Re: appropriate length of fe80:: prefix and new IP-over-foo drafts

[Fernando Gont <[e-mail]>]

On 31/1/19 09:25, Alexandre Petrescu wrote:
> 
> 
> Le 31/01/2019 à 12:41, Fernando Gont a écrit :
>> On 31/1/19 08:33, Alexandre Petrescu wrote:
>>>
>>>
>>> Le 31/01/2019 à 12:17, Fernando Gont a écrit :
>>>> On 31/1/19 07:24, Alexandre Petrescu wrote:
>>>>>
>>>>>
>>>>> Le 30/01/2019 à 23:14, Brian E Carpenter a écrit :
>>>>>> Section 5.3 "Creation of Link-Local Addresses" of RFC4862 refers to
>>>>>> an interface identifier of N bits and an fe80::0 prefix "of
>>>>>> appropriate length", which according to the addressing architecture
>>>>>> is 128-N. But N is undefined by RFC4862 and cannot be derived from an
>>>>>> RA because all this happens as soon as the interface is enabled.
>>>>>> Therefore N must be predefined.
>>>>>>
>>>>>> As pioneered by RFC2464**, all ipv6-over-foo documents must therefore
>>>>>> specify N for their link type. So far, everybody has specified 64, as
>>>>>> far as I know. Is there a reason to do otherwise for the two drafts
>>>>>> mentioned below?
>>>>>
>>>>> One of the two drafts involves a Network ID of length 24.
>>>>>
>>>>> The question is why not use 24 for InterfaceID?  Just because Ethernet
>>>>> had it that way?
>>>>
>>>> You can certainly do that with OpenBSD. ;-)
>>>
>>> In linux too I can add (and we actually do) link-local addresses with
>>> prefix length 32.
>>
>> They will not work if you try to employ them with BSDs. BSDs use bits in
>> the range /16 /64 to store information such as the interface index.
>> So... don't use prefixes shorter than /64 for link-locals, or you'll be
>> asking for trouble.
> 
> Ah?  according to which spec do the BSD implementers put bits in the
> range 16-64?

They don't send those bits on the wire. But they do use those bits as
described. -- According to no spec. If the link-local prefix is assumed
to be /64, then you're free to use those bits *internally* as you wish.
That's what BSDs do. Jinmei is quite likely in a better position ot
comment on this.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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