Re: Updated IID length text

Lorenzo Colitti <> Thu, 19 January 2017 03:01 UTC

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From: Lorenzo Colitti <>
Date: Thu, 19 Jan 2017 12:00:54 +0900
Message-ID: <>
Subject: Re: Updated IID length text
To: Brian E Carpenter <>
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On Thu, Jan 19, 2017 at 9:37 AM, Brian E Carpenter <> 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.
There are lots of good arguments for this in RFC7421 - solid arguments,
about address scarcity and future flexibility, and so on. We should let
those provide the rationale since they can give a much more complete
picture than we can in two lines here. Since you cite RFC7421 just a couple
of lines down, I would just strike this text entirely, since it looks like
the text is only there to justify the following normative text.

I think I'm fine with the text that precedes it ("   IPv6 routing is based
on prefixes of any valid length up to 128 [BCP198]. For example, [RFC6164]
...)" and the text that follows it.