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

Fernando Gont <fgont@si6networks.com> Fri, 13 January 2017 21:02 UTC

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Subject: Re: Review of draft-ietf-6man-rfc4291bis-06
To: Brian E Carpenter <brian.e.carpenter@gmail.com>, Randy Bush <randy@psg.com>
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From: Fernando Gont <fgont@si6networks.com>
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On 01/13/2017 04:58 PM, Brian E Carpenter wrote:
> 
> On 14/01/2017 08:44, Fernando Gont wrote:
>> On 01/12/2017 10:55 PM, Brian E Carpenter wrote:
>>> 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.)
>>
>> Well, yes and no. With the traditional slaac (embed the mac address) it
>> only works with 64-bit IIDs. With something like RFC7217 (grab as many
>> bits as needed to for an IID), it could work.
> 
> Technically that's true, but you can't mix IID sizes on a given link
> and expect it to work, so any legacy system will force the whole link
> to use /64.

Well, you could say that you don't need to mix IID sizes on the same
link: the IID size is whatever the local router happens to advertise
(i.e., 128-Prefix_Length). And, if you do manual configuration, you're
supposed to know the prefix length...

-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
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