Re: Objection to draft-ietf-6man-rfc4291bis-07.txt

Brian E Carpenter <brian.e.carpenter@gmail.com> Tue, 28 February 2017 19:56 UTC

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Subject: Re: Objection to draft-ietf-6man-rfc4291bis-07.txt
To: Lorenzo Colitti <lorenzo@google.com>, David Farmer <farmer@umn.edu>
References: <20170223134026.GI5069@gir.theapt.org> <9277BC0B-04F3-4FC1-901E-F83A8F0E02D7@google.com> <58AF6429.70809@foobar.org> <902276E9-0521-4D4E-A42B-C45E64763896@google.com> <58AF726A.3040302@foobar.org> <F7C230DE-4759-4B78-ABF2-6799F85B3C62@google.com> <58B014F6.2040400@foobar.org> <6DA95097-8730-4353-A0C9-3EB4719EA891@google.com> <CAKD1Yr0qk_njAGnex_FZsYisCVw=eM8hXTr1v+wqvcfX_09wiQ@mail.gmail.com> <CAN-Dau0ohz3Wp55bs+eoFvSyoUjuKfjzKGSAsJS3wUt3z7TGtA@mail.gmail.com> <CAKD1Yr0wK8EiAbz39EZz-xZLtsSV2JROSzNECKtGo36Zc=RZ0Q@mail.gmail.com>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
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Date: Wed, 1 Mar 2017 08:56:00 +1300
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On 01/03/2017 05:29, Lorenzo Colitti wrote:
...
>> We have defined this as a parameter not as a constant.
>>
> 
> I really don't understand this statement. How can you say that it's a
> parameter, given that every RFC that has been published on this topic
> starting from 1998 states that (most) IIDs are 64 bits long?

Yes, I think we have a long tradition of expressing this badly, and now
is a chance to get it right. It's clear at the point where it's
introduced in RFC4291 that it's a floating boundary:

  "A slightly sophisticated host (but still rather simple) may
   additionally be aware of subnet prefix(es) for the link(s) it is
   attached to, where different addresses may have different values for
   n:

   |          n bits               |           128-n bits            |
   +-------------------------------+---------------------------------+
   |       subnet prefix           |           interface ID          |
   +-------------------------------+---------------------------------+"

but later in the same document we state that (128-n) == 64. That is
inconsistent; what we're trying to do now is fix that inconsistency
in a way that is *also* consistent with running code, SLAAC, and the
newly important privacy issues that require long, unpredictable IIDs.

> Most of the code in most implementations treated this a parameter, but
> there is code that just takes the 64-bit length at face value, and is well
> within its rights to do so, because it's specified by the standard.

And that's our fault, all the way back to RFC3513. (RFC2373 was more
cautious.)

On 27/02/2017 23:22, Alexandre Petrescu wrote:
...
> Has the option of removing this 4291 paragraph altogether been
> considered (instead of improving it)?

This idea has some merit. Fixing a parameter doesn't really belong in
the architecture document. However, I think we've painted ourselves
into a corner here, so we have to say something.

     Brian