Subject: Re: xircom error
To: None <port-i386@netbsd.org>
From: David Laight <David.Laight@btinternet.com>
List: port-i386
Date: 11/28/2001 20:17:53
AARRG I got the number wrong - 1514 bytes between destaddr and end of data.
I've clearly not written enough ethernet drivers recently :-)

Not read 802.1Q - but you can't (surely) use a protocol that adds
4 bytes to the maximum length of an ethrnet frame.
Breaks ALL the other specs...

Of course encapsulations (eg those used by ATM) can have longer frames
- but they NEVER appear on a real ethernet network.

Various ethernet chipsets (eg the lance) will report a 'BABBLE' error
if an attempt is made to transmit more than 1518 bytes (including the CRC).

Any decent driver will refuse to send overlong packets and reject
overlong received ones.  However it isn't unusual to find a buggy
driver - because not enough systems check the received frame length.
(although you might have thought it would have got better over the
last 15 years...).

    David



----- Original Message ----- 
From: Patrick Welche <prlw1@newn.cam.ac.uk>
To: Manuel Bouyer <bouyer@antioche.eu.org>
Cc: David Laight <David.Laight@btinternet.com>; <port-i386@netbsd.org>
Sent: Wednesday, November 28, 2001 7:53 PM
Subject: Re: xircom error


> On Wed, Nov 28, 2001 at 08:46:46PM +0100, Manuel Bouyer wrote:
> > On Wed, Nov 28, 2001 at 05:58:24PM -0000, David Laight wrote:
> > > 
> > > > Is xi0: discarding oversize frame (len=1518) a sign that the ethernet card
> > > > is broken in someway, or just something that happens during normal network
> > > > operation
> > > 
> > > Is is more likely that the device driver of the system that SENT the
> > > packet is broken...
> > > 
> > > No ethernet frame should ever contain more then 1516 bytes (including the
> > > destination address, but excluding the CRC).
> > 
> > It can be 4 bytes larger, if you use 802.1Q VLAN tagging.
> 
> Thank you - that's what also changed :)
> 
> Patrick