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.