Subnet router anycast for FE80/10 ?

Hi,

I noticed that once forwarding has been enabled on an interface, there is a "subnet router anycast address" for the link-local address prefix FE80/10.

This address seems not to be explicitly mentioned in any RFC, but RFC 4291 says "All routers are required to support the Subnet-Router anycast addresses for the subnets to which they have interfaces."

In the sense that a Linux router actually has an address FE80/10 on each ipv6 enabled interface, it seems to be correct to also have FE80:: as an anycast address on all interfaces which have ipv6 and forwarding enabled.

But then, FE80/10 is not actually supposed to be routed at all and so a router cannot not really be a router for that particular subnet ?

Or is "FE80::" just supposed to be the anycast equivalent for the "all routers" multicast address ff02::2 ?

Maybe someone on this list could enlighten me.

Ciao
Andi

(Please note that it is fe80::/64 is used, not /10)

That this directly contradicts RFC 2526 which specifies the subnet-router anycast address to be either ::ffff:ffff:ffff:ff80 or ::fcff:ffff:ffff:ff80 depending on the phase of the moon (well, interface type actually, but same thing. Also, the /64 <> /10 distinction would matter here.)

For even more confusion, look at
http://www.iana.org/assignments/ipv6-anycast-addresses/ipv6-anycast-addresses.xml

but the only point why I'm mentioning this at all is that if someone implemented this, they might've noticed the colliding specifications, and there would be an Errata.

This question isn't really relevant, because...

... it's actually fairly hard to implement this at all. The idea of "Anycast" is that even if you have 1000 routers, only one router will receive the packet. The network is supposed to magically take care of that, but in reality this only works with Layer 3/IPv6 routing.

So, if you're /actually on/ the subnet yourself, the RFC
- either expects the ethernet switch to implement anycast (...)
- or implies the need for some anycast resolution protocol (which is possible of course, but there would first need to actually /be/ some such protocol; while some quick googling tells me such things exist, they certainly belong in the esoterics drawer).

... which turns this entire thing into a really bad joke.

I would recommend forgetting that these anycast addresses exist at all. If you need to reach a local on-subnet router, just use ff02::2 and randomly pick one that answers.

-David

[email protected] wrote on 11/02/2011 08:38:09 AM:

anycast

The subnet-router anycast address is defined in section 2.6.1 of
RFC 4291
to be "all 0's" for the prefix. The definition above is for reserved anycast
addresses. RFC 2526 says "IPv6 defines a required Subnet-Router anycast address [3]
for all routers within a subnet prefix, and allows additional anycast addresses to
be taken from the unicast address space. This document defines an additional set
of reserved anycast addresses...".

The host implementation is very straightforward. Not every host
on a segment has to use the *same* host for an anycast address (it's kind of the point that it won't, in fact). A host simply needs to do a solicitation for the anycast address and keep the first one that answers (by definition, the "closest").

                                                                +-DLS

Argh. I got thoroughly confused. Please ignore everything I said.

Right. Sorry. I was half-asleep when I wrote my previous mail and clearly didn't think through the entire problem. I should probably stop writing mails when not at least 80% awake :).

Neighbor Discovery can indeed select one of the anycast hosts and talk to it using its lower-layer address, and no other host should receive/process the packet.

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.

Hoping I was awake enough this time...

-David

[email protected] wrote on 11/02/2011 02:53:59 PM:

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

[email protected] wrote on 10/31/2011 01:22:41 PM:

I think it's just a matter of not making an exception for LL addresses. Forwarding is for the interface, not for individual prefixes. If forwarding is enabled on the interface, anycast addresses for all its prefixes are added.

It could be used to send to "any router" on a subnet, of course.

                                                        +-DLS