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Keywords: MTI, mandatory-to-implement
Internet Engineering Task Force (IETF) A.B. Roach
Request for Comments: 7742 Mozilla
Category: Standards Track March 2016
ISSN: 2070-1721
WebRTC Video Processing and Codec Requirements
Abstract
This specification provides the requirements and considerations for
WebRTC applications to send and receive video across a network. It
specifies the video processing that is required as well as video
codecs and their parameters.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7742.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 7742 WebRTC Video March 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Pre- and Post-Processing . . . . . . . . . . . . . . . . . . 3
3.1. Camera-Source Video . . . . . . . . . . . . . . . . . . . 3
3.2. Screen-Source Video . . . . . . . . . . . . . . . . . . . 4
4. Stream Orientation . . . . . . . . . . . . . . . . . . . . . 4
5. Mandatory-to-Implement Video Codec . . . . . . . . . . . . . 5
6. Codec-Specific Considerations . . . . . . . . . . . . . . . . 6
6.1. VP8 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. H.264 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
One of the major functions of WebRTC endpoints is the ability to send
and receive interactive video. The video might come from a camera, a
screen recording, a stored file, or some other source. This
specification provides the requirements and considerations for WebRTC
applications to send and receive video across a network. It
specifies the video processing that is required as well as video
codecs and their parameters.
Note that this document only discusses those issues dealing with
video-codec handling. Issues that are related to transport of media
streams across the network are specified in [WebRTC-RTP-USAGE].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following definitions are used in this document:
o A WebRTC browser (also called a WebRTC User Agent or WebRTC UA) is
something that conforms to both the protocol specification and the
Javascript API (see [RTCWEB-OVERVIEW]).
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RFC 7742 WebRTC Video March 2016
o A WebRTC non-browser is something that conforms to the protocol
specification, but it does not claim to implement the Javascript
API. This can also be called a "WebRTC device" or "WebRTC native
application".
o A WebRTC endpoint is either a WebRTC browser or a WebRTC non-
browser. It conforms to the protocol specification.
o A WebRTC-compatible endpoint is an endpoint that is able to
successfully communicate with a WebRTC endpoint but may fail to
meet some requirements of a WebRTC endpoint. This may limit where
in the network such an endpoint can be attached, or it may limit
the security guarantees that it offers to others. It is not
constrained by this specification; when it is mentioned at all, it
is to note the implications on WebRTC-compatible endpoints of the
requirements placed on WebRTC endpoints.
These definitions are also found in [RTCWEB-OVERVIEW] and that
document should be consulted for additional information.
3. Pre- and Post-Processing
This section provides guidance on pre- and post-processing of video
streams.
Unless specified otherwise by the Session Description Protocol (SDP)
or codec, the color space SHOULD be sRGB [SRGB]. For clarity, this
is the color space indicated by codepoint 1 from "ColourPrimaries" as
defined in [IEC23001-8].
Unless specified otherwise by the SDP or codec, the video scan
pattern for video codecs is Y'CbCr 4:2:0.
3.1. Camera-Source Video
This document imposes no normative requirements on camera capture;
however, implementors are encouraged to take advantage of the
following features, if feasible for their platform:
o Automatic focus, if applicable for the camera in use
o Automatic white balance
o Automatic light-level control
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RFC 7742 WebRTC Video March 2016
o Dynamic frame rate for video capture based on actual encoding in
use (e.g., if encoding at 15 fps due to bandwidth constraints, low
light conditions, or application settings, the camera will ideally
capture at 15 fps rather than a higher rate).
3.2. Screen-Source Video
If the video source is some portion of a computer screen (e.g.,
desktop or application sharing), then the considerations in this
section also apply.
Because screen-sourced video can change resolution (due to, e.g.,
window resizing and similar operations), WebRTC-video recipients MUST
be prepared to handle midstream resolution changes in a way that
preserves their utility. Precise handling (e.g., resizing the
element a video is rendered in versus scaling down the received
stream; decisions around letter/pillarboxing) is left to the
discretion of the application.
Note that the default video-scan format (Y'CbCr 4:2:0) is known to be
less than optimal for the representation of screen content produced
by most systems in use at the time of this document's writing, which
generally use RGB with at least 24 bits per sample. In the future,
it may be advisable to use video codecs optimized for screen content
for the representation of this type of content.
Additionally, attention is drawn to the requirements in Section 5.2
of [WebRTC-SEC-ARCH] and the considerations in Section 4.1.1. of
[WebRTC-SEC].
4. Stream Orientation
In some circumstances -- and notably those involving mobile devices
-- the orientation of the camera may not match the orientation used
by the encoder. Of more importance, the orientation may change over
the course of a call, requiring the receiver to change the
orientation in which it renders the stream.
While the sender may elect to simply change the pre-encoding
orientation of frames, this may not be practical or efficient (in
particular, in cases where the interface to the camera returns pre-
compressed video frames). Note that the potential for this behavior
adds another set of circumstances under which the resolution of a
screen might change in the middle of a video stream, in addition to
those mentioned in Section 3.2.
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RFC 7742 WebRTC Video March 2016
To accommodate these circumstances, WebRTC implementations that can
generate media in orientations other than the default MUST support
generating the R0 and R1 bits of the Coordination of Video
Orientation (CVO) mechanism described in Section 7.4.5 of [TS26.114]
and MUST send them for all orientations when the peer indicates
support for the mechanism. They MAY support sending the other bits
in the CVO extension, including the higher-resolution rotation bits.
All implementations SHOULD support interpretation of the R0 and R1
bits and MAY support the other CVO bits.
Further, some codecs support in-band signaling of orientation (for
example, the SEI "Display Orientation" messages in H.264 and H.265
[H265]). If CVO has been negotiated, then the sender MUST NOT make
use of such codec-specific mechanisms. However, when support for CVO
is not signaled in the SDP, then such implementations MAY make use of
the codec-specific mechanisms instead.
5. Mandatory-to-Implement Video Codec
For the definitions of "WebRTC browser", "WebRTC non-browser", and
"WebRTC-compatible endpoint" as they are used in this section, please
refer to Section 2.
WebRTC Browsers MUST implement the VP8 video codec as described in
[RFC6386] and H.264 Constrained Baseline as described in [H264].
WebRTC Non-Browsers that support transmitting and/or receiving video
MUST implement the VP8 video codec as described in [RFC6386] and
H.264 Constrained Baseline as described in [H264].
NOTE: To promote the use of non-royalty-bearing video codecs,
participants in the RTCWEB working group, and any successor
working groups in the IETF, intend to monitor the evolving
licensing landscape as it pertains to the two mandatory-to-
implement codecs. If compelling evidence arises that one of the
codecs is available for use on a royalty-free basis, the working
group plans to revisit the question of which codecs are required
for Non-Browsers, with the intention being that the royalty-free
codec will remain mandatory to implement and the other will become
optional.
These provisions apply to WebRTC Non-Browsers only. There is no
plan to revisit the codecs required for WebRTC Browsers.
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RFC 7742 WebRTC Video March 2016
"WebRTC-compatible endpoints" are free to implement any video codecs
they see fit. This follows logically from the definition of "WebRTC-
compatible endpoint". It is, of course, advisable to implement at
least one of the video codecs that is mandated for WebRTC browsers,
and implementors are encouraged to do so.
6. Codec-Specific Considerations
SDP allows for codec-independent indication of preferred video
resolutions using the mechanism described in [RFC6236]. WebRTC
endpoints MAY send an "a=imageattr" attribute to indicate the maximum
resolution they wish to receive. Senders SHOULD interpret and honor
this attribute by limiting the encoded resolution to the indicated
maximum size, as the receiver may not be capable of handling higher
resolutions.
Additionally, codecs may include codec-specific means of signaling
maximum receiver abilities with regard to resolution, frame rate, and
bitrate.
Unless otherwise signaled in SDP, recipients of video streams MUST be
able to decode video at a rate of at least 20 fps at a resolution of
at least 320 pixels by 240 pixels. These values are selected based
on the recommendations in [HSUP1].
Encoders are encouraged to support encoding media with at least the
same resolution and frame rates cited above.
6.1. VP8
For the VP8 codec, defined in [RFC6386], endpoints MUST support the
payload formats defined in [RFC7741].
In addition to the [RFC6236] mechanism, VP8 encoders MUST limit the
streams they send to conform to the values indicated by receivers in
the corresponding max-fr and max-fs SDP attributes.
Unless otherwise signaled, implementations that use VP8 MUST encode
and decode pixels with an implied 1:1 (square) aspect ratio.
6.2. H.264
For the [H264] codec, endpoints MUST support the payload formats
defined in [RFC6184]. In addition, they MUST support Constrained
Baseline Profile Level 1.2 and SHOULD support H.264 Constrained High
Profile Level 1.3.
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RFC 7742 WebRTC Video March 2016
Implementations of the H.264 codec have utilized a wide variety of
optional parameters. To improve interoperability, the following
parameter settings are specified:
packetization-mode: Packetization-mode 1 MUST be supported. Other
modes MAY be negotiated and used.
profile-level-id: Implementations MUST include this parameter within
SDP and MUST interpret it when receiving it.
max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br:
These parameters allow the implementation to specify that they can
support certain features of H.264 at higher rates and values than
those signaled by their level (set with profile-level-id).
Implementations MAY include these parameters in their SDP, but
they SHOULD interpret them when receiving them, allowing them to
send the highest quality of video possible.
sprop-parameter-sets: H.264 allows sequence and picture information
to be sent both in-band and out-of-band. WebRTC implementations
MUST signal this information in-band. This means that WebRTC
implementations MUST NOT include this parameter in the SDP they
generate.
H.264 codecs MAY send and MUST support proper interpretation of
Supplemental Enhancement Information (SEI) "filler payload" and "full
frame freeze" messages. The "full frame freeze" messages are used in
video-switching MCUs, to ensure a stable decoded displayed picture
while switching among various input streams.
When the use of the video orientation (CVO) RTP header extension is
not signaled as part of the SDP, H.264 implementations MAY send and
SHOULD support proper interpretation of Display Orientation SEI
messages.
Implementations MAY send and act upon "User data registered by Rec.
ITU-T T.35" and "User data unregistered" messages. Even if they do
not act on them, implementations MUST be prepared to receive such
messages without any ill effects.
Unless otherwise signaled, implementations that use H.264 MUST encode
and decode pixels with an implied 1:1 (square) aspect ratio.
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RFC 7742 WebRTC Video March 2016
7. Security Considerations
This specification does not introduce any new mechanisms or security
concerns beyond what is in the other documents it references. In
WebRTC, video is protected using Datagram Transport Layer Security
(DTLS) / Secure Real-time Transport Protocol (SRTP). A complete
discussion of the security considerations can be found in
[WebRTC-SEC] and [WebRTC-SEC-ARCH]. Implementors should consider
whether the use of variable bitrate video codecs are appropriate for
their application, keeping in mind that the degree of inter-frame
change (and, by inference, the amount of motion in the frame) may be
deduced by an eavesdropper based on the video stream's bitrate.
Implementors making use of H.264 are also advised to take careful
note of the "Security Considerations" section of [RFC6184], paying
special regard to the normative requirement pertaining to SEI
messages.
8. References
8.1. Normative References
[H264] ITU-T, "Advanced video coding for generic audiovisual
services (V9)", ITU-T Recommendation H.264, February 2014,
<http://www.itu.int/rec/T-REC-H.264>.
[HSUP1] ITU-T, "Application profile - Sign language and lip-
reading real-time conversation using low bit rate video
communication", ITU-T Recommendation H.Sup1, May 1999,
<http://www.itu.int/rec/T-REC-H.Sup1>.
[IEC23001-8]
ISO/IEC, "Coding independent media description code
points", ISO/IEC 23001-8:2013/DCOR1, 2013,
<http://standards.iso.org/ittf/PubliclyAvailableStandards/
c062088_ISO_IEC_23001-8_2013.zip>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<http://www.rfc-editor.org/info/rfc6184>.
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RFC 7742 WebRTC Video March 2016
[RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image
Attributes in the Session Description Protocol (SDP)",
RFC 6236, DOI 10.17487/RFC6236, May 2011,
<http://www.rfc-editor.org/info/rfc6236>.
[RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
<http://www.rfc-editor.org/info/rfc6386>.
[RFC7741] Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
Galligan, "RTP Payload Format for VP8 Video", RFC 7741,
DOI 10.17487/RFC7741, March 2016,
<http://www.rfc-editor.org/info/rfc7741>.
[SRGB] IEC, "Multimedia systems and equipment - Colour
measurement and management - Part 2-1: Colour management -
Default RGB colour space - sRGB.", IEC 61966-2-1, October
1999, <https://webstore.iec.ch/publication/6169>.
[TS26.114] 3GPP, "IP Multimedia Subsystem (IMS); Multimedia
Telephony; Media handling and interaction", TS 26.114,
Version 13.2.0, December 2015,
<http://www.3gpp.org/DynaReport/26114.htm>.
8.2. Informative References
[H265] ITU-T, "High efficiency video coding",
ITU-T Recommendation H.265, April 2015,
<http://www.itu.int/rec/T-REC-H.265>.
[RTCWEB-OVERVIEW]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", Work in Progress,
draft-ietf-rtcweb-overview-14, June 2015.
[WebRTC-RTP-USAGE]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP",
Work in Progress, draft-ietf-rtcweb-rtp-usage-25, June
2015.
[WebRTC-SEC]
Rescorla, E., "Security Considerations for WebRTC", Work
in Progress, draft-ietf-rtcweb-security-08, February 2015.
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RFC 7742 WebRTC Video March 2016
[WebRTC-SEC-ARCH]
Rescorla, E., "WebRTC Security Architecture", Work in
Progress, draft-ietf-rtcweb-security-arch-11, March 2015.
Acknowledgements
The author would like to thank Gaelle Martin-Cocher, Stephan Wenger,
and Bernard Aboba for their detailed feedback and assistance with
this document. Thanks to Cullen Jennings for providing text and
review and to Russ Housley for a careful final review. This document
includes text that originally appeared in "WebRTC Codec and Media
Processing Requirements" (March 2012).
Author's Address
Adam Roach
Mozilla
Dallas
United States
Phone: +1 650 903 0800 x863
Email: adam@nostrum.com
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