Subject: RFC2526 - actually honoured?

From: Karl Auer <[email-masked]>
Date: Mon, 21 Feb 2011 20:29:54 +1100

Does anyone know if (and if so in what way) RFC2526 is honoured in the
real world?

It's standards track, dating from 1999, but I'm not sure I've seen
anyone avoiding the stipulated reserved addresses. Addresses generated
by SLAAC from a MAC address may well have some of the last seven bits
set, for example.

   "The construction of a reserved subnet anycast address depends on the
   type of IPv6 addresses used within the subnet, as indicated by the
   format prefix in the addresses.  In particular, for IPv6 address
   types required to have 64-bit interface identifiers in EUI-64 format,
   the universal/local bit MUST be set to 0 (local) in all reserved
   subnet anycast addresses, to indicate that the interface identifier
   in the address is not globally unique.  IPv6 addresses of this type
   are currently specified to be those having format prefixes 001
   through 111, except for Multicast Addresses (1111 1111) [3]."

Just interested to know if these supposedly reserved anycast addresses
are something one should actually be avoiding in practice or not.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer ([email-masked])                   +61-2-64957160 (h)
http://www.biplane.com.au/kauer/                   +61-428-957160 (mob)

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From: Mark Smith <[email-masked]>
Date: Mon, 21 Feb 2011 20:31:15 +1030

Hi Karl,

On Mon, 21 Feb 2011 20:29:54 +1100

> are something one should actually be avoiding in practice or not.

Reading through the above text and the subsequent text in the RFC a
few times, I don't think it is saying that the subnet anycast address is
derived from the EUI-64 identifier i.e. your MAC address with SLAAC.
I think the word "format" after "EUI-64" means it isn't talking about
a node's EUI-64 identifiers, rather I think all it is saying that if you
require reserved subnet anycast addresses with 64 bit Interface
Identifiers, then the format is -

   |              64 bits            |      57 bits     |   7 bits   |
   +---------------------------------+------------------+------------+
   |           subnet prefix         | 1111110111...111 | anycast ID |
   +---------------------------------+------------------+------------+
                                     |   interface identifier field  |

where the 71st bit is switched off (i.e. 6 on bits, 1 off bit, then 50
on bits for the 57 bits portion).

(It's a bit scary thinking about the code changes to be deployed if
EUI-64 a.k.a. MAC address end-octets needed to be zeroed before use in
SLAAC!).

Regards,
Mark.

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From: Karl Auer <[email-masked]>
Date: Mon, 21 Feb 2011 21:29:51 +1100

> derived from the EUI-64 identifier i.e. your MAC address with SLAAC.

I think it is saying that the last seven bits of every 64-bit interface
ID where the seventh bit is zero are reserved for anycast addresses! On
the face of it, that conflicts directly with SLAAC.

The only exceptions are multicast addresses and addresses beginning with
(binary) 000.

That seems to be what the actual RFC is saying. What interests me,
though, is not so much what exactly it says but whether anyone actually
(i.e., in practice) *cares* what it says :-)

Regards, K.