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From: neeraj@matrix.UUCP (neeraj sangal)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.misc,comp.protocols.iso
Subject: Questions about IEEE 802 packets on ethernet media
Keywords: Ethernet 802.2 802.3 IP SNAP
Message-ID: <132@matrix.UUCP>
Date: 12 Feb 90 20:57:31 GMT
Organization: Matrix Computer Systems, Nashua, NH
Lines: 43


I understand Ethernet packets and now I am trying to understand IEEE802
packets.  (Please bear with me if these are naive questions).  According
to what I understand by looking at the limited documentation I have,
the 802.3+802.2 packet looks as follows:

	destination	6 bytes
	source		6 bytes
	length		2 bytes	(must be less than 0x0600)

	dsap		1 byte
	ssap		1 byte
	control		1 byte

If dsap = ssap = 170 then the following fields (SNAP header) are also included:

	protid		3 bytes (always 0)
	ethertype	2 bytes


Questions:

1. Is the ethertype same as the ethertype in ethernet packet header?

2. Will dsap and ssap ever be different i.e.  What is the justification for
   having two numbers?

3. In practice which protocols use SNAP. I think IP uses SNAP, do
   other protocols such as DECNET, Apple Talk, Novell etc. which
   have ethertype assigned to them always use SNAP or do they have
   newer dsap and ssap numbers assigned to them?

4. Which protocols do not use SNAP?  What are the current assigned
   values of dsap and ssap?

4. When is protid not zero?

Any answers or pointers to where answers could be found will be
greatly appreciated.  Please email directly to me. Thanks.

Neeraj Sangal
Matrix Computer Systems, Inc.           7 1/2 Harris Rd, Nashua, NH 03062
uunet!matrix!neeraj                     (603) 888-7790