[lug] Multicast packets?

[Atkinson, Chip]

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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