[Q] multicasting product ?

Dave Dykstra dwd at bell-labs.com
Fri Dec 21 01:54:13 EST 2001

On Thu, Dec 20, 2001 at 07:45:55AM +0100, Ph. Marek wrote:
> >>So my question is: does anyone know of a product which does reliable
> >>multicasting? (source available would be preferred)
> >
> >  At our company, we have a mrsync running for a couple of months
> >  now.  mrsync is to transfer files to many machines at the same
> >  time using UDP and multicast.  I attached at the end of this message
> >  the excerpt from the docs of mrync.
> >
> >    If this is what you need, we can contribute this program to the
> >  openSource.
> >  
> >HP
> Well, I'm certainly interested.
> But, as this send whole files, I'm a little bit irritated about the name
> "mrsync" - wouldn't mrcp or mtftp be better?

Or "mrdist".  Maybe they hoped to put in the rsync algorithm into
eventually, but it seems to me that it would be very difficult to robustly
implement the rsync algorithm over multicast, because each receiver could
conceivably require different blocks to be sent.  In practice, however,
this will be mostly be used for mirroring so all the receivers should
normally have the same files to begin with, and maybe they could optimize
it for that case and just have reduced performance for the unusual case.
Also, the way it's implemented in the rsync command at least, the receiver
sends all its checksums to the sender, and the sender decides which blocks
to send; I don't see how that would work with multicast because for that I
would think the receivers would need to decide which blocks are relevant to
them and which ones aren't so they'd need to have access to both sets of
checksums.  I guess one could assume that all receivers are indeed
identical and only decide which blocks to send based on what a single
receiver needs, but I think there'd need to be some way of detecting and
supporting the receivers that aren't identical.

> But otherwise this could be exactly what I need.
> Please let me know as soon as this is available somewhere as GPL or BSD or
> similar license!
> Regards,
> Phil

- Dave Dykstra

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