Path: utzoo!utgpu!attcan!uunet!lll-winken!arisia!sgi!vjs@rhyolite.SGI.COM
From: vjs@rhyolite.SGI.COM (Vernon Schryver)
Newsgroups: comp.protocols.tcp-ip
Subject: TCP/IP over FDDI
Keywords: 13-byte-MAC-header 8-byte-LLC-header
Message-ID: <21741@sgi.SGI.COM>
Date: 7 Nov 88 23:07:17 GMT
Sender: daemon@sgi.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 27

Some time ago, I asked if "everyone" is going to use 802.2 LLC and
RFC-1042 to encapsulate TCP/IP over FDDI.  Only a couple of people
responded, and they said yes.

As I understand (or, more accurately, misunderstand) things, in the
next MAC document 48-bit addresses will be required & 16-bit optional
(good news!).  That means, we have a 13 byte MAC header in front of every
FDDI packet.  If we add an 8-byte 802.2 LLC header in the style of
RFC-1042, then the IP header starts at the wonderous offset of 21.

A (mostly) standard 4.3BSD+VanJacobson/Karels TCP/IP on a machine which
insists on natural alignment requires either byte-copying or aligned
packets.  Byte-copying is generally not a formula for speed.  Copying
from 1-byte to 4-byte alignment is "optimal" if you want to avoid
saturating the network.

If we used an 802.2 LLC encapsulation with a length of 3(mod 4) (e.g.
7 or 11), the IP header would be aligned.  (It would also be aligned
if you use 16-bit addresses with 5-byte MAC headers.)  The only other
evident solutions involve extra hardware.

So, my current questions are: Does "everyone" still think RFC 1042 is a
	good idea?  What have I misunderstood?


Vernon Schryver
Silicon Graphics
vjs@sgi.com