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

From: [email-masked]
Date: Sat, 17 Oct 2020 05:53:00 -0700 (PDT)

A new Request for Comments is now available in online RFC libraries.

        RFC 8925

        Title:      IPv6-Only Preferred Option for DHCPv4 
        Author:     L. Colitti,
                    J. Linkova,
                    M. Richardson,
                    T. Mrugalski
        Status:     Standards Track
        Stream:     IETF
        Date:       October 2020
        Mailbox:    [email-masked], 
                    [email-masked], 
                    [email-masked],
                    [email-masked]
        Pages:      12
        Updates:    RFC 2563

        I-D Tag:    draft-ietf-dhc-v6only-08.txt

        URL:        https://www.rfc-editor.org/info/rfc8925

        DOI:        10.17487/RFC8925

This document specifies a DHCPv4 option to indicate that a host
supports an IPv6-only mode and is willing to forgo obtaining an IPv4
address if the network provides IPv6 connectivity. It also updates
RFC 2563 to specify DHCPv4 server behavior when the server receives a
DHCPDISCOVER not containing the Auto-Configure option but containing
the new option defined in this document.

This document is a product of the Dynamic Host Configuration Working Group of the IETF.

This is now a Proposed Standard.

STANDARDS TRACK: This document specifies an Internet Standards Track
protocol for the Internet community, and requests discussion and suggestions
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---

From: Roy Marples <[email-masked]>
Date: Sun, 25 Oct 2020 20:06:10 +0000

>          Title:      IPv6-Only Preferred Option for DHCPv4

Just reading through this and I find this in section 3.2

-------------------------------------------------------------------------------
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.

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.

Roy

---

From: Lorenzo Colitti <[email-masked]>
Date: Mon, 26 Oct 2020 09:52:16 +0900

> assigned IPv4 address until further DHCPv4 reconfiguration events.

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.

> Or just wait until it naturally expires? It's not clear what the expected

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.

Cheers,
Lorenzo

---

From: Michael Richardson <[email-masked]>
Date: Mon, 26 Oct 2020 07:07:59 -0400

Lorenzo Colitti <lorenzo=[email-masked]> wrote:
    >> 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.

and if an operator wanted to prevent the node to continue to use the IPv4
assigned,  then they'd need to configure to NAK the renew and force the
device back to the beginning.