Windows CLAT Enters Private Preview: A Milestone for IPv6 Adoption

IPv6 Adoption: A Global Shift

Since its standardization in the late 1990s, IPv6 has steadily gained traction. The last 15 years, especially since World IPv6 Launch Day in 2011, have seen significant progress. Global IPv6 adoption now hovers around 43 to 48%, with countries like France (80%), Germany (75%), and India (74%) leading the charge. The United States sits just above 50%, while others like China remain below 5%.

This shift is driven by the exhaustion of IPv4 addresses - only 4.3 billion were ever available. The rise of mobile devices, IoT, and cloud services has accelerated this depletion. Governments are responding: the U.S. Office of Management and Budget (OMB) mandates that 80% of federal assets operate in IPv6-only environments by the end 2025 (OMB M-21-07), and Germany has similarly prioritized IPv6 in its digital infrastructure plans (BDBOS IPv6 Programme).

Transition Mechanisms: From Tunnels to Translation

Over the years, various IPv4-to-IPv6 transition mechanisms have emerged:

• Tunneling (e.g., 6to4, Teredo): Often unreliable due to NAT and relay dependencies.
• Dual Stack: Effective but resource-intensive and not sustainable long-term.
• Translation Technologies: Including NAT64 and DNS64, which enable IPv6-only clients to reach IPv4 services.

Among these, CLAT (Customer-side Translator) stands out as a key enabler of IPv6-only networks. It is part of the 464XLAT architecture, defined in RFC 6877, and combines:

• CLAT: Stateless translation (SIIT, RFC 7915) on the client.
• PLAT: Stateful NAT64 (RFC 6146) on the provider side.

Together, they allow IPv6-only clients to communicate with IPv4-only servers without requiring IPv4 connectivity on the client.

Listening to Our Community

In early 2024, the Windows Core OS Networking team ran an IPv6 migration survey. The results were clear: CLAT was the most requested IPv6 feature for Windows. This feedback helped shape our roadmap and reinforce our commitment to delivering CLAT support.

It’s Here: Windows CLAT Private Preview

It’s been over a year since we last updated our community on CLAT and today, we’re thrilled to announce that Windows CLAT is now in private preview. See below for details on how to participate in the preview.

How CLAT Works

CLAT eases transition to IPv6 by bridging IPv6 networks and IPv4 applications and servers. It leverages 464XLAT, which combines:

1. CLAT (Customer-side Translator)

• Performs stateless IP/ICMP translation (SIIT, RFC 7915).
• Converts private IPv4 addresses to global IPv6 addresses and vice versa.
• Embedded in the client, Windows in this case.

2. PLAT (Provider-side Translator)

• Performs stateful translation (NAT64, RFC 6146).
• Converts global IPv6 addresses to global IPv4 addresses and vice versa.
• Typically deployed on edge devices like routers.

CLAT Activation and Packet Flow

The activation of CLAT on a Windows client follows a logical sequence designed to ensure seamless IPv4 application support over IPv6-only networks:

Figure 1: CLAT Enablement & Prefix Discovery Options

1. 1. IPv6-only connectivity: When the Windows client device initializes its network interface, it may find that no native IPv4 connectivity is available. Or, during DHCPv4 negotiation, the network may send Option 108, signaling that the host can operate in IPv6-only mode temporarily. While DHCPv4 Option 108 (RFC 8925) does not provide a NAT64 prefix, it indicates IPv6-only preference. This triggers the system to consider IPv6-only operation.
   2. Discovery of NAT64 (PLAT) availability: The host then attempts to determine whether a NAT64 translator exists on the network and what its associated NAT64 prefix is. This discovery can occur through two mechanisms:
      • Router Advertisement (RA) PREF64 option (RFC 8781): The network router sends RA messages containing the PREF64 option, explicitly informing the host of the NAT64 prefix.
      • DNS-based discovery (RFC 7050): The host performs AAAA DNS queries for ipv4only.arpa using the networks recursive resolver. The response allows the host to infer the NAT64 prefix.
   3. CLAT activation: Once the host confirms the presence of a valid PREF64, it enables CLAT. This ensures that IPv4-only applications can continue functioning even in an IPv6-only environment.
   4. IPv4 address synthesized: CLAT plumbs a synthetic IPv4 address and IPv4 default route. This allows legacy IPv4 applications to connect as if native IPv4 were present.
   5. Stateless IPv4-to-IPv6 translation: Each IPv4 packet generated by an application is intercepted by CLAT. Using the learned NAT64 prefix and the rules defined in RFC 6052, CLAT synthesizes an IPv6 destination address that embeds the original IPv4 address.
   6. Routing to NAT64 (PLAT): The newly formed IPv6 packet is routed across the IPv6-only network to the NAT64 translator. The PLAT performs stateful translation, converting the IPv6 packet back to IPv4 and forwarding it to the intended IPv4 destination.

Figure 2: One example of CLAT Activation and Packet Flow

This flow ensures that even in environments where IPv4 is no longer natively available, legacy applications can continue to operate transparently over IPv6 infrastructure.

Looking Ahead: Building the Future of IPv6 with CLAT

The private preview of Windows CLAT marks a pivotal moment in our IPv6 journey. It’s not just a technical achievement, it’s a response to community demand, a step toward government compliance, and a foundation for future-ready networking.

We’re excited to collaborate with our partners and customers as we refine CLAT for general availability. If you’re interested in testing Windows CLAT in your environment, please sign up for Private Preview at aka.ms/winclatintake. Stay tuned for updates on Public Preview and thank you for being part of this milestone.

 

- The Windows Core OS Networking Team