draft-ietf-dhc-dhcpv6-28.txt

DHCPv6协议在Linux操作系统下的一个客户端实现。

Droms (ed.), et al.             Expires 30 Apr 2003             [Page v]

Internet Draft              DHCP for IPv6 (-28)               2 Nov 2002


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" [21].

   The operational models and relevant configuration information for
   DHCPv4 [1][6] 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 [19].  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.






Droms (ed.), et al.             Expires 30 Apr 2003             [Page 1]

Internet Draft              DHCP for IPv6 (-28)               2 Nov 2002


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 [8] or NTP servers [22]
   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




Droms (ed.), et al.             Expires 30 Apr 2003             [Page 2]

Internet Draft              DHCP for IPv6 (-28)               2 Nov 2002


   addresses, allowing the client to continue to use those addresses
   without interruption.


2. Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [3].

   This document also makes use of internal conceptual variables
   to describe protocol behavior and external variables that an
   implementation must allow system administrators to change.  The
   specific variable names, how their values change, and how their
   settings influence protocol behavior are provided to demonstrate
   protocol behavior.  An implementation is not required to have them in
   the exact form described here, so long as its external behavior is
   consistent with that described in this document.


3. Background

   The IPv6 Specification provides the base architecture and design of
   IPv6.  Related work in IPv6 that would best serve an implementor
   to study includes the IPv6 Specification [5], the IPv6 Addressing
   Architecture [9], IPv6 Stateless Address Autoconfiguration [21], IPv6
   Neighbor Discovery Processing [17], and Dynamic Updates to DNS [23].
   These specifications enable DHCP to build upon the IPv6 work to
   provide both robust stateful autoconfiguration and autoregistration
   of DNS Host Names.

   The IPv6 Addressing Architecture specification [9] defines the
   address scope that can be used in an IPv6 implementation, and the
   various configuration architecture guidelines for network designers
   of the IPv6 address space.  Two advantages of IPv6 are that support
   for multicast is required and nodes can create link-local addresses
   during initialization.  The availability of these features means that
   a client can use its link-local address and a well-known multicast
   address to discover and communicate with DHCP servers or relay agents
   on its link.

   IPv6 Stateless Address Autoconfiguration [21] specifies procedures
   by which a node may autoconfigure addresses based on router
   advertisements [17], and the use of a valid lifetime to support
   renumbering of addresses on the Internet.  In addition the
   protocol interaction by which a node begins stateless or stateful
   autoconfiguration is specified.  DHCP is one vehicle to perform
   stateful autoconfiguration.  Compatibility with stateless address
   autoconfiguration is a design requirement of DHCP.

   IPv6 Neighbor Discovery [17] is the node discovery protocol in IPv6
   which replaces and enhances functions of ARP [18].  To understand




Droms (ed.), et al.             Expires 30 Apr 2003             [Page 3]

Internet Draft              DHCP for IPv6 (-28)               2 Nov 2002


   IPv6 and stateless address autoconfiguration it is strongly
   recommended that implementors understand IPv6 Neighbor Discovery.

   Dynamic Updates to DNS [23] is a specification that supports the
   dynamic update of DNS records for both IPv4 and IPv6.  DHCP can use
   the dynamic updates to DNS to integrate addresses and name space to
   not only support autoconfiguration, but also autoregistration in
   IPv6.


4. Terminology

   This sections defines terminology specific to IPv6 and DHCP used in
   this document.


4.1. IPv6 Terminology

   IPv6 terminology relevant to this specification from the IPv6
   Protocol [5], IPv6 Addressing Architecture [9], and IPv6 Stateless
   Address Autoconfiguration [21] is included below.

      address                   An IP layer identifier for an interface
                                or a set of interfaces.

      host                      Any node that is not a router.

      IP                        Internet Protocol Version 6 (IPv6).  The
                                terms IPv4 and IPv6 are used only in
                                contexts where it is necessary to avoid
                                ambiguity.

      interface                 A node's attachment to a link.

      link                      A communication facility or medium over
                                which nodes can communicate at the link
                                layer, i.e., the layer immediately
                                below IP. Examples are Ethernet (simple
                                or bridged); Token Ring; PPP links,
                                X.25, Frame Relay, or ATM networks; and
                                Internet (or higher) layer "tunnels",
                                such as tunnels over IPv4 or IPv6
                                itself.

      link-layer identifier     A link-layer identifier for an
                                interface.  Examples include IEEE 802
                                addresses for Ethernet or Token Ring
                                network interfaces, and E.164 addresses
                                for ISDN links.

      link-local address        An IPv6 address having link-only
                                scope, indicated by having the prefix
                                (FE80::/10), that can be used to reach



Droms (ed.), et al.             Expires 30 Apr 2003             [Page 4]

Internet Draft              DHCP for IPv6 (-28)               2 Nov 2002


                                neighboring nodes attached to the same
                                link.  Every interface has a link-local
                                address.

      multicast address         An identifier for a set of interfaces
                                (typically belonging to different
                                nodes).  A packet sent to a multicast
                                address is delivered to all interfaces
                                identified by that address.

      neighbor                  A node attached to the same link.

      node                      A device that implements IP.

      packet                    An IP header plus payload.

      prefix                    The initial bits of an address, or a
                                set of IP addresses that share the same
                                initial bits.

      prefix length             The number of bits in a prefix.

      router                    A node that forwards IP packets not
                                explicitly addressed to itself.

      unicast address           An identifier for a single interface.
                                A packet sent to a unicast address is
                                delivered to the interface identified by
                                that address.


4.2. DHCP Terminology

   Terminology specific to DHCP can be found below.


      appropriate to the link   An address is "appropriate to the link"