctp-sdp-05
</desc>
<doc>XEP-SCTP</doc>
</var>
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:xmpp:jingle:transports:dtls-sctp:1'
xmlns='urn:xmpp:jingle:transports:dtls-sctp:1'
elementFormDefault='qualified'>
<xs:annotation>
<xs:documentation>
The protocol documented by this schema is defined in
XEP-0343: http://www.xmpp.org/extensions/xep-0343.html
</xs:documentation>
</xs:annotation>
<xs:element name='sctpmap'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute name='number' type='xs:unsignedByte' use='required'/>
<xs:attribute name='protocol' type='xs:string' use='required'/>
<xs:attribute name='streams' type='xs:positiveInteger' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:xmpp:jingle:transports:webrtc-datachannel:0'
xmlns='urn:xmpp:jingle:transports:webrtc-datachannel:0'
elementFormDefault='qualified'>
<xs:annotation>
<xs:documentation>
The protocol documented by this schema is defined in
XEP-0343: http://www.xmpp.org/extensions/xep-0343.html
</xs:documentation>
</xs:annotation>
<xs:element name='channel'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute name='id' type='xs:unsignedShort' use='optional'/>
<xs:attribute name='maxPacketLifeTime' type='xs:unsignedShort' use='optional'/>
<xs:attribute name='maxRetransmits' type='xs:unsignedShort' use='optional'/>
<xs:attribute name='negotiated' type='xs:boolean' use='optional' default='false'/>
<xs:attribute name='ordered' type='xs:boolean' use='optional' default='true'/>
<xs:attribute name='protocol' type='xs:string' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>
This document in other formats: XML PDF
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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.
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.
Errata can be sent to <editor@xmpp.org>.
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 3758: Stream Control Transmission Protocol (SCTP) Partial Reliability Extension <http://tools.ietf.org/html/rfc3758>.
2. XEP-0234: Jingle File Transfer <https://xmpp.org/extensions/xep-0234.html>.
3. XEP-0176: Jingle ICE-UDP Transport Method <https://xmpp.org/extensions/xep-0176.html>.
4. XEP-0320: Use of DTLS-SRTP in Jingle Sessions <https://xmpp.org/extensions/xep-0320.html>.
5. WebRTC Data Channel Protocol <http://tools.ietf.org/html/draft-jesup-rtcweb-data-protocol-04>.
6. WebRTC 1.0: Real-time Communication Between Browsers <http://www.w3.org/TR/2013/WD-webrtc-20130910/>.
7. 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/>.
8. 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/>.
9. 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/
Add optional explicit signaling of channels to the transport element.
Initial published version approved by the XMPP Council.
First draft.
END