Subject: Re: LL11: Is there an elephant in the room?

From: Stuart Cheshire <[email-masked]>
Date: Sat, 15 Feb 2003 14:19:13 -0800

>However, I think section 2.5 is already saying too much when it
>states "the host MUST immediately cease using this address and
>configure a new link-local IP address". In our stack it would make
>more sense to mark the old address as "deprecated" (see definition
>in RFC2462) and pick a new one.

I have said this before, so I'll try to avoid having to say it too many 
times again.

Keeping an address (even in "deprecated" state) while some other host is 
using the same address may sound like a good idea until you try it, but 
it simply doesn't work. ARP caches flap, correspondent hosts send TCP 
packets to the wrong peer, causing resets, and in no time at all, both 
hosts have lost all their active TCP connections.

The current text is a result of something that I think Erik Nordmark 
originally requested -- that when two authority-less networks merge 
(rare, but lets consider it anyway) and two hosts have previously picked 
the same address (also rare, but lets consider it anyway), that at least 
one of them should be able to continue working without disruption, 
instead of both being disrupted. For that to work, one of the hosts has 
to gracefully defer to the other. The alternative is that if they both 
try to keep the address, they both end up losing all their connections 
anyway.

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

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From: Mika Liljeberg <[email-masked]>
Date: 16 Feb 2003 00:58:28 +0200

> hosts have lost all their active TCP connections.

Our implementation doesn't flap ARP entries quite that easily. All
implementations not being similarly resistant, there is a fair chance
that what you say would happen. I just think that SHOULD would have been
strong enough for what is a corner case in the first place.

	MikaL