Internet DRAFT - draft-alvestrand-constraints-resolution
draft-alvestrand-constraints-resolution
Network Working Group H. Alvestrand
Internet-Draft Google
Intended status: Standards Track August 26, 2013
Expires: February 27, 2014
Resolution Constraints in Web Real Time Communications
draft-alvestrand-constraints-resolution-03
Abstract
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.
It also discusses the possible use of SDP to carry that information
between browsers.
Requirements Language
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 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 February 27, 2014.
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
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(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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Disposition of this text . . . . . . . . . . . . . . . . 3
2. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Scenario: Resolution change . . . . . . . . . . . . . . . 3
2.2. Scenario: Constrained bandwidth . . . . . . . . . . . . . 4
2.3. Scenario: Limited processing capacity . . . . . . . . . . 4
3. Models for resolution manipulation . . . . . . . . . . . . . 4
3.1. Sender-side constraint manipulation . . . . . . . . . . . 5
3.2. Receiver-side constraint manipulation . . . . . . . . . . 6
4. Constraints for specifying resolution . . . . . . . . . . . . 7
5. Syntax and Mapping Examples . . . . . . . . . . . . . . . . . 7
5.1. Examples with GetUserMedia . . . . . . . . . . . . . . . 7
5.2. SDP mappings . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Changes from -00 to -01 . . . . . . . . . . . . . . 9
Appendix B. Changes from -01 to -02 . . . . . . . . . . . . . . 9
Appendix C. Changes from -02 to -03 . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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
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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
constraint
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
distortion
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. Some 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 ----------------------->|
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
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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 "Media Capture And Streams" document
[W3C.WD-mediacapture-streams-20130516] 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 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.
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If both mechanisms are available, the choices available to the writer
of application B in the "increase screen area" above are:
1. 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
1280x720.
2. 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.
3. Execute a renegotiation with Application A, adding 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 both the implementation of constraints and that
browser B has the ability to generate the proper constraints in the
SDP.
The third method requires SDP parsing in browser A, but not SDP
generation in browser B. It does require SDP manipulation in
Javascript at application B.
3.1. Sender-side constraint manipulation
The following Javascript code (somewhat pseudocode) will achieve the
"increase screen area" according to method 1 above.
The examples use APIs from "Media Capture And Streams as well as from
WebRTC [W3C.WD-webrtc-20120821].
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// B side
function needResolutionToChange(newWidth, newHeight) {
message = makeMessage("resolution", newWidth, newHeight);
remote.send(message)
}
// A side
function handleMessage(message) {
if (message.verb === "resolution") {
constraints = video.constraints();
// The function below can be a polyfill.
constraints.replaceOrAddOptConstraint("width", message.arg1);
constraints.replaceOrAddOptConstraint("height", message.arg2);
}
}
3.2. Receiver-side constraint manipulation
This implements version 2 of the constraint manipulation above. Note
that the handing of "onnegotiationneeded" is the same as for any
other renegotiation.
// B side
function needResolutionToChange(newWidth, newHeight) {
constraints = video.constraints();
constraints.replaceOrAddOptConstraint("width",
{ "max": message.arg1 });
constraints.replaceOrAddOptConstraint("height",
{ "max": message.arg2 });
video.applyCoinstraints(constraints);
}
pc.onnegotiationneeded = function() {
offer = pc.CreateOffer();
message = makeMessage("offer", offer);
remote.send(message);
}
// Functions to handle answer from A side omitted.
// A side
function handleMessage(message) {
if (message.verb === "offer") {
pc.SetRemoteDescription(message.arg1, success, failure)
}
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}
// Functions to return answer to B side omitted.
4. Constraints for specifying resolution
All constraints needed are registered in the IANA registry by
[W3C.WD-mediacapture-streams-20130516]. In summary, they are:
o width - unsigned long or MinMaxConstraint - ie width: { min: 640,
max: 1024 }
o height - unsigned long or MinMaxConstraint
o frameRate - float or MinMaxConstraint
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:
getUserMedia({
video: { mandatory: { width: { min: 1024 }, height: { min: 768 }}}
}, successCallback, errorCallback);
A constraint saying that we'd prefer 60 frames per second, if
available, and if we can get that, we'd like to lock the resolution
to 640x480, 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
application:
getUserMedia({
video: {
mandatory: { aspectRatio: { min: 1.333, max: 1.334 } },
optional [
{ frameRate: 60 },
{ width: 640 },
{ heigth: 480 }
]
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}
}, successCallback, errorCallback);
5.2. SDP mappings
The examples below are based on [I-D.roach-mmusic-unified-plan] and
[I-D.ietf-mmusic-msid].
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 16. The incoming stream has MediaStream ID aaaa, and
MediaStreamTrack id bbbb.
Escaped line breaks are added for readability.
m=video
a=imageattr:* [x=320,y=200,q=1.0] \
[x=[120:16:1024],y=[100:16:768],q=0.2]
a=msid: aaaa bbbb
6. IANA Considerations
This document makes no requests of IANA:
Note to RFC Editor: This section can be deleted before publication as
an RFC.
7. Security Considerations
No security considerations particular to these specific constraints
have so far been identified.
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
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9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[I-D.ietf-mmusic-msid]
Alvestrand, H., "Cross Session Stream Identification in
the Session Description Protocol", draft-ietf-mmusic-
msid-01 (work in progress), August 2013.
[I-D.ietf-rtcweb-jsep]
Uberti, J. and C. Jennings, "Javascript Session
Establishment Protocol", draft-ietf-rtcweb-jsep-03 (work
in progress), February 2013.
[I-D.roach-mmusic-unified-plan]
Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
Using SDP with Large Numbers of Media Flows", draft-roach-
mmusic-unified-plan-00 (work in progress), July 2013.
[W3C.WD-mediacapture-streams-20130516]
Burnett, D., Bergkvist, A., Jennings, C., and A.
Narayanan, "Media Capture and Streams", World Wide Web
Consortium WD WD-mediacapture-streams-20130516, May 2013,
<http://www.w3.org/TR/2013/WD-mediacapture-
streams-20130516>.
[W3C.WD-webrtc-20120821]
Bergkvist, A., Burnett, D., Narayanan, A., and C.
Jennings, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD WD-
webrtc-20120821, August 2012,
<http://www.w3.org/TR/2012/WD-webrtc-20120821>.
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
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Moved a bit of the text around between sections, and referred to the
"settings API" proposal from the Media Capture task force.
Appendix C. Changes from -02 to -03
Retargeted document to be a "here's how you can do it" draft.
Updated constraints format to be as per the May 16 W3C draft of
"media capture and streams".
Author's Address
Harald Alvestrand
Google
Email: harald@alvestrand.no
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