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Versions: (draft-cbran-rtcweb-codec) 00 01 02 03 04 05 06 RFC 7742

Network Working Group                                         A.B. Roach
Internet-Draft                                                   Mozilla
Intended status: Standards Track                           June 12, 2015
Expires: December 14, 2015


             WebRTC Video Processing and Codec Requirements
                       draft-ietf-rtcweb-video-06

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 14, 2015.

Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Pre and Post Processing . . . . . . . . . . . . . . . . . . .   2
     3.1.  Camera Source Video . . . . . . . . . . . . . . . . . . .   3
     3.2.  Screen Source Video . . . . . . . . . . . . . . . . . . .   3
   4.  Stream Orientation  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Mandatory to Implement Video Codec  . . . . . . . . . . . . .   4
   6.  Codec-Specific Considerations . . . . . . . . . . . . . . . .   5
     6.1.  VP8 . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  H.264 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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
   [I-D.ietf-rtcweb-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].

3.  Pre and Post Processing

   This section provides guidance on pre- and post-processing of video
   streams.

   Unless specified otherwise by the SDP or codec, the color space
   SHOULD be sRGB [SRGB].  For clarity, this is the color space



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   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

   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 mid-stream 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 publication,
   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
   [I-D.ietf-rtcweb-security-arch] section 5.2 and the considerations in
   [I-D.ietf-rtcweb-security] section 4.1.1.




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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 under "Screen Sourced Video," above.

   To accommodate these circumstances, RTCWEB 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).
   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 [I-D.ietf-rtcweb-overview].

   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



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      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.

   "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 regards 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 [I-D.ietf-payload-vp8].

   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.



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   Unless otherwise signaled, implementations that use VP8 MUST encode
   and decode pixels with a 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 they SHOULD support H.264 Constrained
   High Profile Level 1.3.

   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 signalled by their level (set with profile-level-id).
      Implementations MAY include these parameters in their SDP, but
      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 SEI
   "filler payload" and "full frame freeze" messages.  "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



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   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 a implied 1:1 (square) aspect ratio.

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 DTLS/SRTP.  A complete discussion of
   the security considerations can be found in
   [I-D.ietf-rtcweb-security] and [I-D.ietf-rtcweb-security-arch].
   Implementors should consider whether the use of variable bit rate
   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 bit rate.

   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.  IANA Considerations

   This document requires no actions from IANA.

9.  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 draft
   includes text from draft-cbran-rtcweb-codec.

10.  References

10.1.  Normative References

   [H264]     ITU-T Recommendation H.264, "Advanced video coding for
              generic audiovisual services (V9)", February 2014,
              <http://www.itu.int/rec/T-REC-H.264>.

   [HSUP1]    ITU-T Recommendation H.Sup1, "Application profile - Sign
              language and lip-reading real-time conversation using low
              bit rate video communication", May 1999,
              <http://www.itu.int/rec/T-REC-H.Sup1>.



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   [I-D.ietf-payload-vp8]
              Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
              Galligan, "RTP Payload Format for VP8 Video", draft-ietf-
              payload-vp8-16 (work in progress), June 2015.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-13
              (work in progress), November 2014.

   [IEC23001-8]
              ISO/IEC 23001-8:2013/DCOR1, "Coding independent media
              description code points", 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, March 1997.

   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184, May 2011.

   [RFC6236]  Johansson, I. and K. Jung, "Negotiation of Generic Image
              Attributes in the Session Description Protocol (SDP)", RFC
              6236, May 2011.

   [RFC6386]  Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
              Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
              Guide", RFC 6386, November 2011.

   [SRGB]     IEC 61966-2-1, "Multimedia systems and equipment - Colour
              measurement and management - Part 2-1: Colour management -
              Default RGB colour space - sRGB.", October 1999, <http://
              www.colour.org/tc8-05/Docs/colorspace/61966-2-1.pdf>.

   [TS26.114]
              3GPP TS 26.114 V12.8.0, "3rd Generation Partnership
              Project; Technical Specification Group Services and System
              Aspects; IP Multimedia Subsystem (IMS); Multimedia
              Telephony; Media handling and interaction (Release 12)",
              December 2014, <http://www.3gpp.org/DynaReport/26114.htm>.

10.2.  Informative References

   [I-D.ietf-rtcweb-rtp-usage]






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              Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
              Communication (WebRTC): Media Transport and Use of RTP",
              draft-ietf-rtcweb-rtp-usage-24 (work in progress), May
              2015.

   [I-D.ietf-rtcweb-security-arch]
              Rescorla, E., "WebRTC Security Architecture", draft-ietf-
              rtcweb-security-arch-11 (work in progress), March 2015.

   [I-D.ietf-rtcweb-security]
              Rescorla, E., "Security Considerations for WebRTC", draft-
              ietf-rtcweb-security-08 (work in progress), February 2015.

Author's Address

   Adam Roach
   Mozilla
   \
   Dallas
   US

   Phone: +1 650 903 0800 x863
   Email: adam@nostrum.com



























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