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DHCPv6 Operation Overview
The operation of DHCPv6 is similar to that of DHCPv4, but the protocol itself has been completely rewritten. It is not based on the older DHCP or on BOOTP, except in conceptual terms. It still uses UDP but uses new port numbers, a new message format, and restructured options. All of this means that the new protocol is not strictly compatible with DHCPv4 or BOOTP, though I believe work is underway on a method to allow DHCPv6 servers to work with IPv4 devices.
Key Concept: Since DHCP works with IP addresses and other configuration parameters, the change from IPv4 to IPv6 requires a new version of DHCP commonly called DHCPv6. This new DHCP represents a significant change from the original DHCP, and is still under development. DHCPv6 is used for IPv6 “stateful” autoconfiguration; the alternative is stateless autoconfiguration, a feature of IPv6 that allows a client to determine its IP address without need for a server.
DHCPv6 is also oriented around IPv6 methods of addressing, especially the use of link-local scoped multicast addresses. This allows efficient communication even before a client has been assigned an IP address. Once a client has an address and knows the identity of a server it may communicate with the server directly using unicast addressing.
There are two basic client/server message exchanges that are used in DHCPv6: the four-message exchange and the two-message exchange. The former is used when a client needs to obtain an IPv6 address and other parameters. This process is similar to the regular DHCP address allocation process; highly simplified, it involves these steps:
There is also a shorter variation of the four-message process above, where a client sends a Solicit message and indicates that a server should respond back immediately with a Reply message.
If the client already has an IP address, either assigned manually or obtained in some other way, a simpler process can be undertaken, similar to how in regular DHCP the DHCPINFORM message is used:
As in regular DHCP, a DHCPv6 client renews its lease after a period of time by sending a Renew message. DHCPv6 also supports relay agent functionality as in DHCPv4.