Jingle RTP Sessions (XEP-0167) [1] defines the Jingle (XEP-0166) [2] signalling exchanges needed to establish video sessions using the Real-time Transport Protocol RFC 3550 [3]; however, it does not say which video codecs are mandatory-to-implement, since the state of codec technologies is more fluid than the signalling interactions. This document fills that gap by providing guidance to Jingle developers regarding voice and video codecs.
Because codec technologies are typically subject to patents, the topics discussed here are controversial. This document attempts to steer a middle path between (1) specifying mandatory-to-implement technologies that realistically will not be implemented and deployed and (2) providing guidelines that, while realistic, do not encourage the implementation and deployment of patent-clear technologies.
This document does not yet provide binding recommendations to the XMPP developer community regarding mandatory-to-implement technologies; however, it provides input that the XMPP Standards Foundation (XSF) [4] could use in making such recommendations.
The ideal codec would meet the following criteria:
Unfortunately, not all codecs meet those criteria. In the remainder of this document we discuss the video codecs that are most appropriate for implementation in Jingle RTP applications.
Dirac is a general-purpose video compression technology developed by the BBC that has been licensed in the open. It is used for everything from Internet streaming to HDTV. To date there is no RTP packetization deveintion for Dirac; however, such a format is under development.
| Quality | Packetization | Availability | Patents |
|---|---|---|---|
| High quality. | Not yet defined. | Freely downloadable under both GPL and LGPL at <http://diracvideo.org/>; commonly deployed but not yet in video over IP systems because of the lack of an RTP packetization format. | Diract is patent-clear, and the BBC has allowed its related patents to lapse. |
H.264 is a technology for video compression jointly designed by the ITU and the International Organization for Standardization (ISO) [6]. The following table summarizes the available information about H.264.
| Quality | Packetization | Availability | Patents |
|---|---|---|---|
| High quality. | See RFC 3984 [7]. | Commonly deployed in commercial video systems. Not freely downloadable; both software implementations and service deployments can be subject to royalty payments for commercial use. | Patented. |
According to the theora.org website, the Theora codec is "a free and open video compression format". Theora is based on the VP3 codec originally developed by On2 Technologies and is now maintained by the Xiph.org Foundation. The following table summarizes the available information about Theora.
| Quality | Packetization | Availability | Patents |
|---|---|---|---|
| Acceptable quality. | See RTP Payload Format for Theora Encoded Video [8]. | Freely downloadable under BSD license at <http://theora.org/>; not yet commonly deployed, especially on devices that have deployed H.264 instead. | On2's patents over VP3 were contributed to the Xiph.org Foundation in 2001. |
VP8 is an open video compression format originally developed (as was Theora) by On2 Technologies and released by Google after it acquired On2 Technologies in 2010. The following table summarizes the available information about Theora.
| Quality | Packetization | Availability | Patents |
|---|---|---|---|
| High quality. | See RTP Payload Format for VP8 Video [9]. | Freely downloadable under BSD-like license at <http://webmproject.org/>; not yet commonly deployed, especially on devices that have deployed H.264 instead. | Google released the VP8 bitstream format under an irrevocable free patent license in 2010. |
The situation regarding video codecs is more murky, and implementers face difficult tradeoffs. Although Theora is patent-clear and freely implementable, it is not yet commonly deployed. Although Dirac is patent-clear and deployed fairly widely, no RTP packetization format has been defined for it. Although deployment of H.264 is fairly common, it is not patent-clear or freely implementable. For many open-source / free software projects and smaller technology vendors, implementation of H.264 is either impossible (because of patents and licensing restrictions) or prohibitively expensive (because of royalty payments). These developers are strongly encouraged to implement Theora or Dirac and also to urge wider adoption of Theora and Dirac among larger technology vendors. However, this document acknowledges that it may take some time before Theora and Dirac are commonly deployed (especially on mobile devices) and that systems based on H.264 might be dominant in the marketplace for several years. This situation is unfortunate but cannot be directly changed by the XMPP developer community.
Because video codecs are not as mature as audio codecs, it is not yet possible for the XSF to recommend a mandatory-to-implement technology for Jingle video. However, in the future it might be possible to recommend one of the codecs described in this document.
For security considerations related to Jingle RTP sessions, refer to XEP-0167. This document introduces no new security considerations. See also the security considerations described in the relevant codec specifications.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [10].
This document requires no interaction with the XMPP Registrar [11].
Thanks to Olivier Crête, Dave Cridland, Florian Jensen, Justin Karneges, Evgeniy Khramtsov, Marcus Lundblad, Tobias Markmann, Pedro Melo, Jack Moffitt, Jeff Muller, Jehan Pagès, Arc Riley, Kevin Smith, Remko Tronçon, Justin Uberti, and Paul Witty for their feedback.
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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.
There exists a special venue for discussion related to the technology described in this document: the <jingle@xmpp.org> mailing list.
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. XEP-0167: Jingle RTP Sessions <https://xmpp.org/extensions/xep-0167.html>.
2. XEP-0166: Jingle <https://xmpp.org/extensions/xep-0166.html>.
3. RFC 3550: RTP: A Transport Protocol for Real-Time Applications <http://tools.ietf.org/html/rfc3550>.
4. The XMPP Standards Foundation (XSF) is an independent, non-profit membership organization that develops open extensions to the IETF's Extensible Messaging and Presence Protocol (XMPP). For further information, see <https://xmpp.org/about/xmpp-standards-foundation>.
5. The term patent-clear does not necessarily mean that no patents have ever been applied for or granted regarding a technology, or that the technology is completely free from patents (since such a judgment is nearly impossible to make, and is outside the purview of the XMPP developer community and the XMPP Standards Foundation); the term means only that those who implement the technology are generally understood to be relatively safe from the threat of patent litigation, either because any relevant patents have expired, were filed in a defensive manner, or are made available under suitable royalty-free licenses.
6. The International Organization for Standardization develops standards a wide variety of technical domains. For further information, see <http://www.iso.org/>.
7. RFC 3984: RTP Payload Format for H.264 Video <http://tools.ietf.org/html/rfc3984>.
8. RTP Payload Format for Theora Encoded Video <http://tools.ietf.org/html/draft-barbato-avt-rtp-theora>. Work in progress.
9. RTP Payload Format for VP8 Video <http://tools.ietf.org/html/draft-ietf-payload-vp8>. Work in progress.
10. 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/>.
11. 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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Initial published version, split from XEP-0266.
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