Re: Question on anycast IID range(s)

[Suresh Krishnan <[e-mail]>]

<AD Hat off>

Hi Brian/Erik/Kerry,

> On Jan 2, 2019, at 4:52 PM, Brian E Carpenter <[e-mail]> wrote:
> 
> On 2019-01-03 09:37, Kerry Lynn wrote:
>> On Wed, Jan 2, 2019 at 2:57 PM Brian E Carpenter <
>> [e-mail]> 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. If we do want to cover the non 64-bit cases then the range Brian suggested above is insufficient (because the IIDs will not fit in the 64-bit range suggested) and would require a more considered change.

Thanks
Suresh