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

[Fernando Gont <[e-mail]>]

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.


> I do not understand why there is insistance that it must be 64.

The least of my concerns would be the /64 for link-locals. A more
interesting question is probably why people insist on classfull globals ;-)


-- 
Fernando Gont
SI6 Networks
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