IPv6 Address Auto Configuration Process

IPv6 address can be configured on the host devices either manually or using the auto configuration method. With the Auto configuration feature the devices in the network get addressed by its own and they receive a link-local unicast address.

The concept of Router Advertisements (RA’s) is used in this mechanism.

There below are different types of Auto configuration Types:

Stateless Auto configuration ( SLAAC : Stateless Address Auto Configuration)
Stateful Auto Configuration ( SFAAC : Stateful Address Auto Configuration)

1. Stateless Auto configuration: This configuration relies on the Router Advertisement messages. In these messages the Stateful configuration flag is set to zero (0), the Autonomous flag is set as 1. This message contains managed Address configuration and also includes some Prefix information options as well.

Here the IPv6 address is configured on an interface using the Router Advertisement Message. The first 64 bits of the Router Advertisement Source Address and using the EUI-64 process the 64-bit Interface ID is created. The main aim of creating a stateless auto configuration is for PDS, cell phones, where address can be automatically be assigned without using any DHCP infrastructure.

2. Stateful Auto configuration: This configuration is achieved with the help of protocol like DHCPv6, which share information about address and other setting that is helpful for configuration. When a Router Advertisement message with no Prefix information and the flag for Managed Address Configuration and other stateful info is set to 1, the Stateful auto configuration is used by the host. If no routes are present in the local link, the Stateful configuration can be used.

In the stateful configuration when a router is detected by a client, the router advertisement message is examined to check if the DHCPv6 is configured or setup. The router responds if the DHCPv6 is configured on it or even if the client does not receive any router advertisement, the client generates a DHCP solicit message to find a DHCPv6 Server.

This DHCP solicit message is sent using the link-local scope multicast address to all DHCP– agents. The agent can either be the Router configured with DHCP or a DHCP server itself.

Steps to Configure Auto Configuration

Auto configured IP addresses can be in one of the following address State:

a) Tentative: This is when the address is still being verified of it being unique. With the help of Duplicate address detection the verification is done. When in this state, Unicast traffic cannot be received by the node but can receive and also process any Multicast Neighbor Advertisement messages that are sent to detect duplicate address using Neighbor solicitation Process.

b) Valid: Unicast traffic can be sent and received when on this state.

c) Preferred: When the address is considered Valid, verified for its uniqueness then this address can be used for any communications. To and Fro traffic can be sent if a Node’s address is in this state. Based on the Preferred Lifetime field in the Prefix Information option of a router advertisement message.

d) Deprecated: This address can be used in the current communication as it is valid and unique but it’s not encouraged to use it for new communication. The unicast traffic can be sent and received for this address state.

e) Invalid: This unicast traffic cannot be sent or received using this address. After the lifetime expires, the address comes in this state.

Auto configuration is mentioned in detail in the RFC 4862 and the process would look like below:

1. Using the Link-Local prefix FE80::/64 and interface identified derived from EUI-64 a tentative link-local address is created.

2. The uniqueness of this tentative link-local address is verified using duplicate address Detection. A Neighbor solicitation message is sent having the tentative link-local address set as the target field.

3.In response to the Neighbor Solicitation Message if the Neighbor advertisement message is received, it considers that the tentative link-local address is being used somewhere in the network and no further steps are continued for address auto configuration and then the node must be manually configured.

4. In response to the Neighbor Solicitation Message if the Neighbor advertisement message is NOT received, the tentative link-local address is considered Valid and unique and the address is configured on the interface. The link-layer multicast address gets registered to the network adapter.

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