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draft-v6only-dhc.xml
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draft-v6only-dhc.xml
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<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY RFC2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC2131 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2131.xml">
<!ENTITY RFC2132 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2132.xml">
<!ENTITY RFC2563 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2563.xml">
<!ENTITY RFC3927 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3927.xml">
<!ENTITY RFC4039 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4039.xml">
<!ENTITY RFC4861 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4861.xml">
<!ENTITY RFC4957 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4957.xml">
<!ENTITY RFC6052 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6052.xml">
<!ENTITY RFC6146 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6146.xml">
<!ENTITY RFC6147 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6147.xml">
<!ENTITY RFC6877 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6877.xml">
<!ENTITY RFC8174 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.8174.xml">
]>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<!-- used by XSLT processors -->
<!-- For a complete list and description of processing instructions (PIs),
please see http://xml.resource.org/authoring/README.html. -->
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(Here they are set differently than their defaults in xml2rfc v1.32) -->
<?rfc strict="yes" ?>
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<?rfc tocdepth="4"?>
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<?rfc symrefs="yes"?>
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(using these PIs as follows is recommended by the RFC Editor) -->
<?rfc compact="yes" ?>
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<rfc ipr="trust200902"
updates="2563"
obsoletes=""
category="std"
docName="draft-ietf-dhc-v6only-08">
<!-- category values: std, bcp, info, exp, and historic -->
<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title>IPv6-Only-Preferred Option for DHCPv4</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<author fullname="Lorenzo Colitti" initials="L." surname="Colitti">
<organization>Google</organization>
<address>
<postal>
<street>Shibuya 3-21-3</street>
<city>Shibuya</city>
<region>Tokyo</region>
<code>150-0002</code>
<country>JP</country>
</postal>
<phone></phone>
<email>[email protected]</email>
</address>
</author>
<author fullname="Jen Linkova" initials="J." surname="Linkova">
<organization>Google</organization>
<address>
<postal>
<street>1 Darling Island Rd</street>
<city>Pyrmont</city>
<region>NSW</region>
<code>2009</code>
<country>AU</country>
</postal>
<phone></phone>
<email>[email protected]</email>
</address>
</author>
<author fullname="Michael C. Richardson" initials="M."
surname="Richardson">
<organization abbrev="Sandelman">Sandelman Software Works</organization>
<address>
<email>[email protected]</email>
<uri>http://www.sandelman.ca/</uri>
</address>
</author>
<author fullname="Tomek Mrugalski" initials="T." surname="Mrugalski">
<organization abbrev="ISC">Internet Systems Consortium, Inc.</organization>
<address>
<postal>
<street>950 Charter Street</street>
<city>Redwood City</city>
<region>CA</region>
<code>94063</code>
<country>USA</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<date/>
<!-- If the month and year are both specified and are the current ones, xml2rfc will fill
in the current day for you. If only the current year is specified, xml2rfc will fill
in the current day and month for you. If the year is not the current one, it is
necessary to specify at least a month (xml2rfc assumes day="1" if not specified for the
purpose of calculating the expiry date). With drafts it is normally sufficient to
specify just the year. -->
<!-- Meta-data Declarations -->
<area>Internet</area>
<workgroup>Dynamic Host Configuration</workgroup>
<!-- WG name at the upperleft corner of the doc,
IETF is fine for individual submissions.
If this element is not present, the default is "Network Working Group",
which is used by the RFC Editor as a nod to the history of the IETF. -->
<keyword>template</keyword>
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files in a meta tag but they have no effect on text or nroff
output. If you submit your draft to the RFC Editor, the
keywords will be used for the search engine. -->
<abstract>
<t>
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 RFC2563 to specify the DHCPv4 server behavior when the server receives a DHCPDISCOVER not containing the Auto-Configure option but containing the new option defined in this document.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
One of the biggest challenges of deploying IPv6-only LANs is that such networks might contain rather heterogeneous collection of hosts.
While some hosts are capable of operating in IPv6-only mode (either because the OS and all applications are IPv6-only capable or because the host has some form of 464XLAT <xref target="RFC6877"/> deployed), others might still have IPv4 dependencies and need IPv4 addresses to operate properly.
To incrementally rollout IPv6-only, network operators might need to provide IPv4 on demand whereby a host receives an IPv4 address if it needs it, while IPv6-only capable hosts (such as modern mobile devices) are not allocated IPv4 addresses.
Traditionally that goal is achieved by placing IPv6-only capable devices into a dedicated IPv6-only network segment or WiFi SSID, while dual-stack devices reside in another network with IPv4 and DHCPv4 enabled.
However such an approach has a number of drawbacks, including but not limited to:
<list style="symbols">
<t>
Doubling the number of network segments leads to operational complexity and performance impact, for instance due to high memory utilization caused by an increased number of ACL entries.
</t>
<t>
Placing a host into the correct network segment is problematic.
For example, in the case of 802.11 Wi-Fi the user might select the wrong SSID.
In the case of wired 802.1x authentication the authentication server might not have all the information required to make the correct decision and choose between an IPv6-only and a dual-stack VLAN.
</t>
</list>
</t>
<t>
It would be beneficial for IPv6 deployment if operators could implement IPv6-mostly (or IPv4-on-demand) segments where IPv6-only hosts co-exist with legacy dual-stack devices.
The trivial solution of disabling IPv4 stack on IPv6-only capable hosts is not feasible as those clients must be able to operate on IPv4-only networks as well.
While IPv6-only capable devices might use a heuristic approach to learning if the network provides IPv6-only functionality and stop using IPv4 if it does, such an approach might be practically undesirable.
One important reason is that when a host connects to a network, it does not know if the network is IPv4-only, dual-stack or IPv6-only.
To ensure that the connectivity over whatever protocol is present becomes available as soon as possible the host usually starts configuring both IPv4 and IPv6 immediately.
If hosts were to delay requesting IPv4 until IPv6 reachability is confirmed, that would penalize IPv4-only and dual-stack networks, which does not seem practical.
Requesting IPv4 and then releasing it later, after IPv6 reachability is confirmed, might cause user-visible errors as it would be disruptive for applications which have started using the assigned IPv4 address already.
Instead it would be useful to have a mechanism which would allow a host to indicate that its request for an IPv4 address is optional and a network to signal that IPv6-only functionality (such as NAT64, <xref target="RFC6146"/>) is available.
The proposed solution is to introduce a new DHCPv4 option which a client uses to indicate that it does not need an IPv4 address if the network provides IPv6-only connectivity (as NAT64 and DNS64).
If the particular network segment provides IPv4-on-demand such clients would not be supplied with IPv4 addresses, while on IPv4-only or dual-stack segments without NAT64 services IPv4 addresses will be provided.
</t>
<t>
<xref target="RFC2563"/> introduces the Auto-Configure DHCPv4 option and describes DHCPv4 servers behavior if no address is chosen for a host. This document updates <xref target="RFC2563"/> to modify the server behavior if the DHCPOFFER contains the IPv6-only Preferred option.
</t>
<section title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.</t>
</section>
<section title="Terminology">
<t>
Dual-stack network or device: a network or device which has both versions of the Internet Protocol (IPv4 and IPv6) enabled and operational.
</t>
<t>
IPv6-only capable host: a host which does not require an IPv4 address and can operate on IPv6-only networks.
More precisely, IPv6-only capability is specific to a given interface of the host: if some application on a host require IPv4 and 464XLAT CLAT <xref target="RFC6877"/> is only enabled on one interface, the host is IPv6-only capable if connected to a NAT64 network via that interface. This document implies that IPv6-only capable hosts reach IPv4-only destinations via a NAT64 service provided by the network. <xref target="v6onlydef" /> discusses hypothetical scenarios of other transition technologies being used.
</t>
<t>
IPv4-requiring host: a host which is not IPv6-only capable and can not operate in an IPv6-only network providing NAT64 service.
</t>
<t>
IPv4-on-demand: a deployment scenario where end hosts are expected to operate in IPv6-only mode by default and IPv4 addresses can be assigned to some hosts if those hosts explicitly opt-in to receiving IPv4 addresses.
</t>
<t>
IPv6-mostly network: a network which provides NAT64 (possibly with DNS64) service as well as IPv4 connectivity and allows coexistence of IPv6-only, dual-stack and IPv4-only hosts on the same segment.
Such deployment scenario allows operators to incrementally turn off IPv4 on end hosts, while still providing IPv4 to devices which require IPv4 to operate.
But, IPv6-only capable devices need not be assigned IPv4 addresses.
</t>
<t>
IPv6-only mode: a mode of operation when a host acts as an IPv6-only capable host and does not have IPv4 addresses assigned (except that IPv4 link-local addresses <xref target="RFC3927"/> may have been configured).
</t>
<t>
IPv6-only network: a network which does not provide routing functionality for IPv4 packets.
Such networks may or may not allow intra-LAN IPv4 connectivity.
IPv6-only network usually provides access to IPv4-only resources via NAT64 <xref target="RFC6146"/>.
</t>
<t>
NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers <xref target="RFC6146"/>.
</t>
<t>
RA: Router Advertisement, a message used by IPv6 routers to advertise their presence together
with various link and Internet parameters <xref target="RFC4861"/>.
</t>
<t>
DNS64: a mechanism for synthesizing AAAA records from A records <xref target="RFC6147"/>.
</t>
<t>
Network attachment event: A Link Up event, as described by <xref target="RFC4957" /> which results in a host detecting an available network.
</t>
<t>
Disabling IPv4 stack on the host interface: the host behavior when the host:
<list style="symbols">
<t>
does not send any IPv4 packets from that interface,
</t>
<t>
drops all IPv4 packets received on that interface and
</t>
<t>
does not forward any IPv4 packets to that interface.
</t>
</list>
</t>
</section>
</section>
<section title="Reasons to Signal IPv6-Only Support in DHCPv4 Packets">
<t>
For networks which contain a mix of both IPv6-only capable hosts and IPv4-requiring hosts, and which utilize DHCPv4 for configuring the IPv4 network stack on hosts, it seems natural to leverage the same protocol to signal that IPv4 is discretional on a given segment.
An ability to remotely disable IPv4 on a host can be seen as a new denial-of-service attack vector.
The proposed approach limits the attack surface to DHCPv4-related attacks without introducing new vulnerable elements.
</t>
<t>
Another benefit of using DHCPv4 for signaling is that IPv4 will be disabled only if both the client and the server indicate IPv6-only capability.
It allows IPv6-only capable hosts to turn off IPv4 only upon receiving an explicit signal from the network and operate in dual-stack or IPv4-only mode otherwise.
In addition, the proposed mechanism does not introduce any additional delays to the process of configuring IP stack on hosts.
If the network does not support IPv6-only/IPv4-on-demand mode, an IPv6-only capable host would configure an IPv4 address as quickly as on any other host.
</t>
<t>
Being a client/server protocol, DHCPv4 allows IPv4 to be selectively disabled on a per-host basis on a given network segment.
Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the same LAN would not only allow network administrators to preserve scarce IPv4 addresses but would also drastically simplify incremental deployment of IPv6-only networks, positively impacting IPv6 adoption.
</t>
</section>
<section title="IPv6-Only Preferred Option">
<section anchor="Format" title="Option format">
<figure align="center" anchor="fig_Option"
title="IPv6-Only Preferred Option Format">
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (contd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>Fields:</t>
<texttable style="none">
<ttcol></ttcol>
<ttcol></ttcol>
<c>Code: </c> <c> 8-bit identifier of the IPv6-Only Preferred option code as assigned by IANA: TBD.
The client includes the Code in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST messages as described in <xref target="v4client"/>.</c>
<c>Length:</c> <c> 8-bit unsigned integer. The length of the option excluding the Code and Length Fields. The server MUST set the length field to 4. The client MUST ignore the IPv6-Only Preferred option if the length field value is not 4.</c>
<c>Value:</c> <c> 32-bit unsigned integer.
The number of seconds the client should disable DHCPv4 for (V6ONLY_WAIT configuration variable).
If the server pool is explicitly configured with a V6ONLY_WAIT timer the server MUST set the field to that configured value. Otherwise the server MUST set it to zero.
The client MUST process that field as described in <xref target="v4client"/>.
The client never sets this field as it never sends the full option but includes the option code in the Parameter Request List as described in <xref target="v4client"/>.
</c>
<c></c><c></c>
</texttable>
</section>
<section anchor="v4client" title="DHCPv4 Client Behavior">
<t>
A DHCPv4 client SHOULD allow a device administrator to configure IPv6-only preferred mode either for a specific interface (to indicate that the device is IPv6-only capable if connected to a NAT64 network via that interface) or for all interfaces.
If only a specific interface is configured as IPv6-only capable the DHCPv4 client MUST NOT consider the host to be an IPv6-only capable for the purpose of sending/receiving DHCPv4 packets over any other interfaces.
</t>
<t>
The DHCPv4 client on an IPv4-requiring host MUST NOT include the IPv6-only Preferred option in the Parameter Request List of any DHCPv4 packets and MUST ignore that option in packets received from DHCPv4 servers.
</t>
<t>
DHCPv4 clients running on IPv6-only capable hosts SHOULD include the IPv6-only Preferred option code in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST messages for interfaces so enabled and follow the processing as described below on a per enabled interface basis.
</t>
<t>
If the client did not include the IPv6-only Preferred option code in the Parameter Request List option in the DHCPDISCOVER or DHCPREQUEST message it MUST ignore the IPv6-only Preferred option in any messages received from the server.
</t>
<t>
If the client includes the IPv6-only Preferred option in the Parameter Request List and the DHCPOFFER message from the server contains a valid IPv6-only Preferred option, the client SHOULD NOT request the IPv4 address provided in the DHCPOFFER.
If the IPv6-only Preferred option returned by the server contains a value greater or equal to MIN_V6ONLY_WAIT, the client SHOULD set the V6ONLY_WAIT timer to that value.
Otherwise, the client SHOULD set the V6ONLY_WAIT timer to MIN_V6ONLY_WAIT.
The client SHOULD stop the DHCPv4 configuration process for V6ONLY_WAIT seconds or until a network attachment event, whichever happens first.
The host MAY disable the IPv4 stack completely on the affected interface for V6ONLY_WAIT seconds or until the network attachment event, whichever happens first.
</t>
<t>
The IPv6-only Preferred option SHOULD be included in the Parameter Request List option in DHCPREQUEST messages (after receiving a DHCPOFFER without this option, for a 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 behaviour depends on the client's state.
If the client is in the INIT-REBOOT state it SHOULD stop the DHCPv4 configuration process or disable IPv4 stack completely for V6ONLY_WAIT seconds or until the network 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.
</t>
<t>
If the client includes the IPv6-only Preferred option in the Parameter Request List and the server responds with DHCPOFFER message without a valid IPv6-only Preferred option, the client MUST proceed as normal with a DHCPREQUEST.
</t>
<t>
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 criteria.
</t>
<t>
When an IPv6-only capable client receives the IPv6-Only Preferred option from the server, the client MAY configurean IPv4 link-local address <xref target="RFC3927"/>.
In that case IPv6-only capable devices might still be able to communicate over IPv4 to other devices on the link.
The Auto-Configure Option <xref target="RFC2563"/> can be used to control IPv4 link-local addresses autoconfiguration.
<xref target="autoconf"/> discusses the interaction between the IPv6-only Preferred and the Auto-Configure options.
</t>
</section>
<section anchor="v4srv" title="DHCPv4 Server Behavior">
<t>
The DHCPv4 server SHOULD be able to configure all or individual pools to include the IPv6-only preferred option in DHCPv4 responses if the client included the option code in the Parameter Request List option.
The DHCPv4 server MAY have a configuration option to specify the V6ONLY_WAIT timer for all or individual IPv6-mostly pools.
</t>
<t>
The server MUST NOT include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK message if the YIADDR field in the message does not belong to a pool configured as IPv6-mostly.
The server MUST NOT include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK message if the option was not present in the Parameter Request List sent by the client.
</t>
<t>
If the IPv6-only Preferred option is present in the Parameter Request List received from the client and the corresponding DHCPv4 pool is explicitly configured as belonging to an IPv6-mostly network segment, the server MUST include the IPv6-only Preferred option when responding with the DHCPOFFER or DHCPACK message.
If the server responds with the IPv6-only Preferred option and the V6ONLY_WAIT timer is configured for the pool, the server MUST copy the configured value to the IPv6-only Preferred option value field.
Otherwise it MUST set the field to zero.
The server SHOULD NOT assign an address from the pool.
Instead it SHOULD return 0.0.0.0 as the offered address.
Alternatively, if offering 0.0.0.0 is not feasible, for example due to some limitations of the server or the network infrastructure, the server MAY include an available IPv4 address from the pool into the DHCPOFFER as per recommendations in <xref target="RFC2131"/>.
In this case, the offered address MUST be a valid address that is not committed to any other client.
Because the client is not expected ever to request this address, the server SHOULD NOT reserve the address and SHOULD NOT verify its uniqueness.
If the client then issues a DHCPREQUEST for the address, the server MUST process it per <xref target="RFC2131"/>, including replying with a DHCPACK for the address if in the meantime it has not been committed to another client.
</t>
<t>
If a client includes both a Rapid-Commit option <xref target="RFC4039"/> and IPv6-Only Preferred option in the DHCPDISCOVER message the server SHOULD NOT honor the Rapid-Commit option if the response would contain the IPv6-only Preferred option to the client.
It SHOULD instead respond with a DHCPOFFER as indicated above.
</t>
<t>
If the server receives a DHCPREQUEST containing the IPv6-only Preferred option for the address from a pool configured as IPv6-mostly, the server MUST process it per <xref target="RFC2131"/>.
</t>
<section anchor="autoconf" title="Interaction with RFC2563">
<t>
<xref target="RFC2563"/> defines an Auto-Configure DHCPv4 option to disable IPv4 link-local address configuration for IPv4 clients. Clients can support both, neither or just one of IPv6-Only Preferred and Auto-Configure options.
If a client sends both IPv6-Only Preferred and Auto-Configure options the network administrator can prevent the host from configuring an IPv4 link-local address on an IPv6-mostly network.
To achieve this the server needs to send DHCPOFFER which contains a 'yiaddr' of 0x00000000, and the Auto-Configure flag saying "DoNotAutoConfigure".
</t>
<t>
However special care should be taken in a situation when a server supports both options and receives just IPv6-Only Preferred option from a client.
Section 2.3 of <xref target="RFC2563"/> states that if no address is chosen for the host (which would be the case for IPv6-only capable clients on IPv6-mostly network) then:
"If the DHCPDISCOVER does not contain the Auto-Configure option, it is not answered."
Such behavior would be undesirable for clients supporting the IPv6-Only Preferred option without supporting the Auto-Configure option as they would not receive any response from the server and would keep asking, instead of disabling DHCPv4 for V6ONLY_WAIT seconds.
Therefore the following update is made to Section 2.3 of <xref target="RFC2563"/>"
</t>
<t>
OLD TEXT:
</t>
<t>
---
</t>
<t>
However, if no address is chosen for the host, a few additional steps MUST be taken.
</t>
<t>
If the DHCPDISCOVER does not contain the Auto-Configure option, it is not answered.
</t>
<t>
---
</t>
<t>
NEW TEXT:
</t>
<t>
---
</t>
<t>
However, if no address is chosen for the host, a few additional steps MUST be taken.
</t>
<t>
If the DHCPDISCOVER does not contain the Auto-Configure option and the IPv6-Only Preferred option is not present, it is not answered.
If the DHCPDISCOVER does not contain the Auto-Configure option but contains the IPv6-Only Preferred option, the processing rules for the IPv6-Only Preferred option apply.
</t>
<t>
---
</t>
</section>
</section>
<section anchor="vars" title="Constants and Configuration Variables">
<texttable style="none">
<ttcol></ttcol>
<ttcol></ttcol>
<c>V6ONLY_WAIT</c> <c>The time for which the client SHOULD stop the DHCPv4 configuration process. The value MUST NOT be less than MIN_V6ONLY_WAIT seconds. Default: 1800 seconds</c>
<c>MIN_V6ONLY_WAIT</c> <c>The lower boundary for V6ONLY_WAIT. Value: 300 seconds</c>
<c></c><c></c>
</texttable>
</section>
</section>
<section anchor="v6onlydef" title="IPv6-Only Transition Technologies Considerations">
<t>
Until IPv6 adoption in the Internet reaches 100%, communication between an IPv6-only host and IPv4-only destination requires some form of transition mechanism deployed in the network.
At the time of writing, the only such mechanism that is widely supported by end hosts is NAT64 <xref target="RFC6146"/> (either with or without 464XLAT).
Therefore the IPv6-only Preferred option is only sent by hosts capable of operating on NAT64 networks.
In a typical deployment scenario, a network administrator would not configure the DHCPv4 server to return the IPv6-only Preferred option unless the network provides NAT64 service.
</t>
<t>
Hypothetically, it is possible for multiple transition technologies to coexist. In such scenario some form of negotiation would be required between a client and a server to ensure that the transition technology supported by the client is the one the network provides.
However it seems unlikely that any new transition technology would arise and be widely adopted in any foreseeable future.
Therefore adding support for non-existing technologies seems to be suboptimal and the proposed mechanism implies that NAT64 is used to facilitate connectivity between IPv6 and IPv4.
In the unlikely event that a new transition mechanism becomes widely deployed, the applicability of the IPv6-Only-Preferred option to that mechanism will depend on the nature of the new mechanism.
If the new mechanism is designed in such a way that it's fully transparent for hosts that support NAT64 and the IPv6-Only-Preferred option, then the option can continue to be used with the new mechanism.
If the new mechanism is not compatible with NAT64, and implementation on the host side is required to support it, then a new DHCPv4 option needs to be defined.
</t>
<t>
It should be also noted that declaring a host (technically, a host interface) IPv6-only capable is a policy decision. For example,
<list style="symbols">
<t>
An operating system vendor may make such decision and configure their DHCPv4 clients to send the IPv6-Only Preferred option by default if the OS has 464XLAT CLAT <xref target="RFC6877"/> enabled.
</t>
<t>
An enterprise network administrator may provision the corporate hosts as IPv6-only capable if all applications users are supposed to run have been tested in an IPv6-only environment (or if 464XLAT CLAT is enabled on the devices).
</t>
<t>
IoT devices may be shipped in IPv6-only capable mode if they are designed to connect to IPv6-enabled cloud destination only.
</t>
</list>
</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>The IANA is requested to assign a new DHCPv4 Option code for the IPv6-Only Preferred option
from the BOOTP Vendor Extensions and DHCPv4 Options registry, located at
https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options .
If possible, please assign option code 108.
</t>
<texttable anchor="option_table">
<ttcol align="left">Tag</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Data Length</ttcol>
<ttcol align="left">Meaning</ttcol>
<ttcol align="left">Reference</ttcol>
<c>TBD (proposed value: 108)</c>
<c>IPv6-only Preferred option</c>
<c>4</c>
<c>Number of seconds to disable DHCPv4 for</c>
<c>draft-ietf-dhc-v6only</c>
</texttable>
</section>
<section anchor="Security" title="Security Considerations">
<t>
An attacker might send a spoofed DHCPOFFER containing IPv6-only Preferred option with the value field set to a large number, such as 0xffffffff, effectively disabling DHCPv4 on clients supporting the option.
If the network is IPv4-only such clients would lose connectivity, while on a dual-stack network without NAT64 service only connectivity to IPv4-only destinations would be affected.
The recovery would require triggering a network attachment event.
However it should be noted that if the network does not provide protection from a rogue DHCPv4 server the similar attack vector can be executed by offering an invalid address and setting the Lease Time option value field to 0xffffffff.
The latter attack would affect all hosts, not just hosts that support the IPv6-only Preferred option.
Therefore the security measures against rogue DHCPv4 servers would be sufficient to prevent the attacks specific to IPv6-only Preferred option.
Additionally such attacks can only be executed if the victim prefers the rogue DHCPOFFER over the legitimate ones.
Therefore for the attack to be successful the attacker needs to know the selection criteria used by the client and to be able to make its rogue offer more preferable.
</t>
<t>
It should be noted that disabling IPv4 on a host upon receiving the IPv6-only Preferred option from the DHCPv4 server protects the host from IPv4-related attacks and therefore could be considered a security feature as it reduces the attack surface.
</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>
Thanks to the following people (in alphabetical order) for their review and feedback: Mohamed Boucadair, Martin Duke, Russ Housley, Sheng Jiang, Benjamin Kaduk, Murray Kucherawy, Ted Lemon, Roy Marples, Bjorn Mork, Alvaro Retana, Peng Shuping, Pascal Thubert, Bernie Volz, Eric Vyncke, Robert Wilton.
Authors would like to thank Bob Hinden and Brian Carpenter for the initial idea of signaling IPv6-only capability to hosts.
Special thanks to Erik Kline, Mark Townsley and Maciej Zenczykowski for the discussion which led to the idea of signalling IPv6-only capability over DHCPv4.
</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<references title="Normative References">
&RFC2119;
&RFC2131;
&RFC2563;
&RFC3927;
&RFC4039;
&RFC8174;
</references>
<references title="Informative References">
<!-- &RFC6052; -->
&RFC4861;
&RFC4957;
&RFC6146;
&RFC6147;
&RFC6877;
</references>
</back>
</rfc>