Re: Objection to draft-ietf-6man-rfc4291bis-07.txt

james woodyatt <jhw@google.com> Tue, 28 February 2017 20:09 UTC

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From: james woodyatt <jhw@google.com>
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Subject: Re: Objection to draft-ietf-6man-rfc4291bis-07.txt
Date: Tue, 28 Feb 2017 12:09:23 -0800
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To: David Farmer <farmer@umn.edu>
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On Feb 28, 2017, at 07:46, David Farmer <farmer@umn.edu>; wrote:
> 
> I think we should be saying OS MAY allow configurations other than /64, at least with manual configuration, and maybe DHCPv6 too.

This statement puzzles me because I see the distinction between subnet prefix assignment and interface address assignment as A) really important, and B) one of the ways that IPv6 improves greatly on IPv4. I really really really don’t want to lose that improvement.

p1. DHCPv6 does not assign subnet prefixes. DHCPv4 does, but that was a feature that we deliberately left out of DHCPv6. We put prefix *delegation* to routers into DHCPv6, which is a whole other discussion I don’t want to have right now, but we left subnet prefix assignment to the Router Discovery protocol, where I would say it rightly belongs.

p2. Manual configuration of IPv6 interface addresses and IPv6 subnet prefixes are separate and distinct processes. Either of them can be manually or automatically configured independently from the other in several of the host OS implementations I know about. This is not so often the case in IPv4 implementations.

In all these debates we are having about I-D.ietf-6man-rfc4291-07, I have been very careful to maintain that it is only *subnet* prefix assignment where I am insisting that /64 is a general requirement, with known exceptions for things like subnets that consist entirely of routers.

For that reason, I really don’t know how to respond when people insist that DHCPv6 is somehow involved in the discussion about subnet prefix assignment. Are they confused about how subnet prefixes are assigned in IPv6?

Yes, I recognize that Router Advertisement messages are defined with a packet format that allows routers to advertise PIO with L=1 and a Prefix Length > 64. I continue to maintain those should not be used. There are a lot of IPv6 applications, including quite a few Standards Track protocols published by IETF, that will simply truncate those prefixes to a /64 in every place where an IPv6 interface address must be mapped to a subnet prefix. And there are many SLAAC implementations that will decline to assign statelessly any interface addresses in a prefix advertised with A=1 and Prefix Length > 64. There are also a few implementations [broken ones, admittedly] kicking around that will simply ignore those PIO entirely. I think the successor to RFC 4291 needs to recognize that is reality and provide appropriate requirements language.


--james woodyatt <jhw@google.com <mailto:jhw@google.com>>