Re: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt

JINMEI Tatuya / 神明達哉 <Jinmei_Tatuya@isc.org> Fri, 18 July 2008 02:27 UTC

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Date: Thu, 17 Jul 2008 19:28:06 -0700
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From: JINMEI Tatuya / =?ISO-2022-JP?B?GyRCP0BMQEMjOkgbKEI=?= <Jinmei_Tatuya@isc.org>
To: Erik Nordmark <erik.nordmark@sun.com>
Subject: Re: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt
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At Mon, 14 Jul 2008 03:30:50 -0700,
Erik Nordmark <erik.nordmark@sun.com>; wrote:

> there are currently two places where we treat autoconfigured addresses 
> differently than on-link prefixes.
> One is the text about storing the address and associated lifetime in RFC 
> 4862 that we are debating here.
> The other one is the 2 hour rule that was very explicitly added only for 
> the addrconf property of the prefix and not the on-link property.
> 
> I don't recall the different treatment for the first case.
> But for the second case I clearly recall the argument that loosing the 
> IP address (by an accidental or malicious RA with a zero valid lifetime 
> for the prefix) was considered a lot worse than loosing the on-link 
> property. Basically that loosing the IP address for a short time (it 
> might be re-added by the next RA) is something we must avoid, but that 
> "routing" (the on-link property) is something which must be able to 
> recover from temporary failures in any case.

Yes, I recall that discussion, too.

> [I don't know if the same argument was present when the storing of 
> addresses was discussed.]
> 
> > I'd accept, if
> > 
> > 1. we prohibit both address caching and on-link caching (with
> >    reasonable argument, of course)
> > or,
> > 2. we don't talk about on-link caching (either allow or prohibit) if
> >    we don't talk about address caching
> > 
> > but it would be unfair if
> > 3. we prohibit on-link caching while ignoring its effect on address
> >    caching.
> 
> If we are going to require that all aspects of a prefix should be 
> treated identically then to be consistent we must also make the 2 hour 
> rule be consistent (by either removing it for the addrconf property or 
> adding it to the on-link property).
> 
> FWIW I think forcing such consistency is not necessary; it would just 
> result in unneeded changes to already deployed standards.

There's a subtle difference.  In the two-hour-rule case, we assume we
have a working router.  So even if the host looses its on-link prefix
information, it can still send packets to the router, having it
forward the packets and/or send a redirect back, etc.

In the address storing case, (at least in my understanding) we
consider the case when a router is temporarily out of service.  And
since we dropped the on-link-by-default rule, the cached address will
effectively be of no use.

> Is your concern that there are deployed implementations which store 
> on-link prefixes across reboots?

No, it's rather a meta level concern.  What I wanted to say (in case I
was not clear) is that we should not pretend that killing on-link
prefix caching is independent from address caching.  If we kill the
former, we will effectively also kill the latter (which has
implementors - note: we don't implement it).  So, IMO, if this
document wants to kill on-link caching, it should at least note that
it also kills address caching in effect.  If implementors of address
caching still think it's acceptable, then I have no objection to that
(I have no stake in that area, anyway).

I hope I'm clear this time.

---
JINMEI, Tatuya
Internet Systems Consortium, Inc.
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