Re: Question on anycast IID range(s)

David Farmer <farmer@umn.edu> Fri, 04 January 2019 00:36 UTC

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From: David Farmer <farmer@umn.edu>
Date: Thu, 03 Jan 2019 18:35:44 -0600
Message-ID: <CAN-Dau2rJBNhgH7VOsN8BASnN1vLFDX0HfH_nhmy4XANc+XOGw@mail.gmail.com>
Subject: Re: Question on anycast IID range(s)
To: Suresh Krishnan <suresh.krishnan@gmail.com>
Cc: Brian E Carpenter <brian.e.carpenter@gmail.com>, 6man 6man <ipv6@ietf.org>, ek@loon.co
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On Thu, Jan 3, 2019 at 5:56 PM Suresh Krishnan <suresh.krishnan@gmail.com>
wrote:

> Hi Brian,
>
> On Jan 2, 2019, at 11:15 PM, Brian E Carpenter <
> brian.e.carpenter@gmail.com> wrote:
>
> On 2019-01-03 17:01, Suresh Krishnan wrote:
>
> <AD Hat off>
>
> Hi Brian/Erik/Kerry,
>
> On Jan 2, 2019, at 4:52 PM, Brian E Carpenter <brian.e.carpenter@gmail.com>
> wrote:
>
> On 2019-01-03 09:37, Kerry Lynn wrote:
>
> On Wed, Jan 2, 2019 at 2:57 PM Brian E Carpenter <
> brian.e.carpenter@gmail.com> wrote:
>
> On 2019-01-03 07:15, Kerry Lynn wrote:
>
> Thanks Erik,
>
> My question was ill-posed *and* contained a typo.  I'm really trying to
> figure out
> which range(s) of IIDs RFC 2526 is trying to reserve for anycast use.  I
> now think
> the answer is fdff:ffff:ffff:ff80-fdff:ffff:ffff:ffff based on RFC 5453
>
> and
>
>
>
> https://www.iana.org/assignments/ipv6-interface-ids/ipv6-interface-ids.xhtml
>
> If I take RFC2526 literally, ffff:ffff:ffff:ff80-ffff:ffff:ffff:ffff
> is also reserved, for IIDs not in modified EUI-64 format.
>
> That's the problem with RFC2526; at the time it was written there was a
> class of
> IPv6 address that required IIDs to be 64-bits AND in EUI-64 format.  Given
> that the
> latter requirement no longer seems to hold, it would seem the basis for the
> range
> fdff:ffff:ffff:ff80-fdff:ffff:ffff:ffff no longer exists.  Yet, this range
> is now enshrined in
> RFC5453 and
>
> https://www.iana.org/assignments/ipv6-interface-ids/ipv6-interface-ids.xhtml
>
> But RFC7217 doesn't mention RFC2526, which might be a bug.
>
>
> RFC7217 (and any other proposal for IID generation) should take RFC5453 and
> its associated registry into consideration.
>
>
> It does. But the gap is that RFC5453 doesn't call out
> ffff:ffff:ffff:ff80-ffff:ffff:ffff:ffff
>
>
> Looking back at my notes on what became RFC5453, this is not a gap but
> something I had intentionally left out of RFC5453 based on the addressing
> usage then. I will try to explain my reason why and we can see if this
> still makes sense or not.
>
> According to RFC2526, "for IPv6 address types required to have to have
> 64-bit interface identifiers in EUI-64 format” the reserved anycast range
> was only
>
> FDFF:FFFF:FFFF:FF80-FDFF:FFFF:FFFF:FFFF
>
> Since RFC4291 defined all the space other than ::/3 to be used only with
> 64-bit IIDs, and the goal of RFC5453 was to avoid address conflicts for
> SLAAC (which used 64 bit IIDs due to reasons explored in great detail in
> RFC7421), this is the range that was put into RFC5453.
>
> Yes. I've posted an erratum to 5453. At the time, ffff:etc might
> have seemed like a corner case, but 2526 did actually cover it.
>
>
> 2526 covered this *only* for non 64-bit non EUI-64 IIDs.
>
>
> That's not how I read it. IMHO, it covered it for non-EUI-64 IIDs
> of length N, and there's nothing to say that N may not be 64.
> (In fact, our current addressing architecture states that N==64,
> as we all know only too well.)
>
>
> :-). Yes, we do. The addressing architecture also states in Section 2.5.1.
> that
>
> "  For all unicast addresses, except those that start with the binary
>    value 000, Interface IDs are required to be 64 bits long and to be
>    constructed in Modified EUI-64 format."
>

Well, after the RFC7136 update it says;

      For all unicast addresses, except those that start with the binary
      value 000, Interface IDs are required to be 64 bits long.  If
      derived from an IEEE MAC-layer address, they must be constructed
      in Modified EUI-64 format.

And RFC8064 effective makes Modified EUI-64 format OPTIONAL, or a MAY at
best, one could even argue it depercated Modified EUI-64, A.K.A. NOT
RECOMMENDED.


> So if we want to add the omitted range, we need to restrict it to non ::/3
> prefixes as well.
>
> If we do want to cover the non 64-bit cases then the range Brian suggested
> ave is insufficient (because the IIDs will not fit in the 64-bit range
> suggested) and would require a more considered change.
>
> For the specific case of N==64 I don't see that as necessary.
>
> Agree that the N==64 case works fine but I was talking about whether the
> WG wanted to cover the non-64-bit cases as well.
>

So, I believe ffff:ffff:ffff:ff80-ffff:ffff:ffff:ffff should be added to
the reserved list and RECOMMENDED for use as Anycast addresses.
And, fdff:ffff:ffff:ff80-fdff:ffff:ffff:ffff should remain on the reserved
list, but NOT RECOMMENDED for use as Anycast addresses, as a result of
RFC8064.

They both should have been on the list originally.  Further, I believe the
original intent for Modified EUI-64 is the way RFC7136 updates it to,
especially if you take the paragraphs following that talk about "Modified
EUI-64 format-based interface identifiers". Talking about them that way
kind of implies there are interface identifiers that are not based on
Modified EUI-64 format, despite the paragraph above originally said.

And yes we should assume N==64. But as Ole said, it is quite clear even if
N!=64 that RFC2526 say "the highest 128 interface identifier values are
reserved."

Thanks.


> Thanks
> Suresh
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