The architecture and message exchanges in DHCPv6 are similar to DHCPv4. A DHCPv6 client initiates a DHCPv6 transaction by first locating a DHCPv6 server, and then making a request for configuration information from that server. As in DHCPv4, an IPv6 address is assigned to a host with a lease, and the host can initiate a transaction with the DHCPv6 server to extend the lease on an address.
A DHCPv6 client uses a link-local address when exchanging messages with a DHCPv6 server. To avoid the requirement that a DHCPv6 server be attached to every link, DHCPv6 relay agents forward DHCPv6 messages between hosts and off-link servers. The mechanism through which relay agents forward DHCPv6 messages allows for the use of multiple relay agents between a host and a server. Relay agent options, through which a relay agent can provide additional information to the DHCPv6 server, are included as a design feature in the base DHCPv6 specification.
The address assignment mechanism in DHCPv6 allows for the assignment of multiple addresses to an interface, and allows for the dynamic assignment of additional addresses over time. Addresses are assigned to a host with a lease, a preferred lifetime and a valid lifetime. The mechanism can support renumbering through the assignment of new addresses whose lifetimes overlap existing addresses to allow for graceful transition. Addresses are grouped together for management into an “identity association”, which the host and server exchange for address assignment. DHCPv6 can also be used for assignment of temporary addresses [RFC3041].
Each DHCPv6 host has a “DHCP Unique Identifier” (DUID), which remains unchanged throughout the lifetime of the host. Servers use this DUID to identify hosts reliably even if the host roam between links.
Security is included in the DHCPv6 base specification. The security mechanism uses a framework similar to the security mechanism for DHCPv4 defined in RFC 3118 [RFC3118]. In addition, security for messages exchanged between relay agents and servers is provided by the use of IPSec.
A DHCPv6 server can trigger a message exchange with a host through the Reconfigure message. Security is included for the Reconfigure message to prevent intruder attacks against DHCPv6 clients. Stateless DHCPv6 uses a two-message exchange between a client and a server. To obtain configuration information without address assignment through stateless DHCPv6, the host sends an Information-request message. The DHCPv6 server responds with the requested configuration information. The DHCPv6 server can be configured with host-specific configuration, to allow for customized configuration of different classes of hosts. Stateless DHCPv6 service requires only a subset of the mechanism and messages of the full DHCPv6 protocol, and is easier to implement and deploy.
An ISP wishing to delegate a prefix or prefixes to a customer can use the prefix delegation option. To use prefix delegation, the CPE initiates a DHCPv6 transaction with the ISP edge router. The ISP router selects the prefix or prefixes to be assigned to the customer, through the ISP’s policy or customer provisioning process, and returns those prefixes to the CPE. The prefixes are then available for use in the customer’s network. For example, the customer may be assigned a /48 prefix, which is delegated to the CPE through DHCPv6 prefix delegation. The CPE can then assign /64 prefixes from the delegated /48 prefix to links in the customer’s network.