Autoconfiguration Process

This section provides an overview of the typical steps that are performed by an interface during autoconfiguration. Autoconfiguration is performed only on multicast-capable links. Autoconfiguration 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.

A node must attempt to verify that a tentative link-local address is not already in use by another node on the link. After verification, the link-local address can be assigned to an interface. Specifically, the node sends a neighbor solicitation message that contains the tentative address as the target. If another node is already using that address, the node returns a neighbor advertisement saying that the node is using that address. If another node is also attempting to use the same address, the node also sends a neighbor solicitation for the target. The number of neighbor solicitation transmissions or retransmissions, and the delay between consecutive solicitations, are link specific. These parameters 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 instance, an administrator can supply an alternate interface identifier that overrides the default identifier. Then, the autoconfiguration mechanism can be applied by using the new, presumably unique, interface identifier. Alternatively, link-local and other addresses need to be configured manually.

After a node determines that its tentative link-local address is unique, the node assigns the address 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, the routers send router advertisements that specify what type of autoconfiguration a host should perform. If no routers are present, stateful autoconfiguration is invoked.

Routers send router advertisements periodically. However, the delay between successive advertisements is generally longer than a host that performs autoconfiguration can 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 that indicate 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 that stateless address autoconfiguration uses to generate site-local and global addresses. The stateless address and stateful address autoconfiguration fields in router advertisements are processed independently. A host can use both stateful address and stateless address autoconfiguration simultaneously. One option field that contains prefix information, the autonomous address-configuration flag, indicates whether the option even applies to stateless autoconfiguration. If the option field does apply, additional option fields contain a subnet prefix with lifetime values. These values indicate how long addresses that are created from the prefix remain preferred and valid.

Because routers generate router advertisements periodically, hosts continually receive new advertisements. Hosts process the information that is contained in each advertisement as described previously. Hosts add to the information. Hosts also refresh the information that is received in previous advertisements.

Address Uniqueness

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

To accelerate the autoconfiguration process, a host can generate its link-local address, and verify its uniqueness, while the host waits for a router advertisement. A router might delay a response to a router solicitation for a few seconds. Consequently, the total time necessary to complete autoconfiguration can be significantly longer if the two steps are done serially.