Re: Review of draft-ietf-6man-rfc4291bis-06

Brian E Carpenter <> Fri, 13 January 2017 01:56 UTC

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Subject: Re: Review of draft-ietf-6man-rfc4291bis-06
To: Randy Bush <>
References: <> <> <> <> <> <>
From: Brian E Carpenter <>
Organization: University of Auckland
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Date: Fri, 13 Jan 2017 14:55:59 +1300
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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.)