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Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                       February 25, 2013
Expires: August 29, 2013

         Resolution Constraints in Web Real Time Communications


   This document specifies the constraints necessary for a Javascript
   application to successfully indicate to a browser that supports
   WebRTC what resolutions it desires on a video stream.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 August 29, 2013.

Copyright Notice

   Copyright (c) 2013 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

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Disposition of this text . . . . . . . . . . . . . . . . .  3
   2.  Usage Scenarios  . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Scenario: Resolution change  . . . . . . . . . . . . . . .  4
     2.2.  Scenario: Constrained bandwidth  . . . . . . . . . . . . .  4
     2.3.  Scenario: Limited processing capacity  . . . . . . . . . .  4
   3.  Models for resolution manipulation . . . . . . . . . . . . . .  4
   4.  Constraints for specifying resolution  . . . . . . . . . . . .  6
   5.  Syntax and Mapping Examples  . . . . . . . . . . . . . . . . .  6
     5.1.  Examples with GetUserMedia . . . . . . . . . . . . . . . .  6
     5.2.  SDP mappings . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Changes from -00 to -01 . . . . . . . . . . . . . . . 10
   Appendix B.  Changes from -01 to -02 . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   There are a number of scenarios where it's useful for a WebRTC
   application to indicate to the WebRTC implementation in the supported
   browser what the desired characteristics of a video stream are.
   These include, but are not limited to:

   o  Specifying a minimum desired resolution for a given application,
      in order to control the user experience or resource tradeoffs made
      by the browser to favour a particular stream

   o  Specifying a maximum desired resolution for a given stream, in
      order to save some resource (bandwidth, CPU....), possibly outside
      of the browser where the browser can't tell that it's exceeding a

   o  Specifying resolutions that are a reasonable fit for the current
      usage of the video stream, for instance fitting with the number of
      pixels available on the part of a device's display surface that is
      devoted to displaying this video stream

   o  Specifying the shape of a video stream, in order to fit the video
      onto a display surface without the need for black bars or image

   Similar considerations apply for framerate.

1.1.  Disposition of this text

   This draft is written in order to get something specific out to refer
   to during spec-writing and implementation.  The text may eventually
   get merged into the JSEP specification, [I-D.ietf-rtcweb-jsep].

2.  Usage Scenarios

   Consider the following (simplified) model of a video stream through a
   WebRTC application:

      |<-------------- Browser A -------------------->|
     Camera ---> MediaStream A ---> Peerconnection A ------+
             |<------- Application A ---------->|          |
                             v  ^                          v
                    Signalling channel              Internet (media)
                             v  ^                          |
             |<------- Application B ---------->|          |
     <video> tag <-- MediaStream B <--- Peerconnection B --+
      |<-------------Browser B ----------------------->|

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   Both applications are running in browsers, with Application A
   connected to a camera that is able to deliver video streams up to HD
   quality (1280x720).

2.1.  Scenario: Resolution change

   At one particular moment in time, the <video> tag in Application B is
   rendered as a thumbnail, and video is flowing to it in a 160x100
   resolution; there is no need to send any more data, since no more
   pixels are available for its display anyway.

   Then the user of Application B hits the "full-screen" button.  There
   are now 1600x1200 pixels available for display.

   Initially, Application B will splay the 160x100 image across the
   larger surface, because there is no other choice, but it will desire
   to have as many pixels as possible available to provide a high
   quality image.

2.2.  Scenario: Constrained bandwidth

   At one particular moment in time, the camera is generating 1280x720,
   resulting in a 2 Mbits/second data flow from A to B. Congestion
   control signals that this data rate is no longer available; rather
   than letting the browser reduce the bandwidth of some flow of its
   choice, Application A decides that the high definition video is the
   feature that is least valuable.  It can then apply a new constraint
   to Mediastream A, specifying that resolution should be at most
   640x360; browser A is then responsible for making sure this decision
   is communicated to browser B (if it needs to be).

2.3.  Scenario: Limited processing capacity

   If application B is running on a slow machine (2000-class PC or 2010-
   class mobile phone), the maximum capacity of the video decoder may be
   320x200 - Application B may then wish to indicate that application A
   should limit the stream sent across the network to that resolution -
   sending more bits isn't useful, because the receiver doesn't have
   enough capacity to decode and downscale the video stream.

3.  Models for resolution manipulation

   As specified in the "v6 Settings model" being developed in the Media
   Capture Task Force (snapshot: http://dvcs.w3.org/hg/dap/raw-file/tip/
   media-stream-capture/proposals/SettingsAPI_proposal_v6.html), the
   consumer of a video track in a MediaStream will have a "native
   resolution", which indicates what size video it's useful to push to

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   it.  The application can also set (and change) constraints on the
   video MediaStreamTrack, indicating which range of properties it sees
   useful for the purposes of the application.

   In SDP, the "a=imageattr" attribute is available to provide
   information on the resolution of video streams described by an SDP
   m-line; a proposal [I-D.lennox-mmusic-sdp-source-selection] has been
   floated to provide similar information on a per-SSRC basis.

   If both mechanisms are available, the choices available to the writer
   of application B in the "increase screen area" above are:

   o  Signal (by non-standard means) to Application A that more pixels
      are needed.  Application A will then modify the constraints on
      Mediastream A to say that the desired (not mandatory) min
      resolution is 1600x1200; Browser A will then reconfigure the
      camera to generate the closest available resolution, which is

   o  Apply a new constraint set to Mediastream B's video track, saying
      that the desired resolution is now 1600x1200.  Browser B will then
      have to figure out that this is an incoming track via
      Peerconnection B, and that the resolution needs to be signalled;
      it will then fire a NegotationNeeded event at Application B, which
      will then renegotiate the desired resolutions using an SDP
      exchange with Browser A; Browser A will then figure out from the
      SDP that it's useful to generate a higher resolution video stream,
      and reconfigure the camera as above.

   o  Execute a renegotiation with Application A, adding a=remote-ssrc:
      attributes as described in Section 5 by modifying the SDP
      generated by CreateOffer, and triggering the behaviour in the
      previous alternative inside Browser A. API-wise, this is perhaps
      the most complex method.

   The advantage of the first method is that it does not require any SDP
   parsing or generation.

   The advantage of the second method is that it will work when
   appliation A and application B are different applications; there is
   no need for them to have any private agreement on how to set bitrate.
   It does require boht the implementation of constraints and that
   browser B has the ability to generate the proper constraints in the

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4.  Constraints for specifying resolution

   In order to have either of the approaches to work, the specific
   constraints to use have to be defined.  This section defines the
   constraints for resolution and framerate needed.

   These constraints are usable in several places:

   o  As constraints to the getUserMedia call
      [W3C.WD-mediacapture-streams-20120628], where they serve to guide
      the configuration of the camera obtained, and may influence the
      choice of camera.

   o  As constraints to the addStream call on a PeerConnection
      [W3C.WD-webrtc-20120821], where they serve to guide the
      configuration of the codec that encodes the video content for

   o  As constraints applied to an existing local video stream using the
      "change constraints" API, where it may cause the video engine to
      reconfigure the device or codec for that particular stream.

   o  As constraints applied to an incoming video stream using the
      "change constrains" API on a MediaStreamTrack, where it serves to
      inform the video engine about the desirable properties of the
      video track, which may lead to the video engine choosing to
      reencode the video and/or signal a remote video source that it
      wishes certain constraints to be put in place.

   All of the constraints may be meaningful in both "mandatory" and
   "optional" forms.

5.  Syntax and Mapping Examples

   See Section 6 for the actual definition of the constraints used here.

5.1.  Examples with GetUserMedia

   A constraint saying that we absolutely must have a minimum resolution
   of 1024x768:

      video: { mandatory: { minWidth: 1024, minHeight: 768 } }
   }, successCallback, errorCallback);

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   A constraint saying that we'd prefer 60 frames per second, if
   available, and if we can get that, we'd like to limit the max
   resolution, but in all cases, the screen must be clamped to a 4:3
   aspect ratio - 16:9 or odd aspect ratios are not acceptable to this

      video: {
        mandatory: { minAspectRatio: 1.333, maxAspectRatio: 1.334 },
        optional [
          { minFrameRate: 60 },
          { maxWidth: 640 },
          { maxHeigth: 480 }
   }, successCallback, errorCallback);

5.2.  SDP mappings

   This document does not specify the exact mapping of constraints into
   imageattr values; this will have to be done before this mechanism can
   be depended on.

   The examples below are thought exercises, based on
   [I-D.lennox-mmusic-sdp-source-selection] and

   An optional constraint has been applied to an incoming stream where
   both upper and lower are constrained to 320x200.  The stream has been
   assigned to a hardware video decoder that can decode most resolutions
   up to 1024x768, in any aspect ratio, but only if all divisions are
   divisible by 4.  The incoming stream has SSRC 1234.

   Escaped line breaks are added for readability.

   a=remote-ssrc:1234 imageattr:* [x=320,y=200,q=1.0] \

6.  IANA Considerations

   This document requests IANA to register constraints in the "RTCWeb
   Media Constraints" registry created by
   [I-D.burnett-rtcweb-constraints-registry].  NOTE: The registrations

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   assume that this document is updated to no longer have "video" as
   part of the name, but have "video" as a field-of-use in the

   The definitions of width, height and aspect ratio are taken from

   o  minWidth - valid for video.  Corresponds to the "x" value (pixel
      count) from RFC 6236.  Only integer values are valid.

   o  maxWidth - valid for video.  Definition as for minWidth.

   o  minHeight - valid for video.  Corresponds to the "y" value (pixel
      count) from RFC 6236.  Only integer values are valid.

   o  maxHeight - valid for video.  Definition as for minHeight.

   o  minAspectRatio - valid for video.  Corresponds to the "par"
      (picture aspect ratio), with "sar" set to 1.0.  A 4:3 format
      display corresponds to an AspectRatio of 1.3333.  Floating point
      values are valid.

   o  maxAspectRatio - valid for video.  Definition as for

   o  minFramerate - valid for video.  Corresponds to the framerate
      defined in [RFC4566], the "a=framerate" attribute.

   o  maxFramerate - valid for video.  Definition as for minFramerate.

   The contact person is Harald Alvestrand <hta@google.com>.

   Change control for the registration is with the IETF, as designated
   by the IESG.

   Note that minFramerate defines a lower bound for the a=framerate
   attribute, which is itself defined as an upper limit; this means that
   even if a high framerate is negotiated, the actual framerate used may
   be lower due to temporary considerations (for instance CPU or
   bandwidth, or simply lack of movement in the picture).

7.  Security Considerations

   No security considerations particular to these specific constraints
   have so far been identified.

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8.  Acknowledgements

   Special thanks are given to Dan Burnett, Cullen Jennings, the IETF
   RTCWEB WG and the W3C WEBRTC WG for strongly influencing this memo,
   and to Per Kjellander for being the first to implement the
   constraints in getUserMedia.

9.  References

9.1.  Normative References

              Burnett, D., "IANA Registry for RTCWeb Media Constraints",
              draft-burnett-rtcweb-constraints-registry-02 (work in
              progress), October 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

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

9.2.  Informative References

              Alvestrand, H., "RTCWEB Resolution Negotiation",
              draft-alvestrand-rtcweb-resolution-00 (work in progress),
              April 2012.

              Uberti, J. and C. Jennings, "Javascript Session
              Establishment Protocol", draft-ietf-rtcweb-jsep-02 (work
              in progress), October 2012.

              Lennox, J. and H. Schulzrinne, "Mechanisms for Media
              Source Selection in the Session Description Protocol
              (SDP)", draft-lennox-mmusic-sdp-source-selection-05 (work
              in progress), October 2012.

              Burnett, D. and A. Narayanan, "Media Capture and Streams",
              World Wide Web Consortium WD WD-mediacapture-streams-

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              20120628, June 2012, <http://www.w3.org/TR/2012/

              Bergkvist, A., Burnett, D., Jennings, C., and A.
              Narayanan, "WebRTC 1.0: Real-time Communication Between
              Browsers", World Wide Web Consortium WD WD-webrtc-
              20120821, August 2012,

Appendix A.  Changes from -00 to -01

   Added the "Usage Scenarios" chapter.

   Repointed the eventual target to be incorporation in the JSEP draft.

   Made sure the constraints are consistently spelled in camelCase, with
   a small initial letter.

Appendix B.  Changes from -01 to -02

   Moved a bit of the text around between sections, and referred to the
   "settings API" proposal from the Media Capture task force.

Author's Address

   Harald Alvestrand

   Email: harald@alvestrand.no

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