Re: [v6ops] A proposal for draft-ietf-6man-rfc4291bis-07 - LL notation

Alexandre Petrescu <alexandre.petrescu@gmail.com> Wed, 08 March 2017 08:00 UTC

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Subject: Re: [v6ops] A proposal for draft-ietf-6man-rfc4291bis-07 - LL notation
To: Mark Andrews <marka@isc.org>
References: <CAN-Dau17q_BrUuzfvB1mLDt6p5UxYikphWaHpa8VQ2L-3kx-DA@mail.gmail.com> <m1ck5mu-0000GaC@stereo.hq.phicoh.net> <5B4AFF50-8CA9-4134-8CE2-A383DB5F8BF5@google.com> <m1ckxfo-0000IMC@stereo.hq.phicoh.net> <225F639E-27C1-4408-BC2B-26500929049B@google.com> <CAOSSMjUR203+hYFBrFBrj9Xkjux3o7fYNd4y9kNyxwpLxF11ew@mail.gmail.com> <6D825351-7F43-4540-89AB-48DC2B5E92E3@google.com> <CAOSSMjUP6m-L1iNhE=BxHW+7hvt4YsZgxxtVn+qmgEVS9HeStA@mail.gmail.com> <CAJE_bqfpE-NWwG12S4CXM+ZnHdHHH31-y+_+pYhuCuq2FtqZ4w@mail.gmail.com> <6f532a72-a142-5d84-f351-c38cba5230d! d@gmail.c om> <5773AF65-4E7E-43B6-BCB1-26F6E9CD3C60@steffann.nl! > <fdc3fa b6-fdcb-f390-e6ea-d9e49edacf52@gmail.com> <7A10E8D9-02F4-4D1C-B422-86ACCAABCB38@steffann.nl> <b86627e7-366! 8-304f-f1 2e-71ad3bfd6f7a@gmail.com> <1F674D50-1C89-444C-A617-C528951F80EC@steffann.nl> <6f2b4f35-8501-f9d8-e2f6-9e545419fb76@gmail.com> <a11c6b9b-ae5c-7c7d-a71c-c! c5d8b80d0 47@gmail.com> <20170308032826.9A8FE663A11A@rock.dv.isc.org>
From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
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Date: Wed, 8 Mar 2017 09:00:27 +0100
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Le 08/03/2017 à 04:28, Mark Andrews a écrit :
>
> In message <a11c6b9b-ae5c-7c7d-a71c-cc5d8b80d047@gmail.com>;, Alexandre Petrescu writes:
>> Thanks to this discussion, I would like to promote the following fact
>> that I did not know:
>>
>> If you say "FE80::" without telling the prefix length, that actually
>> means "FE80::/9" (and not FE80::/10, nor FE80::/64).  Because 0xFE80  is
>> "1111 1110 1000 0000" in binary representation.
>>
>> Alex
>
> No.
>
> FE80:: is always 128 bits.
> FE80::/9 says take the first 9 bits from those 128 bits.

YEs, and so it is if we just say "prefix FE80::" without specifying a plen.

> FE80::/10 says take the first 10 bits from those 128 bits.

YEs, and thus FE81::, FE82:: ... FEBF:: are all LL addresses.

I wonder why people only say FE80:: when talking LL address?

> FE80::/64 says take the first 64 bits from those 128 bits.
> FE80::/128 says take the first 128 bits from those 128 bits.
>
> All link local addresses have a prefix of FE80::/10.

But that does not mean that the notation of all LLs must start precisely 
with "FE80".

These notations could start with "FE81", "FE82" ... "FEBF".  Up to 
"FEC0" which is a GUA not an LL.

As such, a better way would be to say that LL addresses are of the form 
"FE##" where ## range from 81 to BF hexa.

> Link local addresses on ethernet are further restricted to
> have a prefix of FE80::/64.  They are a strict subset of
> all valid link local addresses.

YEs, and moreover, in this particular LL case, I suppose not only 
IP-over-Ethernet requires FE80::/64 but also SLAAC.  I.e. SLAAC on _any_ 
link (be it Ethernet or any other future link) seems to require the use 
of FE80::/64 prefix and does not allow for "FE81" or "FEBF".

An I-D describing the formation of LL addresses out of "FE##:://10" 
prefixes could make sense.

Alex