AW: mulitcast copy or snowball copy

Mitchell Skinner mitchskin at comcast.net
Mon Aug 18 13:40:59 EDT 2003


On Mon, 2003-08-18 at 09:50, Donald Becker wrote:
> This is costly.  "Open loop" multicast protocols work by having the
> receiver track the missing blocks, and requesting (or interpolating)
> them later.  Here you are discarding that information and doing much
> extra work on both the sending and receiving side by later locating the
> missing blocks.

Some possible google terms include: reliable multicast, forward error
correction

There's an ietf working group on reliable multicast that wasn't making a
whole lot of progress the last time I checked.  At that time, I recall
there being some acknowledgment-based implementations as well as one
forward error correction-based implementation using reed-solomon codes,
from an academic in Italy whose name I forgot.

It's been a little while, but when I looked at the code for that
FEC-based reliable multicast program (rmdp?) I think it could only
handle pretty small files.  My understanding is that FEC-based
approaches should scale better in terms of the number of receiving
nodes, but the algorithms can be very time/space intensive.  There's a
patented algorithm from Digital Fountain that's supposed to be pretty
efficient (google tornado codes, michael luby, digital fountain) but I'm
not aware that they have a cluster-oriented product.  My impression of
them was that they were pretty WAN-oriented.

If I was less lazy I'd give some links instead of google terms, but
hopefully that's some food for thought.

Mitch

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