IEEE 802.2, 802.3 & Ethernet questions


John A. Shriver (jas@proteon.com)
Thu, 11 Aug 88 09:41:59 EDT


IEEE 802.3 != Ethernet.

There are no frame format or protocol differences between Ethernet
Version 1 and 2. The only differences are subtle (but important) ones
in the electrical interface over the transceiver cable between the
host and transceiver.

IEEE 802.3 does not have the same frame format as Ethernet. What was
the type field in Ethernet becomes a length field. Thus, a new
header has to be added to indicate what Network layer protocol the
packet is for. This (among other things) is provided by IEEE 802.2.

One backwards-compatability has been provided in 802.3 for Ethernet
format frames. It turns out that almost all of the values of the type
field were longer than a maximum length Ethernet packet. So, if the
length is too long, it must be a type.

Electrically, IEEE 802.3 is quite similar to Ethernet Version 2,
mostly stricter. Again, the differences are primarily in the
transceiver cable. On the coax, all three standards are fully
compatible.

Almost nobody runs IP over "Ethernet" using the RFC1042 encapsulation.
Instead, the format defined in RFC894 "A Standard for the Transmission
of IP Datagrams over Ethernet Networks" is used. (Note that RFC894 is
older than those 4.3bsd sources, whereas RFC1042 is much newer.) I
guess the existence of any "Ethernet" RFC1042 implmenations stems from
an obsession with standards (eg. IEEE) over interoperability.

Of course, the other 802 networks (802.4 and 802.5) most definitely
use the RFC1042 encapsulation.



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