Re: Last Call: <draft-ietf-6man-rfc4291bis-07.txt> (IP Version 6 Addressing Architecture) to Internet Standard

[David Farmer <[e-mail]>]

On Thu, Feb 23, 2017 at 9:13 PM, Fernando Gont <[e-mail]>
wrote:
>
> I'd remove a few sentences here, as in:
>
>    IPv6 unicast routing is based on prefixes of any valid length up to
>    128 [BCP198]. Subnet prefixes of /64 are RECOMMENDED for general
>    purpose use, subnet prefixes of /127 are RECOMMENDED for point-
>    to-point router links [RFC6164]. The rationale for the 64 bit
>    boundary in IPv6 addresses can be found in [RFC7421].


The problem is you have stripped out all the implementation guidance and
only left operational guidance.  But maybe the the right idea is to
separate the two, putting the operational guidance in Section 2.4 where we
are talking about prefixes and the implementation guidance in section 2.4.1
where we are talking about IIDs.
2nd Paragraph of 2.4;

   IPv6 unicast routing is based on prefixes of any valid length up to
   and including 128 [BCP198].  However, subnet prefixes of 64 bits in
   length are REQUIRED for use with Stateless Address Autoconfiguration
   (SLAAC)[RFC4862] and are RECOMMENDED for all other general purpose
   use. The rationale for the 64 bit boundary in IPv6 addresses can be
   found in [RFC7421].

4th paragraph of 2.4.1

   For all unicast addresses, except those that start with the binary
   value 000, support for Interface IDs that are 64 bits long is
   REQUIRED, support for other Interface IDs lengths is OPTIONAL. The
   rationale for the 64 bit boundary in IPv6 addresses can be found in
   [RFC7421].

This clearly say that implementations that only support 64 bit IID lengths
are just fine, but also says implementations that allow IID lengths other
than 64 bits are just fine too.  I think the current and historic text
actually implies implementations are not to allow other IID lengths, is
that what we really intended to say?  A lot of implementations seem to
allow other IID lengths, are they wrong?  I don't think so.

This also gives strong operational guidance that 64 bit length subnet
prefixes are expected in most situations.  Reinforcing the 64 bit boundary,
however without outlawing the use of other subnet prefix lengths when
implemented and they could be useful.  This is done without distracting
from the 64 bit boundary, by not directly calling attention to RFC6164 or
the other longer prefix lengths. Since BCP198 and RFC7421 both reference
RFC6164 calling it out here doesn't seem necessary, and would unnecessarily
weaken the focus on the 64 bit boundary that I'm trying to maintain.

I don't see how this text would require changes in any code, nor does it
imply other IID lengths are not allowed operationally, again which a lot of
implementations seem to allow.

Thanks.
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