Subject: Re: Time to Publish draft-ietf-zeroconf-ipv4-linklocal

From: Stuart Cheshire <[email-masked]>
Date: Fri, 3 May 2002 09:24:26 -0700

>Hubs and repeaters were just seen as parts of the wire, they
>amplified signals, allowed longer cable lengths (affected
>collisions on LANS where collisions occur) and stuff - but had
>no effect on where packets were transmitted.
>
>The arrival of bridges (and later switches) changed all that -
>now the link layer only sends packets to nodes that are likely
>to actually want to receive them, usually, so the definition of
>a link layer changed.

I agree with what you are saying, but I disagree that the definition 
changed. Bridges and switches took the filtering algorithm that every 
Ethernet card implements and moved it up the cable to the hub, so that 
packets the Ethernet card would throw away anyway got filtered *before* 
they were sent down the cable, but the burden was on the switch vendors 
*not* to change the operating model. Vendors could never have sold 
"switches" if they didn't preserve the exact same operational model as 
standard Ethernet, because there would have been some higher level 
application software that stopped working. (Packet sniffers arguably fall 
outside the "standard Ethernet operating model".)

However, I don't think you are advocating any change to the document. 
We're disagreeing over tiny nuances of wording. This is what the document 
says:

   Two hosts are considered to be on the same link if, when one host
   sends packets to the other, using unicast, multicast, or broadcast...

My intent by use of the phrase, "sends packets to the other," was
to mean when one host sends packets which, according to rules of
Ethernet, the other host should receive. (I think that is reasonably
conventional use of the word "to". Of course that depends on what the
meaning of the word "is" is :-)

If the other host should receive those packets, but doesn't, then the 
network is broken.

If the other host receives those packets, but they arrive modified, then 
something in the middle modified them, therefore they are on separate 
links.

If the other host receives all such packets unmodified, then the hosts 
are on the same link.

Stuart Cheshire <[email-masked]>
 * Wizard Without Portfolio, Apple Computer
 * Chairman, IETF ZEROCONF
 * www.stuartcheshire.org

---

From: Robert Elz <[email-masked]>
Date: Sat, 04 May 2002 20:00:52 +1000

Date:        Fri, 3 May 2002 09:24:26 -0700
    From:        Stuart Cheshire <[email-masked]>
    Message-ID:  <[email-masked]>

  | If the other host receives all such packets unmodified, then the hosts 
  | are on the same link.

Yes, if it gets all of the unicast/multicast/broadcast packets, that's
probably close enough ... as long as it is clear that all need to get
through, I guess when I read your words it seemed to me that if any of
them could be delivered, it would be OK - but I have seen equipment that
forwards (unaltered - ie: bridges) unicast traffic, but blocks broadcast.
It is really where the broadcast packets go that matters most.

kre