netdev - Re: change the way e1000 is handling short VLAN frames

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Date:	Fri, 21 Sep 2007 14:50:15 +0200
From:	Emil Micek <emil.micek@...jasek.cz>
To:	hadi@...erus.ca
Cc:	"auke-jan.h.kok@...el.com" <auke-jan.h.kok@...el.com>,
	netdev mailing list <netdev@...r.kernel.org>
Subject: Re: change the way e1000 is handling short VLAN frames

On Fri, 2007-09-21 at 07:59 -0400, jamal wrote:
> > Current e1000 (according to our observations) first appends 4 bytes of
> > VLAN tag and then pads the frame to 64 bytes with zeroes if necessary
> > before transmiting it.
> 
> Which would make it a bug. AFAIK, the minimum VLAN tagged packet going
> out is 68 bytes.
Are you sure about this? We did some extensive testing capturing VLAN
frames going out of our Intel PRO/1000 MT Dual Port Server Adapter
(82571EB chipset) and we definitely captured VLAN frames shorter then 68
bytes. Here is text dump of such frame:
http://www.tkrjasek.cz/mie/cmts_tests/between_linux_and_cmts.txt
(note that wireshark captures the frames without 4 bytes FCS (frame
check sequence) so the above packet is in fact 66 bytes long).

What is the right behaviour according to specification? In iee802.3,
minFrameSize is 64bytes. I've never seen any document which'd say that
VLAN frames should be 68 bytes minimum.


> > I'd like to change it so it first pads the frame to 64 B and appends the
> > VLAN tag afterwards, so the resulting VLAN frame would be 68 bytes
> > minimum.
> 
> If you were able to figure this much out: What have you tried and what
> do you still need help on? CCing some of the e1000 people(done) will
> probably help you get this resolved faster.
Well to make long story short. While testing VLAN capabilities of our C3
CMTS, we discovered that some frames are corupted after passing trough
the CMTS. We discovered, that affected frames are VLAN frames 64 - 67
bytes long. 
We tried several NIC's in our linux box mainly with e1000 driver but
also some NIC's from 3com, broadcom and Planet. All of them suffer from
this issue except NIC from Planet (RTL-8169 chipset), which pads VLAN
frames to 68 bytes. The other cards pad VLAN frames only to 64 bytes. 
We also tried to tag the traffic on some VLAN enabled switches (d-link,
edge-core, cisco catalyst) and they also pad VLAN frames to 68 bytes.

Now i'm little confused by your reply, becaouse you write, that VLAN
frames generated by e1000 should be 68 bytes minimum, which contradicts
witch our observations.

thanks for your reply
Emil

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