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Jingle Video via RTP This document defines methods for negotiating Jingle video sessions that use the Real-time Transport Protocol (RTP) for media exchange. &LEGALNOTICE; 0180 Proposed Standards Track Standards Council XMPP Core XEP-0166 TO BE ASSIGNED &stpeter; Milton Chen Milton.Chen@vseelab.com 0.10 2007-11-27 psa

Further editorial review.

0.9 2007-11-15 psa

Editorial review and consistency check.

0.8 2007-05-23 psa

Corrected examples to use video codecs; added clockrate attribute.

0.7 2007-05-23 psa

More completely specified how to include SDP parameters and codec-specific parameters (same approach as in XEP-0167); added and corrected Theora examples.

0.6 2007-04-17 psa

Specified Jingle conformance, including the need to use a lossy transport and the process of sending and receiving video content.

0.5 2007-03-23 psa

Added negotiation flow and SDP mapping; renamed to mention RTP as the associated transport; corrected negotiation flow to be consistent with SIP/SDP (each party specifies a list of the payload types it can receive); added profile attribute to content element in order to specify RTP profile in use.

0.4 2006-12-21 psa

Modified spec to use provisional namespace before advancement to Draft (per XEP-0053).

0.3 2006-08-23 psa

Modified namespace to track XEP-0166.

0.2 2006-07-12 psa

Updated to use content type instead of media type.

0.1 2006-03-23 psa/mc

Initial version.

0.0.1 2006-03-20 psa/mc

First draft.

&xep0166; can be used to initiate and negotiate a wide range of peer-to-peer sessions. One session type of interest is video chat. This document specifies a format for describing Jingle video sessions, where the media exchange occurs using the Real-time Transport Protocol (see &rfc3550;).

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

  1. Enable negotiation of parameters necessary for video chat.
  2. Map these parameters to the Session Description Protocol (SDP; see &rfc4566;) to enable interoperability.
  3. Define informational messages related to video chat.

In accordance with Section 8 of XEP-0166, this document specifies the following information related to the Jingle Video via RTP application type:

  1. The application format negotiation process is defined in the Negotiating a Jingle Video Session section of this document.

  2. The semantics of the &DESCRIPTION; element are defined in the Application Format section of this document.

  3. A mapping of Jingle semantics to the Session Description Protocol is provided in the Mapping to Session Description Protocol section of this document.

  4. A Jingle video session MUST use a lossy transport method such as &xep0177; or the "ice-udp" method specified in &xep0176;.

  5. Content is to be sent and received as follows:

    • Outbound video content shall be encoded into RTP packets and each packet shall be sent individually over the transport. Each inbound packet received over the transport is an RTP packet.

A Jingle video session is described by a content type that contains one application format and one transport method. The application format consists of one or more encodings contained within a wrapper <description/> element qualified by the 'http://www.xmpp.org/extensions/xep-0180.html#ns' namespace &NSNOTE;. In the language of RFC 4566 each encoding is a payload-type; therefore, each <payload-type/> element specifies an encoding that can be used for the audio stream, as illustrated in the following example.

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The &DESCRIPTION; element is intended to be a child of a &CONTENT; element as specified in XEP-0166.

The &CONTENT; element SHOULD possess a 'profile' attribute that specifies the exact protocol in use as would be encapsulated in SDP (e.g., "RTP/AVP" or "UDP/TLS/RTP/SAVP").

The encodings SHOULD be provided in order of preference by placing the most-preferred &PAYLOADTYPE; element as the first child of the &DESCRIPTION; element (etc.).

The allowable attributes of the &PAYLOADTYPE; element are as follows:

Attribute Description Datatype/Units Inclusion
channels The number of channels (e.g., 2 for stereoscopic video) positiveInteger (defaults to 1) OPTIONAL
clockrate The sampling frequency in Hertz positiveInteger RECOMMENDED
height The vertical extent of the displayed video, in pixels positiveInteger RECOMMENDED
id A unique identifier for the payload type positiveInteger REQUIRED
layer The relationship of a layer to the "bottom" of the stack, where 0 = bottom (the first layer) nonNegativeInteger OPTIONAL
name A name for the payload type string RECOMMENDED for static payload types, REQUIRED for dynamic payload types
transparent Whether or not a layer is transparent boolean OPTIONAL
width The horizontal extent of the displayed video, in pixels positiveInteger RECOMMENDED
x The horizontal starting point of a tile, in pixels from the origin point positiveInteger OPTIONAL
y The vertical starting point of a tile, in pixels from the origin point positiveInteger OPTIONAL

In Jingle Video, the encodings are used in the context of RTP. The most common encodings for the Audio/Video Profile (AVP) of RTP are listed in &rfc3551; (these "static" types are reserved from payload ID 0 through payload ID 95), although other encodings are allowed (these "dynamic" types use payload IDs 96 to 127) in accordance with the dynamic assignment rules described in Section 3 of RFC 3551. The payload IDs are represented in the 'id' attribute.

Each <payload-type/> element MAY contain one or more child elements that specify particular parameters related to the payload. For example, as described in &rtpspeex;, the "cng", "mode", and "vbr" parameters may be specified in relation to usage of the Speex See <http://www.speex.org/>. codec. Where such parameters are encoded via the "fmtp" SDP attribute, they shall be represented in Jingle via the following format:

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Note: The parameter names are effectively guaranteed to be unique, since &IANA; maintains a registry of SDP parameters (see <http://www.iana.org/assignments/sdp-parameters>).

When the initiator sends a session-initiate stanza to the responder, the &DESCRIPTION; element includes all of the payload types that the initiator can receive for Jingle video (each one encapsulated in a separate &PAYLOADTYPE; element):

action='session-initiate' initiator='romeo@montague.net/orchard' sid='v1d30k1ll3dth3r4d10st4r'> ]]>

Upon receiving the session-initiate stanza, the responder determines whether it can proceed with the negotiation. The general Jingle error cases are specified in XEP-0166 and illustrated &xep0167;. In addition, the responder must determine if it supports any of the payload types advertised by the initiator; if it supports none of the offered payload types, it must reject the session by returning a ¬acceptable; error with a Jingle-Video-specific condition of <unsupported-codecs/>:

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If there is no error, the responder acknowledges the session initiation request:

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The responder then SHOULD send a list of the payload types that it can receive via a Jingle "content-accept" (or "session-accept") action. The list that the responder sends MAY include any payload types (not a subset of the payload types sent by the initiator) but SHOULD retain the ID numbers specified by the initiator. The order of the &PAYLOADTYPE; elements indicates the responder's preferences, with the most-preferred types first.

action='content-accept' initiator='romeo@montague.net/orchard' sid='v1d30k1ll3dth3r4d10st4r'> ]]>

The initiator acknowledges the 'content-accept' with an empty IQ result:

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After successful transport negotiation (not shown here), the responder then accepts the session:

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And the initiator acknowledges session acceptance:

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Note: Because a "session-accept" action implicitly indicates acceptance of the application format (i.e., "content-accept"), it is not necessary to send a separate "content-accept" action. This flow is shown for completeness only.

The SDP media type for Jingle Video via RTP is "video" (see Section 8.2.1 of RFC 4566).

If the payload type is static (payload-type IDs 0 through 95 inclusive), it MUST be mapped to a media field defined in RFC 4566. The generic format for the media field is as follows:

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In the context of Jingle video sessions, the <media> is "video", the <port> is the preferred port for such communications (which may be determined dynamically), the <transport> is whatever profile is negotiated via the 'profile' attribute of the &CONTENT; element in the Jingle negotiation (e.g., "RTP/AVT"), and the <fmt list> is the payload-type ID.

For example, consider the following static payload-type:

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That Jingle-formatted information would be mapped to SDP as follows:

If the payload type is dynamic (payload-type IDs 96 through 127 inclusive), it SHOULD be mapped to an SDP media field plus an SDP attribute field named "rtpmap".

For example, consider a VC-1 payload such as that described in &rfc4425;:

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That Jingle-formatted information would be mapped to SDP as follows:

As noted, if additional parameters are to be specified, they shall be represented as attributes of the <payload-type/> element or its child <parameter/> element, as in the following example.

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That Jingle-formatted information would be mapped to SDP as follows:

The Jingle-Video-specific error conditions are as follows:

Jingle Video Condition XMPP Condition Description
<unsupported-codecs/> ¬acceptable; The recipient does not support any of the offered video encodings.

If an entity supports Jingle video exchanges via RTP, it MUST advertise that fact by returning a feature of "http://www.xmpp.org/extensions/xep-0180.html#ns" in response to &xep0030; information requests &NSNOTE;.

]]> ... ... ]]>

Naturally, support may also be discovered via the dynamic, presence-based profile of service discovery defined in &xep0115;.

Informational messages may be sent by either party within the context of Jingle to communicate the status of a Jingle video session, device, or principal. The informational message MUST be an IQ-set containing a &JINGLE; element of type "session-info". No informational message payload elements have yet been defined for Jingle Video via RTP, but they may be specified in a future version of this document.

Support for the Theora codec is RECOMMENDED.

In order to secure the data stream, implementations SHOULD use encryption methods appropriate to the transport method and media being exchanged; for example, in the case of UDP, that would include Datagram Transport Layer Security (DTLS) as specified in &rfc4347;. &sdpdtls; defines such methods for the Session Description Protocol; the relevant RTP profile (e.g., "UDP/TLS/RTP/SAVP" for transporting the RTP stream over DTLS with UDP) shall be specified as the value of the &CONTENT; element's 'profile' attribute.

This document requires no interaction with &IANA;.

Until this specification advances to a status of Draft, its associated namespaces shall be:

  • http://www.xmpp.org/extensions/xep-0180.html#ns
  • http://www.xmpp.org/extensions/xep-0180.html#ns-errors

Upon advancement of this specification, the ®ISTRAR; shall issue permanent namespaces in accordance with the process defined in Section 4 of &xep0053;.

The following namespaces are requested, and are thought to be unique per the XMPP Registrar's requirements:

  • urn:xmpp:jingle:app:video-rtp
  • urn:xmpp:jingle:app:video-rtp:errors

The XMPP Registrar shall include "video-rtp" in its registry of Jingle application formats. The registry submission is as follows:

video-rtp Jingle sessions that support video exchange via the Real-time Transport Protocol lossy XEP-0180 ]]>
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