[lug] Multicast packets?

[John Hernandez]

The IP source address of the multicast packets should probably reveal what they're all about.

"Atkinson, Chip" wrote:
> 
> I'll check into the cause of the storms, but I think the queing up packets
> on the switch/hub is probably a likely candidate since I'm connected to a
> 10/100 hub and the rest of the machines that are connected to it are 100Mb
> as is the rest of the network.
> 
> What happens is that whenever the connection goes "down", most packets are
> dropped and the round trip time gets long (1500 ms or so from a hub mate).
> 
> I am using a program, ntop, to watch what is happening, which gave me the
> clues to pursue the problem.
> 
> I believe (ha ha) that I'm getting pretty close to the problem, but want to
> understand the multicast clue.
> 
> Thanks for your help.
> 
> Chip
> > -----Original Message-----
> > From: John Hernandez [mailto:John.Hernandez at noaa.gov]
> > Sent: Monday, February 12, 2001 3:09 PM
> > To: lug at lug.boulder.co.us
> > Subject: Re: [lug] Multicast packets?
> >
> >
> > 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
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