Re: [v6ops] DHCP Option 108 Issue with Mac and iOS devices

David Farmer <> Fri, 24 November 2023 15:14 UTC

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From: David Farmer <>
Date: Fri, 24 Nov 2023 09:14:25 -0600
Message-ID: <>
To: Ole Troan <>
Cc: Lorenzo Colitti <>,
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Subject: Re: [v6ops] DHCP Option 108 Issue with Mac and iOS devices
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On Fri, Nov 24, 2023 at 08:42 Ole Troan <otroan=> wrote:

> > A full blown health check, isn’t easy, and may or may not be advisable.
> Nevertheless, if I understand the reported issue correctly, the IPv6 stack
> wasn’t even getting as far as configuring a globally scoped address.
> Therefore, even without a full blown health check, it seems advisable that
> if after some timeout, the IPv6 stack hasn’t successfully configured a
> globally scoped address, then maybe it should reconsider it’s DHCP Option
> 108 status.
> >
> > If there is no globally scoped IPv6 address available, both NAT64 and
> Native IPv6 are NOT going to work. So, then retrying IPv4 seems like a
> reasonable next step, especially since it got a DHCP Option 108 response,
> that is a clue that there is at least some level of IPv4 functionality
> available.
> >
> > So rather than calling this a health check, I’d call it a sanity check.
> For a host implementation trying to optimise the user experience, why
> wouldn’t always grabbing an IPv4 address if one is available be the right
> thing to do(tm)?
> I.e. not use 108.

You are assuming that native IPv4 is somehow more optimal than NAT64.
However, in most cases, native IPv4 is being provided by NAT44, and in my
opinion NAT44 and NAT64 are mostly equivalent. There are slight
differences, that in certain circumstances could produce an advantage for
one over the other. But overall those differences and therefore any
advantages are mostly trivial.

The biggest difference I see, is full dual stack with NAT44 has a higher
overall operational cost at scale than NAT64. However, at small scale even
this difference is trivial.

The reason to provide DHCP Option 108 is backwards compatibility with IPv4
only devices.  If the hosts make that too expensive by always using an IPv4
address, that could prematurely drive large scale infrastructure providers
to stop providing that backwards compatibility.

There are always trade offs, and what you say could be true if you only
look at it from the host’s perspective. However, looking at the total
system, I think DHCP Option 108 provides a nice solution to support, IPv4
only, dual stack, and IPv6 only, all on the same network and therefore a
long term evolution for the Internet.