Subject: Re: New Version Notification for draft-farmer-6man-exceptions-64-05.txt

From: David Farmer <[email-masked]>
Date: Tue, 14 Aug 2018 17:40:56 -0500

Ok, then now I think I'm hearing;
A=1, L=1 is a subnet prefix as discussed in RFC4291
A=1, L=0 is a subnet prefix as discussed in RFC4291
A=0, L=1 is a subnet prefix as discussed in RFC4291
A=0, L=0 clearly not a subnet prefix, its meaning is defined RFC8028

There is some debate about A=1, L=0 at least as related to Section 2.1 in
RFC4291.  But I don't think that matters for my issue below.

When you combined;

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

With;

   For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long

It seems to me to be saying the first and third options with L=1 are /64s,
and with some debate about the second option.

Where I think we want the IID length to apply the first and second options
with A=1 requiring them to be /64s. And then for the third option, we want
it to say nothing or to only recommend /64s.

This brings me back to my suggestion that we say something like;

For all unicast addresses, except those that start with the binary value
000, Interface Identifiers are required to be 64 bits long. The rationale
for using 64-bit Interface Identifiers can be found in [RFC7421]. The
Interface Identifier length only implies a recommendation as to the subnet
prefixes that are valid for routing or on-link determination.

Thanks

-- 
===============================================
David Farmer               Email:[email-masked]
Networking & Telecommunication Services
Office of Information Technology
University of Minnesota
2218 University Ave SE        Phone: 612-626-0815
Minneapolis, MN 55414-3029   Cell: 612-812-9952
===============================================

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From: 神明達哉 <[email-masked]>
Date: Wed, 15 Aug 2018 08:18:36 -0700

At Tue, 14 Aug 2018 17:40:56 -0500,

> A=0, L=1 is a subnet prefix as discussed in RFC4291

The third one is not a subnet prefix as discussed in RFC4291, at least
in my interpretation.  RFC4291 talks about the *addressing*
architecture, while the A=0,L=1 case is about routing (from the end
hosts' perspective).

> RFC4291.  But I don't think that matters for my issue below.

So, it doesn't seem to me to say that.  At least to me, that's a
misunderstanding due to the confusing text in RFC4291:

   Though a very simple router may have no knowledge of the internal
   structure of IPv6 unicast addresses, routers will more generally have
   knowledge of one or more of the hierarchical boundaries for the
   operation of routing protocols.  The known boundaries will differ
   from router to router, depending on what positions the router holds
   in the routing hierarchy.

placed immediately after the above diagram.

But in any case,

> it to say nothing or to only recommend /64s.

that matches my interpretation of the sense of RFC4291 (and RFC4861
regarding on-link prefixes).

> prefixes that are valid for routing or on-link determination.

Regarding the last sentence, I'd rather avoid the term "subnet prefix"
for routing or on-link determination (see above about
"misunderstanding") and say, e.g., "note that the subnet prefix length
derived from the Interface Identifier length has nothing to do with
routing or on-link determination from the addressing architecture
point of view, even if the same or shorter prefix is often used for
these purposes in practice."  I think it's more consistent with the
latest text in rfc4291bis-09:

   IPv6 unicast routing is based on prefixes of any valid length up to
   128 [BCP198].

--
JINMEI, Tatuya

---

From: David Farmer <[email-masked]>
Date: Wed, 15 Aug 2018 11:13:32 -0500

>    IPv6 unicast routing is based on prefixes of any valid length up to

I like that! However, I think saying they have nothing to do with each
other is maybe a little bit too strong, but they are clearly independent of
each other, one is not in control of the other.

So reformulating a little bit, how about;

Note the subnet prefix length derived from the Interface Identifier length
is independent of the prefixes that are used for routing or on-link
determination, even if identical prefixes are often used in practice.

Thanks.

---

From: 神明達哉 <[email-masked]>
Date: Wed, 15 Aug 2018 10:12:02 -0700

At Wed, 15 Aug 2018 11:13:32 -0500,

> determination, even if identical prefixes are often used in practice.

This one also works for me.  In fact I wondered whether "independent"
might be a better choice of word when I suggested the text using "has
nothing to do with".

--
JINMEI, Tatuya