On 13/01/2017 13:50, Randy Bush wrote:
>> RFC7421 (which is Informational) calls out RFC 6164 (not 6141!) as an exception.
>> To be precise it says:
>>
>> The de facto length of almost all IPv6 interface identifiers is
>> therefore 64 bits. The only documented exception is in [RFC6164],
>> which standardizes 127-bit prefixes for point-to-point links between
>> routers, among other things, to avoid a loop condition known as the
>> ping-pong problem.
>>
>> I would suggest adding a similar exception statement in 4291bis.
>
> and then next year we will go through another draft and have another
> exception. just get rid of classful addressing. we went through this
> in the '90s.
The problem is (and why we wrote 7421) is that stuff breaks with subnet
prefixes longer than 64, *except* for the point-to-point case covered
by 6164. Yes, I see the problem in enshrining this but I think we face
signifcant issues if we do otherwise.
What we could conceivably say is that /64 is mandatory except for
links where SLAAC will never be used. (SLAAC itself is designed
to work with any reasonable length of IID, but again in practice it
only works with /64, because we need mix-and-match capability. So
although IID length is a parameter in the SLAAC design, it's a
parameter whose value needs to be fixed globally.)
Brian