Re: Updated IID length text

Fernando Gont <fgont@si6networks.com> Thu, 19 January 2017 06:01 UTC

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Subject: Re: Updated IID length text
To: Lorenzo Colitti <lorenzo@google.com>, Brian E Carpenter <brian.e.carpenter@gmail.com>
References: <148406593094.22166.2894840062954191477.idtracker@ietfa.amsl.com> <m2fukqbbwv.wl-randy@psg.com> <F6953234-3F85-4E28-9861-433ADD01A490@gmail.com> <m2wpdzhncn.wl-randy@psg.com> <82245ef2-cd34-9bd6-c04e-f262e285f983@gmail.com> <m2d1frhjfn.wl-randy@psg.com> <18e6e13c-e605-48ff-4906-2d5531624d64@gmail.com> <CAKD1Yr1cvZ8Y3+bHeML=Xwqr+YgDspZGnZi=jqQj4qe2kMc4zw@mail.gmail.com> <m2lguffnco.wl-randy@psg.com> <CAKD1Yr1TrTiPRdyutobmb_77XJ7guNzLrg=H_p7qi4BfQ8V=GA@mail.gmail.com> <m2d1frfm6m.wl-randy@psg.com> <CAKD1Yr2Njjd8_Mr+6TRFF6C5pdcX4yFgpFVyEkykDuytu2B8mg@mail.gmail.com> <2A5073777007277764473D78@PSB> <4596c3d4-a337-f08e-7909-f14270b7085f@gmail.com> <CAN-Dau06R3iYRpYLADhvHox4C9qdsJCuxFsJapRhOQcWT4qk_g@mail.gmail.com> <CAO42Z2weZcoHiBzN94QAQ9WGhWR16PmMMFNg=5YLmr_dhPjjpA@mail.gmail.com> <fcc7f136-b5da-527e-b495-5a2d7f7a3ce8@gmail.com> <CAKD1Yr2Y8yY5=E3VUNuJqPsxeEJ2AMJM2ShKyQhQJRiO7fq3HA@mail.gmail.com>
From: Fernando Gont <fgont@si6networks.com>
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Date: Thu, 19 Jan 2017 02:48:37 -0300
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On 01/19/2017 12:00 AM, Lorenzo Colitti wrote:
> On Thu, Jan 19, 2017 at 9:37 AM, Brian E Carpenter
> <brian.e.carpenter@gmail.com <mailto:brian.e.carpenter@gmail.com>> wrote:
> 
>     However, consistent use of Stateless Address Autoconfiguration
>        (SLAAC)[RFC4862] requires that all interfaces on a link use the
>     same length
>        of Interface ID. To guarantee interoperability of SLAAC, a fixed
>     length of
>        Interface ID is necessary.
> 
> 
> I'm not a fan of this text, because it's a weak argument. A possible
> (uninformed) response to it might be "why do we need this SLAAC thing
> anyway? I want to use /120 prefixes and DHCPv6, just like I do in IPv4".
> And really, SLAAC is only one of the reasons why we have a 64-bit IID.

Please enlighten me. The 64-bit length has to do with SLAAC, and the
fact that the IID had to be able to embed 64-bit link-layer addresses.

If you were to start with a clean slate, you wouldn't embed link-layer
addresses in the layer-3 address, and hence wouldn't need to do
fixed-length 64-bit IIDs. With manual configuration or DHCPv6, you would
support "any size that can fit all your systems and allow your network
to grow". And with SLAAC (+ something like RFC7217), it would be the
same, with the contraint that "the size results in low host density, so
that IID collisions are not a concern".



> There are lots of good arguments for this in RFC7421 - solid arguments,
> about address scarcity and future flexibility, and so on. 

That translates well into "well... we have so many bits that... let's
burn them in any possible way" (fwiw, I don't think that's the argument
being made in RFC7421, though)

An artifact of history need not and does not automagically become a
design principle.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492