On Aug 2, 2006, at 2:27 PM, stef mientki wrote:

> Sorry I made a little mistake, instead of checking the ZERO bit, I
> checked the Length check error,
> which is not the standard 08 00 (ARP) or 08 06 (IP),
> but 00 2C which apparently ??? triggers the Length check error.
>
When ethernet was first created, the 2 byte field after the
ethernet addresses was a "type" field.  Assorted people got
type values assigned.  IEEE came along and in the habit of
standards bodies everywhere, decided that they HAD to change
SOMETHING in the existing spec, so they made it a length field.
But then there wasn't any type field anymore, so they had to
add some additional bytes at the beginning of the packet to
identify the protocol (SAP, SNAP) Novell sort of showed up
between those two steps, and is one of the few protocols that
includes a native packet immediately following the length field,
making it a PITA.  Meanwhile, the TYPE field was so intrenched
that it wouldn't go away, and software implementations had to
deal multiple encapsulations, sometimes for the same protocol
(there were three ways that IP might have appeared, and the hacks
to ARP to figure out which one was appropriate for each destination
were... extensive and good for router sales.)

I'm not sure why the packets would trigger the Length check error,
though.  2C should be a fine length (44 bytes, yes?)  Perhaps
it doesn't match the length received by the hardware (which
ought to be OK since small ethernet frames get padded, I think (I
don't recall whether it's supposed to actual length or padded
length))  (Hmm.  I read the manual; looks like the error happens
for the mismatch; but it also says the length is the unpadded
data length, so it looks like a pretty useless error bit, unless
you also compare to the actual min length)

BillW




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