How does HTTP Adaptive Bitrate Streaming work on the iPhone?

Question

Apple has included HTTP Adaptive Bitrate Streaming in the iPhone OS 3.0, in particular Safari handles this automatically.

I\'d like to play with this in a low cost manne

Answer 1

Update

Looks like Apple made an IETF draft proposal, and some people are already working on segmenters:

HTTP Live Streaming - draft-pantos-http-live-streaming-01
http://tools.ietf.org/id/draft-pantos-http-live-streaming-01.txt

iPhone HTTP Streaming with FFMpeg and an Open Source Segmenter
http://www.ioncannon.net/programming/452/iphone-http-streaming-with-ffmpeg-and-an-open-source-segmenter/

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Looks like the HTTP server acts simply as a dumb HTTP server. Poking around the example website provided by Akamai gives me enough info to get started with static content streaming.

http://iphone.akamai.com/

The whitepaper ( http://www.akamai.com/dl/akamai/iphone_wp.pdf ) provides information about the transport stream encoding, so the .ts streams are straightforward.

The encoder (or a separate segmenter process) will produce H.264/AAC content in a sequence of small content segments, in MPEG-2 TS format (.ts). There is also an M3U8 index file that references the segments; in the case of live content the M3U8 is continuously updated to reflect the latest content.

H.264 Encoding should be single-pass Baseline Profile, frame re-ordering disabled. Key frames are suggested every 5 seconds, ideally an even divisor of the chosen segment length.

The website provides an M3U8 file, which is simply an M3U playlist, but in the UTF-8 character encoding format.

That file then links to an M3U8 file for each bitrate. I assume they must all have cuts at the same positions (every 2 or 10 seconds, for instance) so that switching can be seamless. It appears to be completely client driven - the client decides how to measure bandwidth and which version it's going to get.

The contents of the main file are:

#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1, BANDWIDTH=860000
hi/prog_index.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1, BANDWIDTH=512000
med/prog_index.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1, BANDWIDTH=160000
lo/prog_index.m3u8

Then each of the other files are:

hi/prog_index.m3u8

#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXTINF:10, 
fileSequence0.ts
#EXTINF:10, 
fileSequence1.ts
#EXTINF:10, 
fileSequence2.ts
#EXTINF:10, 
fileSequence3.ts
#EXTINF:1,  
fileSequence4.ts
#EXT-X-ENDLIST

med/prog_index.m3u8

#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXTINF:10, 
fileSequence0.ts
#EXTINF:10, 
fileSequence1.ts
#EXTINF:10, 
fileSequence2.ts
#EXTINF:10, 
fileSequence3.ts
#EXTINF:1,  
fileSequence4.ts
#EXT-X-ENDLIST

lo/prog_index.m3u8

#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXTINF:10, 
fileSequence0.ts
#EXTINF:10, 
fileSequence1.ts
#EXTINF:10, 
fileSequence2.ts
#EXTINF:10, 
fileSequence3.ts
#EXTINF:1,  
fileSequence4.ts
#EXT-X-ENDLIST

This works with the HTML 5 video tag:

There are still a lot of unanswered questions, but this is probably enough to get started.