[lug] Multicast packets?

[John Hernandez]

IP multicast packets are commonly used for audio/video conferencing. 
Another source that comes to mind is the OSPF routing protocol.  But I
don't think there's much to worry about there, unless your routers are
seriously misbehaving.

An ethernet bus (shared segment) can operate at 10Mbps OR 100Mbps. 
Switches can generally bridge segments of different speeds, and they
generally have the capability to buffer frames to some degree.  TCP/IP
traffic will generally self-throttle -- in an over-simplified sense, the
sender won't blast out more unicast packets until it knows your station
is ready.

Broadcast (or multicast) "storms" can cause major network slow-downs and
in some cases isolation as switches become too busy flooding all ports
with broadcast traffic; your data waits in a queue and gets dropped
if/when the buffer can't handle the load.  You should be able to track
these events with a tcpdump-style monitor.

When you ping another station on your same IP network (no router hops)
for extended periods of time, do you see large fluctuations in your
Round-Trip Time?  What's the packet loss ratio over the working hours? 
Perhaps run a script and log the output.  By correlating events, you
might figure out what's going on.

"Atkinson, Chip" wrote:
> 
> Does anyone know what things use multicast packets?  Specifically I'm trying
> to find out why my machine has outages with the network.  Upon looking at
> the problem I see blasts of multicast packets as reported by ntop.  I'm just
> in a "typical" corporate network.  I believe that the outage is caused
> because the network is 100Mb/s whereas my machine is 10Mb/s and the
> translation is done by the hub I'm connected to.  This then means that the
> data being sent out by the 100Mb/s machines basically overwhelms my machine.
> 
> Thanks in advance,
> 
> Chip
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug