1.3 Protocol Overview (Synopsis)

DHCP uses the following basic steps to automatically configure a network address and configuration information on a DHCP client. The application of DHCP discussed here is an illustrative example of an IPv4 network, where the DHCP client and the DHCP server are on the same subnet and the client machine has no prior IP address configured on the network interface. DHCP may also be used by a client to obtain configuration parameters (other than the IP address) from the DHCP server. For further details, see section 3.4 of [RFC2131].

1. When the TCP/IP protocol initializes and DHCP has been enabled on any of the client machine's interfaces, the DHCP client sends a DHCPDISCOVER message to find the DHCP servers on the network and to obtain a valid IPv4 address configuration. The DHCP client includes a Vendor Class Identifier Option that contains "vendor-class identifier" information about the host, such as the operating system version.
2. All DHCP servers that receive the DHCPDISCOVER message and have been configured with valid IPv4 address configuration for the client send a DHCPOFFER message back to the DHCP client. The DHCP servers optionally include other configuration information for the client in the DHCPOFFER message, in case the client wants to select the specific configuration information it desires. If configuration information is included, then based on the vendor class identifier that the client included in the message, the DHCP servers also include any specific standard options or vendor-specific options appropriate to hosts running that operating system version. If no specific standard options or vendor-specific options are defined for hosts running that operating system version, the server ignores the Vendor Class Identifier Option sent by the client.
3. The DHCP client selects an IPv4 address configuration to use from the DHCPOFFER messages that it receives. The DHCP client then sends a DHCPREQUEST message to the selected DHCP server by using the Server ID option, requesting the use of the selected configuration. The client again includes its vendor class identifier in the message.
4. The DHCPREQUEST message identifies the server that sent the offer that the DHCP client selected. The DHCP servers for which the Server Identifier sent by the client in the DHCP REQUEST does not match the Server ID put the offered IPv4 address back into the available pool of addresses. The selected DHCP server assigns the IPv4 address configuration to the DHCP client and sends a DHCPACK (acknowledgment) message to the DHCP client. The DHCP servers include configuration information, including any specific standard options or vendor-specific options based on the vendor class identifier sent by the client in the DHCPREQUEST message.

The DHCP client computer completes the TCP/IP initialization. It then repeats the preceding steps for other interfaces, if present, for which DHCP is enabled. (See the following figure.) After this is complete, the client can use all TCP/IP services and applications for normal network communications and connectivity to other IPv4 hosts.

Figure 1: Basic DHCP process

The DHCP client may decline an offer from a DHCP server if it determines that the IP address included in the DHCPOFFER message sent by the server is already in use on that network. If so, the DHCP client sends a DHCPDECLINE message and restarts the configuration process by sending a DHCPDISCOVER message again.

The DHCP server may send a DHCPNAK message in response to the client's DHCPREQUEST message if one or more of the desired configuration options sent by the client in that message are unacceptable. In this case, the DHCP client restarts the configuration process by sending a DHCPDISCOVER message again.

The DHCP client may relinquish its lease on the IP address by sending a DHCPRELEASE message to the server.

In some cases, the DHCP client may remember and want to reuse an IP address that was previously allocated by the DHCP server to it. In this case, the client may begin the initialization process by sending a DHCPREQUEST message to the server containing that network address as the "requested IP address". The DHCP server sends a DHCPACK message to the client if it chooses to allow the client to continue to use that IP address. Otherwise, the DHCP server may send a DHCPNAK message to the client.

For further details on the DHCP protocol overview, refer to section 3 of [RFC2131].

DHCPv6 uses the following basic steps to automatically configure a network address on a DHCPv6 client. The application of DHCPv6 discussed here is an illustrative example of an IPv6 network. The DHCPv6 client and the DHCPv6 server are on the same subnet, and the client machine has no prior IPv6 address configured on the network interface. DHCPv6 may also be used by a client to obtain configuration parameters (other than the IP address) from the DHCPv6 server. Details are as specified in [RFC3315] sections 1, 18.1.5, and 18.2.5.

1. When the TCP/IP protocol initializes and DHCPv6 has been enabled on any of the client machine's interfaces, the DHCPv6 client sends a DHCPv6 Solicit message to the All_DHCP_Relay_Agents_and_Servers multicast address specified in [RFC3315] to discover the available DHCPv6 servers. The DHCPv6 client includes a Vendor Class Option that contains information about the host, such as the operating system version.
2. All DHCPv6 servers that receive the DHCPv6 Solicit message from the client and have been configured with valid IPv6 address configuration information for the client send a DHCPv6 Advertise message in response to the DHCPv6 client. The DHCPv6 servers optionally include other configuration information for the client in the DHCPv6 Advertise message, in case the client wants to select the specific configuration information it desires. If configuration information is included, then based on the vendor class information that the client included in the message, the DHCPv6 servers also include any specific standard options or vendor-specific options appropriate to hosts running that operating system version. If no specific standard options or vendor-specific options are defined for hosts running that operating system version, the DHCPv6 servers ignore the Vendor Class Option sent by the client.
3. The DHCPv6 client selects an IPv6 address configuration to use from the DHCPv6 Advertise messages that it receives. The DHCPv6 client then sends a DHCP Request message to the selected DHCPv6 server by using the Server Identifier option, requesting the use of the selected configuration. The client again includes a Vendor Class Option in the message.
4. The DHCPv6 Request message identifies the server that sent the offer that the DHCPv6 client selected. The DHCPv6 servers for which the Server Identifier sent by the client in DHCPv6 Request does not match the Server Identifier put the offered IPv6 address back into the available pool of addresses. The selected DHCPv6 server assigns the IPv6 address configuration to the DHCPv6 client and sends a DHCPv6 Reply message with no Status code option or with a Status code option with the value Success to the DHCPv6 client. The DHCPv6 servers include the configuration information, including any specific standard options or vendor-specific options based on the vendor class information sent by the client in the DHCPv6 Request message.

The presence of a Status code option with any value other than Success in a DHCPv6 message from the server to the client is construed as a failure, and the DHCPv6 client then restarts the initialization process by sending the DHCPv6 Solicit message again.

The DHCPv6 client computer completes the TCP/IP initialization as described in the preceding steps. It then repeats the preceding steps for other interfaces, if present, for which DHCPv6 is enabled. (See the following figure.) After this is complete, the client can use all TCP/IP services and applications for normal network communications and connectivity to other IPv6 hosts.

Figure 2: Basic DHCPv6 process

The DHCPv6 client may decline an offer from a DHCP server if it finds that the IPv6 address included in the DHCPv6 Advertise message sent by the server is already in use on that network. If so, the DHCPv6 client sends a DHCPv6 Decline message and restarts the configuration process by sending a DHCPv6 Solicit message again.

The DHCPv6 server may send a DHCPv6 Reply message with a Status Code option with a value other than Success in response to the client's DHCPv6 Request message if one or more of the desired configuration options sent by the client in that message are unacceptable. In this case, the DHCPv6 client restarts the configuration process by sending a DHCPv6 Solicit message again.

The DHCPv6 client may relinquish its lease on the IP address by sending a DHCPv6 Release message to the server.

In some cases, the DHCPv6 client may remember and want to reuse an IP address that was previously allocated by the DHCPv6 server to it. In this case, the client may begin the initialization process by sending a DHCPv6 Renew or Rebind message to the server containing that network address as the "requested IP address." The DHCPv6 server sends a DHCPv6 Reply message with no Status Code option or with a Status Code option with the value Success to the client if it chooses to allow the client to continue to use that IPv6 address. Otherwise, the DHCPv6 server may send a DHCP Reply message to the client with a value other than Success.

For further details of the DHCPv6 protocol overview, refer to Section 3 of [RFC3315].