[Bridge] Re: Small packets

Andy Gospodarek andy at greyhouse.net
Mon Oct 29 09:15:22 PDT 2007


On 10/29/07, Ben Greear <greearb at candelatech.com> wrote:
> Andy Gospodarek wrote:
> > On 10/29/07, Benny Amorsen <benny+usenet at amorsen.dk> wrote:
> >>>>>>> "LS" == Leigh Sharpe <lsharpe at pacificwireless.com.au> writes:
> >> LS> Standard e1000 hardware. The packets being bridged contain a VLAN
> >> LS> tag, which is included in the 60 bytes.
> >>
> >> The e1000 has VLAN acceleration. The VLAN tag is sent in a separate
> >> register. If you do packet capture on the sender, the packet will
> >> likely look 60 bytes long, even if it is 64 bytes on the wire.
> >>
> >> The same thing happens on receive. Packet dumping with VLAN's is a bit
> >> of a mess in Linux. If you're lucky you can find a card without VLAN
> >> acceleration to do the packet dump.
> >>
> >>
> >> /Benny
> >>
> >
> > Are these the lengths on the wire or when captured on the host?   The
> > smallest VLAN tagged frame should be 68 bytes IIRC.  A tagged frame
> > that is 64 bytes seems too small.
>
> That is not correct per the 802.1Q VLAN RFC, though
> I don't have the reference handy at the moment.
>
> 64 bytes is fine, vlan tagged or otherwise.
>
>
> Thanks,
> Ben
>
>

Ah, you are correct, Ben.  Thanks for pointing that out.  Section
C.4.4.1 of the IEEE 802.1Q spec does state that 64-byte tagged frames
are OK, but that is only if you are able to drop padding from a
64-byte untagged one.  Transmitting a 60-byte frame on the wire is
incorrect.


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