



IPv6 operations                                               J. Linkova
Internet-Draft                                                    Google
Updates: 6877, 8585 (if approved)                              T. Jensen
Intended status: Standards Track                            25 July 2025
Expires: 26 January 2026


     464XLAT Customer-side Translator (CLAT): Node Recommendations
                       draft-ietf-v6ops-claton-06

Abstract

   464XLAT [RFC6877] defines an architecture for providing IPv4
   connectivity across an IPv6-only network.  The solution contains two
   key elements: provider-side translator (PLAT) and customer-side
   translator (CLAT).  This document complements [RFC6877] and updates
   Requirements for IPv6 Customer Edge Routers to Support IPv4-as-
   a-Service (RFC8585) by providing recommendations for the node
   developers on enabling and disabling CLAT functions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 January 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.










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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Multiple Interfaces Considerations  . . . . . . . . . . . . .   4
   5.  Enabling CLAT . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Disabling CLAT  . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  CLAT Addresses Considerations . . . . . . . . . . . . . . . .   6
     7.1.  CLAT IPv4 Addresses . . . . . . . . . . . . . . . . . . .   6
     7.2.  CLAT IPv6 Addresses . . . . . . . . . . . . . . . . . . .   7
   8.  CLAT and Multiple Prefixes per Interface  . . . . . . . . . .  10
     8.1.  Link Renumbering  . . . . . . . . . . . . . . . . . . . .  12
   9.  MTU Considerations  . . . . . . . . . . . . . . . . . . . . .  13
   10. Updates to RFC8585  . . . . . . . . . . . . . . . . . . . . .  13
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     14.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Enabling and Disabling CLAT: Flowchart . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   464XLAT is widely deployed in 3GPP networks (as described in
   Section 4.2 of [RFC6877]) where User Equipment (UE) devices (such as
   mobile phones and CE routers) perform the CLAT function, providing a
   private IPv4 address and default route for the applications and
   tethered devices.  Enabling 464XLAT allowed mobile operators to
   transition UE devices to IPv6-only mode, where UE devices WAN
   interfaces have only IPv6 addresses, and no IPv4 addresses.

   Until recently, IPv6-only hosts were rather uncommon outside of
   mobile networks and datacenters.  Even if the network provides PLAT
   in the form of NAT64 ([RFC6146]), hosts (desktops, laptops, etc.)
   still needed the network to provide IPv4 addresses, as otherwise



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   applications which require IPv4 would fail.  However, as more and
   more operating systems outside of the 3GPP world support CLAT, it
   becomes possible to migrate those devices to IPv6-only mode, while
   still providing IPv4 as a service via 464XLAT.  Networks such as
   public Wi-Fi, enterprise networks, or even home networks can deploy
   464XLAT as described in Section 4.2 of [RFC6877]:

   *  PLAT functions are performed by NAT64 network devices.

   *  CLAT is performed by the UA itself.

   Another 464XLAT deployment model is a Wireline one (section 4.1 of
   [RFC6877]), when a CPE router is connected to an IPv6-only network
   and provides CLAT functions for IPv4-enabled downstream devices.
   [RFC8585] specifies 464XLAT support requirements for such devices.

   For both scenarios to work, the node performing CLAT (such as a host
   or a CPE router) need to enable the CLAT when connecting to an
   IPv6-only network.  This document complements [RFC6877] and updates
   [RFC8585] by providing recommendations for the node developers on
   enabling and disabling CLAT functions.

2.  Requirements Language

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   This document reuses most of Terminology section from [RFC6877].

   *  CLAT Node: a node (a host or a router) which performs CLAT
      functions by running one or multiple CLAT instances (e.g. one CLAT
      instance per interface).

   *  IPv4-only application: An application which requires the presence
      of an IPv4 address and/or IPv4 default route to operate.  Examples
      include but are not limited to applications using IPv4 literals or
      opening IPv4-only sockets.

   *  IPv6-only network: A network that does not assign IPv4 addresses
      to hosts and facilitates connectivity to IPv4-only destinations
      using NAT64 ([RFC6146]).  In this document, the term "IPv6-only
      network" specifically refers to networks that provide NAT64 as
      it's required by the 464XLAT architecture ([RFC6877]).



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   *  Native IPv4 (such as in 'native IPv4 connectivity' or 'native IPv4
      default gateway'): IPv4 connectivity or default gateway provided
      by the network without using any form of IPv4-as-a-service or
      translation mechanisms (such as 464XLAT).

   *  ULA: Unique Local Addresses, [RFC4193].

   Currently the only avalable form of PLAT for 464XLAT deployments is
   NAT64.  Therefore this document uses those terms interchangeably.

4.  Multiple Interfaces Considerations

   A node may have multiple IPv6-only interfaces (for example, a mobile
   phone can be connected to an IPv6-only Wi-Fi network and to an
   IPv6-only mobile network).  In that case the node SHOULD run an
   independent, dedicated CLAT instance on each interface connected to a
   network equipped with PLAT.  Consequently, each CLAT instance SHOULD
   install a separate default IPv4 route on each CLAT-enabled interface.
   The metrics of IPv4 routes SHOULD be consistent with the metrics of
   IPv6 default routes.

5.  Enabling CLAT

   For performance and security reasons CLAT MUST NOT be enabled if the
   node has IPv4 native connectivity over the given interface.
   Therefore recommendations provided in this section are only
   applicable to the IPv6-only interfaces of a given node (the node has
   no native IPv4 default route pointing to that interface).

   To enable CLAT, an IPv6-only node needs to discover the PLAT-side
   translation IPv6 prefix, also known as the NAT64 prefix (see
   Section 6.3 of [RFC6877]).  The PREF64 Router Advertisement (RA)
   option ([RFC8781]) provides that information and can be used as a
   strong indication that the network supports PLAT (NAT64)
   functionality.  Therefore an IPv6-only node SHOULD enable CLAT as
   soon as a Router Advertisement containing PREF64 option is received.

   If RAs received by the node do not contain a PREF64 option, the node
   MAY use other mechanisms to detect the PLAT presence and obtain the
   NAT64 prefix (such as [RFC7050]).  When discovering the NAT64 prefix
   using the mechanism defined in [RFC7050], the node MUST follow
   recommendations provided in [RFC8880].  Specifically, the node MUST
   send the query to the DNS servers for the specific network interface
   per Section 7.1 of [RFC8880].  In particular, queries for AAAA
   resource records of "ipv4only.arpa."  MUST be sent to the recursive
   resolvers provided by the network, not to the resolvers configured
   manually, local recursive resolvers etc.




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   Any delay in enabling CLAT on an IPv6-only node would be impactful
   for IPv4-only applications, as such applications cannot benefit from
   464XLAT until CLAT is operational.  Therefore it's desirable that the
   node enables CLAT as soon as network support for PLAT is detected
   while IPv4 connectivity is not yet detected.  The node SHOULD enable
   CLAT after discovering the NAT64 prefix, unless by that time the node
   has already obtained a non-link-local IPv4 address.  The node SHOULD
   NOT wait for an explicit (DHCP Option 108) or an implicit (DHCP
   timeouts) indication that native IPv4 connectivity is not available.
   However, to mitigate attacks described in Section 7 of [RFC7050], the
   node MAY delay enabling CLAT if the NAT64 prefix was discovered via
   DNS ([RFC7050]) only.  The delay is implementation specific.  If IPv4
   connectivity becomes available later, the node MUST disable CLAT
   (unless explicitly configured to keep it running) as discussed in the
   following section.

   If the node supports multiple IPv4 continuity solutions, the node
   MUST follow recommendations from Section 4 of [RFC7335] to avoid IPv4
   address space conflicts.

6.  Disabling CLAT

   It is possible that after the CLAT instance has started, native IPv4
   becomes available (e.g. an IPv4 address received via DHCP).  Unless
   explicitly configured otherwise, the node MUST disable CLAT
   immediately upon obtaining a native IPv4 default gateway.

   While disabling CLAT is impactful for all applications and traffic
   flows already utilizing CLAT, it is recommended not only from a
   performance perspective, but also from a security point of view.
   Because IPv4-only networks are inherently IPv6-ignorant, they might
   lack IPv6 layer2 security features, such as RA Guard, that would
   prevent spoofed RAs.  An attacker can send an RA containing the
   PREF64 option, while the network doesn't provide any PLAT
   functionality.  If the node keeps CLAT enabled and uses it for
   IPv4-only destinations, that traffic could be dropped (availability
   attack) or routed through an attacker-controlled route (active attack
   on insecure traffic or hoarding secure traffic for "Harvest Now,
   Decrypt Later" attacks for non-PQC secure traffic, see Section 8 of
   [I-D.ietf-pquip-pqc-engineers]).  Rogue RAs can also disturb traffic
   to destinations that support both IPv4 and IPv6 by causing IPv6
   through an attacker's PLAT to be used instead of the legitimate
   network owner's IPv4 path.

   There are some corner cases when the administrator might prefer the
   node to use CLAT even if the native IPv4 connectivity is available
   (e.g. for performance reasons, if IPv4 as a Service performs better
   than native IPv4).  However for the reasons described above such



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   behaviour MUST be explicitly enabled by the administrator via a
   configuration knob and MUST NOT be a default behaviour, especially
   for unmanaged nodes.

7.  CLAT Addresses Considerations

7.1.  CLAT IPv4 Addresses

   There are two different 464XLAT deployments models:

   *  A dedicated prefix model, which Section 4.1 of [RFC6877] calls
      "wireline network architecture".  In that case, the node
      performing CLAT functions also extends the network downstream and
      provides network connectivity services to other connected systems.
      Those systems can be physical (e.g. various clients connected to a
      CPE router), or logical (e.g. virtual systems running on a node,
      while the host system acts as a router and performs CLAT).  In all
      those cases, systems behind the CLAT node usually use [RFC1918]
      addresses.  Despite the word "wireline", this architecture is
      applicable for wireless or 3GPP routers: such router can provide
      IPv4 connectivity to connected devices, performing CLAT functions
      for traffic sent/received via the IPv6-only uplink interface.

   *  A single-address model, which section 4.2 of [RFC6877] calls
      "wireless network architecture".  In that case, the CLAT instance
      provides an IPv4 address and the default route to the local node's
      network stack only.  It should be noted that [RFC6877] implies
      that the second deployment scenario is limited to 3GPP cases,
      while currently it is also deployed in other types of networks,
      such as enterprise networks and Wi-Fi hotspots, where hosts (as
      mobile phones, laptops and desktops) use CLAT to provide
      connectivity to IPv4-only local applications.  Despite the word
      "wireless" in the name, this architecture is applicable for wired
      networks as well (e.g. desktops or servers using wired
      connections).
















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   In the single-address model, the CLAT instance needs a single IPv4
   CLAT address and a single CLAT-only IPv6 address (which is distinct
   from the one or more IPv6 addresses used by the node running CLAT for
   its own native IPv6 connectivity, see Section 7.2).  The node
   providing CLAT functions to local applications SHOULD use IPv4
   addresses from the dedicated 192.0.0.0/29 range ([RFC7335]), reserved
   for IPv4 continuity solutions including but not limited to 464XLAT.
   If the node runs multiple CLAT instances (see Section 4), the node
   SHOULD use different local IPv4 addresses for each CLAT instance.
   This approach limits the number of CLAT instances per node to 8,
   which seems to be more that sufficient at the time of writing.  If in
   the future some deployment scenarios require more that 8 CLAT
   instances per node, a new larger IPv4 range will be requested from
   IANA.

   The node MUST NOT send packets on wire from the local CLAT address.

   The host SHOULD use 255.255.255.255 as a netmask for the CLAT
   address.  That allows all 8 addresses from 192.0.0.0/29 to be used,
   if needed.

   It should be noted that 192.0.0.0/29 is shared between multiple IPv4
   continuity solutions such as 464XLAT and DS-Lite (see [RFC7335].  For
   example, Section 10 of [RFC6333] reserves 192.0.0.1 for the Dual-
   Stack Lite default router.  However, as per Section 4 of [RFC7335],
   the host MUST NOT enable two active IPv4 continuity solutions
   simultaneously in a way that would cause a node to have overlapping
   192.0.0.0/29 address space.  Therefore, as long as the host is not
   using DS-Lite, it MAY use 192.0.0.1 as a CLAT address.

7.2.  CLAT IPv6 Addresses

   Section 6.3 of [RFC6877] recommends that the CLAT instance acquires a
   dedicated /64 for translating between IPv4 and IPv6, and only uses a
   single interface IPv6 address if a dedicated prefix is not available
   via DHCPv6-PD.  However, deployments where each node can obtain a
   dedicated /64 just for CLAT are rather uncommon, especially in
   environments like enterprise networks, Wi-Fi hotspots, etc.  Quite
   often the CLAT instance uses a single IPv6 address as a source for
   all IPv4 traffic translated by CLAT.  In particular, in a single
   address model (see Section 7.1) the CLAT instance only need a single
   CLAT IPv6 address, so obtaining a /64 is wasteful.  For instance, a
   home network that gets a /60 from its ISP can only connect up to 15
   CLAT-enabled devices before it runs out of available prefixes.  Even
   in a dedicated prefix model, the CLAT instance can first perform
   stateful NAT44 to translate all IPv4 addresses from the dedicated
   prefix to a single IPv4 address, and then perform stateless CLAT.




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   This document updates [RFC6877] by removing the requirement to
   acquire a dedicated /64 prefix for the purpose of sending and
   receiving statelessly translated packets.  The following
   recommendations are made instead:

   *  In a single-address model, the CLAT instance SHOULD NOT obtain a
      dedicated /64 for the purpose of sending and receiving statelessly
      translated packets.

   *  In a dedicated prefix model, the CLAT instance MAY do one of the
      following:

      -  Perform stateful NAT44 to translate all IPv4 addresses from the
         dedicated prefix to a single IPv4 address, then perform
         stateless CLAT.

      -  Obtain a dedicated IPv6 address for each CLAT IPv4 address.

      -  Obtain a dedicated /64 prefix via DHCPv6-PD.

   In a single-address model, the CLAT instance SHOULD obtain a
   dedicated IPv6 address used exclusively for CLAT functions.  This is
   required as when the node receives a packet from a NAT64 source, the
   node needs to differentiate between native IPv6 traffic and traffic
   which needs to be passed to the CLAT instance.  For example, an
   ICMPv6 Echo Reply packet from 64:ff9b::203.0.113.1 can be a response
   to either an IPv6 ping to 64:ff9b::203.0.113.1, or an IPv4 ping to
   203.0.113.1, translated by CLAT.  Using a dedicated IPv6 source
   address for CLAT traffic allows the node to make that distinction
   without keeping state and operate in the stateless mode (see
   Section 1.3 of [RFC7915].

   The node SHOULD treat its CLAT IPv6 addresses as any other IPv6
   address and comply with [RFC4861] and [RFC4862].  In particular:

   *  Performing Duplicate Address Detection for each dedicated CLAT
      address (Section 5.4 of [RFC4862]);

      -  Justification: performing DAD minimizes loss of connectivity in
         the unlikely event of address collision.  Additionally, real
         world deployment experience shows that network infrastructure
         devices mandate a DAD packet from the client before enabling
         network access.

   *  Processing received unicast Neighbor Solicitations (NSes) as well
      as multicast ones sent to the solicited-node multicast address
      ([RFC4861]) for the node CLAT addresses.




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      -  Justification: If a node doesn't respond to unicast NSes,
         anytime the first-hop router gets a packet for the CLAT address
         and its Neighbor Cache entry is 'STALE' (Section 7.3.2 of
         [RFC4861]), the Neighbor Unreachability Detection process
         (Section 7.3.3 of [RFC4861]) will delete that CLAT address's
         cache entry.  This forces the address resolution process to
         restart from scratch.  Until resolution finishes, traffic for
         the CLAT address might drop, leading to a degraded user
         experience, especially for applications sensitive to jitter and
         packet loss.

   *  Sending Gratuitous Neighbor Advertisements ([RFC9131]) for the
      CLAT addresses.

      -  Justification: not following this recommendation leads to user-
         visible packet loss when the CLAT instance starts receiving
         traffic after period of inactivity, or when connected to the
         network for the first time.  The problem is discussed in
         Section 2 of [RFC9131].

   *  Registering the CLAT addresses assigned via SLAAC or statically
      using DHCPv6 ([RFC9686]), if the network supports the
      registration.

      -  Justification: not registering CLAT addresses reduces traffic
         visibility for network operators, which complicates
         troubleshooting and forensics, as discussed in [RFC9686].

   If the dedicated CLAT address is obtained via Stateless Address
   Autoconfiguration (SLAAC, [RFC4862]), the CLAT instance SHOULD ensure
   that the address is checksum-neutral for the given local IPv4 CLAT
   address and the NAT64 prefix (this means that the local IPv4 address
   needs to be assigned/known before the IPv6 address is configured).
   Using a checksum-neutral CLAT address provides the following
   benefits:

   *  Better performance as CLAT doesn't need to recalculate the
      checksum.

   *  If a protocol uses the standard IP checksum, CLAT doesn't need to
      recalculate the checksum.  That improves the chances of the
      protocol working via CLAT even if CLAT is not aware of the
      protocol's semantics.








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   To protect user privacy and prevent user tracking through CLAT
   addresses, the node SHOULD generate a different interface id for the
   CLAT address when connecting to different networks, even if the NAT64
   prefix and the local IPv4 CLAT address do not change.  In particular,
   the node SHOULD generate a random CLAT address every time the network
   attachement changes to another network.

8.  CLAT and Multiple Prefixes per Interface

   IPv6 multihoming, particularly when multiple routers on the same link
   advertise different prefixes PIOs, presents a complex and not yet
   fully resolved challenge.

   When routers on a given link are managed independently (e.g., by
   different ISPs), the resulting set of configuration parameters
   received by a host can be difficult to utilize without creating a
   complex and fragile state machine.  For example, if router_A
   advertises a PIO with prefix_A and PREF64_A, while router_B
   advertises a PIO with prefix_B and a PREF64_B, it is crucial that the
   CLAT bundles the information received from each router.  A CLAT
   instance must use PREF64_A and generate a CLAT address from prefix_A,
   sending translated packets to router_A.  Alternatively, it must use
   PREF64_B, generate an address from prefix_B, and send translated
   packets to router_B.  Mixing configuration information from different
   routers (e.g., generating a CLAT address from prefix_A but using
   PREF64_B for translation) can lead to packet loss.  For example, if
   packets with source addresses from prefix_A are sent to router_B,
   that router (or the uplink network) might drop the packets according
   to BCP 38 ([RFC2827]).  Similarly, if the CLAT instance uses
   PREF64_A, advertised by router_A, but those packets are sent to
   router_B, that router might not be configured to translate packets
   for that prefix.

   This document does not aim to define CLAT behavior for every possible
   multi-router/multi-prefix scenario.  Instead, this section provides
   recommendations for common scenarios, leaving numerous corner cases
   out of scope.

   This section implies that a router is identified by its link-local
   address, used as a source address for RAs.  For example, "detecting
   multiple routers" means that the node received RAs from multiple
   link-local addresses.

   A node discovering multiple routers on the same interface advertising
   the _same PIOs and NAT64 prefix_, SHOULD only create one CLAT
   instance using one of the PIOs to form a CLAT address.





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   A node discovering multiple routers on the same interface signalling
   the _different PIOs and NAT64 prefixes_, MAY create one CLAT instance
   for each tuple of PIOs and NAT64 prefix (both PIO and NAT64 prefix in
   a given tuple MUST be advertised by the same router), or only a
   single CLAT instance using the NAT64 prefix discovered through the
   selected IPv6 default router and the address formed from a PIO
   advertised by that router.

   When a node creates a single CLAT instance and must choose between
   multiple PIOs, the node SHOULD select a single PIO using the same
   algorithm as for choosing the source address for a destination within
   the selected NAT64 prefix ([RFC6724], updated by
   [I-D.ietf-6man-rfc6724-update]).

   Discussion: This approach, leveraging the default source address
   selection algorithm (Section 5 of [RFC6724]), typically results in
   the policy table (rule 6) and longest prefix match (rule 8) being
   used for prefix selection.  This ensures CLAT address selection
   aligns with default source address selection for native IPv6 flows,
   offering the following advantages:

   *  When using the well-known NAT64 prefix (64:ff9b::/96), non-ULA
      prefixes are preferred over ULA prefixes by default.  This is
      beneficial as ULA source packets may not reach PLAT devices.

   *  For network-specific NAT64 prefixes within the known-local ULA
      range ([I-D.ietf-6man-rfc6724-update]), the ULA prefix is
      preferred.  This can be advantageous in home and enterprise
      environments where administrators intend to perform NAT64 for
      specific source prefixes only.

   *  For network-specific NAT64 prefixes within the operator's global
      non-ULA range, the longest prefix match selects the PIO, ensuring
      CLAT uses the operator's source address for traffic to the
      operator's PLAT in multi-prefix environments.

   *  In managed environments, operators can customize CLAT behavior by
      modifying the policy table if the default prefix selection is
      unsuitable.

   Creating a single CLAT instance significantly simplifies the CLAT
   state machine.  However, this approach may concentrate all traffic
   from that instance onto the same first-hop router and NAT64 device in
   some multihomed topologies.  As traffic shifts from CLAT to native
   IPv6, this drawback becomes less significant and does not justify the
   added complexity of multiple instances.





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8.1.  Link Renumbering

   As discussed above, a single CLAT instance per interface, using a
   single PIO, is typically sufficient, even if the link has multiple
   assigned subnets.  However, PIO selection can significantly impact
   user experience during link renumbering.

   [RFC8978] discusses various examples of "flash renumbering," where
   the IPv6 prefix assigned to the link changes without explicit host
   notification.  [I-D.ietf-6man-slaac-renum] and [I-D.link-6man-gulla]
   discuss methods to mitigate the impact of flash renumbering.  These
   methods generally rely on hosts with addresses from both old and new
   prefixes ceasing use of the old prefix and adopting the new prefix.
   For nodes running CLAT instances, this requires disabling instances
   using addresses from the old prefix and creating an instance using an
   address from the new prefix.

   The CLAT node SHOULD use at least the following signals to detect
   link renumbering events:

   *  A prefix used to form the CLAT address becomes deperecated or
      invalid ([RFC4862]).

   *  The router (or routers) advertizing the PIO used to form the CLAT
      address

      -  has changed it state from being reachable or probably reachable
         to being unknown or suspect (i.e., it's neighbor cache entry
         moved to the INCOMPLETE state or ceased to exist, see
         Section 6.3.6 of [RFC4861]).

      -  ceased to be a router (see Section 7.3.3 of [RFC4861]).

   Upon receiving a signal indicating a possible renumbering event, the
   node SHOULD disable the CLAT instance(s) affected by the renumbering,
   and create new instance(s).  In case of implicit signals (provided by
   the Neighbor Unreachability Detection, [RFC4861], rather than by a
   Router Advertisement deprecating or invalidating a prefix), the node
   MAY send Router Solicitations to obtain the most up-to-date network
   configuration information.  When sending Router Solicitations the
   node MUST follow recommendations specified in Section 6.3.7 of
   [RFC4861].  The node MAY react to a potential renumebring event in a
   "make-before-break" manner, when old instances are still running
   until all required information to enable new ones becomes available.







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9.  MTU Considerations

   The IPv4 header is 20 bytes long (or longer if IP options are
   present), while the IPv6 header is 40 bytes.  This means that when
   CLAT translates an IPv4 packet to IPv6, it usually adds 20 bytes to
   the packet size.  However, when CLAT transates a fragmented IPv4
   packet, then Fragment Header needs to be added to the resulting IPv6
   packet (Section 4.1 of [RFC7915]).  The length of IPv6 Fragment
   Extension header is 8 bytes (Section 4.5 of [RFC8200]).  Therefore,
   to minimize undesirable IP fragmentation ([RFC8900]), the CLAT
   instance in a single-address mode SHOULD present IPv4-only
   applications with an IPv4 MTU which is 28 bytes smaller than the IPv6
   MTU of the interface the instance is running on.

10.  Updates to RFC8585

   This document makes the following changes to Section 3.2.1 of
   [RFC8585]:

   OLD TEXT:

   ===

   464XLAT requirements:

   ===

   NEW TEXT:

   ===

   464XLAT requirements:

   464XLAT-0: The IPv6 Transition CE Router SHOULD follow
   recommendations provided in draft-link-v6ops-claton.

   ===

   OLD TEXT:

   ===

   464XLAT-4: The IPv6 Transition CE Router MUST implement [RFC7050]
   ("Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis") in
   order to discover the provider-side translator (PLAT) translation
   IPv4 and IPv6 prefix(es)/suffix(es).

   ===



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   NEW TEXT:

   ===

   464XLAT-4: The IPv6 Transition CE Router MUST implement [RFC8781]
   ("Discovering PREF64 in Router Advertisements") and SHOULD implement
   [RFC7050] ("Discovery of the IPv6 Prefix Used for IPv6 Address
   Synthesis") in order to discover the provider-side translator (PLAT)
   translation IPv4 and IPv6 prefix(es)/suffix(es).

   ===

   OLD TEXT:

   ===

   464XLAT-6: If the network provides several choices for the discovery/
   learning of the NAT64 prefix, the priority to use one or the other
   MUST follow this order: 1) [RFC7225] and 2) [RFC7050].

   The NAT64 prefix could be discovered by means of the method defined
   in [RFC7050] only if the service provider uses DNS64 [RFC6147].  It
   may be the case that the service provider does not use or does not
   trust DNS64 [RFC6147] because the DNS configuration at the CE (or
   hosts behind the CE) can be modified by the customer.  In that case,
   the service provider may opt to configure the NAT64 prefix by means
   of the option defined in [RFC7225].  This can also be used if the
   service provider uses DNS64 [RFC6147].

   ===

   NEW TEXT

   ===

   464XLAT-6: If the network provides several choices for the discovery/
   learning of the NAT64 prefix, the priority to use one or the other
   MUST follow this order: 1)[RFC7225] 2)[RFC8781] and 3) [RFC7050].

   464XLAT-7: If the IPv6 Transition CE Router performs CLAT functions
   it SHOULD also include the PLAT prefix in Router Advertisements
   ([RFC8781]) sent via the LAN interfaces.  If the IPv6 Transition CE
   Router acts as a DHCP server it SHOULD enable DHCP Option 108
   ([RFC8925]) processing.  The router SHOULD have a configuration knob
   to disable DHCP Option 108 processing.






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   [RFC8781] allows the service provider to signal NAT64 prefix
   independently from DNS64 presence.  At the same time the NAT64 prefix
   could be discovered by means of the method defined in [RFC7050] only
   if the service provider uses DNS64 [RFC6147].  It may be the case
   that the service provider does not use or does not trust DNS64
   [RFC6147] because the DNS configuration at the CE (or hosts behind
   the CE) can be modified by the customer.  In that case, the service
   provider may opt to configure the NAT64 prefix by means of the option
   defined in [RFC7225].  This can also be used if the service provider
   uses DNS64 [RFC6147].

   ===

11.  Security Considerations

   If a malicious actor spoofs PLAT presence signals (such as an RA with
   PREF64 option) or DNS responses for DNS-based NAT64 prefix detection
   ([RFC7050]), traffic of IPv4-only applications using CLAT can be
   affected:

   *  if there is no PLAT (NAT64) devices, traffic to NAT64 destinations
      would be dropped.

   *  If the attacker intercepts traffic for the NAT64 prefix (e.g. by
      providing the victim with a bogus NAT64 prefix and steering
      traffic for those destinations towards themselves), the attacker
      might be able to perform man-in-the-middle attacks.

   Using the PREF64 RA option to detect PLAT presence and the NAT64
   prefix is less prone to such attacks than DNS-based detection
   ([RFC7050]), as the attacker needs to be on-link and be able to
   bypass layer-2 security features such as RA Guard.  Therefore
   Section 5 recommends the PREF64 RA option as a preferred way to
   detect PLAT presence.

   In networks lacking explicit IPv6 deployment and consequently IPv6
   security features, administrators may inadvertently expose link-local
   IPv6 connectivity when endpoints have IPv6 enabled.  This unintended
   exposure can facilitate PLAT presence signal falsification, enabling
   malicious actors to conduct Man-in-the-Middle (MitM) or Denial-of-
   Service (DoS) attacks.  This document mitigates this risk by
   requiring endpoints to disable CLAT when the network provides non-
   link-local IPv4 connectivity, as outlined in Section 6.








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12.  Privacy Considerations

   If the instance utilizes the same CLAT address for an extensive
   period of time or, much worse, uses the same CLAT address when
   connecting to different networks, eavesdroppers and information
   collectors can correlate various network activity to the same node.
   To mitigate that risk and make address-based network-activity
   correlation more difficult, the node SHOULD generate a different
   interface id for the CLAT address when connecting to different
   networks (see Section 7.2).

   It should be noted that the node's CLAT IPv6 address is only used
   (and visible to observers) when the traffic is carried from the CLAT
   node to the PLAT device.  In the vast majority of the cases it means
   that address is never visible outside of the network Internet edge,
   so to perform address-based network-activity correlation the observer
   needs to be located in the same network as the CLAT node.

   To summarize, this document does not introduce any new privacy
   considerations.

13.  IANA Considerations

   This memo does not introduce any requests to IANA.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.




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   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
              <https://www.rfc-editor.org/info/rfc6333>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <https://www.rfc-editor.org/info/rfc7050>.

   [RFC7335]  Byrne, C., "IPv4 Service Continuity Prefix", RFC 7335,
              DOI 10.17487/RFC7335, August 2014,
              <https://www.rfc-editor.org/info/rfc7335>.

   [RFC7915]  Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
              "IP/ICMP Translation Algorithm", RFC 7915,
              DOI 10.17487/RFC7915, June 2016,
              <https://www.rfc-editor.org/info/rfc7915>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8880]  Cheshire, S. and D. Schinazi, "Special Use Domain Name
              'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
              2020, <https://www.rfc-editor.org/info/rfc8880>.



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   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8781]  Colitti, L. and J. Linkova, "Discovering PREF64 in Router
              Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
              2020, <https://www.rfc-editor.org/info/rfc8781>.

   [RFC8585]  Palet Martinez, J., Liu, H. M.-H., and M. Kawashima,
              "Requirements for IPv6 Customer Edge Routers to Support
              IPv4-as-a-Service", RFC 8585, DOI 10.17487/RFC8585, May
              2019, <https://www.rfc-editor.org/info/rfc8585>.

   [RFC8925]  Colitti, L., Linkova, J., Richardson, M., and T.
              Mrugalski, "IPv6-Only Preferred Option for DHCPv4",
              RFC 8925, DOI 10.17487/RFC8925, October 2020,
              <https://www.rfc-editor.org/info/rfc8925>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [RFC9131]  Linkova, J., "Gratuitous Neighbor Discovery: Creating
              Neighbor Cache Entries on First-Hop Routers", RFC 9131,
              DOI 10.17487/RFC9131, October 2021,
              <https://www.rfc-editor.org/info/rfc9131>.

   [RFC9686]  Kumari, W., Krishnan, S., Asati, R., Colitti, L., Linkova,
              J., and S. Jiang, "Registering Self-Generated IPv6
              Addresses Using DHCPv6", RFC 9686, DOI 10.17487/RFC9686,
              December 2024, <https://www.rfc-editor.org/info/rfc9686>.

   [I-D.ietf-6man-rfc6724-update]
              Buraglio, N., Chown, T., and J. Duncan, "Prioritizing
              known-local IPv6 ULAs through address selection policy",
              Work in Progress, Internet-Draft, draft-ietf-6man-rfc6724-
              update-23, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              rfc6724-update-23>.

14.2.  Informative References



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   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC7225]  Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
              Port Control Protocol (PCP)", RFC 7225,
              DOI 10.17487/RFC7225, May 2014,
              <https://www.rfc-editor.org/info/rfc7225>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC8978]  Gont, F., Žorž, J., and R. Patterson, "Reaction of IPv6
              Stateless Address Autoconfiguration (SLAAC) to Flash-
              Renumbering Events", RFC 8978, DOI 10.17487/RFC8978, March
              2021, <https://www.rfc-editor.org/info/rfc8978>.

   [I-D.ietf-pquip-pqc-engineers]
              Banerjee, A., Reddy.K, T., Schoinianakis, D., Hollebeek,
              T., and M. Ounsworth, "Post-Quantum Cryptography for
              Engineers", Work in Progress, Internet-Draft, draft-ietf-
              pquip-pqc-engineers-13, 1 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
              pqc-engineers-13>.

   [I-D.ietf-6man-slaac-renum]
              Gont, F., Zorz, J., Patterson, R., and J. Linkova,
              "Improving the Robustness of Stateless Address
              Autoconfiguration (SLAAC) to Flash Renumbering Events",
              Work in Progress, Internet-Draft, draft-ietf-6man-slaac-
              renum-09, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              slaac-renum-09>.

   [I-D.link-6man-gulla]
              Linkova, J., "Using Prefix-Specific Link-Local Addresses
              to Improve SLAAC Robustness", Work in Progress, Internet-
              Draft, draft-link-6man-gulla-01, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-link-6man-
              gulla-01>.



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Appendix A.  Enabling and Disabling CLAT: Flowchart

                    +-----------------------------------------+
                    |                                         |
                    |                                         |
                    v                                         |
+----------------------------------+        +-----------------+------------+
|                                  | YES    |                              |
| Has native IPv4 default route?   +------> |        CLAT Disabled         |
|                                  |        |                              |
+------------------+---------------+        +------------------------------+
                   |  NO                                 ^               ^
         +---------+-------------+                       | NO            |
         |                       |                       |               |
+--------v--------+  +-----------v------------+       +--+----------+    |
|                 |  |                        |  NO   | PLAT prefix |    |
|   Start DHCP    |  | Received PREF64 in RA? +------>| discovered  |    |
|                 |  |                        |       | via RFC7050?|    |
+---------+-------+  +-------------------+----+       +-------+-----+    |
          |                 ^            |                    |          |
          |                 |            | YES                |          |
          v                 |            v                    | YES      |
+---------------------+     |      +---------------+          |          |
|                     |     |      |               |          |          |
| Wait for DHCP Offer |<----+------+ Enable CLAT   |<---------+          |
|                     |     |      |               |                     |
+----------+----------+     |      +---------------+                     |
           |                |                                            |
           |                |                                            |
           v                | YES                                        |
+---------------------+     |                                            |
|                     |     |                                            |
|                     +-----+                                            |
| Option 108 Present? |                                                  |
|                     |                             NO                   |
|                     +--------------------------------------------------+
+---------------------+

            Figure 1: Enabling and Disabling CLAT Instance

Acknowledgements

   Thanks to Ondrej Caletka, Stuart Cheshire, Lorenzo Colitti, Jeremy
   Duncan, Jason Healy, Ed Horley, KAWASHIMA Masanobu, Ted Lemon, George
   Michaelson, Jordi Palet, Dieter Siegmund, Philipp S.  Tiesel, Eric
   Vyncke for the discussions, the input, and all contribution.





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Authors' Addresses

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont NSW 2009
   Australia
   Email: furry13@gmail.com, furry@google.com


   Tommy Jensen
   Email: tojens.ietf@gmail.com







































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