Re: [dhcwg] RFC 8925 on IPv6-Only Preferred Option for DHCPv4

Lorenzo Colitti <> Mon, 26 October 2020 00:52 UTC

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From: Lorenzo Colitti <>
Date: Mon, 26 Oct 2020 09:52:16 +0900
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To: Roy Marples <>
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Subject: Re: [dhcwg] RFC 8925 on IPv6-Only Preferred Option for DHCPv4
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On Mon, Oct 26, 2020 at 5:06 AM Roy Marples <roy=> wrote:

> The IPv6-Only Preferred option SHOULD be included in the Parameter Request
> List
> in DHCPREQUEST messages (after receiving a DHCPOFFER without this option,
> for an
> INIT-REBOOT, or when renewing or rebinding a leased address). If the
> DHCPv4
> server responds with a DHCPACK that includes the IPv6-Only Preferred
> option, the
> client's behavior depends on the client's state. If the client is in the
> INIT-REBOOT state, it SHOULD stop the DHCPv4 configuration process or
> disable
> the IPv4 stack completely for V6ONLY_WAIT seconds or until the network
> attachment event, whichever happens first. It also MAY send a DHCPRELEASE
> message. If the client is in any other state, it SHOULD continue to use
> the
> assigned IPv4 address until further DHCPv4 reconfiguration events.
> -------------------------------------------------------------------------------
> It's the last line of the paragraph which is of interest. It implies that
> this
> option only works in the INIT-REBOOT state. The lack of the SELECTING
> state is
> pretty glaring but a minor omission.

While it's true that the SELECTING state is not explicitly mentioned, I
think it's implicitly dealt with by this text:

   If the client waits for multiple DHCPOFFER responses and selects one
   of them, it MUST follow the processing for the IPv6-Only Preferred
   option based on the selected response.  A client MAY use the presence
   of the IPv6-Only Preferred option as a selection criterion.

> Am I correct in thinking that RENEWING and REBINDING states pretty much
> ignore
> this option if it was absent and is now present? Or is the intent to stop
> using
> the address when in these states as they are "DHCPv6 reconfiguaration
> events"?
> Or just wait until it naturally expires? It's not clear what the expected
> action
> here should be.

I think this is documented in the text that you quoted above. If the client
is in RENEWING or REBINDING, it SHOULD continue to use the assigned IPv4
address. That means accept the DHCPACK, continue to use the address, and
later, when the appropriate timers fire, enter RENEWING and REBINDING as
specified by RFC 2131.