Faster Multicast [was Re: WatchMe-1, a VNC activity]

Benjamin M. Schwartz bmschwar at
Wed Aug 12 10:43:21 EDT 2009

Martin Langhoff wrote:
>  - can we use multicast frames... and get the APs speeds bumped up to
> do a "fast multicast" so as to not use up all the airtime?

This would be great... and I just realized that it might also be
tremendously beneficial for Telepathy-Salut.

Salut uses multicast for two things: presence (mDNS) and D-Bus Tubes (a
homebrew network protocol called Clique).  mDNS is a classic
multicast/broadcast system, where if you miss a packet, you just have to
wait for the next round.  Clique, however, is a "reliable multicast"
system, guaranteeing in-order delivery, etc.  In other words, Clique can
tolerate some packet loss, because it knows how to request retransmission.

My understanding is that APs use the basic rate for multicast because they
presume it is an unreliable transport, and the basic rate is the least
likely to drop a packet.  However, for Clique, it might actually be more
effective to use the highest available rate, even if it causes occasional
retransmissions due to loss.

I don't know how to test this, but I think there's a chance it could be a
big win.  Of course, in situations where the mDNS presence information
itself is overloading the network, there's not much we can do.


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