Re: [ieee-ietf-coord] Rationale for 0xFFFE as octets 4 and 5 pf EUI-64?

"Pascal Thubert (pthubert)" <pthubert@cisco.com> Tue, 26 June 2018 10:57 UTC

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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: Charlie Perkins <charles.perkins@earthlink.net>
CC: "ieee-ietf-coord@ietf.org" <ieee-ietf-coord@ietf.org>
Thread-Topic: [ieee-ietf-coord] Rationale for 0xFFFE as octets 4 and 5 pf EUI-64?
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Subject: Re: [ieee-ietf-coord] Rationale for 0xFFFE as octets 4 and 5 pf EUI-64?
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Hello Charlie, 

As I understand RFC 4944, the 48bits address is built on pan-id (2 octets) concatenated with 0s (2 octets) concatenated with short address (2 octets).

So no, two devices on different PANs that happen to have the same 16-bit short address cannot end up with the same IPv6 address.

Take care,

Pascal

> -----Original Message-----
> From: Charlie Perkins <charles.perkins@earthlink.net>
> Sent: lundi 25 juin 2018 20:45
> To: Pascal Thubert (pthubert) <pthubert@cisco.com>
> Cc: ieee-ietf-coord@ietf.org
> Subject: Re: [ieee-ietf-coord] Rationale for 0xFFFE as octets 4 and 5 pf EUI-64?
> 
> Hello Pascal,
> 
> Yes, I saw that specification, but it seemed to me that it still has the same
> problem as I mentioned in the previous email.  By RFC 4944, two devices on
> different PANs that happen to have the same 16-bit short address could end
> up with the same IPv6 address.
> 
> It doesn't have to be that way, to my understanding.
> 
> Regards,
> Charlie P.
> 
> 
> On 6/25/2018 11:20 AM, Pascal Thubert (pthubert) wrote:
> > Hello Charlie
> >
> > Section 6 of RFC 4944 builds an EUI 48 out of panid :0: short address and
> from there an interface ID for IPv6. Is that what you are after?
> >
> >
> > Pascal
> >
> >> Le 25 juin 2018 à 19:46, Charlie Perkins <charles.perkins@earthlink.net> a
> écrit :
> >>
> >> Hello folks,
> >>
> >> The reason why I asked about why 0xFFFE was chosen, was because I am
> trying to understand how best to do something similar for 802.15 devices that
> have a PAN-ID and a 16-bit short address as in IEEE 1901.2.  Or, if someone
> has already done it, then even better.
> >>
> >> What I saw was to make the PAN-ID into the leading 16 bits, by analogy to
> making the OUI into the leading 24 bits.  But the OUI already had bits set
> aside for U/L and I/G, whereas the PAN ID does not.  So, setting the U/L bit
> would effectively change the PAN-ID and that seems wrong to me.  A similar
> problem exists already in IEEE 1901.1 because the NID (Network ID) is made
> into the leading 24 bits of the EUI-64.  So, two devices on different Networks
> that happen to have the same 16-bit equipment identifier could end up with
> the same IPv6 address.
> >>
> >> I have looked in a number of places for an existing design, or for
> information to guide the design, so far coming up empty handed.
> >>
> >> Thanks for any help!
> >>
> >> Regards,
> >> Charlie P.
> >>
> >>
> >>> On 6/24/2018 9:22 PM, Donald Eastlake wrote:
> >>> OK, it would have been better if I had said "converts the format of
> >>> X to Y" instead of "converts X to Y".
> >>>
> >>> In any case, the original question from Charlie had nothing to do
> >>> with why or how a larger 64 bit MAC address space would be of
> >>> benefit or what its goals were. I believe he was just asking where
> >>> the 0xFFFE came from that is actually and currently used in, for
> >>> example, construction of some IPv6 addresses from 48 bit MAC
> >>> addresses. As far as I know it came from the IEEE. See for example
> >>> http://standards.ieee.org/develop/regauth/tut/eui.pdf which, while
> >>> it deprecates this "mapping", still documents FF-FF and FF-FE as
> >>> insertions. An earlier IEEE tutorial which, as I call, documented
> >>> this mapping without any deprecation, seems to no longer be on the web.
> >>>
> >>> Thanks,
> >>> Donald
> >>> ===============================
> >>>   Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
> >>>   155 Beaver Street, Milford, MA 01757 USA
> >>>   d3e3e3@gmail.com
> >>>
> >>>
> >>>> On Sun, Jun 24, 2018 at 9:27 PM, Geoff Thompson
> <thompson@ieee.org> wrote:
> >>>> Inserting "0xFFFF to convert a MAC-48 to and EUI-64" or "0xFFFE to
> >>>> convert an EUI-48 to an EUI-64"
> >>>> does not actually "convert" anything in a useful way except to to
> >>>> make a "EUI-48" readable in a 64 bit system.
> >>>>
> >>>> The purpose of developing EUI-64 was to have a larger address space
> >>>> that could be used for (among other things) software instances.
> >>>> Having a fixed 16 bit value in a 64 bit address does nothing
> >>>> towards achieving that goal or slowing down the usage of 48 bit
> >>>> addresses to extend the life of 802 physical networks.
> >>>>
> >>>> Geoff Thompson
> >>>>
> >>>>
> >>>> On Jun 24, 2018, at 5:52 PMPDT, Donald Eastlake <d3e3e3@gmail.com>
> wrote:
> >>>>
> >>>> Hi Charlie,
> >>>>
> >>>> As I recall, there is/was this distinction between MAC-48 and
> >>>> EUI-48 addresses. I think MAC-48 was just for hardware and EUI-48
> >>>> was for other devices and software. Anyway, you inserted 0xFFFF to
> >>>> convert a MAC-48 to and
> >>>> EUI-64 and 0xFFFE to convert an EUI-48 to an EUI-64. The RFCs that
> >>>> talk about extending a 48 bit address to 64 bits to use as the low
> >>>> order bits of an IPv6 address say that 0xFFFE was used by mistake
> >>>> and that 0xFFFF should have been used (see for example the Note on
> >>>> page 22 of RFC 4291) but it was decided to stick with 0xFFFE for that
> purpose. Hope this helps.
> >>>>
> >>>> Thanks,
> >>>> Donald
> >>>> ===============================
> >>>>   Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
> >>>>   155 Beaver Street, Milford, MA 01757 USA
> >>>>   d3e3e3@gmail.com
> >>>>
> >>>> On Fri, Jun 22, 2018 at 9:54 PM, Charlie Perkins
> >>>> <charles.perkins@earthlink.net> wrote:
> >>>>> Hello folks,
> >>>>>
> >>>>> Does anyone here remember why 0xFFFE were chosen to be the filler
> >>>>> bits (i.e., bytes 4 and 5 of 8) when expanding a 48-bit MAC address to
> be EUI-64?
> >>>>> It is not explained in RFC 2464.
> >>>>>
> >>>>> Or maybe there was not a reason...?
> >>>>>
> >>>>> Thanks in advance,
> >>>>> Charlie P.
> >>>>>
> >>>>> _______________________________________________
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