1. Introduction and Overview

1. Introduction and Overview

   This document describes DHCP for IPv6 (DHCP), a client/server protocol that provides managed configuration of devices.

   DHCP can provide a device with addresses assigned by a DHCP server and other configuration information, which are carried in options. DHCP can be extended through the definition of new options to carry configuration information not specified in this document.

   DHCP is the "stateful address autoconfiguration protocol" and the "stateful autoconfiguration protocol" referred to in "IPv6 Stateless Address Autoconfiguration" [17].

   The operational models and relevant configuration information for DHCPv4 [18][19] and DHCPv6 are sufficiently different that integration between the two services is not included in this document. If there is sufficient interest and demand, integration can be specified in a document that extends DHCPv6 to carry IPv4 addresses and configuration information.

   The remainder of this introduction summarizes DHCP, explaining the message exchange mechanisms and example message flows. The message flows in sections 1.2 and 1.3 are intended as illustrations of DHCP operation rather than an exhaustive list of all possible client-server interactions. Sections 17, 18, and 19 explain client and server operation in detail.

1.1. Protocols and Addressing

Clients and servers exchange DHCP messages using UDP [15]. The client uses a link-local address or addresses determined through other mechanisms for transmitting and receiving DHCP messages.

DHCP servers receive messages from clients using a reserved, link-scoped multicast address. A DHCP client transmits most messages to this reserved multicast address, so that the client need not be configured with the address or addresses of DHCP servers.

To allow a DHCP client to send a message to a DHCP server that is not attached to the same link, a DHCP relay agent on the client's link will relay messages between the client and server. The operation of the relay agent is transparent to the client and the discussion of message exchanges in the remainder of this section will omit the description of message relaying by relay agents.

Once the client has determined the address of a server, it may under some circumstances send messages directly to the server using unicast.

1.2. Client-server Exchanges Involving Two Messages

When a DHCP client does not need to have a DHCP server assign it IP addresses, the client can obtain configuration information such as a list of available DNS servers [20] or NTP servers [21] through a single message and reply exchanged with a DHCP server. To obtain configuration information the client first sends an Information-Request message to the All_DHCP_Relay_Agents_and_Servers multicast address. Servers respond with a Reply message containing the configuration information for the client.

This message exchange assumes that the client requires only configuration information and does not require the assignment of any IPv6 addresses.

When a server has IPv6 addresses and other configuration information committed to a client, the client and server may be able to complete the exchange using only two messages, instead of four messages as described in the next section. In this case, the client sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers requesting the assignment of addresses and other configuration information. This message includes an indication that the client is willing to accept an immediate Reply message from the server. The server that is willing to commit the assignment of addresses to the client

immediately responds with a Reply message. The configuration information and the addresses in the Reply message are then immediately available for use by the client.

Each address assigned to the client has associated preferred and valid lifetimes specified by the server. To request an extension of the lifetimes assigned to an address, the client sends a Renew message to the server. The server sends a Reply message to the client with the new lifetimes, allowing the client to continue to use the address without interruption.

1.3. Client-server Exchanges Involving Four Messages

To request the assignment of one or more IPv6 addresses, a client first locates a DHCP server and then requests the assignment of addresses and other configuration information from the server. The client sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers address to find available DHCP servers. Any server that can meet the client's requirements responds with an Advertise message. The client then chooses one of the servers and sends a Request message to the server asking for confirmed assignment of addresses and other configuration information. The server responds with a Reply message that contains the confirmed addresses and configuration.

As described in the previous section, the client sends a Renew message to the server to extend the lifetimes associated with its addresses, allowing the client to continue to use those addresses without interruption.