Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!wuarchive!decwrl!ucbvax!ASYLUM.SF.CA.US!romkey
From: romkey@ASYLUM.SF.CA.US (John Romkey)
Newsgroups: comp.protocols.tcp-ip
Subject: choosing ethernet packet type automatically
Message-ID: <9009240047.AA25144@asylum.sf.ca.us>
Date: 24 Sep 90 07:47:38 GMT
References: <1990Sep24.150326@aussie.enet.dec.com>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: romkey@asylum.sf.ca.us
Organization: The Internet
Lines: 22

The Hosts Requirements group talked about the issue and decided that
if you want to run both encapsulations on one ethernet, they must be
two different subnets; you should not try to automatically negotiate
which encapsulation to use. You need a router to switch packets, even
though they're the same cable, logically they're two separate subnets.

I was a proponent of using ARP to negotiate encapsulation, similarly
to the way 4BSD TCP now uses ARP to negotiate trailers, but I agree with
the decision. The broadcast problem is that you need to decide which
format to broadcast in, and you don't want to broadcast twice (I don't
recall there being more to it than that). Another problem is that the
MTU is different between ethernet and IEEE 802. And this scheme is
also idealogically more pure: you want to switch packets between
encapsulations, you use a router.

I would recommend that if you want to support both, you create two
different logical network interfaces that use the same physical
interface, and have them use different encapsulations and different
network numbers. That's the way I'm handling it in an IP
implementation I'm working on now.
			- john romkey
USENET/UUCP: romkey@asylum.sf.ca.us	Internet: romkey@ftp.com