Subject: Re: Confirmation to advance: draft-ietf-6man-ipv6only-flag-05
From: Mark Smith <[email-masked]>
To: Sander Steffann <[email-masked]>
Cc: David Farmer <[email-masked]>, 6man WG <[email-masked]>
Date: Tue, 28 May 2019 10:31:46 +1000
List-Id: "IPv6 Maintenance Working Group \(6man\)" <ipv6.ietf.org>

Hi Sander,

On Tue, 28 May 2019 at 09:05, Sander Steffann <[email-masked]> wrote:
>
> Hi David,
>
<snip>

> > Again this is a real problem is some at least a few situations, and they are fairly common situations, just not in the home broadband world at least today.
>
> And there are solutions already available. Just pick one instead of inventing a new one...
>

That's really arguing against ever making things better. Plenty of
things exist that replaced existing, harder or more time costly and
usually manual mechanisms; DHCPv4, DNS, automatic transmissions and
power windows in cars, dishwashers and clothes washing machines etc.

Blocking 0x800/0x806 on individual link-layer ports would prevent IPv4
traffic including link-layer broadcasts on the link, but it doesn't
stop hosts initialising their IPv4 stack and trying to acquire an IPv4
address via DHCP. It requires link-layer per-host port configuration,
which would involve configuring 100s if not 1000s of individual ports.

RFC2563 should stop an IPv4 host sending IPv4 packets after the
initial DHCP transaction. However, it doesn't prevent link-layer
broadcast DHCPDISCOVERS, which is flooded to all other hosts on the
link and has to be processed by their main OS CPU to be discarded,
because the NIC can't filter them using the DA. This is where the main
battery cost will be when mobile nodes join the link, likely
continuously on big Wifi networks at e.g. conferences.

A link with a DHCPv4 server on it supporting RFC2563 is not an IPv6
Only link, because IPv4 is still present on the link. There should be
no IPv4 services/infrastructure and no IPv4 packets on an IPv6 Only
link, including IPv4 zero-conf link-local traffic. An IPv4 only
zero-conf printer should not work on an IPv6 Only link, intentionally.

The combination of those two mechanisms would go close to achieving
what this flag would, except that the first mechanism prevents the
second from working.

So individually or in combination, neither of these are a solution to
the problem of achieving an IPv6 only link.

Since the flooded link-layer broadcasts is probably more of a problem
than hosts trying to acquire addresses via DHCPv4, say the choice is
to block 0x800, 0x806 on all host ports.

Is that easier than setting a flag on one or usually no more than two
IPv6 routers to set this flag? I think it is far harder and far more
prone to error.

An IPv6 Only RA flag is better mechanism because it is both easier and
quicker to use than the existing alternatives, and also achieves
something that the existing potential alternatives can't.

Regards,
Mark.

> Cheers,
> Sander
>
>
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