[opendtv] Re: Apple to Provide Live Video Streaming of September 1 Event
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 2 Sep 2010 16:48:04 -0500
Craig Birkmaier wrote:
> Perhaps Bert would like to comment on the approach Apple has taken
> with HTTP Live streaming, as it appears to be a viable work around
> to UDP based streaming, making it possible to deliver multiple levels
> of quality through the existing HTTP router infrastructure...
I never paid a lot of attention to this, because it seems to be specifics added
to streaming mechanisms that have been around a long time. This is not a
fundamentally new protocol. It seems more like nitty gritty specified to make
sure everything plays together as Apple intends. Others have done similar
things in the past, thir own way.
Among the specifics are encryption of the streams and encyption of the
playlists. Cl;ients need to download the encrypted playlist before they can
start a session. The precise mechanisms for this are described.
There is an Internet Draft, dated 2009, and updated most recently in June 2010,
that explains this new protocol.
http://tools.ietf.org/html/draft-pantos-http-live-streaming-04
It is based on HTTP, meaning RFC 2616. As such, the video streams are sent over
TCP only, which means they cannot use IP multicast. Since IP multicast is,
let's say, non-existent among different ISPs, I don't think that limitation
amounts to much. But surely, we have all used other variants of HTTP live
streaming, including the possibility of viewing at various quality levels,
haven't we?
One of the specifics is that Apple is restricting this to MPEG-2 TS formatting.
For example, if they had based the streaming on RTP/RTCP, that MPEG-2 TS would
not have been required. But that's a tradeoff, because doing it this way allows
leveraging off HTTP, which must use a TCP Transport Layer. So, to keep the
packets flowing at a steady rate, while using TCP, might as well go with MPEG-2
TS.
Bert
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