Flow control frames: will I see them with tcpdump?

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I'm troubleshooting an application problem in which I think that
Ethernet flow control may be a contributing factor.

I think my switch may be sending PAUSE messages to particularly
talkative hosts because I'm seeing brief (sub-second) stoppages in the
multicast data streams coming from these hosts at the instant that a
new multicast sender / receiver box joins the lan.  Link establishment,
IGMP host report and new (S,G) happen more or less simultaneously.

All of my talkative hosts stop transmitting for about the same duration
and at precisely the same time.

Anyhoo, I don't *know* that Ethernet flow control is the problem, but
it is enabled for RX and TX on both ends of each host-switch link.

So, the question is this:  Will I see the PAUSE frames when I run
tcpdump on the hosts in question, or are those flow control frames a
special case handled only by the NIC, and which won't make it up to

I'm running tcpdump in promiscuous mode on each of these hosts.  I have
never observed a pause frame in the tcpdump output.  The boxes are
running linux 2.4.something.

Many thanks!


Re: Flow control frames: will I see them with tcpdump?

Anno Domini 30-01-2006 16:05, googlegroups@marget.com wrote:
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Don't know about tcpdump, but Ethereal will. Check this discussion
<http://www.ethereal.com/lists/ethereal-users/200510/msg00037.html .

---- GnuPG public key - http://home.ifi.uio.no/~svenar/gpg.asc ----
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  binary and those who don't."
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Re: Flow control frames: will I see them with tcpdump?

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Have you checked for "Unknown flooding"?

This could stop certain hosts from receiving some frames
especially if there are transmittters on faster intefaces than
some receivers.

Re: Flow control frames: will I see them with tcpdump?

anybody43@hotmail.com wrote:
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Thank you both for the replies.

I've found the source of the problem in the application -- it had
nothing to do with Ethernet flow control.  That's why I wasn't seeing
those frames :-)

My original (poorly stated) question wasn't so much about whether
Ethereal / tcpdump would be able to interpret and display the frames,
but rather, whether I should expect the NIC to pass them up the stack.

I was expecting flow control frames but didn't see them, because they
werent there.  Expecting them, and not seeing them led me to all kinds
of crazy guessing about whether the NIC might intercept them,
preventing higher layers from knowing about the frames.  The frames are
*for* the NIC afterall.

My guess (someone has seen them!) is that they *will* make their way up
to tcpdump, but since I'd never caught one, I wasn't sure.

Thanks very much.


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