4. Deployment Scenarios

4.1. Access Model

Instead of relying on a cascade of NATs, the Dual-Stack Lite model is built on IPv4-in-IPv6 tunnels to cross the network to reach a carrier-grade IPv4-IPv4 NAT (the AFTR), where customers will share IPv4 addresses. There are a number of benefits to this approach:

This technology decouples the deployment of IPv6 in the service provider network (up to the customer premise equipment or CPE) from the deployment of IPv6 in the global Internet and in customer applications and devices.
The management of the service provider access networks is simplified by leveraging the large IPv6 address space. Overlapping private IPv4 address spaces are not required to support very large customer bases.
As tunnels can terminate anywhere in the service provider network, this architecture lends itself to horizontal scaling and provides some flexibility to adapt to changing traffic load. More discussion of horizontal scaling can be found in Appendix A.
Tunnels provide a direct connection between B4 and the AFTR. This can be leveraged to enable customers and their applications to control how the NAT function of the AFTR is performed.

A key characteristic of this approach is that communications between end-nodes stay within their address family. IPv6 sources only communicate with IPv6 destinations, and IPv4 sources only communicate with IPv4 destinations. There is no protocol family translation involved in this approach. This simplifies greatly the task of applications that may carry literal IP addresses in their payloads.

4.2. CPE

This section describes home Local Area networks characterized by the presence of a home gateway, or CPE, provisioned only with IPv6 by the service provider.

A DS-Lite CPE is an IPv6-aware CPE with a B4 interface implemented in the WAN interface.

A DS-Lite CPE SHOULD NOT operate a NAT function between an internal interface and a B4 interface, as the NAT function will be performed by the AFTR in the service provider's network. This will avoid accidentally operating in a double-NAT environment.

However, it SHOULD operate its own DHCP(v4) server handing out [RFC1918] address space (e.g., 192.168.0.0/16) to hosts in the home. It SHOULD advertise itself as the default IPv4 router to those home hosts. It SHOULD also advertise itself as a DNS server in the DHCP Option 6 (DNS Server). Additionally, it SHOULD operate a DNS proxy to accept DNS IPv4 requests from home hosts and send them using IPv6 to the service provider DNS servers, as described in Section 5.5.

Note: If an IPv4 home host decides to use another IPv4 DNS server, the DS-Lite CPE will forward those DNS requests via the B4 interface, the same way it forwards any regular IPv4 packets. However, each DNS request will create a binding in the AFTR. A large number of DNS requests may have a direct impact on the AFTR's NAT table utilization.

IPv6-capable devices directly reach the IPv6 Internet. Packets simply follow IPv6 routing, they do not go through the tunnel, and they are not subject to any translation. It is expected that most IPv6-capable devices will also be IPv4 capable and will simply be configured with an IPv4 [RFC1918]-style address within the home network and access the IPv4 Internet the same way as the legacy IPv4-only devices within the home.

Pure IPv6-only devices (i.e., devices that do not include an IPv4 stack) are outside of the scope of this document.

4.3. Directly Connected Device

In broadband home networks, some devices are directly connected to the broadband service provider. They are connected straight to a modem, without a home gateway. Those devices are, in fact, acting as CPEs.

Under this scenario, the customer device is a dual-stack-capable host that is provisioned by the service provider with IPv6 only. The device itself acts as a B4 element, and the IPv4 service is provided by an IPv4-in-IPv6 tunnel, just as in the home gateway/CPE case. That device can run any combinations of IPv4 and/or IPv6 applications.

A directly connected DS-Lite device SHOULD send its DNS requests over IPv6 to the IPv6 DNS server it has been configured to use.

Similarly to the previous sections, IPv6 packets follow IPv6 routing, they do not go through the tunnel, and they are not subject to any translation.

The support of IPv4-only devices and IPv6-only devices in this scenario is out of scope for this document.