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From: koning@koning.enet.dec.com (Paul Koning)
Newsgroups: comp.dcom.lans
Subject: Re: 802.2 issues
Message-ID: <21106@shlump.nac.dec.com>
Date: 14 Mar 91 19:57:06 GMT
References: <7160@emory.mathcs.emory.edu> <91117@sgi.sgi.com>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: koning@koning.enet.dec.com
Organization: Digital Equipment Co., distributed systems architecture
Lines: 47


|>...
|>| 2) I know informally about the SAP "AA", which goes with a SNAP
|>| discriminator. Is "AA" one of these code points? Where can I find a
|>| formal description of the "AA" SAP and SNAP?
|>+---------------
|>
|>Here's an informal one...
|>
|>When 802.2 was being defined, there were some various interests who lobbied
|>succesfully for an "escape hatch" to LLC, which became SNAP. [I've heard
|>various explanations for the acronym, but all of them seem to agree that
|>it's some variation of {Sub-,Sudden,Special} Network Access Protocol.]
|>
|>If the SSAP/DSAP are each 0xAA, then the next three bytes after the LLC
|>header are an OUI (Organization-Unique Identifier) -- I could be wrong, but
|>I think the upper 24 bits of your registered 48-bit MAC address group will
|>do nicely here -- and the two bytes after that are "reserved for organization 
|>definition". That is, each "organization" (OUI number) is allowed to define
|>those two bytes (and the rest of the packet!) any way they like.

Yes, the OUI is what's used in the first 24 bits of the Ethernet address,
and also the first 24 bits of the SNAP Protocol Identifier (that's what
the 40-bit field after the LLC header is called).

For an official definition of SNAP, as well as assorted other useful
things, refer to IEEE 802.1a, "Architecture and Overview".

|>...
|>See RFC 1042 and RFC 1103 for a more formal description of the LLC/SNAP/OUI==0
|>encapsulation.

1103 has been superseded (by 1188, I think).  Also, if you do what 1042
talks about on Ethernet, please be sure to follow the rules for that 
case which are mentioned in RFC 1122.

|>...
|>+---------------
|>| 3) What SNAPs have been defined to date?
|>+---------------
|>
|>Besides OUI==0 (RFC 1042/1103), who knows...?  Try the IEEE.

IEEE doesn't assign Protocol Identifiers.  It does assign OUIs, of course,
but as far as I know those assignments are not made public by IEEE.

	paul