[e2e] Ethernet Encapsulation

[Craig Partridge]

In message <3BEAE744.B9C0CD03 at cpsc.ucalgary.ca>, Tianbo Kuang writes:

>> 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?

I believe Ethernet specs used to permit padding to 16-bit boundaries.
In general, link layers are permitted to pad (and, for instance, ATM does).

Truncation indicates an error somewhere in the transmission path
(not necessarily a higher layer -- it may have been a previous network
hop, or even an bug in the sending or receiving Ethernet hardware).
[I can't think of a case where truncation is not caused by error but
others may remember odd cases]

Craig