Subject: longest matching in default address selection

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Wed, 12 Dec 2001 09:43:13 +0900

I just implemented the destination address ordering described in
draft-ietf-ipngwg-default-addr-select-06.txt, and have another comment
about the longest matching.

The draft has the following text:

   Rule 9: Use longest matching prefix. 
   If CommonPrefixLen(DA, Source(DA)) > CommonPrefixLen(DB, 
   Source(DB)), then sort DA before DB. Similarly, if 
   CommonPrefixLen(DA, Source(DA)) < CommonPrefixLen(DB, Source(DB)), 
   then sort DB before DA. 

It is not clear to me how we should compare the match length when the
address family of A and the family of B.  If we converted IPv4
addresses to IPv4-mapped IPv6 addresses before the comparison, and the
policy table did not prefer a particular address family, then the
combination of IPv4 src and dst would tend to match longer than the
IPv6 src and dst (because the former would match at least 96 bits).

In any event, I don't see it meaningful to compare match lengths
between different families of addresses.  So, I'd propose to revise
the text like this:

   Rule 9: Use longest matching prefix. 
   When the address families of DA and DB are same,
   if CommonPrefixLen(DA, Source(DA)) > CommonPrefixLen(DB,
   Source(DB)), then sort DA before DB. Similarly, if
   CommonPrefixLen(DA, Source(DA)) < CommonPrefixLen(DB, Source(DB)),
   then sort DB before DA.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]
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