Figure 8-3 The address autoconfiguration process for a host (Part 2)

DHCPv6

DHCPv6 is defined in RFC 3315 to provide stateful address configuration or stateless configuration settings for IPv6 hosts. An IPv6 host uses a configuration protocol, such as DHCPv6, based on the following flags in the Router Advertisement message sent by a neighboring router:

■Managed Address Configuration flag This is also known as the M flag. When set to 1, this flag instructs the host to use a configuration protocol to obtain stateful addresses.

■Other Stateful Configuration flag This is also known as the O flag. When set to 1, this flag instructs the host to use a configuration protocol to obtain other configuration settings.

The combinations of the values of the M and O flags are the following:

■Both M and O flags are set to 0 This combination corresponds to a network without a DHCPv6 infrastructure. Hosts use router advertisements for non-link-local addresses, and they use other methods (such as manual configuration) to configure other settings.

■Both M and O flags are set to 1 In this combination, DHCPv6 is used for both addresses and other configuration settings. This combination is known as DHCPv6 stateful, in which a DHCPv6 server is assigning stateful addresses to IPv6 hosts.

be configured with other settings, such as the IPv6 addresses of Domain Name System (DNS) servers, this is an unlikely combination.

Like DHCP for IPv4, the components of a DHCPv6 infrastructure consist of DHCPv6 clients that request configuration, DHCPv6 servers that provide configuration, and DHCPv6 relay agents that relay messages between clients and servers when clients are on subnets that do not include a DHCPv6 server.

More Info You can configure an IPv6 router running Windows Server 2008 or Windows Vista to set the M flag to 1 in router advertisements with the netsh interface ipv6 set interface InterfaceNameOrIndex managedaddress=enabled command. Similarly, you can set the O flag to 1 in router advertisements with the netsh interface ipv6 set interface InterfaceNameOrIndex otherstateful=enabled command. For more information about configuring an IPv6 router running Windows Server 2008 or Windows Vista, see Chapter 10, “IPv6 Routing.”

DHCPv6 Messages

As in DHCP for IPv4, DHCPv6 messages are User Datagram Protocol (UDP) messages. DHCPv6 clients listen for DHCP messages on UDP port 546. DHCPv6 servers and relay agents listen for DHCPv6 messages on UDP port 547. The structure for DHCPv6 messages is much simpler than for DHCP for IPv4, which had its historical origins in the BOOTP protocol to support diskless workstations. Figure 8-4 shows the structure of DHCPv6 messages sent between client and server.

198 Understanding IPv6, Second Edition

Msg-Type

Transaction-ID

Options

• • •

Figure 8-4 DHCPv6 messages between client and server

The following list describes the fields in a DHCPv6 message:

■Msg-Type This 1-byte field indicates the type of DHCPv6 message. See Table 8-1.

■Transaction-ID This 3-byte field is determined by a client and used to group the messages of a DHCPv6 message exchange together. DHCPv6 servers copy the value of the Transaction-ID field from request messages to their corresponding reply messages.

■Options This variable-sized field contains one or more options that are used to contain client and server identification information, stateful IPv6 addresses, and other configuration settings.

Table 8-1 lists the DHCPv6 message types defined in RFC 3315.

Information-Request, Confirm,

Release, or Decline message.

contains a server message encapsulated as the DHCPv6 Relay-Message option.

DHCPv6 options are formatted in type-length-value (TLV) format. Figure 8-5 shows the structure of DHCPv6 options.

Figure 8-5 DHCPv6 options

The following list describes the fields in a DHCPv6 option:

■Option-Code This 2-byte field indicates the type of DHCPv6 option.

■Option-Len This 2-byte field indicates the length of the Option-Data field in bytes.

■Option-Data This variable-sized field contains the data for the option.

200 Understanding IPv6, Second Edition

There is a separate message structure for the messages exchanged between relay agents and servers to record additional information, such as the subnet from which the message originated so that the DHCPv6 server can determine the appropriate subnet prefix to assign to the client. Figure 8-6 shows the structure of DHCPv6 messages sent between relay agents and servers.

Msg-Type

Hop-Count

Link-Address

Peer-Address

Options

• • •

Figure 8-6 DHCPv6 messages between relay agent and server

The following list describes the additional fields in the DHCPv6 messages between relay and server:

■Hop-Count This 1-byte field indicates the number of relay agents that have received the message. A receiving relay agent can discard the message if it exceeds a configured maximum hop count.

■Link-Address This 16-byte field contains a non-link-local address that is assigned to an interface of the relay agent that is connected to the subnet on which the client is located. From the Link-Address field, the server can determine the correct address IPv6 scope from which to assign an address.

■Peer-Address This 16-byte field contains the IPv6 address of the client that originally sent the message or the previous relay agent that relayed the message.

Beyond the Peer-Address field are DHCPv6 options that include the Relay Message option, which contains the message being relayed, and other relay options. The Relay Message option provides an encapsulation of the DHCPv6 messages being exchanged between the client and the server.

Because there are no broadcast addresses defined for IPv6, the use of the limited broadcast address for some DHCP for IPv4 messages has been replaced with the use of the All_DHCP_Relay_Agents_and_Servers address of FF02::1:2 for DHCPv6. For example, a DHCPv6 client attempting to discover the location of the DHCPv6 server on the network sends a Solicit message from its link-local address to FF02::1:2. If there is a DHCPv6 server on the host’s subnet, it receives the Solicit message and sends an appropriate reply. More typically, a DHCPv6 relay agent on the host’s subnet receives the Solicit message and forwards it to a DHCPv6 server.