[Udpcast] multi computer cast problem

Alain Knaff alain at knaff.lu
Tue Sep 23 23:09:55 CEST 2008

PaweĊ‚ Ufnalewski // BSS wrote:
> Problem is that, when I'm doing it one-by-one (I mean server-1 client) I
> get full speed - ~70MB/s, but when I'm trying to to this with >= 2
> computers transfer drops to >0,5Mb/s. Why?

This is most likely a switch issue. Many switches limit (throttle)
multicast traffic, as they consider it "not important". Fortunately,
this can be configured on most switches.

Another thing to watch out for is the lonely network printer (or other
ancient device) that can't take data at more than one Mb/s. If you
broadcast or multicast, most switches send out the data to all ports,
and adapt the speed to the slowest device connected. Try to find out if
that is the case by only connecting a bunch of "known-good" PC's to a
switch (no uplink, no unknown devices).

Some high-end switches do actually understand the IGMP messages by which
receivers inform that they are interested in the transmission. However,
these devices are rare, and even if they do support IGMP snooping, it is
often switched off by default.

Another thing you can try is switch off flow control. Some switches even
allow to do this on a per port basis.



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