Re: Proposal to further clarify prefix length issues in I-D.ietf-6man-rfc4291bis

james woodyatt <jhw@google.com> Thu, 09 March 2017 16:42 UTC

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From: james woodyatt <jhw@google.com>
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Subject: Re: Proposal to further clarify prefix length issues in I-D.ietf-6man-rfc4291bis
Date: Thu, 9 Mar 2017 08:42:41 -0800
In-Reply-To: <m1clw44-0000I4C@stereo.hq.phicoh.net>
To: Philip Homburg <pch-ipv6-ietf-3@u-1.phicoh.com>
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On Mar 9, 2017, at 03:16, Philip Homburg <pch-ipv6-ietf-3@u-1.phicoh.com> wrote:
> 
> It seems to me that RFC 4291 uses the term interface identifier for what
> are now two separate concepts. 
> 
> The first interface identifier concept is that for any prefix of length n,
> the length of the interface identifier is 128-n. Let me call this prefix-iid.
> 
> The second iid concept is in auto configuration of addresses. I'll call this
> slaac-iid.
> 
> The way I see i […]

I see it differently.

Predecessors of I-D.ietf-6man-rfc4192bis going back to RFC 2373 have provided text in the Interface Identifiers subsection defining the uniqueness properties of IIDs, and usage of that term has always been clearly limited to the part of unicast address following a subnet prefix. The part following any other unicast routing prefix has always been unnamed and unstructured, and its uniqueness properties are not those of an IID unless it is long enough to include the whole IID within it.

In my proposed clarification (both liberal and conservative versions), I preserve the distinction that subnet prefixes precede IIDs, whereas other routing prefixes, e.g. on-link prefixes, do not. If that doesn’t seem clear in my proposal for §2.4.8 Neighbor Discovery, then I could see adding text to that effect. Right now, it seems clear enough to me, so I’m holding off.

--james woodyatt <jhw@google.com <mailto:jhw@google.com>>