[theora] Http adaptive streaming for html5

[Michael Dale]

There has been some light talk of http adaptive streaming support for 
html5 but I wanted to ensure that is making progress and get feedback 
from Mozilla video tag developers and theora community members before 
pushing the html5 spec to include the necessary information.

I think we will want to target near zero server side configuration. The 
server should just have to support normal http byte-range request 
serving, and host multiple bit-rate files. The adaptive streaming should 
be supported by multiple <source> children each with a given "bitrate". 
And a attribute on the parent "video" tag that expresses if the player 
should automatically switch bitrates.

Something like:
<video adaptiveSource="true">
    <source src="http://cdn1.cat.com/myVid.256k.ogg" bitrate="256k" 
codecs="theora, vorbis"/>
    <source src="http://cdn2.cat.com/myVid.386k.ogg" bitrate="386k" 
codecs="theora, vorbis"/>
    <source src="http://cdn1.cat.com/myVidSD.ogg" bitrate="512k"  
codecs="theora, vorbis"/>
    <source src="http://cdn3.cat.com/myVidHD_480P.ogg" bitrate="1024k" 
codecs="theora, vorbis"/>
    <source src="http://cdn5.cat.com/myVidHD_720p.ogg" bitrate="2048k" 
codecs="theora, vorbis"/>
    <source src="http://cdn5.cat.com/myVidHD_1080p.ogg" bitrate="4096k" 
codecs="theora, vorbis"/>
</video>


All the logic of bitrate switching would be implemented in the browser. 
The video element would also have to support some events like 
"onswichsource" with the source element in the callback so that user 
interfaces could be updated as the stream switched sources if necessary.

~ not necessary for the whatwg ~

Implementation wise it could work by dynamically switch streams based on 
how fast the next key-frame byte offset chunk was downloaded. Byte 
offset chunks would be defined as the byte rage between two key frames. 
This would be very dependent on the ogg index support so that frame 
accurate seeks in switching chunks would happen seamlessly.

Ideally content providers would encode each stream with the same 
keyframe settings resulting in similar time offsets for keyframe 
switching. But it would not be required, and the system would obviously 
have to seek to the right time not just some random keyframe ;)

When the browser first starts buffering the clip it would target a 
default speed based on previous stored client download rate. It would 
then measure how fast it downloaded a given byte offset chunk and switch 
up or down. If the download rate supported the bitrate of a higher 
quality stream it would download the oggIndex and byteoffset chunk a few 
"byte offset chunks" ahead. Or if a chunk was downloaded slower than 
necessary to maintain playback it would switch downward. Optimizations 
could include cashing the byte offsets of the stream below and stream 
above on initial load.


--michael