1. INTRODUCTION

     This document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6. The autoconfiguration process includes creating a link-local address and verifying its uniqueness on a link, determining what information should be autoconfigured (addresses, other information, or both), and in the case of addresses, whether they should be obtained through the stateless mechanism, the stateful mechanism, or both. 

    This document defines the process for generating a link-local address, the process for generating site-local and global addresses via stateless address   autoconfiguration, and the Duplicate Address Detection procedure.

   IPv6 defines both a stateful and stateless address autoconfiguration mechanism. Stateless autoconfiguration requires no manual configuration of hosts, minimal (if any) configuration of routers, and no additional servers. 
 

The stateless mechanism allows a host to generate its own addresses using a combination of locally available information, called "interface identifier", and information received from the routers that are used as prefixes in order to identify the subnet\s.

In the absence of routers, a host can only generate link-local addresses. However, link-local addresses are sufficient for allowing communication among nodes attached to the same link.

   In the stateful autoconfiguration model, hosts obtain interface addresses and/or configuration information and parameters from a server.  Servers maintain a database that keeps track of which addresses have been assigned to which hosts. The stateful autoconfiguration protocol allows hosts to obtain addresses, other configuration information or both from a server. 
Stateless and stateful autoconfiguration complement each other. For example, a host can use stateless autoconfiguration to configure its own addresses,  but use stateful autoconfiguration to obtain other information.

When would we use each approach?

    The stateless approach is used when a site is not particularly concerned with the exact addresses hosts use, so long as they are unique and properly routable.
   The stateful approach, on the other hand, is used when a site requires tighter control over exact address assignments. 
Both stateful and stateless address autoconfiguration may be used simultaneously.  The site administrator specifies which type of autoconfiguration to use through the setting of appropriate fields in Router Advertisement messages.
 

The lifetime of an IPv6 address:

   IPv6 addresses are leased to an interface for a fixed (possibly infinite) length of time. Each address has an associated lifetime that indicates how long the address is bound to an interface. When a lifetime expires, the binding (and address) become invalid and the address may be reassigned to another interface elsewhere in the Internet.

    To handle the expiration of address bindings correctly, an address goes through two distinct phases while assigned to an interface:

    - At first, the address is "preferred", meaning that it's available for use in arbitrary communication.

    - Later, the address becomes "deprecated" in anticipation that its current interface binding will become invalid.

While in a deprecated state, the use of the address is discouraged, but not strictly forbidden. 
New communication (e.g., the opening of a new TCP connection) should use a preferred address when possible.  A deprecated address should be used only by applications that have been using it and would have difficulty switching to another address without a service disruption.

   To insure that all configured addresses are likely to be unique on a given link, nodes run a "duplicate address detection" algorithm on addresses before assigning them to an interface.  The Duplicate Address Detection algorithm, explained later on this tutorial, is performed on all addresses, independent of whether they are obtained via stateless or stateful autoconfiguration.

   The autoconfiguration process specified in this tutorial applies only to hosts and not routers.
Since host autoconfiguration uses information advertised by routers, routers will need to be configured by some other means. However, it is expected that routers will generate link-local addresses using the mechanism described in this tutorial. In addition, routers are expected to successfully pass the Duplicate Address Detection procedure described in this tutorial on all addresses prior to assigning them to an interface.