[e2e] Ethernet Encapsulation

[Dennis Rockwell]

On 8 Nov, Tianbo Kuang wrote:

> Dennis Rockwell wrote:
> 
> > So, if the length
> > supplied by the link layer is is too short, the packet
> > should be discarded.  If it is longer, the excess data is
> > simply ignored.
> 
> I thought the packets handed to IP layer by link layer are "clean" as the MAC
> layer has CRC. The situation you mentioned can only be caused by an error of
> higher layers. Am I wrong?

Stuff happens.  There are errors that the CRC doesn't catch,
the CRC computation requires intact hardware (been there,
got bit by that).  Bridges, routers, and hosts have memory
failures (ask any oldtimer about the IMP with the zeroed
memory).  Remember that (dumb) bridges don't know from IP
and wires and fibers and other links *never* have absolutely
zero errors.

A clean CRC doesn't mean anything about the validity of the
upper-level data, just that the controller thinks that all
is OK.  Verification must happen at each layer.

Dennis