[vlc-devel] Re: Multicast streaming over wifi

[Greg Shepherd]

On 11/6/06, Rémi Denis-Courmont <rdenis at simphalempin.com> wrote:
> On Mon, Nov 06, 2006 at 04:50:17AM -0800, Greg Shepherd wrote :
> > >most-likely-failed checksum will cause the packet to be discarded in the
> > >IP stack, before being passed to the application.
>
> > Sure you can, and it is done in many ways today. Some of the more
> > effiecient FEC solutions today do not affect the original data stream.
> > They simply create an additional stream of repair symbols to a
> > different port.
>
> That does ABSOLUTELY NOT address the UDP checksum problem. Both streams
> (data and error correction) will have UDP checksumming, which by
> definition prevents error correction.

I think you may missunderstand the application of an FEC. FEC's do not
correct random bit errors per-packet - if the checksum fails, drop the
packet. FECs repair data over a protect period; ie, some number of
packets as determined by the particular FEC and it's tunable
parameters. So when a packet is lost (or dropped due to failed
checksum) the data in that packet can be calculated by using the
received data in the other packets of the protect window and the data
in the symbol packets for that same window.

Greg

> You need either a transport with
> no payload checksum (and AFAIK, UDP-Lite is the only one on the
> Internet), or you must do error correction below IP (which 802.11 does
> by the way, though maybe not efficiently enough for multicast(?)).
>
> Now, it certainly would be a good idea to add UDP-Lite to the official
> VLC tree (and I plan on doing so), but I'd rather have it merged into
> the official Linux kernel first, and if you want FEC on Windows or OSX,
> I don't see any solution.
>
> --
> Rémi Denis-Courmont
> looking for a job
> http://www.simphalempin.com/home/infos/CV-en.pdf
>
> --
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>

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