1. Introduction

1. Introduction

The common thinking for more than 10 years has been that the transition to IPv6 will be based solely on the dual-stack model and that most things would be converted this way before we ran out of IPv4. However, this has not happened. The IANA free pool of IPv4 addresses has now been depleted, well before sufficient IPv6 deployment had taken place. As a result, many IPv4 services have to continue to be provided even under severely limited address space.

This document specifies the Dual-Stack Lite technology, which is aimed at better aligning the costs and benefits in service provider networks. Dual-Stack Lite will enable both continued support for IPv4 services and incentives for the deployment of IPv6. It also de-couples IPv6 deployment in the service provider network from the rest of the Internet, making incremental deployment easier.

Dual-Stack Lite enables a broadband service provider to share IPv4 addresses among customers by combining two well-known technologies: IP in IP (IPv4-in-IPv6) and Network Address Translation (NAT).

This document makes a distinction between a dual-stack-capable and a dual-stack-provisioned device. The former is a device that has code that implements both IPv4 and IPv6, from the network layer to the applications. The latter is a similar device that has been provisioned with both an IPv4 and an IPv6 address on its interface(s). This document will also further refine this notion by distinguishing between interfaces provisioned directly by the service provider from those provisioned by the customer.

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

This document will first present some deployment scenarios and then define the behavior of the two elements of the Dual-Stack Lite technology: the Basic Bridging BroadBand (B4) element and the Address Family Transition Router (AFTR) element. It will then go into networking and NAT-ing considerations.