Re: IPv6 Formal Anycast Addresses and Functional Anycast Addresses (Fwd: New Version Notification for draft-smith-6man-form-func-anycast-addresses-01.txt)

Toerless Eckert <> Sun, 03 November 2019 21:27 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 06ED9120123 for <>; Sun, 3 Nov 2019 13:27:21 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -3.95
X-Spam-Status: No, score=-3.95 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.25, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id odOCI1qzdQkK for <>; Sun, 3 Nov 2019 13:27:18 -0800 (PST)
Received: from ( []) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id C990E120120 for <>; Sun, 3 Nov 2019 13:27:17 -0800 (PST)
Received: from ( [IPv6:2001:638:a000:4134::ffff:52]) by (Postfix) with ESMTP id 875A054802E; Sun, 3 Nov 2019 22:27:12 +0100 (CET)
Received: by (Postfix, from userid 10463) id 7C0B8440015; Sun, 3 Nov 2019 22:27:12 +0100 (CET)
Date: Sun, 3 Nov 2019 22:27:12 +0100
From: Toerless Eckert <>
To: Fred Baker <>
Cc: Mark Smith <>, 6MAN <>
Subject: Re: IPv6 Formal Anycast Addresses and Functional Anycast Addresses (Fwd: New Version Notification for draft-smith-6man-form-func-anycast-addresses-01.txt)
Message-ID: <>
References: <> <> <> <>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <>
User-Agent: Mutt/1.10.1 (2018-07-13)
Archived-At: <>
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "IPv6 Maintenance Working Group \(6man\)" <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Sun, 03 Nov 2019 21:27:21 -0000

It is somewhat architecturally dissatisfying that (AFAIK) we seem to need to
resolve limitations of anycast addresses at the transport layer,
e.g.: redirecting connection requests to an anycast address to a
unicast address of the transport responder. If initiators would know an address is
an anycast address, they could use some TBD network layer (ICMP) extension
to do that resolution independent of individual transport protocols.

And the network layer would only know it needed to do this if there was
a way for the initiator to identify an address as an anycast address
AFAIK (can't think of a simpler way).


On Sun, Nov 03, 2019 at 01:59:24PM -0500, Fred Baker wrote:
> On Nov 3, 2019, at 9:23 AM, Nick Hilliard <> wrote:
> > If you create an anycast protocol which has characteristics which are sufficiently different to unicast that it requires a separate addressing schema, then by all means it would be appropriate to create an addressing schema to fit in with this.  The determinant here would be that global unicast addresses would not be usable for this protocol. Until then, a separate address block is mostly a matter of aesthetics.
> I would agree. I did some poking around to identify anycast address groups. The IANA has records for three. RFC 4291 has a fourth, which is subnet anycast which is supposed to get a packet to a router I'm not sure I can say how widely deployed any of those are. 
> RFC 2526             Mobile IPv6 Home-Agents anycast
> ETSI EN 302 636-6-1  IPv6 over GeoNetworking geographic anycast	
> RFC 4291             IPv6 Anycast Subnet-Router Anycast Address
> On the other hand, there are a number of unicast addresses in daily use worldwide as anycast, which are the addresses one uses to access the DNS root. Collected statistics tell us that on the order of 10% of DNS requests to the root use IPv6, and the rest are IPv4. So I would say that the use of unicast addresses as anycast has a strong supporting case.
> The one use case that your draft mentions that seemed to be new was that of a network operator that wanted to deploy an anycast service, but only to its customers. It, however, seemed to be hypothetical. Do you know of operators or services that have that requirement?
> In other words, I'm wondering whether there is a problem being solved, or an architectural preference.
> --------------------------------------------------------------------
> IETF IPv6 working group mailing list
> Administrative Requests:
> --------------------------------------------------------------------