Re: question about draft-ietf-6man-rfc3484bis-06.txt and CommonPrefixLen()

Karl Auer <kauer@biplane.com.au> Sun, 29 July 2012 01:14 UTC

Return-Path: <kauer@biplane.com.au>
X-Original-To: ipv6@ietfa.amsl.com
Delivered-To: ipv6@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id B750A21F84F5 for <ipv6@ietfa.amsl.com>; Sat, 28 Jul 2012 18:14:59 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.999
X-Spam-Level:
X-Spam-Status: No, score=-1.999 tagged_above=-999 required=5 tests=[BAYES_00=-2.599, J_CHICKENPOX_21=0.6]
Received: from mail.ietf.org ([12.22.58.30]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id APfu9zLzwTHE for <ipv6@ietfa.amsl.com>; Sat, 28 Jul 2012 18:14:59 -0700 (PDT)
Received: from ipmail06.adl6.internode.on.net (ipmail06.adl6.internode.on.net [IPv6:2001:44b8:8060:ff02:300:1:6:6]) by ietfa.amsl.com (Postfix) with ESMTP id BD65D21F84B2 for <ipv6@ietf.org>; Sat, 28 Jul 2012 18:14:56 -0700 (PDT)
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: ApMBAJeNFFCWZX+7/2dsb2JhbAANOIVztxMBAQEDASNbCwsaAiYCAlcZiAemPW6SFIEhgSGMVIIKgRIDoFaHcQ
Received: from eth4284.nsw.adsl.internode.on.net (HELO [192.168.1.200]) ([150.101.127.187]) by ipmail06.adl6.internode.on.net with ESMTP; 29 Jul 2012 10:44:53 +0930
Subject: Re: question about draft-ietf-6man-rfc3484bis-06.txt and CommonPrefixLen()
From: Karl Auer <kauer@biplane.com.au>
To: ipv6@ietf.org
In-Reply-To: <1343085852.2796.201.camel@karl>
References: <1343085852.2796.201.camel@karl>
Content-Type: text/plain; charset="UTF-8"
Date: Sun, 29 Jul 2012 11:14:49 +1000
Message-ID: <1343524489.3497.94.camel@karl>
Mime-Version: 1.0
X-Mailer: Evolution 2.30.3
Content-Transfer-Encoding: 7bit
X-BeenThere: ipv6@ietf.org
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: "IPv6 Maintenance Working Group \(6man\)" <ipv6.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/ipv6>, <mailto:ipv6-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/ipv6>
List-Post: <mailto:ipv6@ietf.org>
List-Help: <mailto:ipv6-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/ipv6>, <mailto:ipv6-request@ietf.org?subject=subscribe>
X-List-Received-Date: Sun, 29 Jul 2012 01:14:59 -0000

A few days ago, I asked about this draft as below. I haven't seen a
response, but the question still seems fair.

> I don't fully understand this change (from section Appendix B):
> 
> 1.  Changed the definition of CommonPrefixLen() to only compare bits
>        up to the source address's prefix length.  The previous
>        definition used the entire source address, rather than only its
>        prefix. As a result, when a source and destination addresses
>        had the same prefix, common bits in the interface ID would
>        previously result in overriding DNS load balancing [RFC1794] by
>        forcing the destination address with the most bits in common to
>        be always chosen.  The updated definition allows DNS load
>        balancing to continue to be used as a tie breaker.
> 
> I can see this for destination address selection, where you are
> working from a candidate set provided by a DNS server.
> 
> But I don't see it for source address selection. If you have multiple
> source address candidates in the same prefix, they will "fall through"
> Rule 8 and the implementation then has to make a choice anyway, and
> that choice doesn't seem able to be related to DNS load balancing.
> 
> That is, the design rationale for the new CommonPrefixLen() doesn't
> seem to apply to source address selection, which leads me to think
> that for source address selection, CommonPrefixLen() should not stop
> at the source address prefix length.
> 
> Am I missing something obvious here?
> 
> Regards, K.
>  
-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer@biplane.com.au)
http://www.biplane.com.au/kauer
http://www.biplane.com.au/blog

GPG fingerprint: AE1D 4868 6420 AD9A A698 5251 1699 7B78 4EEE 6017
Old fingerprint: DA41 51B1 1481 16E1 F7E2 B2E9 3007 14ED 5736 F687