Thursday, November 3, 2011

IPv6 Autoconfiguration

IPv6 supports autoconfiguration of globally unique addresses. With stateless autoconfiguration, a router sends periodical router advertisement messages to all nodes on the local link for them to autoconfigure their IPv6 addresses. An IPv6 host uses the 48-bit global routing prefix and the 16-bit SLA advertised by the router as the first 64 bits for its address, and its 48-bit MAC address in Extended Universal Identifier 64-bit (EUI-64) format as the last 64 bits for its address.

Autoconfiguration provides the plug-and-play feature which allows devices to connect to the Internet without any IP address configuration or DHCP server. Plug-and-play is the key feature to provide Internet connectivity for devices such as cordless phones and even bread toasters.

The stateless autoconfiguration process is initiated when a host sends a router solicitation message upon system startup to request for an immediate transmission of a router advertisement message, which contains the autoconfiguration information from an IPv6 router on the local link, without waiting for the next scheduled router advertisement message from an IPv6 router.

An IPv6 router must be configured with router advertisement retransmission timer and other parameters (eg: router lifetime, reachable time) for the operation of stateless autoconfiguration.

Comparison of Stateful and Stateless DHCPv6, as well as Stateless Autoconfiguration:

Feature Stateful DHCPv6 Stateless DHCPv6 Stateless Autoconfiguration
Dynamically assign IPv6 address to client Yes No Yes
Supplies useful information, eg: domain name, DNS and NTP servers IP addresses, etc. Yes Yes No

Note: Stateless autoconfiguration often works in conjunction with Stateless DHCPv6.

IPv6 provides a mechanism to detect duplicate addresses and prevent address collision, although the use of MAC address to derive the Interface ID and eventually the IPv6 address almost guarantees a unique address. Neighbor solicitation messages are used to detect duplicate address on the link. Duplicate address detection occurs during the autoconfiguration process.

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