RE: IEEE 802.2, 802.3 & Ethernet questions
11 Aug 88 09:16:00 PST


>I have a few questions regarding the use of TCP/IP on an Ethernet.
>From reading RFC 1042, I'm led to believe that "all communication
>is performed using 802.2 type 1 communication." That would seem to
>imply that the 8-byte LLC header and SNAP header is being used,
>but going through our 4.3 BSD sources, I can't find any refernces
>to these headers. Later in the RFC, they talk about "Interoperation
>with Ethernet", which makes me think that what we're using is the
>"Ethernet" instead of the 802.3 link level control.

We tend to get a little lax in our use of terminology. People tend to
interchange the use of the terms "Ethernet" and "802.3" even though they
are not really quite the same thing.

>I can't find many references that describe 802.2 at all - I don't
>want to read the IEEE standard (if possible) and Stallings Volume 2
>(which the local bookstore doesn't have) appears to be the only
>reference I've seen.

The IEEE 802 standards are much easier reading than some of the ISO or
CCITT documents.

>(1) What are the differences between Ethernet V1.0 and V2.0 and 802.3 ?
> From seeing how our hardware is set up, it appears that all our
> interface cards are set for 802.3.

First keep in mind that these specifications have mechanical, electrical and
protocol components.

Ethernet V1.0 is the original, derived from the 3mb/sec CSMA/CD work done
at Xerox PARC. This standard was pushed by the joint efforts of DEC, INTEL,
and Xerox.

Ethernet V2.0 was released a few years later and mainly changed the electrical
interface between the host and transceiver. The MAC headers did not change
between V1.0 and V2.0 and both define the last word of the MAC header to be
the protocol type.

When IEEE took up standardization of LANs, 802.3 was developed for CSMA/CD
(Ethernet type) LANS. 802.3 is really broader than Ethernet and covers more
speeds and media types (and is still being extended). 802.3 and Ethernet V2.0
are close enough at mechanical and electrical levels to be functionally
equivalent. IEEE 802 defines many MAC technologies (CSMA/CD, token bus,
token ring) but attempts to provide a common link level service. To do this,
802 protocols are broken into two layers, Media Access Control (MAC) and
Logical Link Control (LLC or 802.2). The MAC headers tend to differ amoung
the LAN types, but each LAN type has at least one type of "Information" MAC
frame. These MAC frames carry LLC packets whose length must be derived from
the MAC header. For 802.3 there is only one MAC frame type. The 802.3 MAC
header consists of <Dest, Src, Len>, where Dest and Src are either 2 byte
or 6 byte MAC addresses and Len is the LLC packet length. For networks
using 6 byte addresses, the Ethernet and 802.3 headers only differ in the
last word being a protocol id or an LLC length respectively.

In 802 protocols, the LLC header identifies the next layer "user" using
the "Service Access Point" (SAP) fields. Unfortuneately, they only defined
6 usable bits for LLC SAPs and refused to assign standard SAP values for
protocols like ARP (though IP was assigned a value). To solve this problem,
the Sub-Network Access Protocol (SNAP) was defined to expand protocol
identification. One variant of the SNAP header carries Ethernet protocol
type codes. RFC 1042 is basically describing this situation.

>(2) Do any TCP/IP implementations on an "ethernet" use the 802.2 LLC ?
> Looking at the RT PC AIX V2.1 manual (pp. 1-17 to 1-19) leads
> me to belive that this implementation does use the 802.2 LLC.

Most TCP/IP vendors still use Ethernet MAC headers, but there are one or
two that have adopted LLC and SNAP. It is expected that when ISO protocols
get established that 802.2 will be more widespread. Note that for "Etherenet"
it is possible to distinguish between Ethernet and 802.3 MAC headers because
the Ethernet protocol types are illegal LLC length values and vice versa.

>(3) Why do some documents refer to the 2-byte field in the Ethernet
> MAC (immediately following the destination address and source
> address) as "length" (RFC 1042, for example) while others call
> it "packet type" (Comer's new book, for example) ?

As above, in Ethernet it is "type" and in 802.3 it is "length".

>(4) Are there any implementations of TCP/IP on a Token-Ring (802.5)
> other than the RT PC ?

I believe that IBM's VM TCP/IP supports TR. I would assume Proteon does
in their routers and some universities are probably running TCP/IP TRs.
(Of course there is the whole TR source routing controversy :-> ).

>Thanks for any light anyone can shed on this.

I hope this helps.

> Richard Stevens
> Health Systems International, New Haven, CT
> { uunet | yale } ! hsi ! stevens

                                                Art Berggreen


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