IPv6 Stateless Address Autoconfiguration

A host takes several steps to decide how to autoconfigure its interfaces in IPv6. 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 section describes the process for generating a link-local address, the process for generating site-local and global addresses by stateless address autoconfiguration, and the duplicate address detection procedure.

Stateless Autoconfiguration Requirements

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 and information advertised by routers. Routers advertise prefixes that identify the subnet(s) associated with a link, while hosts generate an interface identifier that uniquely identifies an interface on a subnet. An address is formed by combining the two. In the absence of routers, a host can generate only link-local addresses. However, link-local addresses are only sufficient for allowing communication among nodes attached to the same link.

Stateful Autoconfiguration Model

In the stateful autoconfiguration model, hosts obtain interface addresses 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 to Use Stateless and Stateful Approaches

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 is used when a site requires tighter control over exact address assignments. Both stateful and stateless address autoconfiguration can be used simultaneously. The site administrator specifies which type of autoconfiguration to use through the setting of appropriate fields in router advertisement messages.

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 can be reassigned to another interface elsewhere. To handle the expiration of address bindings gracefully, an address goes through two distinct phases while assigned to an interface. Initially, an address is preferred, meaning that its use in arbitrary communication is unrestricted. Later, an address becomes deprecated in anticipation that its current interface binding will become invalid. While in a deprecated state, the use of an address is discouraged, but not strictly forbidden. New communication (for example, 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.

Duplicate Address Detection Algorithm

To ensure 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 is performed on all addresses, independent of whether they are obtained by stateless or stateful autoconfiguration.

The autoconfiguration process specified in this document applies only to hosts and not routers. Because host autoconfiguration uses information advertised by routers, routers 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 document. In addition, routers are expected to pass successfully the duplicate address detection procedure on all addresses, prior to assigning them to an interface.

IPv6 Protocol Overview

This section provides an overview of the typical steps that take place when an interface autoconfigures itself. Autoconfiguration is performed only on multicast-capable links and begins when a multicast-capable interface is enabled, for example, during system startup. Nodes (both hosts and routers) begin the autoconfiguration process by generating a link-local address for the interface. A link-local address is formed by appending the interface's identifier to the well-known link-local prefix.

Before the link-local address can be assigned to an interface and used, however, a node must attempt to verify that this tentative address is not already in use by another node on the link. Specifically, it sends a neighbor solicitation message containing the tentative address as the target. If another node is already using that address, it returns a neighbor advertisement saying so. If another node is also attempting to use the same address, it sends a neighbor solicitation for the target as well. The number of neighbor solicitation transmissions or retransmissions, and the delay between consecutive solicitations, are link-specific and can be set by system management.

If a node determines that its tentative link-local address is not unique, autoconfiguration stops and manual configuration of the interface is required. To simplify recovery in this case, it should be possible for an administrator to supply an alternate interface identifier that overrides the default identifier in such a way that the autoconfiguration mechanism can then be applied using the new (presumably unique) interface identifier. Alternatively, link-local and other addresses will need to be configured manually.

After a node ascertains that its tentative link-local address is unique, it assigns it to the interface. At this point, the node has IP-level connectivity with neighboring nodes. The remaining autoconfiguration steps are performed only by hosts.

Obtaining Router Advertisement

The next phase of autoconfiguration involves obtaining a router advertisement or determining that no routers are present. If routers are present, they send router advertisements that specify what sort of autoconfiguration a host should perform. If no routers are present, stateful autoconfiguration is invoked.

Routers send router advertisements periodically, but the delay between successive advertisements are generally longer than a host performing autoconfiguration wants to wait. To obtain an advertisement quickly, a host sends one or more router solicitations to the all-routers multicast group. Router advertisements contain two flags indicating what type of stateful autoconfiguration (if any) should be performed. A managed address configuration flag indicates whether hosts should use stateful autoconfiguration to obtain addresses. An other stateful configuration flag indicates whether hosts should use stateful autoconfiguration to obtain additional information (excluding addresses).

Prefix Information

Router advertisements also contain zero or more prefix information options that contain information used by stateless address autoconfiguration to generate site-local and global addresses. It should be noted that the stateless and stateful address autoconfiguration fields in router advertisements are processed independently of one another, and a host can use both stateful and stateless address autoconfiguration simultaneously. One prefix information option field, the autonomous address-configuration flag, indicates whether or not the option even applies to stateless autoconfiguration. If it does, additional option fields contain a subnet prefix with lifetime values, indicating how long addresses created from the prefix remain preferred and valid.

Because routers generate router advertisements periodically, hosts continually receive new advertisements. Hosts process the information contained in each advertisement as described above, adding to and refreshing information received in previous advertisements.

Address Uniqueness

For safety, all addresses must be tested for uniqueness prior to their assignment to an interface. In the case of addresses created through stateless autoconfiguration, however, the uniqueness of an address is determined primarily by the portion of the address formed from an interface identifier. Thus, if a node has already verified the uniqueness of a link-local address, additional addresses created from the same interface identifier need not be tested individually. In contrast, all addresses obtained manually or by stateful address autoconfiguration should be tested individually for uniqueness. To accommodate sites that believe the overhead of performing duplicate address detection outweighs its benefits, the use of duplicate address detection can be disabled through the administrative setting of a per-interface configuration flag.

To speed up the autoconfiguration process, a host can generate its link-local address (and verify its uniqueness) in parallel with waiting for a router advertisement. Because a router might delay responding to a router solicitation for a few seconds, the total time needed to complete autoconfiguration can be significantly longer if the two steps are done serially.