Flow control frames: will I see them with tcpdump?

Jan 30, 2006 3 Replies
G

Greetings,



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 tcpdump?



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!



/chris



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S

Don't know about tcpdump, but Ethereal will. Check this discussion .

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A

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.

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G

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.

/chris

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