netdev - Re: Subnet router anycast for FE80/10 ?

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Date:	Wed, 2 Nov 2011 16:27:43 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	David Lamparter <equinox@...c24.net>
Cc:	Andreas Hofmeister <andi@...lax.com>,
	David Lamparter <equinox@...c24.net>, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org
Subject: Re: Subnet router anycast for FE80/10 ?

netdev-owner@...r.kernel.org wrote on 11/02/2011 02:53:59 PM:

> From: David Lamparter <equinox@...c24.net>

> Going back to Andreas's original question about Subnet-Router Anycast
> for fe80::/64 (or /10), RFC 4291 says
>    +------------------------------------------------+----------------+
>    |                   subnet prefix                | 00000000000000 |
>    +------------------------------------------------+----------------+
> 
>    The "subnet prefix" in an anycast address is the prefix that
>    identifies a specific link.
> 
> But fe80::/64 does not identify a specific link, as it is link-local and
> would specify all links but not one specifically. So, fe80:: is not a
> Subnet-Router anycast address, I'd say.

fe80:: is in fact a subnet-router anycast address because it's a valid
prefix with all-0's host part. Adding it, as linux does,  simply means
a host can use "fe80::" to get an answer from any router on a particular
link. That might be useful, e.g., if a host wants routing protocol 
information
from a directly attached router.

It isn't unique on a host, but all LL addresses require a scope_id to
identify an interface, anyway, so there is no ambiguity. Any multihomed
v6 host will have multiple fe80/10 routes -- one for each interface--
too, used for receiving packets. Those routes and those anycast addresses
only matter for input processing, as is true for all LL addresses.

So, there's no particular reason I see to treat LL as a special case and
exclude them. I haven't seen any RFC wording that forbids it, though the
implementation predates RFC4291.

You certainly don't have to use them in any application if you don't like 
it. :-)

                                                                +-DLS

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