Wireshark · Wireshark-users: Re: [Wireshark-users] Ethernet flow control

["Jake Peavy" <djstunks@xxxxxxxxx>]

On 12/4/08, Bill Meier <wmeier@xxxxxxxxxxx> wrote:

Jake Peavy wrote:
> Thanks Jaap,
>
> I should probably start off by saying that I'm not sure I understand
> flow control completely; I'm a little out of my depth.
>
> I've also tried capturing off the full duplex 10/100 switch, but don't
> see them there either.
>
> Should I just be able to look for eth.type == 0x8808 in order to see
> flow control information or do I need to look inside the Frame/Ethernet
> headers?
>
> I did look at the sample capture:
> http://wiki.wireshark.org/SampleCaptures?action="">
> <http://wiki.wireshark.org/SampleCaptures?action="">>
> but I'm not sure if that's what to expect.  If it is, and if they're
> there, I'm definitely not seeing them.


My "taking a step back" question:

I'm curious as to what leads you to believe that you're

"running into an issue caused by one device sending PAUSE frames onto
the wire and impairing the bandwidth delivered by the upstream switch".

Hi Bill,

Here's what I've got:

        10/100 full-duplex
         unmanaged switch
              +---+
              |   | 
              |   +----- destination 1
source ------ +   |
              |   |
              |   +----- destination 2
              |   |
              +---+

In this case, if the source provides more than ~7mbps of multicast data to the switch, the buffers on dest1 can't handle the rate and begin to send ethernet flow control messages back to the switch.  The switch pauses either the source itself or the incoming source port (I don't know which).  This affects dest2 which has sufficient buffer and would like to continue to receive the full rate stream.  

I tried snooping on the switch - dest1 line in order to catch the flow control messaging, but, as per my other email in this thread, the kernel never seems to see the frames.  I would still like to see if I can get wireshark/tcpdump to pick up these frames, but it sounds like I need different hardware to do so.

-- 
-jp

Is there anything more beautiful than a beautiful, beautiful flamingo, flying across in front of a beautiful sunset? And he's carrying a beautiful rose in his beak, and also he's carrying a very beautiful painting with his feet. And also, you're drunk.

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