Internet Engineering Task Force                                 J. Bound                    R. Droms (ed.), Cisco
INTERNET DRAFT                                 J. Bound, Hewlett Packard
DHC Working Group                                              M. Carney                                  Bernie Volz, Ericsson
Obsoletes:  draft-ietf-dhc-dhcpv6-24.txt           Sun Microsystems, Inc  draft-ietf-dhc-dhcpv6-25.txt              Ted Lemon, Nominum
                                       C. Perkins Perkins, Nokia Research Center
                                                               Ted Lemon
                                                                 Nominum
                                                             Bernie Volz
                                                                Ericsson
                                                           R. Droms(ed.)
                                                           Cisco Systems
                                                             May 24
                                             M. Carney, Sun Microsystems
                                                            June 9, 2002

         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
                      draft-ietf-dhc-dhcpv6-25.txt
                      draft-ietf-dhc-dhcpv6-26.txt

Status of This Memo

   This document is a submission by the Dynamic Host Configuration
   Working Group of the Internet Engineering Task Force (IETF). Comments
   should be submitted to the dhcwg@ietf.org mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at:
         http://www.ietf.org/ietf/1id-abstracts.txt
    The list of Internet-Draft Shadow Directories can be accessed at:
         http://www.ietf.org/shadow.html.

Abstract

   The Dynamic Host Configuration Protocol for IPv6 (DHCP) enables
   DHCP servers to pass configuration parameters such as IPv6 network
   addresses to IPv6 nodes.  It offers the capability of automatic
   allocation of reusable network addresses and additional configuration
   flexibility.  This protocol is a stateful counterpart to "IPv6
   Stateless Address Autoconfiguration" (RFC2462), and can be used
   separately or concurrently with the latter to obtain configuration
   parameters.

                                Contents

Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction and Overview                                          1                                          2
     1.1. Protocols and addressing  . . . . . . . . . . . . . . . .    2
     1.2. Protocol implementation . . . . . . . . . . . . . . . . .    2
     1.3. Client-server exchanges involving two messages  . . . . .    3
     1.4.
     1.3. Client-server exchanges involving four messages . . . . .    3

 2. Requirements                                                       4

 3. Background                                                         4

 4. Terminology                                                        5
     4.1. IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .    5
     4.2. DHCP Terminology  . . . . . . . . . . . . . . . . . . . .    6

 5. DHCP Constants                                                     8
     5.1. Multicast Addresses . . . . . . . . . . . . . . . . . . .    8
     5.2. UDP ports . . . . . . . . . . . . . . . . . . . . . . . .    8
     5.3. DHCP message types  . . . . . . . . . . . . . . . . . . .    8
     5.4. Status Codes  . . . . . . . . . . . . . . . . . . . . . .   10
     5.5. Configuration Transmission and Retransmission Parameters  . . . . . . . . . . . . . . . .   10   11

 6. Message Formats                                                   11

 7. Relay agent messages                                              12
     7.1. Relay-forward message . . . . . . . . . . . . . . . . . .   12   13
     7.2. Relay-reply message . . . . . . . . . . . . . . . . . . .   13

 8. Representation and use of domain names                            13

 9. DHCP unique identifier (DUID)                                     13                                     14
     9.1. DUID contents . . . . . . . . . . . . . . . . . . . . . .   14
     9.2. DUID based on link-layer address plus time  . . . . . [DUID-LLT] . .   14
     9.3. Vendor-assigned unique ID DUID assigned by vendor based on Enterprise Number
             (VUID-EN) number
             [DUID-EN]  . . . . . . . . . . . . . . . . . . . . . .   15   16
     9.4. Link-layer DUID based on link-layer address [DUID-LL]  . . . . . . . . . . . . . . . . . . .   16   17

10. Identity association                                              17

11. Selecting addresses for assignment to an IA                       17                       18

12. Management of temporary addresses                                 18                                 19

13. Transmission of messages by a client                              19

14. Reliability of Client Initiated Message Exchanges                 19                 20

15. Message validation                                                21
    15.1. Use of Transaction-ID field Transaction IDs  . . . . . . . . . . . . . . . . .   21
    15.2. Solicit message . . . . . . . . . . . . . . . . . . . . .   21   22
    15.3. Advertise message . . . . . . . . . . . . . . . . . . . .   21   22
    15.4. Request message . . . . . . . . . . . . . . . . . . . . .   22
    15.5. Confirm message . . . . . . . . . . . . . . . . . . . . .   22
    15.6. Renew message . . . . . . . . . . . . . . . . . . . . . .   22   23
    15.7. Rebind message  . . . . . . . . . . . . . . . . . . . . .   22   23
    15.8. Decline messages  . . . . . . . . . . . . . . . . . . . .   23
    15.9. Release message . . . . . . . . . . . . . . . . . . . . .   23
   15.10. Reply message . . . . . . . . . . . . . . . . . . . . . .   23   24
   15.11. Reconfigure message . . . . . . . . . . . . . . . . . . .   24
   15.12. Information-request message . . . . . . . . . . . . . . .   24   25
   15.13. Relay-forward message . . . . . . . . . . . . . . . . . .   24   25
   15.14. Relay-reply message . . . . . . . . . . . . . . . . . . .   24   25

16. Client Source Address and Interface Selection                     24                     25

17. DHCP Server Solicitation                                          25
    17.1. Client Behavior . . . . . . . . . . . . . . . . . . . . .   25   26
          17.1.1. Creation of Solicit messages  . . . . . . . . . .   25   26
          17.1.2. Transmission of Solicit Messages  . . . . . . . .   26   27
          17.1.3. Receipt of Advertise messages . . . . . . . . . .   27   28
          17.1.4. Receipt of Reply message  . . . . . . . . . . . .   28   29
    17.2. Server Behavior . . . . . . . . . . . . . . . . . . . . .   28   29
          17.2.1. Receipt of Solicit messages . . . . . . . . . . .   28   29
          17.2.2. Creation and transmission of Advertise messages .   29
          17.2.3. Creation and Transmission of Reply messages . . .   30

18. DHCP Client-Initiated Configuration Exchange                      30                      31
    18.1. Client Behavior . . . . . . . . . . . . . . . . . . . . .   31
          18.1.1. Creation and transmission of Request messages . .   31   32
          18.1.2. Creation and transmission of Confirm messages . .   32   33
          18.1.3. Creation and transmission of Renew messages . . .   33   34
          18.1.4. Creation and transmission of Rebind messages  . .   35
          18.1.5. Creation and Transmission of Information-request
                          messages . . . . . . . . . . . . . . . . .  36
          18.1.6. Receipt of Reply message in response to a Request,
                          Confirm, Renew, Rebind or Information-request
                          message  . . . . . . . . . . . . . . . . .  36
          18.1.7. Creation and transmission of Release messages . .   37
          18.1.8. Receipt of Reply message in response to a Release
                          message  . . . . . . . . . . . . . . . . .  39
          18.1.9.
          18.1.7. Creation and transmission of Decline messages . .   39
         18.1.10.   38
          18.1.8. Receipt of Reply message in response to a Decline
                          message  . . . . . messages . . . . . . . . . . . .  40   38
    18.2. Server Behavior . . . . . . . . . . . . . . . . . . . . .   40
          18.2.1. Receipt of Request messages . . . . . . . . . . .   40
          18.2.2. Receipt of Confirm messages . . . . . . . . . . .   41
          18.2.3. Receipt of Renew messages . . . . . . . . . . . .   42   41
          18.2.4. Receipt of Rebind messages  . . . . . . . . . . .   43   42
          18.2.5. Receipt of Information-request messages . . . . .   43
          18.2.6. Receipt of Release messages . . . . . . . . . . .   44
          18.2.7. Receipt of Decline messages . . . . . . . . . . .   45   44
          18.2.8. Transmission of Reply messages  . . . . . . . . .   45

19. DHCP Server-Initiated Configuration Exchange                      45
    19.1. Server Behavior . . . . . . . . . . . . . . . . . . . . .   46   45
          19.1.1. Creation and transmission of Reconfigure messages   46   45
          19.1.2. Time out and retransmission of Reconfigure
                          messages . . . . . . . . . . . . . . . . .  47
          19.1.3.  46
    19.2. Receipt of Renew messages . . . . . . . . . . . . . . . .   47
    19.2.
    19.3. Receipt of Information-request messages . . . . . . . . .   47
    19.3.
    19.4. Client Behavior . . . . . . . . . . . . . . . . . . . . .   47
          19.3.1.
          19.4.1. Receipt of Reconfigure messages . . . . . . . . .   48
          19.3.2.   47
          19.4.2. Creation and transmission of Renew messages . . .   48
          19.3.3.
          19.4.3. Creation and transmission of Information-request
                          messages . . . . . . . . . . . . . . . . .  49
          19.3.4.  48
          19.4.4. Time out and retransmission of Renew or
                          Information-request messages . . . . . . .  49
          19.3.5.
          19.4.5. Receipt of Reply messages . . . . . . . . . . . .   49

20. Relay Agent Behavior                                              49
    20.1. Relaying of Forwarding a client messages message or a Relay-forward message  .   49
          20.1.1. Forwarding a message from a client  . . . . . . .   49
          20.1.2. Forwarding a message from a relay agent . . . . .   50
    20.2. Forwarding a Relay-reply message  . .   49
    20.2. Relaying of server messages . . . . . . . . . .   50
    20.3. Construction of Relay-reply messages  . . . . .   50 . . . . .   51

21. Authentication of DHCP messages                                   50                                   51
    21.1. DHCP threat model . . . . . . . . . . . . . . . . . . . .   51
    21.2. Security of messages sent between servers and relay agents  51  52
    21.3. Summary of DHCP authentication  . . . . . . . . . . . . .   51   52
    21.4. Replay detection  . . . . . . . . . . . . . . . . . . . .   52   53
    21.5. Delayed authentication protocol . . . . . . . . . . . . .   52   53
          21.5.1. Management issues in the delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  52
          21.5.2. Use of the Authentication option in the delayed
                          authentication protocol  . . . . . . . . .  53
          21.5.3.
          21.5.2. Message validation  . . . . . . . . . . . . . . .   54
          21.5.4.   55
          21.5.3. Key utilization . . . . . . . . . . . . . . . . .   54
          21.5.5.   55
          21.5.4. Client considerations for delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  55
          21.5.6.
          21.5.5. Server considerations for delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  56  57

22. DHCP options                                                      57
    22.1. Format of DHCP options  . . . . . . . . . . . . . . . . .   57   58
    22.2. Client Identifier option  . . . . . . . . . . . . . . . .   58
    22.3. Server Identifier option  . . . . . . . . . . . . . . . .   58   59
    22.4. Identity Association option . . . . . . . . . . . . . . .   59   60
    22.5. Identity Association for Temporary Addresses option . . .   61
    22.6. IA Address option . . . . . . . . . . . . . . . . . . . .   62   63
    22.7. Option Request option . . . . . . . . . . . . . . . . . .   63   64
    22.8. Preference option . . . . . . . . . . . . . . . . . . . .   64   65
    22.9. Elapsed Time option . . . . . . . . . . . . . . . . . . . . . .   64   65
   22.10. Client message Relay Message option  . . . . . . . . . . . . . . . . . .   65   66
   22.11. Server message option . . . . . . . . . . . . . . . . . .   65
   22.12. Authentication option . . . . . . . . . . . . . . . . . .   66
   22.13.
   22.12. Server unicast option . . . . . . . . . . . . . . . . . .   67
   22.14.
   22.13. Status Code Option  . . . . . . . . . . . . . . . . . . .   67
   22.15.   68
   22.14. Rapid Commit option . . . . . . . . . . . . . . . . . . .   68
   22.16.   69
   22.15. User Class Option option . . . . . . . . . . . . . . . . . . . .   69
   22.17.   70
   22.16. Vendor Class Option . . . . . . . . . . . . . . . . . . .   70
   22.18.   71
   22.17. Vendor-specific Information option  . . . . . . . . . . .   71
   22.19.   72
   22.18. Interface-Id Option . . . . . . . . . . . . . . . . . . .   72
   22.20.   73
   22.19. Reconfigure Message option  . . . . . . . . . . . . . . .   73
   22.21.   74
   22.20. Reconfigure Nonce option  . . . . . . . . . . . . . . . .   73   74

23. Security Considerations                                           74                                           75

24. IANA Considerations                                               74                                               76
    24.1. Multicast addresses . . . . . . . . . . . . . . . . . . .   75   77
    24.2. DHCP message types  . . . . . . . . . . . . . . . . . . .   75   77
    24.3. DHCP options  . . . . . . . . . . . . . . . . . . . . . .   76   78
    24.4. Status codes  . . . . . . . . . . . . . . . . . . . . . .   76   79
    24.5. DUID  . . . . . . . . . . . . . . . . . . . . . . . . . .   77
    24.6. Authentication option . . . . . . . . . . . . . . . . . .   77   79

25. Acknowledgments                                                   77                                                   79

26. Changes in draft-ietf-dhc-dhcpv6-25.txt                           78

References                           80

27. Changes in draft-ietf-dhc-dhcpv6-26.txt                           82

References                                                            83

Chair's Address                                                       81                                                       84

Authors' Addresses                                                    81                                                    85

 A. Appearance of Options in Message Types                            83                            86

 B. Appearance of Options in the Options Field of DHCP Options        83        86

 C. Full Copyright Statement                                          84                                          87

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.  The addresses and additional
   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 RFC2462, "IPv6 Stateless
   Address Autoconfiguration". Autoconfiguration" [20].

   The remainder of this introduction summarizes DHCP, explaining
   the message exchange mechanisms operational models and example message flows.  The
   message flows in sections 1.3 relevant configuration information
   for DHCPv4 and 1.4 DHCPv6 are intended as illustrations
   of DHCP operation 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 [16]. [18].  The
   client uses its a link-local address or addresses determined through stateless
   autoconfiguration
   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 forward 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 forwarding 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. Protocol implementation

   This specification for DHCP includes Client-server exchanges involving two messages and descriptions of
   client and server behavior for several different functions.  Some
   clients and servers will be deployed in situations in which not all
   of the functions will be required.  For example,

   When a client that uses
   stateless autoconfiguration to determine its IPv6 addresses would
   use only the Information-request and Reply messages to obtain other
   configuration information.

   Clients and servers that do not use all of the functions of DHCP
   need not implement processing for those DHCP messages that will not
   be used.  A client or server that receives a message that it is does not
   prepared need to process may simply discard that message.  For example, have a DHCP server that only provides configuration information and does not
   do IPv6 address assignment can respond to only Information-request
   messages and discard other messages such as Solicit or Request
   messages.

1.3. Client-server exchanges involving two messages

   A DHCP assign
   it IP addresses, the client can obtain configuration information
   such as a list of available DNS servers [7] 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.  The server responds 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.  Because the server need not keep any dynamic state
   information about individual clients to support this two message
   exchange, a server that provides just configuration information can
   be realized with a relatively simple and small implementation.

   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.

1.4.

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
   messages to the server to extend the lifetimes associated with its
   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 [2].

   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 [3], [4], the IPv6 Addressing
   Architecture [6], [8], IPv6 Stateless Address Autoconfiguration [18], [20], IPv6
   Neighbor Discovery Processing [14], [16], and Dynamic Updates to DNS [19]. [22].
   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 [6] [8] 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.  This means that a client can immediately use
   its link-local address and a well-known multicast address to begin
   communications to discover neighbors on the link.  For instance, a
   client can send a Solicit message and locate a server or relay agent.

   IPv6 Stateless Address Autoconfiguration [18] [20] specifies procedures
   by which a node may autoconfigure addresses based on router
   advertisements [14], [16], 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 [14] [16] is the node discovery protocol in IPv6
   which replaces and enhances functions of ARP [15]. [17].  To understand
   IPv6 and stateless address autoconfiguration it is strongly
   recommended that implementors understand IPv6 Neighbor Discovery.

   Dynamic Updates to DNS [19] [22] 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 [3], [4], IPv6 Addressing Architecture [6], [8], and IPv6 Stateless
   Address Autoconfiguration [18] [20] 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::0000/10), that can be used to
                                reach 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"
                                when the address is consistent with the
                                DHCP server's knowledge of the network
                                topology, prefix assignment and address
                                assignment policies

      binding                   A binding (or, client binding) is a
                                group of server data records containing
                                the information the server has about
                                the addresses in an IA and any other or configuration
                                information explicitly assigned to the
                                client.  Configuration information that
                                has been returned to a client through a
                                policy - for example, the information
                                returned to all clients on the same
                                link - does not require a binding.  A
                                binding containing information about
                                an IA is indexed by the tuple <DUID,
                                IA-type, IAID> (where IA-type is the
                                type of address in the IA; for example, temporary)
                                temporary).  A binding containing
                                configuration information for a client
                                is indexed by <DUID>.

      configuration parameter   An element of the configuration
                                information set on the server and
                                delivered to the client using DHCP.
                                Such parameters may be used to carry
                                information to be used by a node to
                                configure its network subsystem and
                                enable communication on a link or
                                internetwork, for example.

      DHCP                      Dynamic Host Configuration Protocol
                                for IPv6.  The terms DHCPv4 and DHCPv6
                                are used only in contexts where it is
                                necessary to avoid ambiguity.

      DHCP client (or client)   A node that initiates requests on a link
                                to obtain configuration parameters from
                                one or more DHCP servers.

      DHCP domain               A set of links managed by DHCP and
                                operated by a single administrative
                                entity.

      DHCP relay agent (or relay agent) A node that acts as an
                                intermediary to deliver DHCP messages
                                between clients and servers, and is on
                                the same link as the client.

      DHCP server (or server)   A node that responds to requests from
                                clients, and may or may not be on the
                                same link as the client(s).

      DUID                      A DHCP Unique IDentifier for a DHCP
                                participant; each DHCP client and server
                                has exactly one DUID. See section 9 for
                                details of the ways in which a DUID may
                                be constructed.

      Identity association (IA) A collection of addresses assigned to
                                a client.  Each IA has an associated
                                IAID. A client may have more than one
                                IA assigned to it; for example, one for
                                each of its interfaces.

      Identity association identifier (IAID) An identifier for an IA,
                                chosen by the client.  Each IA has an
                                IAID, which is chosen to be unique among
                                all IAIDs for IAs belonging to that
                                client.

      message                   A unit of data carried as the payload
                                of a UDP datagram, exchanged among DHCP
                                servers, relay agents and clients.

      reconfiguration nonce     A 64 bit     An opaque value used to provide security
                                for Reconfigure messages.  A
                                server generates a cryptographically
                                strong random number as a nonce and
                                sends that nonce value to the client.
                                The server then includes the nonce in
                                any Reconfigure messages it sends to the
                                client.

      transaction-ID

      transaction ID            An opaque value used to match responses
                                with replies initiated either by a
                                client or server.

5. DHCP Constants

   This section describes various program and networking constants used
   by DHCP.

5.1. Multicast Addresses

   DHCP makes use of the following multicast addresses:

      All_DHCP_Relay_Agents_and_Servers (FF02::1:2) A link-scoped
                  multicast address used by a client to communicate with
                  neighboring (i.e., on-link) relay agents and servers.
                  All servers and relay agents are members of this
                  multicast group.

      All_DHCP_Servers (FF05::1:3) A site-scoped multicast address used
                  by a client or relay agent to communicate with servers, either
                  because the client or relay agent wants to send messages to
                  all servers or because it does not know the unicast
                  addresses of the servers.  Note that in order for
                  a client or relay agent to use this address, it must have an
                  address of sufficient scope to be reachable by the
                  servers.  All servers within the site are members of
                  this multicast group.

5.2. UDP ports

   Clients listen for DHCP messages on UDP port 546.  Servers and relay
   agents listen for DHCP messages on UDP port 547.

5.3. DHCP message types

   DHCP defines the following message types.  More detail on these
   message types can be found in Section 6.  Message types not listed
   here are reserved for future use.  The message code for each message
   type is shown with the message name.

      SOLICIT (1)        A client sends a Solicit message to locate
                         servers.

      ADVERTISE (2)      A server sends an Advertise message to indicate
                         that it is available for DHCP service, in
                         response to a Solicit message received from a
                         client.

      REQUEST (3)        A client sends a Request message to request
                         configuration parameters parameters, including IP
                         addresses, from a specific server.

      CONFIRM (4)        A client sends a Confirm message to any
                         available server when it detects that it may
                         have moved to a new link to request that the
                         servers verify that determine whether the
                         addresses and current
                         configuration parameters it was assigned by are still appropriate
                         to the server link to which the client are still valid. is connected.

      RENEW (5)          A client sends a Renew message to the server
                         that originally provided the client's addresses
                         and configuration parameters to extend the
                         leases on the addresses assigned to the client
                         and to update other configuration parameters.

      REBIND (6)         A client sends a Rebind message to any
                         available server to extend the leases on the
                         addresses assigned to the client and to update
                         other configuration parameters; this message is
                         sent after a client receives no response to a
                         Renew message.

      REPLY (7)          A server sends a Reply message containing
                         assigned addresses and configuration parameters
                         in response to a Solicit, Request, Renew,
                         Rebind or Information-request message received from a client.  A
                         server sends a Reply message containing
                         configuration parameters in response to an
                         Information-request message.  A server sends a
                         Reply message confirming or denying the validity of that
                         the client's addresses and configuration parameters are appropriate to
                         the link to which the client is connected in
                         response to a Confirm message.  A server sends
                         a Reply message to acknowledge receipt of a
                         Release or Decline message.

      RELEASE (8)

                         A client server sends a Release Reply message to the server
                         that containing
                         assigned addresses and configuration parameters
                         in response to the client to
                         indicate that the client will no longer use one
                         or more of the assigned addresses.

      DECLINE (9)        A client sends a Decline message to Solicit, Request, Renew,
                         Rebind message received from a client.  A
                         server sends a Reply message containing
                         configuration parameters in response to an
                         Information-request message.  A server sends a
                         Reply message in response to a Confirm message
                         confirming or denying that the addresses
                         assigned to the client are appropriate to the
                         link to which the client is connected.  A
                         server sends a Reply message to acknowledge
                         receipt of a Release or Decline message.

      RELEASE (8)        A client sends a Release message to the server
                         that assigned addresses to the client to
                         indicate that the client will no longer use one
                         or more of the assigned addresses.

      DECLINE (9)        A client sends a Decline message to a server to
                         indicate that the client has determined that
                         one or more addresses assigned by the server
                         are already in use on the link to which the
                         client is connected.

      RECONFIGURE (10)   A server sends a Reconfigure message to a
                         client to inform the client that the server has
                         new or updated configuration parameters, and
                         that the client is to initiate a Renew/Reply
                         or Information-request/Reply transaction with
                         the server in order to receive the updated
                         information.

      INFORMATION-REQUEST (11) A client sends an Information-request
                         message to a server to request configuration
                         parameters without the assignment of any IP
                         addresses to the client.

      RELAY-FORW (12)    A relay agent sends a Relay-forward message
                         to forward client messages to servers. servers, either
                         directly or through another relay agent.  The
                         client message is encapsulated in an option in
                         the Relay-forward message.

      RELAY-REPL (13)    A server sends a Relay-reply message to a relay
                         agent
                         agent, either directly or through another relay
                         agent, to send messages to clients through
                         the relay agent.  The server encapsulates the
                         client message as an option in the Relay-reply
                         message, which the relay agent extracts and
                         forwards to the client.

5.4. Status Codes

   DHCPv6 uses status codes to communicate the success or failure of
   operations requested in messages from clients and servers, and to
   provide additional information about the specific cause of the
   failure of a message.  The specific status codes are defined in
   section 24.4.

5.5. Configuration Transmission and Retransmission Parameters

   This section presents a table of configuration parameters values used to describe the message
   transmission behavior of clients and servers.

      Parameter     Default  Description
   -------------------------------------
   MIN_SOL_DELAY
   MAX_SOL_DELAY     1 sec   Min delay of first Solicit
   MAX_SOL_DELAY     5 secs   Max delay of first Solicit
   SOL_TIMEOUT     500 msecs       1 sec   Initial Solicit timeout
   SOL_MAX_RT       30      120 secs  Max Solicit timeout value
   REQ_TIMEOUT     250 msecs       1 sec   Initial Request timeout
   REQ_MAX_RT       30 secs  Max Request timeout value
   REQ_MAX_RC       10       Max Request retry attempts
   CNF_TIMEOUT     250 msecs       1 sec   Initial Confirm timeout
   CNF_MAX_RT        1 sec        4 secs  Max Confirm timeout
   CNF_MAX_RD       10 secs  Max Confirm duration
   REN_TIMEOUT      10 sec   Initial Renew timeout
   REN_MAX_RT      600 secs  Max Renew timeout value
   REB_TIMEOUT      10 secs  Initial Rebind timeout
   REB_MAX_RT      600 secs  Max Rebind timeout value
   INF_TIMEOUT     500 ms       1 sec   Initial Information-request timeout
   INF_MAX_RT       30      120 secs  Max Information-request timeout value
   REL_TIMEOUT     250 msecs       1 sec   Initial Release timeout
   REL_MAX_RT        1 sec        0       Max Release timeout
   REL_MAX_RC        5       MAX Release attempts
   DEC_TIMEOUT     250 msecs       1 sec   Initial Decline timeout
   DEC_MAX_RT        1 sec        0       Max Decline timeout
   DEC_MAX_RC        5       Max Decline attempts
   REC_TIMEOUT       1       2 sec   Initial Reconfigure timeout
   REC_MAX_RC        8       Max Reconfigure attempts
   HOP_COUNT_LIMIT   4       Max hop count in a Relay-forward message

6. Message Formats

   All DHCP messages sent between clients and servers share an identical
   fixed format header and a variable format area for options.

   All values in the message header and in options are in network byte
   order.

   Options are stored serially in the options field, with no padding
   between the options.  Options are byte-aligned but are not aligned in
   any other way such as on 2 or 4 byte boundaries.

   The following diagram illustrates the format of DHCP messages sent
   between clients and servers:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |               transaction-ID               transaction-id                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                            options                            .
     .                           (variable)                          .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type             Identifies the DHCP message type; the
                           available message types are listed in
                           section 5.3.

      transaction-id       An unsigned integer used by a client or
                           server to match a response       The transaction ID for this message to a
                           request message. exchange.

      options              Options carried in this message; options are
                           described in section 22.

7. Relay agent messages

   Relay agents exchange messages with servers to forward messages
   between clients and servers that are not connected to the same link.

   There are two relay agent messages, which share the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |   hop-count   |
     +-+-+-+-+-+-+-+-+                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                         link-address                                                               |
     |                         link-address                          |
     |                                                               |
     |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               |                               |
     +-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                        client-address                                                               |
     |                         peer-address                          |
     |                                                               |
     |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               |                               |
     +-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     .                                                               .
     .            options (variable number and length)   ....        .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The following sections describe the use of the Relay Agent message
   header.

7.1. Relay-forward message

   The following table defines the use of message fields in a
   Relay-forward message.

      msg-type       RELAY-FORW

      hop-count      Number of relay agents that have forwarded this
                     message

      link-address   A global or site-local address that will be used by
                     the server to identify the link on which the client
                     is located.

      client-address

      peer-address   The address of the client or relay agent from which
                     the message to be forwarded was received

      options        MUST include a "Client message "Relay Message option" (see
                     section 22.10); MAY include other options added by
                     the relay agent

7.2. Relay-reply message

   The following table defines the use of message fields in a
   Relay-reply message.

      msg-type       RELAY-REPL

      hop-count      Number of relay agents that have forwarded this
                     message

      link-address     The link-address copied   Copied from the Relay-forward message

      client-address

      peer-address   The client's address, copied from client or relay agent address to which the
                       relay-forward message

      options          MUST include a "Server
                     message option"; contained in the Relay Message option in
                     this Relay-reply message is to be forwarded

      options        MUST include a "Relay Message option"; see
                     section 22.11; 22.10; MAY include other options

8. Representation and use of domain names

   So that domain names may be encoded uniformly, a domain name or a
   list of domain names is encoded using the technique described in
   section 3.1 of RFC1035 [11]. [13].  A domain name or list of domain names
   in DHCP MUST NOT be stored in compressed form as described in section
   4.1.4 of RFC1035.

9. DHCP unique identifier (DUID)

   Each DHCP client and server has a DUID. DHCP servers use DUIDs to
   identify clients for the selection of configuration parameters and
   in the association of IAs with clients.  DHCP clients use DUIDs to
   identify a server in messages where a server needs to be identified.
   See sections 22.2 and 22.3 for the representation of a DUID in a DHCP
   message.

   Clients and servers MUST treat DUIDs as opaque values and MUST only
   compare DUIDs for equality.  Clients and servers MUST NOT in any
   other way interpret DUIDs.  Clients and servers MUST NOT restrict
   DUIDs to the types defined in this document as additional DUID types
   may be defined in the future.

   The DUID is carried in an option because it may be variable length
   and because it is not required in all DHCP messages.  The DUID is
   designed to be unique across all DHCP clients and servers, and
   consistent stable
   for any specific client or server - that is, the DUID used by a
   client or server SHOULD NOT change over time if at all possible; for
   example, a device's DUID should not change as a result of a change in
   the device's network hardware.

   The motivation for having more than one type of DUID is that the DUID
   must be globally unique, and must also be easy to generate.  The sort
   of globally-unique identifier that is easy to generate for any given
   device can differ quite widely.  Also, some devices may not contain
   any persistent storage.  Retaining a generated DUID in such a device
   is not possible, so the DUID scheme must accommodate such devices.

9.1. DUID contents

   A DUID consists of a two octet type code represented in network byte
   order, followed by a variable number of octets that make up the
   actual identifier.  A DUID can be no more than 256 octets long. long (not
   including the type code).  The following types are currently defined:

       1        Link-layer address plus time
       2        Vendor-assigned unique ID based on domain name
       3        Vendor-assigned unique ID based on Enterprise Number
       4
       3        Link-layer address

   Formats for the variable field of the DUID for each of the above
   types are shown below.

9.2. DUID based on link-layer address plus time [DUID-LLT]

   This type of DUID consists of a two octet type field containing the
   value 1, a two octet hardware type code, four octets containing
   a time value, followed by link-layer address of any one network
   interface that is connected to the DHCP device at the time that the
   DUID is generated.  The time value is the time that the DUID is
   generated represented in seconds since midnight (UTC), January 1,
   2000, modulo 2^32.  The hardware type MUST be a valid hardware type
   assigned by the IANA as described in the section on ARP in RFC 826.
   Both the time and the hardware type are stored in network byte order.
   The link-layer address is stored in canonical form, as described in
   RFC2464 [3].

   The following diagram illustrates the format of a DUID based on
   link-layer address plus time: DUID-LLT:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               1               |    Hardware type (16 bits)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Time (32 bits)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .             link-layer address (variable length)              .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The choice of network interface can be completely arbitrary, as long
   as that interface provides a globally unique link-layer address for
   the link type, and the same DUID DUID-LLT should be used in configuring
   all network interfaces connected to the device, regardless of which
   interface's link-layer address was used to generate the DUID. DUID-LLT.

   Clients and servers using this type of DUID MUST store the DUID DUID-LLT
   in stable storage, and MUST continue to use this DUID DUID-LLT even if the
   network interface used to generate the DUID DUID-LLT is removed.  Clients
   and servers that do not have any stable storage MUST NOT use this
   type of DUID.

   Clients and servers that use this DUID SHOULD attempt to configure
   the time prior to generating the DUID, if that is possible, and MUST
   use some sort of time source (for example, a real-time clock) in
   generating the DUID, even if that time source could not be configured
   prior to generating the DUID. The use of a time source makes it
   unlikely that two identical DUIDs DUID-LLTs will be generated if the
   network interface is removed from the client and another client then
   uses the same network interface to generate a DUID. DUID-LLT. A DUID collision
   between two DUID-LLTs is very unlikely even if the clocks haven't
   been configured prior to generating the DUID.

   This method of DUID generation is recommended for all general purpose
   computing devices such as desktop computers and laptop computers, and
   also for devices such as printers, routers, and so on, that contain
   some form of writable non-volatile storage.

   Despite our best efforts, it is possible that this algorithm for
   generating a DUID could result in a client identifier collision.

   A DHCP client that generates a DUID DUID-LLT using this mechanism MUST
   provide an administrative interface that replaces the existing DUID
   with a newly-generated DUID of this type. DUID-LLT.

9.3. Vendor-assigned unique ID based on Enterprise Number (VUID-EN)

   The vendor-assigned unique ID DUID assigned by vendor based on Enterprise Number number [DUID-EN]

   This form of DUID is assigned by the vendor to the device.  It
   consists of the vendor's registered Private Enterprise Number as
   maintained by IANA [7] [9] followed by a unique identifier assigned by
   the value of the identifier. vendor.

   The following diagram summarizes the structure of a VUID-EN: DUID-EN:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               3               2               |       enterprise-number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   enterprise-number (contd)   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   .                           identifier                          .
   .                       (variable length)                       .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The source of the identifier is left up to the vendor defining it,
   but each identifier part of each VUID-EN DUID-EN MUST be unique to the device
   that is using it, and MUST be assigned to the device at the time of
   manufacture and stored in some form of non-volatile storage.  The VUID
   generated DUID SHOULD be recorded in non-erasable storage.  The
   enterprise-number is the vendor's registered Private Enterprise
   Number as maintained by IANA [7]. [9].  The enterprise-number is stored as
   an unsigned 32 bit number.

   An example DUID of this type might look like this:

   +---+---+---+---+---+---+---+---+
   | 0 | 3 2 | 0 | 0 | 0 |  9| 12|192|
   +---+---+---+---+---+---+---+---+
   |132|221| 3 | 0 | 9 | 18|
   +---+---+---+---+---+---+

   This example includes the two-octet type of 3, 2, the Enterprise
   Number (9), followed by eight octets of identifier data. data
   (0x0CC084D303000912).

9.4. Link-layer DUID based on link-layer address [DUID-LL]

   This type of DUID consists of two octets containing the DUID type 4, 3,
   a two octet network hardware type code, followed by the link-layer
   address of any one network interface that is permanently connected to
   the client or server device.  For example, this DUID could be used
   by a host that has a network
   interface implemented in a chip that is unlikely to be removed and
   used elsewhere. elsewhere could use a DUID-LL. The hardware type MUST be a valid
   hardware type assigned by the IANA as described in the section on
   ARP in RFC 826.  The hardware type is stored in network byte order.
   The link-layer address is stored in canonical form, as described in
   RFC2464 [3].

   The following diagram illustrates the format of a DUID based on
   link-layer address: DUID-LL:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               4               3               |    Hardware type (16 bits)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .             link-layer address (variable length)              .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The choice of network interface can be completely arbitrary, as
   long as that interface provides a unique link-layer address and is
   permanently attached to the device on which the DUID DUID-LL is being
   generated.  The same DUID should be used in configuring all network
   interfaces connected to the device, regardless of which interface's
   link-layer address was used to generate the DUID.

   This type of DUID

   DUID-LL is recommended for devices that have a permanently-connected
   network interface with a link-layer address and do not have
   nonvolatile, writable stable storage.  This type of DUID  DUID-LL MUST NOT be used by
   DHCP clients or servers that cannot tell whether or not a network
   interface is permanently attached to the device on which the DHCP
   client is running.

10. Identity association

   An "identity-association" (IA) is a construct through which a server
   and a client can identify, group and manage a set of related IPv6
   addresses.  Each IA consists of an IAID and associated configuration
   information.

   A client must associate at least one distinct IA with each of
   its network interfaces and uses that IA to obtain configuration
   information from a server for that interface.  Each IA must be
   associated with exactly one interface.

   The IAID uniquely identifies the IA and must be chosen to be unique
   among the IAIDs on the client.  The IAID is chosen by the client.
   For any given use of an IA by the client, the IAID for that IA MUST
   be consistent across restarts of the DHCP client.  The client may
   maintain consistency either by storing the IAID in non-volatile
   storage or by using an algorithm that will consistently produce the
   same IAID as long as the configuration of the client has not changed.
   There may be no way for a client to maintain consistency of the IAIDs
   if it does not have non-volatile storage and the client's hardware
   configuration changes.

   The configuration information in an IA consists of one or more IPv6
   addresses and other parameters.  The parameters are specified as DHCP
   options within the IA, and are associated along with the addresses in the
   IA times T1 and T2 for the interface to which IA. See section 22.4
   for the representation of an IA belongs.  Other parameters that
   are not associated with a particular interface may be specified in
   the options section of a DHCP message, outside the scope of any IA. message.

   Each address in an IA has a preferred lifetime and a valid lifetime,
   as defined in RFC2462 [18]. [20].  The lifetimes are transmitted from the
   DHCP server to the client in the IA option.  The lifetimes apply to
   the use of IPv6 addresses as described in section 5.5.4 of RFC2462.

   See section 22.4 for the representation of an IA in a DHCP message.

11. Selecting addresses for assignment to an IA

   A server selects addresses to be assigned to an IA according to the
   address assignment policies determined by the server administrator
   and the specific information the server determines about the client
   from some combination of the following sources:

    -  The link to which the client is attached.  The server determines
       the link as follows:

        *  If the server receives the message directly from the client
           and the source address in the IP datagram in which the
           message was received is a link-local address, then the client
           is on the same link to which the interface over which the
           message was received is attached

        *  If the server receives the message from a forwarding relay
           agent, then the client is on the same link as the one to
           which the interface identified by the link-address field in
           the message from the relay agent is attached

        *  If the server receives the message directly from the client
           and the source address in the IP datagram in which the
           message was received is not a link-local address, then the
           client is on the link identified by the source address in the
           IP datagram (note that this situation can occur only if the
           server has enabled the use of unicast message delivery by the
           client and the client has sent a message for which unicast
           delivery is allowed)

    -  The DUID supplied by the client
    -  Other information in options supplied by the client

    -  Other information in options supplied by the relay agent

   Any unicast address assigned by a server that is based on an
   EUI-64 identifier MUST include an interface identifier with the "u"
   (universal/local) and "g" (individual/group) bits of the interface
   identifier set appropriately, as indicated in section 2.5.1 of RFC
   2373.
   2373 [8].

   A server MUST NOT assign an address that is otherwise reserved for
   some other purpose.  These reserved addresses may be specified as
   128-bit IPv6 addresses or as interface identifiers that are reserved
   for all subnets.  For example, a server MUST NOT assign reserved
   anycast addresses, as defined in RFC2526, from any subnet.

12. Management of temporary addresses

   A client may be assigned temporary addresses (temporary addresses are
   defined in RFC 3041 [13]).  Clients and servers simply identify
   addresses as "temporary". [15]).  DHCPv6 handling of address assignment
   is no different for temporary addresses.  DHCPv6 says nothing about
   details of temporary addresses like lifetimes, how clients use
   temporary addresses, rules for generating successive temporary
   addresses, etc.

   Clients ask for temporary addresses and servers assign them.
   Temporary addresses are carried in the Identity Association for
   Temporary Addresses (IA_TA) option (see section 22.5).  Each IA_TA
   option contains at most one temporary address for each of the
   prefixes on the link to which the client is attached.

   Unless otherwise stated, an IA_TA option is used in the same way in
   as an IA option.  In the protocol specification, unless otherwise
   stated, a reference to an IA should be read as either an IA or an
   IA_TA.

   The IAID number space for the IA_TA option IAID number space is
   separate from the IA option IAID number space.

   The server MAY update the DNS for a temporary address as described in
   section 4 of RFC3041.

13. Transmission of messages by a client

   Unless otherwise specified, specified in this document or in a document that
   describes how IPv6 is carried over a specific type of link (for link
   types that do not support multicast), a client sends DHCP messages to
   the All_DHCP_Relay_Agents_and_Servers.

   If the

   A client is attached uses multicast to a link reach all servers or an individual server.
   An individual server is indicated by specifying that supports multicast
   transmission, server's DUID in
   a Server Identifier option (see section 22.3) in the client sends DHCP messages to client's message
   (all servers will receive this message but only the
   All_DHCP_Relay_Agents_and_Servers address. indicated server
   will respond).  All servers are indicated by not supplying this
   option.

   A client may send some messages directly to a server using unicast,
   as described in section 22.13. 22.12.

14. Reliability of Client Initiated Message Exchanges

   DHCP clients are responsible for reliable delivery of messages in the
   client-initiated message exchanges described in sections 17 and 18.
   If a DHCP client fails to receive an expected response from a server,
   the client must retransmit its message.  This section describes the
   retransmission strategy to be used by clients in client-initiated
   message exchanges.

   Note that the procedure described in this section is slightly
   modified when used with the Solicit message.  The modified procedure
   is described in section 17.1.2.

   The client begins the message exchange by transmitting a message to
   the server.  The message exchange terminates when either the client
   successfully receives the appropriate response or responses from a
   server or servers, or when the message exchange is considered to have
   failed according to the retransmission mechanism described below.

   The client retransmission behavior is controlled and described by the
   following variables:

      RT     Retransmission timeout

      IRT    Initial retransmission time

      MRC    Maximum retransmission count

      MRT    Maximum retransmission time

      MRD    Maximum retransmission duration

      RAND   Randomization factor

   With each message transmission or retransmission, the client sets RT
   according to the rules given below.  If RT expires before the message
   exchange terminates, the client recomputes RT and retransmits the
   message.

   Each of the computations of a new RT include a randomization factor
   (RAND), which is a random number chosen with a uniform distribution
   between -0.1 and +0.1.  The randomization factor is included to
   minimize synchronization of messages transmitted by DHCP clients.
   The algorithm for choosing a random number does not need to be
   cryptographically sound.  The algorithm SHOULD produce a different
   sequence of random numbers from each invocation of the DHCP client.

   RT for the first message transmission is based on IRT:

      RT = IRT + RAND*IRT

   RT for each subsequent message transmission is based on the previous
   value of RT:

      RT = 2*RTprev + RAND*RTprev

   MRT specifies an upper bound on the value of RT. If MRT has a value
   of 0, there is no upper limit on the value of RT. Otherwise:

    if (RT > MRT)
       RT = MRT + RAND*MRT

   MRC specifies an upper bound on the number of times a client may
   retransmit a message.  Unless MRC is zero, the message exchange fails
   once the client has transmitted the message MRC times.

   MRD specifies an upper bound on the length of time a client may
   retransmit a message.  Unless MRD is zero, the message exchange fails
   once MRD seconds have elapsed since the client first transmitted the
   message.

   If both MRC and MRD are non-zero, the message exchange fails whenever
   either of the conditions specified in the previous two paragraphs are
   met.

   If both MRC and MRD are zero, the client continues to transmit the
   message until it receives a response.

15. Message validation

   Clients and servers SHOULD discard any messages that contain options
   that are not allowed to appear in the received message.  For example,
   an Information-request message must not include an IA option.
   Clients and server MAY choose to extract information from such a
   message if the information is of use to the recipient.

   Message validation based on DHCP authentication is discussed in
   section 21.5.3. 21.5.2.

15.1. Use of Transaction-ID field Transaction IDs

   The "transaction-ID" "transaction-id" field holds a value used by clients and servers
   to synchronize server responses to client messages.  A client
   SHOULD
   choose generate a different transaction-ID random number that cannot easily be guessed or
   predicted to use as the transaction ID for each new message it sends.

   Note that if a client generates easily predictable transaction
   identifiers, it may become more vulnerable to certain kinds of
   attacks from off-path intruders.  A client MUST leave the transaction-ID transaction
   ID unchanged in retransmissions of a message.

15.2. Solicit message

   Clients MUST discard any received Solicit messages.

   Servers MUST discard any Solicit messages that do not include a
   Client Identifier option or that do include a Server Identifier
   option.

15.3. Advertise message

   Clients MUST discard any received Advertise messages that meet any of
   the following conditions:

    -  the message does not include a Server Identifier option

    -  the message does not include a Client Identifier option

    -  the contents of the Client Identifier option does not match the
       client's DUID

    -  the "Transaction-ID" "transaction-id" field value does not match the value the
       client used in its Solicit message

   Servers and relay agents MUST discard any received Advertise
   messages.

15.4. Request message

   Clients MUST discard any received Request messages.

   Servers MUST discard any received Request message that meet any of
   the following conditions:

    -  the message does not include a Server Identifier option

    -  the contents of the Server Identifier option do not match the
       server's identifier DUID

    -  the message does not include a Client Identifier option

15.5. Confirm message

   Clients MUST discard any received Confirm messages.

   Servers MUST discard any received Confirm messages received that do not
   include a Client Identifier option or that do include a Server
   Identifier option.

15.6. Renew message

   Clients MUST discard any received Renew messages.

   Servers MUST discard any received Renew message that meets fails to meet
   any of the following conditions:

    -  the message does not MUST include a Server Identifier option

    -  the contents of the Server Identifier option do not MUST match the
       server's identifier

    -  the message does not MUST include a Client Identifier option

15.7. Rebind message

   Clients MUST discard any received Rebind messages.

   Servers MUST discard any received Rebind messages that do not include
   a Client Identifier option or that do include a Server Identifier
   option.

15.8. Decline messages

   Clients MUST discard any received Decline messages.

   Servers MUST discard any received Decline message that meets fails to meet
   any of the following conditions:

    -  the message does not MUST include a Server Identifier option

    -  the contents of the Server Identifier option do not MUST match the
       server's identifier

    -  the message does not MUST include a Client Identifier option

15.9. Release message

   Clients MUST discard any received Release messages.

   Servers MUST discard any received Release message that meets fails to meet
   any of the following conditions:

    -  the message does not MUST include a Server Identifier option
    -  the contents of the Server Identifier option do not MUST match the
       server's identifier

    -  the message does not MUST include a Client Identifier option

15.10. Reply message

   Clients MUST discard any received Reply messages message that fails to meet
   any of the following conditions:

    -  the message does not MUST include a Server Identifier option

    -  the "transaction-ID" "transaction-id" field in the message does not MUST match the value
       used in the original message

    -  the message does not MUST include a Client Identifier option and the
       original message from the client contained a Client Identifier
       option

    -  if the client included a Client Identifier option in the original
       message, the message includes MUST include a Client Identifier option
       and the contents of the Client Identifier option does not MUST match the
       DUID of the client or or, if the client did not include a Client
       Identifier option in the original message, the Reply message MUST
       NOT include a Client Identifier option

   Servers and relay agents MUST discard any received Reply messages.

15.11. Reconfigure message

   Servers and relay agents MUST discard any received Reconfigure
   messages.

   Clients MUST discard any Reconfigure messages that fails any of the
   following conditions:

    -  the message MUST include a Server Identifier option

    -  the message MUST include a Client Identifier option that contains
       the client's DUID

    -  the message MUST include one of the available security
       mechanisms:

        *  the server sends a Reconfigure Nonce option whose value
           matches the current server nonce value known to the client

        *  the server uses DHCP authentication:  beginitemize

        *

            +  the message MUST contain an authentication option

        *
            +  the message MUST pass the authentication validation
               performed by the client

15.12. Information-request message

   Clients MUST discard any received Information-request messages.

   Servers MUST discard any received Information-request message that
   includes a Server Identifier option and the DUID in the option does
   not match the server's DUID.

15.13. Relay-forward message

   Clients MUST discard any received Relay-forward messages.

15.14. Relay-reply message

   Clients and servers MUST discard any received Relay-reply messages.

16. Client Source Address and Interface Selection

   When a client sends a DHCP message to the
   All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the
   message through the interface for which configuration information is
   being requested.  However, the client MAY send the message through
   another interface attached to the same link if and only if the
   client is certain the two interface are attached to the same link.
   In addition, the client MUST SHOULD use the IPv6 link-local an IP address assigned to the
   interface for which it is requesting configuration information as
   the source address in the header of the IP datagram.

   When a  If the client sends a DHCP message directly to
   uses a server using unicast
   (after different address as the source address, the address MUST
   be assigned to an interface that will be attached to the same link
   as the interface for which the client is requesting configuration
   information when the response is received from the server.

   When a client sends a DHCP message directly to a server using unicast
   (after receiving the Server Unicast option from that server), the
   source address in the header of the IP datagram MUST be an address
   assigned to the interface for which the client is interested in
   obtaining configuration and which is suitable for use by the server
   in responding to the client.

17. DHCP Server Solicitation

   This section describes how a client locates servers that will assign
   addresses to IAs belonging to the client.

   The client is responsible for creating IAs and requesting that a
   server assign configuration information, including IPv6 addresses,
   to the IA. The client first creates an IA and assigns it an IAID.
   The client then transmits a Solicit message containing an IA option
   describing the IA. Servers that can assign configuration information
   to the IA respond to the client with an Advertise message.  The
   client then initiates a configuration exchange as described in
   section 18.

   Whenever a client initiates server solicitation with a Solicit
   message, it discards any reconfigure nonce values it may have
   previously recorded.

17.1. Client Behavior

   A client uses the Solicit message to discover DHCP servers configured
   to serve assign addresses or return other configuration parameters on the
   link to which the client is attached.

17.1.1. Creation of Solicit messages

   The client sets the "msg-type" field to SOLICIT. The client generates
   a transaction ID and inserts this value in the "transaction-ID" "transaction-id"
   field.

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client MUST include one or more includes IA options for any IAs to which
   it wants the server to assign addresses.  The client MAY include
   addresses in the IAs as a hint to the server about addresses for
   which the client has a preference.  The client MUST NOT include any
   other options in the Solicit message except as specifically allowed
   in the definition of individual options.

   The client uses IA options to request the assignment of non-temporary
   addresses and uses IA_TA options to request the assignment of
   temporary addresses.  Either IA or IA_TA options, or a combination of
   both can be included in DHCP messages.

   The client MAY request specific options from the server by including MUST include an Option Request option (see section 22.7) as a hint about
   to indicate the options the client is interested in receiving.  If the client
   requires a consistent prioritization of the options it receives, it
   includes an Option Request option indicating the options it needs
   (see section 17.2.2).  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   If the client will accept a Reply message with committed address
   assignments and other resources in response to the Solicit message,
   the client includes a Rapid Commit option (see section 22.15) 22.14) in the
   Solicit message.

17.1.2. Transmission of Solicit Messages

   The first Solicit message from the client on the interface MUST be
   delayed by a random amount of time between MIN_SOL_DELAY 0 and MAX_SOL_DELAY. In
   the case of a Solicit message transmitted when DHCP is initiated
   by IPv6 Neighbor Discovery, the delay gives the amount of time to
   wait after IPv6 Neighbor Discovery causes the client to invoke the
   stateful address autoconfiguration protocol (see section 5.5.3 of
   RFC2462).  This random delay desynchronizes clients which start at
   the same time (for example, after a power outage).

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   SOL_TIMEOUT

      MRT   SOL_MAX_RT

      MRC   0

      MRD   0

   If the client has included a Rapid Commit option and is waiting for
   a Reply in its Solicit
   message, the client terminates the retransmission waiting process as soon as a
   Reply message with a Rapid Commit option is received.  If the client
   receives an Advertise message that includes a Preference option
   with a preference value of 255, the client immediately begins a
   client-initiated message exchange (as described in section 18) by
   sending a Request message to the server from which the Advertise
   message was received.  If the client receives an Advertise message
   that does not include a Preference option with a preference value of
   255, the client continues to wait until the first RT elapses.  If the
   first RT elapses and the client has received an Advertise message,
   the client SHOULD continue with a client-initiated message exchange
   by sending a Request message.

   If the client is waiting for an Advertise message, the mechanism in
   section 14 is modified as follows for use in the transmission of
   Solicit messages.  The message exchange is not terminated by the
   receipt of an Advertise before the first RT has elapsed.  Rather, the
   client collects Advertise messages until the first RT has elapsed.
   Also, the first RT MUST be selected to be strictly greater than IRT
   by choosing RAND to be strictly greater than 0.

   A client MUST collect Advertise messages for the first RT seconds,
   unless it receives an Advertise message with a preference value
   of 255.  The preference value is carried in the Preference option
   (section 22.8).  Any Solicit Advertise that does not include a Preference
   option is considered to have a preference value of 0.  If the client
   receives an Advertise message with a preference value of 255, then
   the client SHOULD act immediately on that Advertise message without
   waiting for any additional Advertise messages.

   If the client does not receive any Advertise messages before
   the first RT has elapsed, it begins the retransmission mechanism
   described in section 14.  The client terminates the retransmission
   process as soon as it receives any Advertise message, and the client
   acts on the received Advertise message without waiting for any
   additional Advertise messages.

   A DHCP client SHOULD choose MRC and MRD to be 0.  If the DHCP client
   is configured with either MRC or MRD set to a value other than
   0, it MUST stop trying to configure the interface if the message
   exchange fails.  After the DHCP client stops trying to configure
   the interface, it SHOULD choose to restart the reconfiguration process after
   some external event, such as user input, system restart, or when the
   client is attached to a new link.

17.1.3. Receipt of Advertise messages

   The client MUST ignore any Advertise message that includes a Status
   Code option containing the value AddrUnavail, NoAddrsAvail, with the exception
   that the client MAY display the associated status message to the
   user.

   Upon receipt of one or more valid Advertise messages, the client
   selects one or more Advertise messages based upon the following
   criteria.

    -  Those Advertise messages with the highest server preference value
       are preferred over all other Advertise messages.

    -  Within a group of Advertise messages with the same server
       preference value, a client MAY select those servers whose
       Advertise messages advertise information of interest to the
       client.  For example, the client may choose a server that
       returned an advertisement with configuration options of interest
       to the client.

    -  The client MAY choose a less-preferred server if that server has
       a better set of advertised parameters, such as the available
       addresses advertised in IAs.

   Once a client has selected Advertise message(s), the client will
   typically store information about each server, such as server
   preference value, addresses advertised, when the advertisement was
   received, and so on.

   If the client needs to select an alternate server in the case that a
   chosen server does not respond, the client chooses the next server
   according to the criteria given above.

17.1.4. Receipt of Reply message

   If the client includes a Rapid Commit option in the Solicit message,
   it will expect a Reply message that includes a Rapid Commit option
   in response.  The client discards any Reply messages it receives
   that do not include a Rapid Commit option.  If the client receives
   a valid Reply message that includes a Rapid Commit option, it
   processes the message as described in section 18.1.6. 18.1.8.  If it does
   not receive such a Reply message and does receive a valid Advertise
   message, the client processes the Advertise message as described in
   section 17.1.3.

17.2. Server Behavior

   A server sends an Advertise message in response to valid Solicit
   messages it receives to announce the availability of the server to
   the client.

17.2.1. Receipt of Solicit messages

   The server determines the information about the client and its
   location as described in section 11 and checks its administrative
   policy about responding to the client.  If the server is not
   permitted to respond to the client, the server discards the Solicit
   message.

   If the client has included a Rapid Commit option in the Solicit
   message and the server has been configured to respond with committed
   address assignments and other resources, the server responds to
   the Solicit with a Reply message as described in section 17.2.3.  If
   Otherwise, the server has been configured to respond to ignores the client but has not been
   configured to respond with committed address assignments Rapid Commit option and other
   resources, processes
   the server responds with an Advertise message.

   Otherwise, remainder of the server generates and sends an Advertise message to the
   client. as if no Rapid Commit option were
   present.

17.2.2. Creation and transmission of Advertise messages

   The server sets the "msg-type" field to ADVERTISE and copies the
   contents of the transaction-ID transaction-id field from the Solicit message
   received from the client to the Advertise message.  The server
   includes its server identifier in a Server Identifier option and
   copies the Client Identifier from the Solicit message into the
   Advertise message.

   The server MAY add a Preference option to carry the preference value
   for the Advertise message.  The server implementation SHOULD allow
   the setting of a server preference value by the administrator.
   The server preference value MUST default to zero unless otherwise
   configured by the server administrator.

   The server MUST include IA includes options in the Advertise message
   containing any addresses that would be assigned to IAs contained in
   the Solicit message from the client.

   If the server will not assign any addresses return to IAs the client in
   a subsequent
   Request from Reply message.  The information in these options may
   be used by the client, client in the selection of a server MUST send an if the client
   receives more than one Advertise message to message.  If the client that includes only a status code has included
   an Option Request option with in the status
   code set to AddrUnavail and a status message for Solicit message, the user.

   The server includes other
   options in the Advertise message containing configuration parameters
   for all of the options identified in the Option Request option
   that the server will has been configured to return to the client.  The
   server MAY return additional options to the client in a subsequent Reply message. if it has been
   configured to do so.  The server SHOULD limit the options returned to
   the client so that the DHCP message header and options do not cause
   fragmentation.  The information in these options
   will be used by

   If the Solicit message from the client in included one or more IA
   options, the selection of a server if MUST include IA or IAID options in the client
   receives more than one Advertise message.
   message containing any addresses that would be assigned to IAs
   contained in the Solicit message from the client.

   If the client has included
   an Option Request option server will not assign any addresses to any IAs in a
   subsequent Request from the Solicit message, client, the server uses MUST send an Advertise
   message to the client that includes only a Status Code option as with
   code NoAddrsAvail, a hint about status message for the options user, a Server Identifier
   option with the client has server's DUID and a preference for
   receiving from Client Identifier option with the server.
   client's DUID.

   If the Solicit message was received directly by the server, the
   server unicasts the Advertise message directly to the client using
   the address in the source address field from the IP datagram in which
   the Solicit message was received.  The Advertise message MUST be
   unicast through the interface on the link from which the Solicit message was received.

   If the Solicit message was received in a Relay-forward message,
   the server constructs a Relay-reply message with the Advertise
   message in the payload of a "server-message" option.  If the
   Relay-forward messages included an Interface-id option, the server
   copies that option to the Relay-reply message.  The server unicasts
   the Relay-reply message directly to the relay agent using the
   address in the source address field from the IP datagram in which the
   Relay-forward message was received.

17.2.3. Creation and Transmission of Reply messages

   The server MUST commit the assignment of any addresses or other
   configuration information message before sending a Reply message to a
   client in response to a Solicit message.

   DISCUSSION:

      When using the Solicit-Advertise Solicit-Reply message exchange, a the server
      need not commit
      commits the assignment of configuration information
      to the client or otherwise keep state about the client
      before the server sends the Advertise message to the client.
      The client will choose one of the responding servers and
      send a Request message to obtain configuration information.
      The other servers can make any addresses they might have
      offered to the client available for assignment to other
      clients.

      When using the Solicit-Reply message exchange, the server
      commits the assignment of any addresses before sending any addresses before sending the
      Reply message.  The client can assume it has been assigned
      the addresses in the Reply message and does not need to send
      a Request message for those addresses.

      Typically, servers that are configured to use the
      Solicit-Reply message exchange will be deployed so that only
      one server will respond to a Solicit message.  If more than
      one server responds, the client will only use the addresses
      from one of the servers and the addresses from the other
      servers will be committed to the client but not used by the
      client.

      The problem of unused addresses can be minimized, for
      example, by designing the DHCP service so that only one
      server responds to the Solicit or by using relatively short
      lifetimes for assigned addresses.

   The server includes a Rapid Commit option in the Reply message to
   indicate that the Reply is in response to a Solicit message.

   The server produces the Reply message as though it had received
   a Request message, as described in section 18.2.1.  The server
   transmits the Reply message as described in section 18.2.8.

18. DHCP Client-Initiated Configuration Exchange

   A client initiates a message exchange with a server or servers
   to acquire or update configuration information of interest.  The
   client may initiate the configuration exchange as part of the
   operating system configuration process, when requested to do
   so by the application layer, when required by Stateless Address
   Autoconfiguration or as required to extend the lifetime of an address
   (Rebind
   (Renew and Renew Rebind messages).

18.1. Client Behavior

   A client will use uses Request, Confirm, Renew, Rebind and Information-request
   messages to acquire and confirm the validity of configuration
   information.  The

   If the client uses has a source address of sufficient scope that can be
   used by the server
   identifier information as a return address and information about IAs the client has received
   a Server Unicast option (section 22.12) from previous
   Advertise the server, the client
   SHOULD unicast any Request, Renew, Release and Decline messages for use to
   the server.

   DISCUSSION:

      Use of unicast may avoid delays due to forwarding of
      messages by relay agents as well as avoid overhead and
      duplicate responses by servers due to delivery of client
      messages to multiple servers.  Requiring the client to
      relay all DHCP messages through a relay agent enables the
      inclusion of relay agent options in constructing these messages. all messages sent by the
      client.  The server should enable the use of unicast only
      when relay agent options will not be used.

18.1.1. Creation and transmission of Request messages

   The client uses a Request message to populate IAs with addresses and
   obtain other configuration information.  The client includes one or
   more IA options in the Request message, with addresses and
   information about the IAs that were obtained from the server in a
   previous Advertise message.  The server then returns
   addresses and other information about the IAs to the client in IA
   options in a Reply message.

   The client generates a transaction ID and inserts this value in the
   "transaction-ID"
   "transaction-id" field.

   The client places the identifier of the destination server in a
   Server Identifier option.

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client adds any other appropriate options,
   including one or more IA options (if the client is requesting that
   the server assign it some network addresses).

   The client MAY request specific options from the server by including MUST include an Option Request option (see section 22.7) as a hint about
   to indicate the options the client is interested in receiving.  If the  The
   client
   requires a consistent prioritization of the options it receives, it
   includes an Option Request option indicating the options it needs
   (see section 18.2.1).  The client MAY include MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   If the client has a source address of sufficient scope that can be
   used by the server as a return address and the client has received
   a Server Unicast option (section 22.13) from the server, the client
   SHOULD unicast the Request message to the server.

   DISCUSSION:

      Use of multicast on a link and relay agents enables the
      inclusion of relay agent options in all messages sent by the
      client.  The server should enable the use of unicast only
      when relay agent options will not be used.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   REQ_TIMEOUT

      MRT   REQ_MAX_RT

      MRC   REQ_MAX_RC

      MRD   0

   If the message exchange fails, the client MAY choose one takes an action based on
   the client's local policy.  Examples of actions the
   following actions: client might take
   include:

    -  Select another server from a list of servers known to the client;
       for example, servers that responded with an Advertise message

    -  Initiate the server discovery process described in section 17

    -  Terminate the configuration process and report failure

18.1.2. Creation and transmission of Confirm messages

   Whenever a client may have moved to a new link, its IPv6 the prefixes from the
   addresses
   and other configuration information assigned to the interfaces on that link may no longer be valid.
   appropriate to the link.  Examples of times when a client may have
   moved to a new link include:

   Whenever a client may have moved to a new link, the prefixes from the
   addresses assigned to the interfaces on that link may no longer be
   appropriate to the link to which the client is attached.  Examples of
   times when a client may have moved to a new link include:

     o The client reboots

     o The client is physically disconnected from a wired connection

     o The client returns from sleep mode

     o The client using a wireless technology changes access points

   In any situation when a client may have moved to a new link, the
   client MUST initiate a Confirm/Reply message exchange.  The client
   includes any IAs, along with the addresses associated with those IAs,
   in its Confirm message.  Any responding servers will indicate whether
   those addresses are appropriate to the
   acceptability of link to which the addresses client is
   attached with the status in the Reply message it returns to the
   client.

   The

   In any situation when a client sets the "msg-type" field may have moved to CONFIRM. The client generates a transaction ID and inserts this value in new link, the "transaction-ID"
   field.

   The
   client MUST include initiate a Client Identifier option to identify itself
   to the server. Confirm/Reply message exchange.  The client adds
   includes any appropriate options, including
   one or more IA options.  The client MUST include IAs, along with the addresses the
   client currently has associated with those IAs.  The client fills IAs,
   in its Confirm message.  Any responding servers will indicate whether
   those addresses are appropriate to the T1 and T2 fields in link to which the IA options and client is
   attached with the preferred-lifetime and
   valid-lifetime fields status in the IA Address options with preferred values
   or 0 if Reply message it returns to the client has no preference about those values.
   client.

   The client MAY request specific options from the server by including
   an Option Request option (see section 22.7) as a hint about the
   options sets the "msg-type" field to CONFIRM. The client is interested generates
   a transaction ID and inserts this value in receiving.  If the "transaction-id"
   field.

   The client
   requires MUST include a consistent prioritization of the options it receives, it
   includes an Option Request Client Identifier option indicating to identify itself
   to the options it needs
   (see section 18.2.2). server.  The client MAY include includes IA options with data
   values as hints to the server about parameter values for all of the client would
   like to have returned.

   When IAs
   currently in use by the client sends client.  The IA options include all of the Confirm message, it MUST use an IPv6
   address that
   addresses the client currently has confirmed to be valid on associated with those IAs.  The
   client SHOULD set the link to
   which it is currently attached T1 and that is assigned to the interface
   for which the client is interested T2 fields in obtaining configuration
   information as the source address in IA options and the IP header of
   preferred-lifetime and valid-lifetime fields in the datagram
   carrying IA Address
   options to 0, and the Confirm message. server will ignore these fields.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   CNF_TIMEOUT
      MRT   CNF_MAX_RT

      MRC   0

      MRD   CNF_MAX_RD

   If the client receives no responses before the message transmission
   process as described in section 14 terminates, the client SHOULD
   continue to use any IP addresses, using the last known lifetimes for
   those addresses, and SHOULD continue to use any other previously
   obtained configuration parameters.

18.1.3. Creation and transmission of Renew messages

   To extend the valid and preferred lifetimes for the addresses
   associated with
   addresses, an IA, the client sends a Renew message to the server containing
   an IA
   from which the client obtained the addresses in the IA containing
   an IA option for the IA. The client includes IA Address options in
   the IA and its option for the addresses associated addresses. with the IA. The server
   determines new lifetimes for the addresses in the IA according to the
   administrative configuration of the server.  The server may also add
   new addresses to the IA. The server may remove addresses from the IA
   by setting the preferred and valid lifetimes of those addresses to
   zero.

   The server controls the time at which the client contacts the server
   to extend the lifetimes on assigned addresses through the T1 and T2
   parameters assigned to an IA.

   If T1 or T2 is set to 0 by the server, the client does not send a
   Renew or Rebind message, respectively, for the IA.

   At time T1 for an IA, the client initiates a Renew/Reply message
   exchange to extend the lifetimes on any addresses in the IA. The
   client includes an IA option with all addresses currently assigned to
   the IA in its Renew message.

   The client sets the "msg-type" field to RENEW. The client generates a
   transaction ID and inserts this value in the "transaction-ID" "transaction-id" field.

   The client places the identifier of the destination server in a
   Server Identifier option.

   The client MUST include a Client Identifier option to identify
   itself to the server.  The client adds any appropriate options,
   including one or more IA options.  The client MUST include the list
   of addresses the client currently has associated with the IAs in the
   Renew message.

   The client MAY request specific options from the server by including MUST include an Option Request option (see section 22.7) as a hint about
   to indicate the options the client is interested in receiving.  If the client
   requires a consistent prioritization of the options it receives, it
   includes an Option Request option indicating the options it needs
   (see section 18.2.3).  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   If the client has a source address of sufficient scope that can be
   used by the server as a return address and the client has received a
   Server Unicast option (see section 22.13) from the server, the client
   SHOULD unicast the Renew message to the server.

   DISCUSSION:

      Use of multicast on a link and relay agents enables the
      inclusion of relay agent options in all messages sent by
      the client.  The server MUST NOT enable the use of unicast
      for a client when relay agent options are required for that
      client.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   REN_TIMEOUT

      MRT   REP_MAX_RT   REN_MAX_RT

      MRC   0

      MRD   Remaining time until T2

   The message exchange is terminated when time T2 is reached (see
   section 18.1.4), at which time the client begins a Rebind message
   exchange.

18.1.4. Creation and transmission of Rebind messages

   At time T2 for an IA (which will only be reached if the server to
   which the Renew message was sent at time T1 has not responded),
   the client initiates a Rebind/Reply message exchange with any
   available server.  The client sends the Rebind message to the
   All_DHCP_Relay_Agents_and_Servers multicast address.  The client
   includes an IA option with all addresses currently assigned to the IA
   in its Rebind message.

   The client sets the "msg-type" field to REBIND. The client generates
   a transaction ID and inserts this value in the "transaction-ID" "transaction-id"
   field.

   The client MUST include a Client Identifier option to identify
   itself to the server.  The client adds any appropriate options,
   including one or more IA options.  The client MUST include the list
   of addresses the client currently has associated with the IAs in the
   Rebind message.

   The client MAY request specific options from the server by including MUST include an Option Request option (see section 22.7) as a hint about
   to indicate the options the client is interested in receiving.  If the client
   requires a consistent prioritization of the options it receives, it
   includes an Option Request option indicating the options it needs
   (see section 18.2.4).  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   REB_TIMEOUT

      MRT   REB_MAX_RT

      MRC   0
      MRD   Remaining time until valid lifetimes of all addresses have
            expired

   The mechanism in section 14 is modified as follows for use in the
   transmission of Rebind messages.  The message exchange is terminated
   when the valid lifetimes of all of the addresses assigned to the
   IA expire (see section 10), at which time the client has several
   alternative actions to choose from:

    -  The client may choose to use a Solicit message to locate a new
       DHCP server and send a Request for the expired IA to the new
       server

    -  The client may have other addresses in other IAs, so the client
       may choose to discard the expired IA and use the addresses in the
       other IAs

18.1.5. Creation and Transmission of Information-request messages

   The client uses an Information-request message to obtain
   configuration information without having addresses assigned to it.

   The client sets the "msg-type" field to INFORMATION-REQUEST. The
   client generates a transaction ID and inserts this value in the
   "transaction-ID"
   "transaction-id" field.

   The client SHOULD include a Client Identifier option to identify
   itself to the server.  If the client does not include a Client
   Identifier option, the server will not be able to return any
   client-specific options to the client, or the server may choose not
   to respond to the message at all.

   The client MAY request specific options from the server by including MUST include an Option Request option (see section 22.7) as a hint about
   to indicate the options the client is interested in receiving.  If the client
   requires a consistent prioritization of the options it receives, it
   includes an Option Request option indicating the options it needs
   (see section 18.2.5).  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   INF_TIMEOUT

      MRT   INF_MAX_RT

      MRC   0

      MRD   0

18.1.6. Receipt Creation and transmission of Reply message in response to a Request, Confirm,
   Renew, Rebind Release messages

   To release one or Information-request message

   Upon the receipt of more addresses, a valid Reply client sends a Release message in response to a Request,
   Confirm, Renew, Rebind or Information-request message,
   the server.

   The client
   extracts the configuration information contained in sets the Reply. "msg-type" field to RELEASE. The client MAY choose to report any status code or message from the
   status code option generates
   a transaction ID and places this value in the Reply message. "transaction-id" field.

   The client SHOULD perform duplicate address detection [18] on each
   of places the addresses in any IAs it receives in identifier of the Reply message before
   using server that address for traffic.  If any of allocated the addresses are found
   to be
   address(es) in use on the link, the a Server Identifier option.

   The client sends MUST include a Decline message Client Identifier option to identify itself
   to the
   server as described in section 18.1.9. server.  The client records includes options containing the T1 and T2 times IAs for each IA
   the addresses it is releasing in the Reply
   message. "options" field.  The client records any addresses
   to be released MUST be included with IAs in the Reply message.  The client updates the preferred and valid
   lifetimes for the IAs.  Any addresses in for the IA from
   IAs the lifetime information client wishes to continue to use MUST NOT be in added to the IA option.
   IAs.

   The client leaves MUST NOT use any addresses that of the client
   has associated with addresses it is releasing as
   the IA that are not included source address in the IA option
   unchanged.

   If the Reply was received in response to a Request, Renew Release message or Rebind
   message, the client must update the information in any IA option
   contained in the Reply subsequently
   transmitted message.  The client adds any new addresses
   from the IA option to the IA, updates lifetimes for existing
   addresses in the IA from the IA option and discards any addresses
   with a lifetime of zero in

   Because Release messages may be lost, the IA option.

   Management of client should retransmit
   the specific configuration information Release if no Reply is detailed in
   the definition of each option, in section 22.

   When received.  However, there are scenarios
   where the client receives a NotOnLink status in an IA from may not wish to wait for the server
   in response normal retransmission
   timeout before giving up (e.g., on power down).  Implementations
   SHOULD retransmit one or more times, but MAY choose to a Confirm message, terminate the
   retransmission procedure early.

   The client can assume it needs to
   send a Request to transmits the server message according to obtain appropriate section 14, using the
   following parameters:

      IRT   REL_TIMEOUT

      MRT   0

      MRC   REL_MAX_MRC

      MRD   0

   The client MUST stop using all of the addresses for being released as
   soon as the
   IA. client begins the Release message exchange process.  If
   addresses are released but the client receives any Reply messages that do not indicate from a NotOnLink status, DHCP server is lost,
   the client can use the addresses in will retransmit the IA Release message, and
   ignore any messages that do indicate the server may
   respond with a NotOnLink status.

   When Reply indicating a status of NoBinding.  Therefore,
   the client receives does not treat a Reply message with a NoBinding status of NoBinding
   in an IA from the server
   for a Renew Release message exchange as if it indicates an error.

   Note that if the client can assume it needs to send a Request fails to reestablish an IA with release the server.

   When addresses, the client receives a NoBinding status in an IA from the server
   for a Rebind message the client can assume it needs to send a Request addresses
   assigned to reestablish an IA with the server or try another server.

   When the client receives an AddrUnavail status in an IA from will be reclaimed by the server for a Rebind message when the client can assume it needs to send a
   Request to reestablish an IA with valid
   lifetime of the server or try another server. address expires.

18.1.7. Creation and transmission of Release Decline messages

   To release

   If a client detects that one or more addresses, addresses assigned to it by a
   server are already in use by another node, the client sends a Release Decline
   message to the server. server to inform it that the address is suspect.

   The client sets the "msg-type" field to RELEASE. DECLINE. The client generates
   a transaction ID and places this value in the "transaction-ID" "transaction-id" field.

   The client places the identifier of the server that allocated the
   address(es) in a Server Identifier option.

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client includes options containing the IAs for
   the addresses it is releasing declining in the "options" field.  The addresses
   to be released declined MUST be included in the IAs.  Any addresses for the
   IAs the client wishes to continue to use should not be in added to
   the IAs.

   The client MUST NOT use any of the addresses if it is releasing declining as
   the source address in the Release Decline message or in any subsequently
   transmitted message.

   The client transmits the message according to section 14, using the
   following parameters:

      IRT   DEC_TIMEOUT

      MRT   DEC_MAX_RT

      MRC   DEC_MAX_RC

      MRD   0

   If addresses are released but the client has Reply from a source address of sufficient scope that can be
   used by the DHCP server as a return address and is lost,
   the client has received
   a Server Unicast option (section 22.13) from will retransmit the server, Decline message, and the client
   SHOULD unicast server may
   respond with a Reply indicating a status of NoBinding.  Therefore,
   the Release client does not treat a Reply message to the server.

   DISCUSSION:

      Use of multicast on with a link and relay agents enables the
      inclusion status of relay agent options NoBinding
   in all a Decline message exchange as if it indicates an error.

18.1.8. Receipt of Reply messages sent by
      the client.  The server MUST NOT enable

   Upon the use receipt of unicast
      for a client when relay agent options are required for that
      client.

   The client SHOULD choose to guarantee the delivery of the Release valid Reply message using the retransmission strategy in section 14.  An example
   of a situation in which response to a client would not guarantee delivery would
   be when the client is powering down Request,
   Confirm, Renew, Rebind or restarting because of some
   error condition.

   The Information-request message, the client transmits
   extracts the message according to section 14, using configuration information contained in the
   following parameters:

      IRT   REL_TIMEOUT

      MRT   0

      MRC   REL_MAX_MRC

      MRD   0 Reply.  The
   client MUST abandon the attempt MAY choose to release addresses if the
   Release report any status code or message exchange fails. from the
   status code option in the Reply message.

   The client MUST stop using all SHOULD perform duplicate address detection [20] on each
   of the addresses being released as
   soon as the client begins in any IAs it receives in the Release Reply message exchange process. before
   using that address for traffic.  If any of the addresses are released but found
   to be in use on the Reply from link, the client sends a DHCP server is lost, Decline message to the
   server as described in section 18.1.7.

   The client will retransmit records the Release message, T1 and T2 times for each IA in the server may
   respond with a Reply indicating a status of NoBinding.  Therefore,
   the
   message.  The client does not treat a Reply message records any addresses included with a status of NoBinding IAs in a Release message exchange as if it indicates an error.

   Note that if the
   Reply message.  The client fails to release updates the addresses, preferred and valid lifetimes
   for the addresses
   assigned to in the IA will be reclaimed by the server when from the lifetime
   of the address expires.

18.1.8. Receipt of Reply message information in response to a Release message

   Upon receipt of a valid Reply message, the client can consider the
   Release event successful.

18.1.9. Creation
   IA option, and transmission of Decline messages

   The client sets the "msg-type" field discards any addresses with valid lifetime set to DECLINE. The client generates
   a transaction ID and places this value 0
   in the "transaction-ID" field. IA option.  The client places leaves any addresses that the identifier of client
   has associated with the server IA that allocated the
   address(es) are not included in a Server Identifier option.

   The client MUST include a Client Identifier the IA option
   unchanged.

   If the Reply was received in response to identify itself
   to a Request, Renew or Rebind
   message, the server.  The client includes options containing must update the information it has recorded about
   IAs for from the addresses it is declining IA options contained in the "options" field.  The Reply message:

    -  Add any new addresses
   to be declined MUST be included in the IAs.  Any addresses for the
   IAs the client wishes to continue to use should not be in added IA option to the IAs.

   The IA as recorded by
       the client MUST NOT use

    -  Update lifetimes for any of the addresses it is declining as
   the source address in the Decline message or in any subsequently
   transmitted message.

   If IA option that the
       client already has a source address of sufficient scope that can be
   used by recorded in the server IA

    -  Discard any addresses from the IA as a return address and recorded by the client has received that
       have a Server Unicast option (section 22.13) from lifetime of 0 in the server, IA Address option

   Management of the client
   SHOULD unicast specific configuration information is detailed in
   the Decline message to the server.

   DISCUSSION:

      Use definition of multicast on each option, in section 22.

   When the client receives a link and relay agents enables Reply message with a Status Code option
   with value UseMulticast, the
      inclusion client records the receipt of relay agent options in all the
   message and sends subsequent messages sent by to the client.  The server MUST NOT enable through the use of unicast
      for a client when relay agent options are required for that
      client.
   interface on which the message was received using multicast.  The
   client transmits resends the original message according to section 14, using multicast.

   When the
   following parameters:

      IRT   DEC_TIMEOUT

      MRT   DEC_MAX_RT
      MRC   DEC_MAX_RC

      MRD   0

   The client MUST abandon receives a NotOnLink status from the attempt server in
   response to decline addresses if a Confirm message, the
   Decline message exchange fails. client performs DHCP server
   solicitation as described in section 17 and client-initiated
   configuration as described in section 18.  If addresses are released but the client receives any
   Reply from messages that do not indicate a DHCP server is lost, NotOnLink status, the client will retransmit
   can use the Decline message, and addresses in the server may
   respond with a Reply indicating IA and ignore any messages that do
   indicate a status of NoBinding.  Therefore, NotOnLink status.

   When the client does not treat a Reply message with receives a status of code with value NoBinding in a Decline message exchange as if it indicates an error.

18.1.10. Receipt of Reply message IA
   from the server in response to a Decline Renew message

   Upon receipt of or a Rebind message,
   the client sends a Request to reestablish an IA with the server.

   When the client receives a valid Reply message in response to a
   Release message, the client can consider considers the
   Decline Release event successful.

18.2. Server Behavior

   For this discussion, completed,
   regardless of the Status Code option(s) returned by the server.

   When the client receives a valid Reply message in response to a
   Decline message, the client considers the Decline event completed,
   regardless of the Status Code option(s) returned by the server.

18.2. Server Behavior

   For this discussion, the Server is assumed to have been configured in
   an implementation specific manner with configuration of interest to
   clients.

   In most instances, the server will send a Reply in response to a
   client message.  This Reply message MUST always contain the Server
   Identifier option containing the server's DUID and the Client
   Identifier option from the client message if one was present.

18.2.1. Receipt of Request messages

   When the server receives a Request message via unicast from a
   client to which the server has not sent a unicast option, the server
   discards the Request message and responds with a Reply message
   containing a status code Status Code option with value UseMulticast UseMulticast, a Server
   Identifier option containing the server's DUID, the Client Identifier
   option from the client message and no other options.

   When the server receives a valid Request message, the client is requesting the
   configuration of IAs by the server.  The server creates
   the bindings for that client according to the server's policy and
   configuration information and records the IAs and other information about
   requested by the client.

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Request message into
   the transaction-ID transaction-id field.

   The server MUST include a Server Identifier option containing the
   server's DUID and the Client Identifier option from the Request
   message in the Reply message.

   If the server finds that the prefix on one or more IP addresses in
   any IA in the message from the client is not a valid prefix for appropriate to the link
   to which the client is connected, the server MUST return the IA to
   the client with the status field set to a Status Code option with value NotOnLink.

   If the server cannot assign any addresses to any of the IAs in the
   message from the client, the server MUST include the IAs in the Reply
   message with the status field Status Code option set to AddrUnavail NoAddrsAvail and no addresses
   in the IA.

   For any IAs to which the server can assign addresses, the server
   includes the IA with addresses and other configuration parameters and
   records the IA as a new client binding.

   If the server will use a reconfigure nonce value for security of
   Reconfigure messages, the server generates a new nonce value for the
   client, records the value and includes it in a Reconfigure Nonce
   option (see section 22.21) 22.20) in the Reply message.

   The server includes other options containing configuration
   information to be returned to the client.  The server SHOULD limit
   the options returned to the client so that the DHCP message header
   and options do not cause fragmentation.  If the client has included
   an Option Request option in the Solicit message, the server uses that includes
   options in the Advertise message containing configuration parameters
   for all of the options identified in the Option Request option as a hint about that
   the server has been configured to return to the client.  The server
   MAY return additional options to the client if it has a preference for
   receiving from the server. been configured
   to do so.

18.2.2. Receipt of Confirm messages

   When the server receives a Confirm message, the client is requesting
   confirmation that server determines
   if the IP addresses it will use is valid and
   requesting in the most current configuration information for Confirm message are appropriate to the client.
   link to which the client is attached.  The server compares ignores the addresses T1
   and T2 fields in the IA Address options and the preferred-lifetime and
   valid-lifetime fields in the IA Address options.

   If all of the addresses in the Confirm message from the client with the addresses in the binding for
   the client.

   If pass this test, the
   server finds that returns a status of Success.  If any of the addresses in the Confirm message do not
   match what is in
   pass this test, the binding for that client or server returns a status of NotOnLink.

   If the server does not find any addresses in
   the Confirm message that are not appropriate for to
   the link from to which the client sent the Confirm message, the server sends a Reply message
   containing a Status Code option with the value ConfNoMatch.

   If the server finds that the addresses in the Confirm message match
   the addresses in the binding for that client, and the configuration
   information is still valid, the server sends a Reply message
   containing a Status Code option with the value Success.

   If the server connected, but cannot determine if
   some of the information in addresses are appropriate to the Confirm
   message is valid link or invalid, not appropriate
   to the link, the server MUST NOT send a reply to the client.  For
   example, if the server does not have a binding for the
   client, but the configuration information in about prefixes on
   the Confirm message
   appears valid, link to which the client is connected, the server does not reply.

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Confirm message into
   the transaction-ID transaction-id field.

   The server MUST include a Server Identifier option containing the
   server's DUID and the Client Identifier option from the Confirm
   message in the Reply message.  The server includes IA options for each of a Status Code
   option indicating the IA options in status of the Confirm message.  The server chooses values for T1, T2 and lifetimes
   for each

18.2.3. Receipt of Renew messages

   When the addresses in the IAs according to the server's
   configured policies.  The values for T1, T2 and the lifetimes sent by
   the server receives a Renew message via unicast from a client are to
   which the client's preferences for those values.  The server
   also includes options for any other configuration information to be has not sent to the client.

   The server includes other options containing configuration
   information to be returned to the client.  The server SHOULD limit
   the options returned to the client so that the DHCP message header
   and options do not cause fragmentation.  If the client has included
   an Option Request option in the Renew message, the server uses that
   option as a hint about the options the client has a preference for
   receiving from the server.

   The Reply message from the server MUST include a Status Code option.

18.2.3. Receipt of Renew messages

   When the server receives a Renew message via unicast from a client to
   which the server has not sent a unicast option, a unicast option, the server discards
   the Renew message and responds with a Reply message containing a
   status code
   Status Code option with value UseMulticast UseMulticast, a Server Identifier
   option containing the server's DUID, the Client Identifier option
   from the client message and no other options.

   When the server receives a Renew and message that contains an IA option
   from a client client, it locates the client's binding and verifies that the
   information in the IA from the client matches the information stored
   for that client.

   If the server cannot find a client entry for the IA the server
   returns the IA containing no addresses with status a Status Code option set
   to NoBinding in the Renew Reply message.

   If the server finds that any of the addresses are no longer valid
   for not appropriate
   to the client, link to which the client is attached, the server returns the
   address to the client with lifetimes of 0.

   If the server finds the addresses in the IA for the client then the
   server sends back the IA to the client with new lifetimes and T1/T2
   times, and includes a Status Code option with value Success.  The
   server may choose to change the list of addresses and the lifetimes
   of addresses in IAs that are returned to the client.

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Renew message into the
   transaction-ID
   transaction-id field.

   The server MUST include a Server Identifier option containing the
   server's DUID and the Client Identifier option from the Renew message
   in the Reply message.

18.2.4. Receipt of Rebind messages

   When the server receives a Rebind and message that contains an IA option
   from a client client, it locates the client's binding and verifies that the
   information in the IA from the client matches the information stored
   for that client.

   If the server cannot find a client entry for the IA the server
   returns the IA containing no addresses with status a Status Code option set
   to NoBinding in the Rebind Reply message.

   If the server finds that the any of the addresses are no longer valid
   for
   appropriate to the client, link to which the client is attache, the server
   returns the address to the client with lifetimes of 0.

   If the server finds the addresses in the IA for the client then the
   server SHOULD send back the IA to the client with new lifetimes and
   T1/T2 times.

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Rebind message into the
   transaction-ID
   transaction-id field.

   The server MUST include a Server Identifier option containing the
   server's DUID and the Client Identifier option from the Rebind
   message in the Reply message.

   The server includes other options containing configuration
   information to be returned to the client.  The server SHOULD limit
   the options returned to the client so that the DHCP message header
   and options do not cause fragmentation.  If the client has included
   an Option Request option in the Rebind Solicit message, the server uses that includes
   options in the Advertise message containing configuration parameters
   for all of the options identified in the Option Request option as a hint about that
   the server has been configured to return to the client .  The server
   MAY return additional options to the client if it has a preference for
   receiving from the server. been configured
   to do so.

18.2.5. Receipt of Information-request messages

   When the server receives an Information-request message, the
   client is requesting configuration information that does not
   include the assignment of any addresses.  The server determines all
   configuration parameters appropriate to the client, based on the
   server configuration policies known to the server.

   The

   If the Information-request message specifies an IA option or an IA_TA
   option, the server constructs responds by sending a Reply message by setting the containing
   a Server Identifier option, a Client Identifier option if one was
   included in the Information-request message and a Status Code option
   with status UnSpecFail.

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Information-request
   message into the transaction-ID transaction-id field.

   The server MUST include a Server Identifier option containing the
   server's DUID in the Reply message.  If the client included a Client
   Identification option in the Information-request message, the server
   copies that option to the Reply message.

   The server includes options containing configuration information
   to be returned to the client.  The server SHOULD limit the options
   returned to the client so that the DHCP message header and options
   do not cause fragmentation.  If the client has included an Option
   Request option in the Information-request Solicit message, the server uses
   that includes options in
   the Advertise message containing configuration parameters for all of
   the options identified in the Option Request option as a hint about that the server
   has been configured to return to the client .  The server MAY return
   additional options to the client if it has a preference
   for receiving from the server. been configured to do so.

   If the Information-request message received from the client did
   not include a Client Identifier option, the server SHOULD respond
   with a Reply message containing any configuration parameters
   that are not determined by the client's identity.  If the server
   chooses not to respond, the client may continue to retransmit the
   Information-request message indefinitely.

18.2.6. Receipt of Release messages

   When the server receives a Release message via unicast from a
   client to which the server has not sent a unicast option, the server
   discards the Release message and responds with a Reply message
   containing a status code Status Code option with value UseMulticast UseMulticast, a Server
   Identifier option containing the server's DUID, the Client Identifier
   option from the client message and no other options.

   Upon the receipt of a valid Release message, the server examines
   the IAs and the addresses in the IAs for validity.  If the IAs in
   the message are in a binding for the client and the addresses in
   the IAs have been assigned by the server to those IAs, the server
   deletes the addresses from the IAs and makes the addresses available
   for assignment to other clients.  The server ignores addresses not
   assigned to the IA (though IA, and it may choose to log an error make a notification if it finds such
   an address). address.

   After all the addresses have been processed, the server generates a
   Reply message and includes a Status Code option with value Success,
   a Server Identifier option with the server's DUID and a Client
   Identifier option with the client's DUID. For each IA in the Release
   message for which the server has no binding information, the server
   adds an IA option using the IAID from the Release message and
   includes a Status Code option with the value NoBinding in the IA
   option.  No other options are included in the IA option.

   A server may choose to retain a record of assigned addresses and IAs
   after the lifetimes on the addresses have expired to allow the server
   to reassign the previously assigned addresses to a client.

18.2.7. Receipt of Decline messages

   When the server receives a Decline message via unicast from a
   client to which the server has not sent a unicast option, the server
   discards the Decline message and responds with a Reply message
   containing a status code Status Code option with value UseMulticast UseMulticast, a Server
   Identifier option containing the server's DUID, the Client Identifier
   option from the client message and no other options.

   Upon the receipt of a valid Decline message, the server examines the
   IAs and the addresses in the IAs for validity.  If the IAs in the
   message are in a binding for the client and the addresses in the IAs
   have been assigned by the server to those IA, the server deletes
   the addresses from the IAs.  The server SHOULD mark the addresses
   declined by the client so that those addresses are not assigned to
   other clients, and MAY choose to make a notification that addresses
   were declined.  The server ignores addresses not assigned to the IA
   (though it may choose to log an error if it finds such an address).

   After all the address have been processed, the server generates a
   Reply message and includes a Status Code option with value Success,
   a Server Identifier option with the server's DUID and a Client
   Identifier option with the client's DUID. For each IA in the Release Decline
   message for which the server has no binding information, the server
   adds an IA option using the IAID from the Release message and
   includes a Status Code option with the value NoBinding in the IA
   option.  No other options are included in the IA option.

18.2.8. Transmission of Reply messages

   If the original message was received directly by the server, the
   server unicasts the Reply message directly to the client using the
   address in the source address field from the IP datagram in which the
   original message was received.  The Reply message MUST be unicast
   through the interface on which the original message was received.

   If the original message was received in a Relay-forward message, the
   server constructs a Relay-reply message with the Reply message in the
   payload of a "server-message" option.  If the Relay-forward messages
   included an Interface-id option, the server copies that option to the
   Relay-reply message.  The server unicasts the Relay-reply message
   directly to the relay agent using the address in the source address
   field from the IP datagram in which the Relay-forward message was
   received.

19. DHCP Server-Initiated Configuration Exchange

   A server initiates a configuration exchange to cause DHCP clients
   to obtain new addresses and other configuration information.  For
   example, an administrator may use a server-initiated configuration
   exchange when links in the DHCP domain are to be renumbered.  Other
   examples include changes in the location of directory servers,
   addition of new services such as printing, and availability of new
   software.

19.1. Server Behavior

   A server sends a Reconfigure message to cause a client to initiate
   immediately a Renew/Reply or Information-request/Reply message
   exchange with the server.

19.1.1. Creation and transmission of Reconfigure messages

   The server sets the "msg-type" field to RECONFIGURE. The server
   sets the transaction-id field to 0.  The server places its identifier in includes a Server
   Identifier option. option containing its DUID and a Client Identifier option
   containing the client's DUID in the Reconfigure message.

   The server MAY include an Option Request option to inform the client
   of what information has been changed or new information that has been
   added.  In particular, the server specifies the IA option in the
   Option Request option if the server wants the client to obtain new
   address information.

   Because of the risk of denial of service attacks against DHCP
   clients, the use of a security mechanism is mandated in Reconfigure
   messages.The
   messages.  The server MUST use one of the following two security
   mechanisms:

    -  The server includes a Reconfigure Nonce option containing the
       reconfiugre
       reconfigure nonce value currently assigned to the client

    -  The server includes an authentication option in the Reconfigure
       message

   The server MUST include a Reconfigure Message option (defined in
   section 22.20) 22.19) to select whether the client responds with a Renew
   message or an Information-Request message.

   The server MUST NOT include any other options in the Reconfigure
   except as specifically allowed in the definition of individual
   options.

   A server sends each Reconfigure message to a single DHCP client,
   using an IPv6 unicast address of sufficient scope belonging to the
   DHCP client.  The server may obtain the address of the client through
   the information that the server has about clients that have been in
   contact with the server, or the server may be configured with the
   address of the client through some external agent.

   To reconfigure more than one client, the server unicasts a separate
   message to each client.  The server may initiate the reconfiguration
   of multiple clients concurrently; for example, a server may
   send a Reconfigure message to additional clients while previous
   reconfiguration message exchanges are still in progress.

   The Reconfigure message causes the client to initiate a Renew/Reply
   or Information-request/Reply message exchange with the server.  The
   server interprets the receipt of a Renew or Information-request
   message (whichever was specified in the original Reconfigure message)
   from the client as satisfying the Reconfigure message request.

19.1.2. Time out and retransmission of Reconfigure messages

   If the server does not receive a Renew or Information-request
   message from the client in REC_TIMEOUT milliseconds, the server
   retransmits the Reconfigure message, doubles the RECREP_TIMEOUT REC_TIMEOUT value
   and waits again.  The server continues this process until REC_MAX_R REC_MAX_RC
   unsuccessful attempts have been made, at which point the server
   SHOULD abort the reconfigure process for that client.

   Default and initial values for REC_TIMEOUT and REC_MAX_RT REC_MAX_RC are
   documented in section 5.5.

19.1.3.

19.2. Receipt of Renew messages

   The server generates and sends a Reply message(s) message to the client as
   described in sections 18.2.3 and 18.2.8, including options for
   configuration parameters.

   The server MAY choose to send a Reply with include options containing the IAs and new values for
   other configuration parameters to be reconfigured, in the Reply message, even if those
   IAs and parameters were not requested in the Renew message from the
   client.

19.2.

19.3. Receipt of Information-request messages

   The server generates and sends a Reply message(s) message to the client as
   described in sections 18.2.5 and 18.2.8, including options for
   configuration parameters.

   The server MAY choose to send a Reply with the include options containing new values for other
   configuration parameters to
   be reconfigured, in the Reply message, even if those
   parameters were not specified requested in the Information-request message from
   the client.

19.3.

19.4. Client Behavior

   A client MUST accept Reconfigure messages sent to UDP port 546 on
   interfaces for which it has acquired configuration information
   through DHCP. These messages may be sent at any time.  Since the
   results of a reconfiguration event may affect application layer
   programs, the client SHOULD log these events, and MAY notify these
   programs of the change through an implementation-specific interface.

19.3.1.

19.4.1. Receipt of Reconfigure messages

   Upon receipt of a valid Reconfigure message, the client initiates a
   transaction with the server by sending a Reply Renew or Information-request
   message.  The client ignores the transaction-id field in the received
   Reconfigure message.  While the transaction is in progress, the
   client silently discards any Reconfigure messages it receives.

   The client responds with either a Renew message or an
   Information-request message as indicated by the Reconfigure
   Message option (as defined in section 22.20). 22.19).

   DISCUSSION:

      The Reconfigure message acts as a trigger that signals the
      client to complete a successful message exchange.  Once
      the client has received a Reconfigure, the client proceeds
      with the message exchange (retransmitting the Renew or
      Information-request message if necessary); the client
      ignores any additional Reconfigure messages (regardless
      of the transaction ID in the Reconfigure message) until the
      exchange is complete.  Subsequent Reconfigure messages
      (again independent of the transaction ID) cause
      the client to initiate a new exchange.

      How does this mechanism work in the face of duplicated or
      retransmitted Reconfigure messages?  Duplicate messages
      will be ignored because the client will begin the exchange
      after the receipt of the first Reconfigure.  Retransmitted
      messages will either trigger the exchange (if the first
      Reconfigure was not received by the client) or will be
      ignored.  The server can discontinue retransmission of
      Reconfigure messages to the client once the server receives
      the Renew or Information-request message from the client.

      It might be possible for a duplicate or retransmitted
      Reconfigure to be sufficiently delayed (and delivered out of
      order) to arrive at the client after the exchange (initiated
      by the original Reconfigure) has been completed.  In this
      case, the client would initiate a redundant exchange.  The
      likelihood of delayed and out of order delivery is small
      enough to be ignored.  The consequence of the redundant
      exchange is inefficiency rather than incorrect operation.

19.3.2.

19.4.2. Creation and transmission of Renew messages

   When responding to a Reconfigure, the client creates and sends
   the Renew message in exactly the same manner as outlined in
   section 18.1.3, with the exception:  if the server included on Reconfigure message
   contains an Option Request option specifying that includes the IA option, option code,
   the client MUST include IA options containing the addresses the
   client currently has assigned to ALL IAs for the interface through
   which the Reconfigure message was received.

19.3.3.

19.4.3. Creation and transmission of Information-request messages

   When responding to a Reconfigure, the client creates and sends the
   Information-request message in exactly the same manner as outlined in
   section 18.1.5, with the exception that the client includes a Server
   Identifier option with the identifier from the Reconfigure message to
   which the client is responding.

19.3.4.

19.4.4. Time out and retransmission of Renew or Information-request
   messages

   The client uses the same variables and retransmission algorithm as
   it does with Renew or Information-request messages generated as part
   of a client-initiated configuration exchange.  See sections 18.1.3
   and 18.1.5 for details.

19.3.5.  If the client does not receive a response
   from the server by the end of the retransmission process, the client
   ignores and discards the Reconfigure message.

19.4.5. Receipt of Reply messages

   Upon the receipt of a valid Reply message, the client extracts the
   contents of processes the "options" field,
   options and sets (or resets) configuration parameters appropriately.
   The client records and updates the lifetimes for any addresses
   specified in IAs in the Reply message.

20. Relay Agent Behavior

   For this discussion, the

   The relay agent MAY be configured to use a list of server destination
   addresses, which MAY include unicast addresses, the All_DHCP_Servers
   multicast address, or other multicast addresses selected by the network
   administrator.  If the relay agent has not been explicitly
   configured, it MUST use the All_DHCP_Servers multicast address as the
   default.

20.1. Relaying of Forwarding a client message or a Relay-forward message

   A relay agent forwards both messages from clients and Relay-forward
   messages from other relay agents.  When a relay agent receives a
   valid client message, message to be forwarded, it constructs a new Relay-forward
   message.  The relay agent copies the source address from the
   header of the IP datagram in which the message was received to the
   peer-address field of the Relay-forward message.  The relay agent
   copies the received DHCP message (excluding any IP or UDP headers)
   into a Relay Message option in the new message.  The relay agent adds
   to the Relay-forward message any other options it is configured to
   include.

20.1.1. Forwarding a message from a client

   If the relay agent received the message to be forwarded from a
   client, the relay agent places a global or site-scoped address with a
   prefix assigned to the link on which the client should be assigned an
   address in the link-address field.  This address will be used by the
   server to determine the link from which the client should be assigned
   an address and other configuration information.  The hop-count in the
   Relay-forward message is set to 0.

   If the relay agent cannot use the address in the link-address field
   to identify the interface through which the response to the client
   will be forwarded, the relay agent MUST include an Interface-id
   option (see section 22.19) 22.18) in the Relay-forward message.  The server
   will include the Interface-id option in its Relay-reply message.
   The relay agent fills in the link-address field as described in the
   previous paragraph regardless of whether the relay agent includes an
   Interface-id option in the Relay-forward message.

20.1.2. Forwarding a message from a relay agent

   If the message received by the relay agent is a Relay-forward
   message and the hop-count in the message is greater than or equal to
   HOP_COUNT_LIMIT, the relay agent discards the received message.

   The relay agent sets the link-address field to 0 and sets the
   hop-count field to the value of the hop-count field in the received
   message incremented by 1.

   The relay agent copies the source address from the IP datagram in
   which the message was received from the client into the
   client-address peer-address
   field in the Relay-forward message.

20.2. Forwarding a Relay-reply message

   The relay agent constructs a Client Message option (see
   section 22.10) that contains processes any options included in the entire DHCP Relay-reply
   message (excluding
   the IP and UDP headers) from the client in addition to the data field of
   the option.  The relay agent places the Client Relay Message option
   along with any "relay agent-specific" options in the options
   field of the Relay-forward message. and then discards
   those options.

   The relay agent sends extracts the
   Relay-forward message to all the servers in from the list of server
   destination addresses with which Relay Message option
   and forwards it has been configured or to the
   All_DHCP_Servers address if it has not been explicitly configured
   with server destination addresses.

20.2. Relaying of server messages

   The relay agent processes any other options included contained in the peer-address field of
   the Relay-reply message as appropriate to those options.  The relay
   agents then discards those options. message.

   If the Relay-reply message includes a an Interface-id option, the
   relay agent forwards the message from the server to the client on
   the link identified by the Interface-id option.  Otherwise, if the
   link-address field is not set to zero, the relay agent forwards the
   message on the link identified by the link-address field.

   In either case, the relay agent extracts

   If the server Relay-reply message from the
   Server Message does not include an Interface-id option (see section 22.11)
   and forwards the message to link-address field is zero, the address in the client-address peer-address
   field must be a global address or a site-local address (and the
   device on which the relay agent is running belongs to only one
   site).  If the address in the peer-address field does not meet this
   condition, the relay agent discards the Relay-reply message.

21. Authentication message and
   SHOULD communicate an error condition.

20.3. Construction of DHCP Relay-reply messages

   Some network administrators may wish

   A server uses a Relay-reply message to provide authentication of
   the source and contents of DHCP messages.  For example, clients may
   be subject return a response to denial of service attacks through a client
   if the use of bogus
   DHCP servers, or may simply be misconfigured due to unintentionally
   instantiated DHCP servers.  Network administrators may wish original message from the client was forwarded to
   constrain the server
   in a Relay-forward message.  The response to the client MUST be
   forwarded through the same relay agents as the original client
   message.  The server causes this to happen by creating a Relay-reply
   message that includes a Relay Message option containing the message
   for the next relay agent in the return path to the client.  The
   contained Relay-reply message contains another Relay Message option
   to be sent to the next relay agent, and so on.  The server must
   record the contents of the peer-address fields in the received
   message so it can construct the appropriate Relay-reply message
   carrying the response from the server.

   For example, if client C sent a message that was forwarded by relay
   agent A to relay agent B and then to the server, the server would the
   following Relay-Reply message to relay agent B:

  msg-type:       RELAY-REPLY
  hop-count:      0
  link-address:   0
  peer-address: A
  Relay Message option, containing:
    msg-type:       RELAY-REPLY
    hop-count:      0
    link-address: address from link to which C is attached
    peer-address: C
    Relay Message option: <response from server>

21. Authentication of DHCP messages

   Some network administrators may wish to provide authentication of
   the source and contents of DHCP messages.  For example, clients may
   be subject to denial of service attacks through the use of bogus
   DHCP servers, or may simply be misconfigured due to unintentionally
   instantiated DHCP servers.  Network administrators may wish to
   constrain the allocation of addresses to authorized hosts to avoid
   denial of service attacks in "hostile" environments where the network
   medium is not physically secured, such as wireless networks or
   college residence halls.

   The DHCP authentication mechanism is based on the design of
   authentication for DHCPv4 [5]. [6].

21.1. DHCP threat model

   The threat to DHCP is inherently an insider threat (assuming a
   properly configured network where DHCPv6 ports are blocked on the
   perimeter gateways of the enterprise).  Regardless of the gateway
   configuration, however, the potential attacks by insiders and
   outsiders are the same.

   The attack specific to a DHCP client is the possibility of the
   establishment of a "rogue" server with the intent of providing
   incorrect configuration information to the client.  The motivation
   for doing so may be to establish a "man in the middle" attack or it
   may be for a "denial of service" attack.

   There is another threat to DHCP clients from mistakenly or
   accidentally configured DHCP servers that answer DHCP client requests
   with unintentionally incorrect configuration parameters.

   The threat specific to a DHCP server is an invalid client
   masquerading as a valid client.  The motivation for this may be for
   "theft of service", or to circumvent auditing for any number of
   nefarious purposes.

   The threat common to both the client and the server is the resource
   "denial of service" (DoS) attack.  These attacks typically involve
   the exhaustion of valid available addresses, or the exhaustion of CPU
   or network bandwidth, and are present anytime there is a shared
   resource.

   This threat model does not consider the privacy of the contents
   of DHCP messages to be important.  DHCP is not used to exchange
   authentication or configuration information that must be kept secret
   from other networks nodes.

21.2. Security of messages sent between servers and relay agents

   Relay agents and servers that choose to exchange messages securely
   use the IPsec mechanisms for IPv6 [8].  The way in which IPsec
   is employed by relay [10].  Relay agents and servers is not specified in this
   document.

21.3. Summary of DHCP authentication

   Authentication
   MUST support manual configuration and installation of DHCP messages static keys.
   If a client message is accomplished forwarded through the use multiple relay agents, each
   of the Authentication option (see section 22.12).  The authentication
   information carried relay agents must have established independent, pairwise trust
   relationships.  That is, if messages from client C will be forwarded
   by relay agent A to relay agent B and then to the server, relay
   agents A and B must be configured to use IPSec for the messages they
   exchange, and relay agent B and the server must be configured to use
   IPSec for the messages they exchange.

21.3. Summary of DHCP authentication

   Authentication of DHCP messages is accomplished through the use of
   the Authentication option (see section 22.11).  The authentication
   information carried in the Authentication option can be used to
   reliably identify the source of a DHCP message and to confirm that
   the contents of the DHCP message have not been tampered with.

   The Authentication option provides a framework for multiple
   authentication protocols.  One such protocol is defined here.
   Other protocols defined in the future will be specified in separate
   documents.

   The protocol field in the Authentication option identifies the
   specific protocol used to generate the authentication information
   carried in the option.  The algorithm field identifies a specific
   algorithm within the authentication protocol; for example, the
   algorithm field specifies the hash algorithm used to generate the
   message authentication code (MAC) in the authentication option.  The
   replay detection method (RDM) field specifies the type of replay
   detection used in the replay detection field.

21.4. Replay detection

   The Replay Detection Method (RDM) field determines the type of replay
   detection used in the Replay Detection field.

   If the RDM field contains 0x00, the replay detection field MUST
   be set to the value of a monotonically increasing counter.  Using
   a counter value such as the current time of day (for example, an
   NTP-format timestamp [10]) [12]) can reduce the danger of replay attacks.
   This method MUST be supported by all protocols.

21.5. Delayed authentication protocol

   If the protocol field is 1, the message is using the "delayed
   authentication" mechanism.  In delayed authentication, the client
   requests authentication in its Solicit message and the server replies
   with an Advertise message that includes authentication information.
   This authentication information contains a nonce value generated by
   the source as a message authentication code (MAC) to provide message
   authentication and entity authentication.

   The use of a particular technique based on the HMAC protocol [9] [11]
   using the MD5 hash [17] [19] is defined here.

21.5.1. Management issues in the delayed authentication protocol

   The "delayed authentication" protocol does not attempt to address
   situations where a client may roam from one administrative domain
   to another, i.e.  interdomain roaming.  This protocol is focused on
   solving the intradomain problem where the out-of-band exchange of a
   shared key is feasible.

21.5.2. Use of the Authentication option in the delayed authentication
   protocol

   In a Solicit message, the Authentication option carries the Protocol,
   Algorithm and fields, but no Replay detection or Authentication
   information.

   In an Advertise, Request, Renew, Rebind, Confirm, Decline, Release
   or Information-request message, the Authentication option carries
   the Protocol, Algorithm, RDM and Replay detection fields and
   Authentication information.

   A DHCP message MUST NOT contain more than one Authentication option
   when using the delayed authentication protocol.

   The format of the Authentication information is:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                            Key ID                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                           HMAC-MD5                            |
  |                          (128 bits)                           |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following definitions will be used in the description of the
   authentication information for delayed authentication, algorithm 1:

   Replay Detection  - as defined by the RDM field
   K                 - a key (secret value) shared
                       between the source and
                       destination of the message;
                       each key has a unique
                       identifier (key ID)
   key ID            - the unique identifier for the key value
                       used to generate the MAC for this message
   HMAC-MD5          - the MAC generating function.

   The sender computes the MAC using the HMAC generation algorithm [9] [11]
   and the MD5 hash function [17]. [19].  The entire DHCP message (setting
   the MAC field of the authentication option to zero), including the
   DHCP message header and the options field, is used as input to the
   HMAC-MD5 computation function.  The 'key ID' field MUST be set to the
   identifier of the key used to generate the MAC.

   DISCUSSION:

      Algorithm 1 specifies the use of HMAC-MD5.  Use of a
      different technique, such as HMAC-SHA, will be specified as
      a separate protocol.

      Delayed authentication requires a shared secret key for each
      client on each DHCP server with which that client may wish
      to use the DHCP protocol.  Each key has a unique identifier
      that can be used by a receiver to determine which key was
      used to generate the MAC in the DHCP message.  Therefore,
      delayed authentication may not scale well in an architecture
      in which a DHCP client connects to multiple administrative
      domains.

21.5.3.

21.5.2. Message validation

   Any DHCP message that includes more than one authentication option
   MUST be discarded.

   To validate an incoming message, the receiver first checks that
   the value in the replay detection field is acceptable according to
   the replay detection method specified by the RDM field.  Next, the
   receiver computes the MAC as described in [9]. [11].  The entire DHCP
   message (except (setting the MAC field of the authentication option itself), to 0),
   is used as input to the HMAC-MD5 computation function.  If the MAC
   computed by the receiver does not match the MAC contained in the
   authentication option, the receiver MUST discard the DHCP message.

21.5.4.

21.5.3. Key utilization

   Each DHCP client has a key, K. The server uses the client's DUID to
   identify the client's key.  The client uses its key to encode any
   messages it sends to the server and to authenticate and verify any
   messages it receives from the server.  The client's key is initially
   distributed to the client through some out-of-band mechanism, and
   is stored locally on the client for use in all authenticated DHCP
   messages.  Once the client has been given its key, it uses that key
   for all transactions even if the client's configuration changes; for
   example, if the client is assigned a new network address.

   Each DHCP server knows, stores or be is able to obtain in a secure manner, manner the
   keys for all authorized clients.  If all clients use the same
   key, clients can perform both entity and message authentication for
   all messages received from servers.  However, the sharing of keys
   is strongly discouraged as it allows for unauthorized clients to
   masquerade as authorized clients by obtaining a copy of the shared
   key and allows for trivial spoofing of an authenticated DHCP server.  To authenticate the identity of
   individual clients, each client must be configured with a unique key
   and a key ID for that key.

21.5.5.

21.5.4. Client considerations for delayed authentication protocol

21.5.5.1.

21.5.4.1. Sending Solicit messages

   When the client sends a Solicit message and wishes to use
   authentication, it includes an Authentication option with the desired
   protocol, algorithm and RDM as described in section 21.5.  The client
   does not include any replay detection or authentication information
   in the Authentication option.

21.5.5.2.

21.5.4.2. Receiving Advertise messages

   The client validates any Advertise messages containing an
   Authentication option specifying the delayed authentication protocol
   using the validation test described in section 21.5.3. 21.5.2.

   Client behavior if no Advertise messages include authentication
   information or pass the validation test is controlled by local policy
   on the client.  According to client policy, the client MAY choose to
   respond to a Advertise message that has not been authenticated.

   The decision to set local policy to accept unauthenticated messages
   should be made with care.  Accepting an unauthenticated Advertise
   message can make the client vulnerable to spoofing and other
   attacks.  If local users are not explicitly informed that the client
   has accepted an unauthenticated Advertise message, the users may
   incorrectly assume that the client has received an authenticated
   address and is not subject to DHCP attacks through unauthenticated
   messages.

   A client MUST be configurable to discard unauthenticated messages,
   and SHOULD be configured by default to discard unauthenticated
   messages if the client has been configured with an authentication
   key or other authentication information.  A client MAY choose to
   differentiate between Advertise messages with no authentication
   information and Advertise messages that do not pass the validation
   test; for example, a client might accept the former and discard the
   latter.  If a client does accept an unauthenticated message, the
   client SHOULD inform any local users and SHOULD log the event.

21.5.5.3.

21.5.4.3. Sending Request, Confirm, Renew, Rebind, Decline or Release
   messages

   If the client authenticated the Advertise message through which the
   client selected the server, the client MUST generate authentication
   information for subsequent Request, Confirm, Renew, Rebind or Release
   messages sent to the server as described in section 21.5.  When the
   client sends a subsequent message, it MUST use the same key used by
   the server to generate the authentication information.

21.5.5.4.

21.5.4.4. Sending Information-request messages

   If the server has selected a key for the client in a previous message
   exchange (see section 21.5.6.1, 21.5.5.1), the client MUST use the same key
   to generate the authentication information.  If the client has not
   previously been given a key with the server, the client MUST use
   a key that has been selected for the client through some external
   mechanism.

21.5.5.5.

21.5.4.5. Receiving Reply messages

   If the client authenticated the Advertise it accepted, the client
   MUST validate the associated Reply message from the server.  The
   client MUST discard the Reply if the message fails to pass validation
   and MAY log the validation failure.  If the Reply fails to pass
   validation, the client MUST restart the DHCP configuration process by
   sending a Solicit message.

   If the client accepted an Advertise message that did not include
   authentication information or did not pass the validation test, the
   client MAY accept an unauthenticated Reply message from the server.

21.5.5.6.

21.5.4.6. Receiving Reconfigure messages

   The client MUST discard the Reconfigure if the message fails to pass
   validation and MAY log the validation failure.

21.5.6.

21.5.5. Server considerations for delayed authentication protocol

21.5.6.1.

21.5.5.1. Receiving Solicit messages and Sending Advertise messages

   The server selects a key for the client and includes authentication
   information in the Advertise message returned to the client as
   specified in section 21.5.  The server MUST record the identifier of
   the key selected for the client and use that same key for validating
   subsequent messages with the client.

21.5.6.2.

21.5.5.2. Receiving Request, Confirm, Renew, Rebind or Release messages
   and Sending Reply messages

   The server uses the key identified in the message and validates the
   message as specified in section 21.5.3. 21.5.2.  If the message fails to pass
   validation or the server does not know the key identified by the 'key
   ID' field, the server MUST discard the message and MAY choose to log
   the validation failure.

   If the message passes the validation procedure, the server responds
   to the specific message as described in section 18.2.  The server
   MUST include authentication information generated using the key
   identified in the received message as specified in section 21.5.

21.5.6.3.

21.5.5.3. Sending Reconfigure messages

   The server MUST include an Authentication option in a Reconfigure
   message, generated as specified in section 21.5 using the key the
   server initially selected for the client to which the Reconfigure
   message is to be sent.

   If the server has not previously selected a key for the client, the
   server MUST use a key that has been selected for the client through
   some external mechanism.

22. DHCP options

   Options are used to carry additional information and parameters
   in DHCP messages.  Every option shares a common base format, as
   described in section 22.1.  All values in options are represented in
   network byte order.

   This document describes the DHCP options defined as part of the base
   DHCP specification.  Other options may be defined in the future in
   separate documents.

   Unless otherwise noted, each option may appear only in the options
   area of a DHCP message and may appear only once.  If an option does
   appear multiple times, each instance is considered separate and the
   data areas of the options MUST NOT be concatenated or otherwise
   combined.

22.1. Format of DHCP options

   The format of DHCP options is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          option-code          |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          option-data                          |
     |                      (option-len octets)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   An unsigned integer identifying the specific option
                    type carried in this option.

      option-len    An unsigned integer giving the length of the
                    option-data field in this option in octets.

      option-data   The data for the option; the format of this data
                    depends on the definition of the option.

   DHCPv6 options are scoped by using encapsulation.  Some options apply
   generally to the client, some are specific to an IA, and some are
   specific to the addresses within an IA. These latter two cases are
   discussed in sections 22.4 and 22.6.

22.2. Client Identifier option

   The Client Identifier option is used to carry a DUID identifying a
   client between a client and a server.

   The format of the Client Identifier option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OPTION_CLIENTID        |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                              DUID                             .
     .                        (variable length)                      .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_CLIENTID (1)

      option-len    Length of DUID in octets

      DUID          The DUID for the client

22.3. Server Identifier option

   The Server Identifier option is used to carry a DUID identifying a
   server between a client and a server.

   The format of the Server Identifier option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OPTION_SERVERID        |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                              DUID                             .
     .                        (variable length)                      .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_SERVERID (2)

      option-len    Length of DUID in octets

      DUID          The DUID for the server

   A server MUST process any message it receives that contains a Server
   Identifier option with a DUID that matches the server's DUID.

22.4. Identity Association option

   The Identity Association option (IA option) is used to carry an
   identity association, the parameters associated with the IA and the
   addresses associated with the IA.

   Addresses appearing in an IA option are not temporary addresses (see
   section 22.5).  The format of the IA option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           OPTION_IA           |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        IAID (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              T1                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              T2                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                           IA-options                          .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_IA (3)

      option-len           12 + length of IA-options field

      IAID                 The unique identifier for this IA; the IAID
                           must be unique among the identifiers for all
                           of this client's IAs.  The number space for
                           IA IAIDs is separate from the number space
                           for IA_TA IAIDs.

      T1                   The time at which the client contacts the
                           server from which the addresses in the IA
                           were obtained to extend the lifetimes of
                           the addresses assigned to the IA; T1 is a
                           time duration relative to the current time
                           expressed in units of seconds

      T2                   The time at which the client contacts any
                           available server to extend the lifetimes of
                           the addresses assigned to the IA; T2 is a
                           time duration relative to the current time
                           expressed in units of seconds

      IA-options           Options associated with this IA.

   The IA-options field encapsulates those options that are specific
   to this IA. For example, all of the IA Address Options carrying the
   addresses associated with this IA are in the IA-options field.

   An IA option may only appear in the options area of a DHCP message.
   A DHCP message may contain multiple IA options.

   The status of any operations involving this IA is indicated in a
   Status Code option in the IA-options field.

   Note that an IA has no explicit "lifetime" or "lease length" of its
   own.  When the valid lifetimes of all of the addresses in an IA
   have expired, the IA can be considered as having expired.  T1 and
   T2 are included to give servers explicit control over when a client
   recontacts the server about a specific IA.

   In a message sent by a client to a server, values in the T1 and
   T2 fields indicate the client's preference for those parameters.
   The client may send 0 if it has no preference for T1 and T2.  In a
   message sent by a server to a client, the client MUST use the values
   in the T1 and T2 fields for the T1 and T2 parameters.  The values in
   the T1 and T2 fields are the number of seconds until T1 and T2.

   The server selects the T1 and T2 times to allow the client to extend
   the lifetimes of any addresses in the IA before the lifetimes expire,
   even if the server is unavailable for some short period of time.
   Recommended values for T1 and T2 are .5 and .8 times the shortest
   preferred lifetime of the addresses in the IA, respectively.  If the
   server does not intend for a client to extend the lifetimes of the
   addresses in an IA, the server sets T1 and T2 to 0.

   T1 is the time at which the client begins the lifetime extension
   process by sending a Renew message to the server that originally
   assigned the addresses to the IA. T2 is the time at which the client
   starts sending a Rebind message to any server.

   T1 and T2 are specified as unsigned integers that specify the time
   in seconds relative to the time at which the messages containing the
   option is received.

22.5. Identity Association for Temporary Addresses option

   The Identity Association for Temporary Addresses (IA_TA) option is
   used to carry an IA, the parameters associated with the IA and the
   addresses associated with the IA. All of the addresses in this option
   are used by the client as temporary addresses, as defined in RFC
   3041.

   The format of the IA_TA option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         OPTION_IA_TA          |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        IAID (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                           IA-options                          .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_IA_TA (4)

      option-len           4 + length of IA-options field

      IAID                 The unique identifier for this IA; the IAID
                           must be unique among the identifiers for all
                           of this client's IAs.  The number space for
                           IA_TA IAIDs is separate from the number space
                           for IA IAIDs.

      IA-options           Options associated with this IA.

   The IA-Options field encapsulates those options that are specific
   to this IA. For example, all of the IA Address Options carrying the
   addresses associated with this IA are in the IA-options field.

   Each IA_TA carries one "set" of temporary addresses; that is, at most
   one address from each prefix assigned to the link to which the client
   is attached.

   An IA_TA option may only appear in the options area of a DHCP
   message.  A DHCP message may contain multiple IA_TA options.

   The status of any operations involving this IA is indicated in a
   Status Code option in the IA-options field.

   Note that an IA has no explicit "lifetime" or "lease length" of its
   own.  When the valid lifetimes of all of the addresses in an IA have
   expired, the IA can be considered as having expired.

   An IA_TA option does not include values for T1 and T2.  A client
   MAY request that the lifetimes on temporary addresses be extended
   by including the addresses in a IA_TA option sent in a Renew or
   Rebind message to a server.  For example, a client would request
   an extension on the lifetime of a temporary address to allow an
   application to continue to use an established TCP connection.

   The client obtains new temporary addresses by sending an IA_TA option
   with a new IAID to a server.  Requesting new temporary addresses from
   the server is the equivalent of generating new temporary addresses
   as described in RFC 3041.  The server will generate new temporary
   addresses and return them to the client.  The client should request
   new temporary addresses before the lifetimes on the previously
   assigned addresses expire.

   A server MUST return the same set of temporary address for the same
   IA_TA (as identified by the IAID) as long as those addresses are
   still valid.  After the lifetimes of the addresses in an IA_TA have
   expired, the IAID may be reused to identify a new IA_TA with new
   temporary addresses.

   This option MAY appear in a Confirm message if the lifetimes on the
   temporary addresses in the associated IA have not expired.

22.6. IA Address option

   The IA Address option is used to specify IPv6 addresses associated
   with an IA. The IA Address option must be encapsulated in the
   Options field of an Identity Association option.  The Options field
   encapsulates those options that are specific to this address.

   The format of the IA Address option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          OPTION_IAADDR        |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      preferred-lifetime                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        valid-lifetime                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                        IAaddr-options                         .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_IADDR   OPTION_IAADDR (5)

      option-len    24 + length of IAaddr-options field
      IPv6 address  An IPv6 address

      preferred-lifetime The preferred lifetime for the IPv6 address in
                    the option, expressed in units of seconds

      valid-lifetime The valid lifetime for the IPv6 address in the
                    option, expressed in units of seconds

      IAaddr-options Options associated with this address

   In a message sent by a client to a server, values in the preferred
   and valid lifetime fields indicate the client's preference for those
   parameters.  The client may send 0 if it has no preference for the
   preferred and valid lifetimes.  In a message sent by a server to a
   client, the client MUST use the values in the preferred and valid
   lifetime fields for the preferred and valid lifetimes.  The values in
   the preferred and valid lifetimes are the number of seconds remaining
   in each lifetime.

   An IA Address option may appear only in an IA option or an IA_TA
   option.  More than one IA Address Options can appear in an IA option
   or an IA_TA option.

   The status of any operations involving this IA Address is indicated
   in a Status Code option in the IAaddr-options field.

22.7. Option Request option

   The Option Request option is used to identify a list of options in
   a message between a client and a server.  The format of the Option
   Request option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           OPTION_ORO          |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    requested-option-code-1    |    requested-option-code-2    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_ORO (6)

      option-len    2 * number of requested options

      requested-option-code-n The option code for an option requested by
                    the client.

   A client MAY include an Option Request option in a Solicit, Request,
   Renew, Rebind, Confirm or Information-request message to inform
   the server about options the client wants the server to send to
   the client.  A server MAY include an Option Request option in a
   Reconfigure option to indicate which options the client should
   request from the server.

22.8. Preference option

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3

   The Preference option is sent by a server to a client to affect the
   selection of a server by the client.  The format of the Preference
   option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OPTION_PREFERENCE       |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  pref-value   |
     +-+-+-+-+-+-+-+-+

      option-code   OPTION_PREFERENCE (7)

      option-len    1.    1

      pref-value    The preference value for the server in this message.

   A server MAY include a Preference option in an Advertise message to
   control the selection of a server by the client.  See section 17.1.3
   for the use of the Preference option by the client and the
   interpretation of Preference option data value.

22.9. Elapsed Time option

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      OPTION_ELAPSED_TIME      |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          elapsed-time         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_ELAPSED_TIME (8)

      option-len    2.

      elapsed-time  The amount of time since the client began its
                    current DHCP transaction.  This time is expressed in
                    hundredths of a second (10^-2 seconds).

   A client MUST include an Elapsed Time option in messages to indicate
   how long the client has been trying to complete a DHCP transaction. message
   exchange.  The elapsed time is measured from the time at which the
   client sent the first message in the message exchange, and the
   elapsed-time field is set to 0 in the first message in the message
   exchange.  Servers and Relay Agents use the data value in this option
   as input to policy controlling how a server responds to a client
   message.  For example, the elapsed time option allows a secondary
   DHCP server to respond to a request when a primary server hasn't
   answered in a reasonable time.

22.10. Client message Relay Message option

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OPTION_CLIENT_MSG       |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                      DHCP-client-message                      .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_CLIENT_MSG (9)

      option-len    Length of DHCP client message.

      DHCP-client-message

   The message received from the client;
                    forwarded verbatim to the server.

   A relay agent forwards a message from a client to a server as the
   contents of a Client Relay Message option carries a DHCP message in a Relay-forward or
   Relay-reply message.

22.11. Server message

   The format of the Relay Message option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OPTION_SERVER_MSG        OPTION_RELAY_MSG       |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                       DHCP-server-message                       DHCP-relay-message                      .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code   OPTION_SERVER_MSG (10)   OPTION_RELAY_MSG (9)

      option-len    Length of DHCP server message.

      DHCP-server-message The message received from DHCP-relay-message

      DHCP-relay-message In a Relay-forward message, the server; received
                    message, forwarded verbatim to the client.

   A server sends next relay agent
                    or server; in a DHCP Relay-reply message, the message to
                    be copied and forwarded to a client by a the relay agent as or client
                    whose address is in the contents peer-address field of a Server Message option in a the
                    Relay-reply
   message.

22.12. message

22.11. Authentication option

   The Authentication option carries authentication information to
   authenticate the identity and contents of DHCP messages.  The use of
   the Authentication option is described in section 21.

   The format of the Authentication option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_AUTH          |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Protocol    |   Algorithm   |      RDM      |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |          Replay Detection (64 bits)           +-+-+-+-+-+-+-+-+
   |                                               |  Auth. Info   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   .                                                               .
   .                   Authentication Information                  .
   .                       (variable length)                        .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code                  OPTION_AUTH (11)

      option-len                   15 + length of Authentication
                                   Information field

      protocol                     The authentication protocol used in
                                   this authentication option

      algorithm                    The algorithm used in the
                                   authentication protocol

      RDM                          The replay detection method used in
                                   this authentication option

      Replay detection             The replay detection information for
                                   the RDM

      Authentication information   The authentication information,
                                   as specified by the protocol and
                                   algorithm used in this authentication
                                   option

22.13.

22.12. Server unicast option

   The server sends this option to a client to indicate to the client
   that it is allowed to unicast messages to the server.

   The server
   specifies the IPv6 address to which the client is to send unicast
   messages in the server-address field.  When a client receives this
   option, where permissible and appropriate, the client sends messages
   directly to the server using the IPv6 address specified in the
   server-address field format of the option.

   Details about when the client may send messages to the server using
   unicast are in section 18. Server Unicast option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_UNICAST       |        option-len             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                       server-address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code     OPTION_UNICAST (12)

      option-len      16

      server-address  The IP address to which the client should send
                      messages delivered using unicast

22.14.

   The server specifies the IPv6 address to which the client is to send
   unicast messages in the server-address field.  When a client receives
   this option, where permissible and appropriate, the client sends
   messages directly to the server using the IPv6 address specified in
   the server-address field of the option.

   Details about when the client may send messages to the server using
   unicast are in section 18.

22.13. Status Code Option

   This option returns a status indication related to the DHCP message
   or option in which it appears.  The format of the Status Code option
   is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OPTION_STATUS_CODE      |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          status-code          |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   .                                                               .
   .                        status-message                         .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_STATUS_CODE (13)
      option-len           2 + length of status-message

      status-code          The numeric code for the status encoded in
                           this option.  The status codes are defined in
                           section 24.4.

      status-message       A UTF-8 encoded text string, string suitable for
                           display to an end user, which MUST NOT be
                           null-terminated.

   A Status Code option may appear in the options field of a DHCP
   message option, or and/or in the options area field of another option.  If the Status
   Code option does not appear in a message in which the option could
   appear, the status of the message is assumed to be Success.

22.15.

22.14. Rapid Commit option

   A client MAY include this

   The Rapid Commit option in a Solicit message if the client is prepared used to perform signal the Solicit-Reply use of the two message
   exchange described
   in section 17.1.1.

   A server MUST include this option in a Reply message sent in response
   to a Solicit message when completing for address assignment.  The format of the Solicit-Reply message
   exchange. Rapid Commit
   option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_RAPID_COMMIT      |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code     OPTION_RAPID_COMMIT (14)

      option-len      0

   DISCUSSION:

      Each server that responds with a Reply to a Solicit that
      includes a Rapid Commit

   A client MAY include this option will commit the assigned
      addresses in the Reply a Solicit message if the client
   is prepared to perform the Solicit-Reply message exchange described
   in section 17.1.1.

   A server MUST include this option in a Reply message sent in response
   to a Solicit message when completing the Solicit-Reply message
   exchange.

   DISCUSSION:

      Each server that responds with a Reply to a Solicit that
      includes a Rapid Commit option will commit the assigned
      addresses in the Reply message to the client, and will not
      receive any confirmation that the client has received the
      Reply message.  Therefore, if more than one server responds
      to a Solicit that includes a Rapid Commit option, some
      servers will commit addresses that are not actually used by
      the client.

      The problem of unused addresses can be minimized, for
      example, by designing the DHCP service so that only one
      server responds to the Solicit or by using relatively short
      lifetimes for assigned addresses.

22.16.

22.15. User Class option

   The User Class Option

   This option is used by a client to identify the type or
   category of user or applications it represents.

   The format of the User Class option is:

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OPTION_USER_CLASS       |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                          user-class-data                      .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_USER_CLASS (15)

      option-len           Length of user class data field

      user-class-data      The user classes carried by the client.

   The information contained in the data area of this option is
   contained in one or more opaque fields that represent the user
   class or classes of which the client is a member.  A server selects
   configuration information for the client based on the classes
   identified in this option.  For example, the User Class option can be
   used to configure all clients of people in the accounting department
   with a different printer than clients of people in the marketing
   department.  The user class information carried in this option MUST
   be configurable on the client.

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OPTION_USER_CLASS       |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                          user-class-data                      .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_USER_CLASS (15)

      option-len           Length of user class data field

      user-class-data      The user classes carried by the client.

   The data area of the user class option MUST contain one or more
   instances of user class data.  Each instance of the user class data
   is formatted as follows:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
     |        user-class-len         |          opaque-data          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+

   The user-class-len is two octets long and specifies the length of the
   opaque user class data in network byte order.

   A server interprets the classes identified in this option according
   to its configuration to select the appropriate configuration
   information for the client.  A server may use only those user
   classes that it is configured to interpret in selecting configuration
   information for a client and ignore any other user classes.  In
   response to a message containing a User Class option, a server
   includes a User Class option containing those classes that were
   successfully interpreted by the server, so that the client can be
   informed of the classes interpreted by the server.

22.17.

22.16. Vendor Class Option

   This option is used by a client to identify the vendor that
   manufactured the hardware on which the client is running.  The
   information contained in the data area of this option is contained
   in one or more opaque fields that identify details of the hardware
   configuration.  The format of the Vendor Class option is:

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      OPTION_VENDOR_CLASS      |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       enterprise-number                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                       vendor-class-data                       .
     .                             . . .                             .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_VENDOR_CLASS (16)

      option-len           4 + length of vendor class data field

      enterprise-number    The vendor's registered Enterprise Number as
                           registered with IANA.

      vendor-class-data    The hardware configuration of the host on
                           which the client is running.

   The vendor-class-data is composed of a series of separate items,
   each of which describes some characteristic of the client's hardware
   configuration.  Examples of vendor-class-data instances might include
   the version of the operating system the client is running or the
   amount of memory installed on the client.

   Each instance of the vendor-class-data is formatted as follows:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
     |       vendor-class-len        |          opaque-data          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
   The vendor-class-len is two octets long and specifies the length of
   the opaque vendor class data in network byte order.

22.18.

22.17. Vendor-specific Information option

   This option is used by clients and servers to exchange
   vendor-specific information.  The definition format of this information is
   vendor specific.  The vendor is indicated in the enterprise-number
   field.  Clients that do not receive desired vendor-specific
   information SHOULD make an attempt to operate without it, although
   they may do so (and announce they are doing so) in a degraded mode. option
   is:

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      OPTION_VENDOR_OPTS       |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       enterprise-number                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                          option-data                          .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_VENDOR_OPTS (17)

      option-len           4 + length of option-data field

      enterprise-number    The vendor's registered Enterprise Number as
                           registered with IANA.

      option-data          An opaque object of option-len octets,
                           interpreted by vendor-specific code on the
                           clients and servers

   The definition of the information carried in this option is vendor
   specific.  The vendor is indicated in the enterprise-number field.
   Use of vendor-specific information allows enhanced operation,
   utilizing additional features in a vendor's DHCP implementation.
   A DHCP client that does not receive requested vendor-specific
   information will still configure the host device's IPv6 stack to be
   functional.

   The encapsulated vendor-specific options field MUST be encoded as a
   sequence of code/length/value fields of identical format to the DHCP
   options field.  The option codes are defined by the vendor identified
   in the enterprise-number field and are not managed by IANA.

   Each of the encapsulated options is formatted as follows.

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          opt-code             |             option-len        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                          option-data                          .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      opt-code             The code for the encapsulated option

      option-len           An unsigned integer giving the length of the
                           option-data field in this encapsulated option
                           in octets.

      option-data          The data area for the encapsulated option

   Multiple instances of the Vendor-specific Information option may
   appear in a DHCP message.  Each instance of the option is interpreted
   according to the option codes defined by the vendor identified by the
   Enterprise Number in that option.  A DHCP message MUST NOT contain
   more than one Vendor-specific Information option with the same
   Enterprise Number.

22.19.

22.18. Interface-Id Option

   The relay agent MAY send the Interface-id option to identify the
   interface on which the client message was received.  If a relay agent
   receives a Relay-reply message with an Interface-id option, the
   relay agent forwards the message to the client through the interface
   identified by the option.

   The format of the Interface ID option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_INTERFACE_ID      |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                         interface-id                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_INTERFACE_ID (18)

      option-len           Length of interface-id field
      interface-id         An opaque value of arbitrary length generated
                           by the relay agent to identify one of the
                           relay agent's interfaces

   The server MUST copy the Interface-Id option from the Relay-Forward
   message into the Relay-Reply message the server sends to the relay
   agent in response to the Relay-Forward message.  This option MUST NOT
   appear in any message except a Relay-Forward or Relay-Reply message.

   Servers MAY use the Interface-ID for parameter assignment policies.
   The Interface-ID SHOULD be considered an opaque value, with policies
   based on exact string match only; that is, the Interface-ID SHOULD NOT be
   internally parsed by the server.  The Interface-ID value for an
   interface SHOULD be stable and remain unchanged, for example, after
   the relay agent is restarted; if the Interface-ID changes, a server
   will not be able to use it reliably in parameter assignment policies.

22.20.

22.19. Reconfigure Message option

   A server includes a Reconfigure Message option in a Reconfigure
   message to indicate to the client whether the client responds with a
   Renew message or an Information-request message.  The format of this
   option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_RECONF_MSG        |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    msg-type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

      option-code          OPTION_RECONF_MSG (19)

      option-len           1

      msg-type             1             5 for Renew message, 2 11 for
                           Information-request message

22.21.

   The Reconfigure Message option can only appear in a Reconfigure
   message.

22.20. Reconfigure Nonce option

   If a server uses a reconfigure nonce to provide security for
   Reconfigure messages, the server maintains a nonce value for each
   client.  It initially informs the client of the nonce value and then
   includes the nonce value in any Reconfigure message sent to the
   client.

   The following figure gives the format of the Reconfigure Nonce
   option:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     OPTION_RECONF_NONCE       |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       reconfigure-nonce                       |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code          OPTION_RECONF_NONCE (20)

      option-len           8

      reconfigure-nonce    reconfigure nonce value sent to the client client;
                           the nonce MUST be a cryptographically strong
                           random number that cannot easily be guessed
                           or predicted.

   The Reconfigure Nonce option MUST NOT appear in any DHCP message
   other than Reply or Reconfigure.

23. Security Considerations

   Section 21 describes a threat model and an option that provides an
   authentication framework to defend against that the DHCP threat model.

24. IANA Considerations

   This document defines several new name spaces associated with DHCPv6
   and DHCPv6 options:

    -  Multicast addresses

    -  Message types

    -  Option codes

    -  Status codes

    -  DUID

    -  Authentication

           *  Protocol

           *  Algorithm

           *  RDM

   IANA is requested  The primary threat
   to manage a registry of values for each DHCP client is the incorrect configuration of these
   name spaces, which are described in the remainder client
   through a DHCP exchange with a malicious DHCP server.  The incorrect
   configuration may present a denial of this section.

   These name spaces are all service attack by causing
   communication with a service to be managed separately from fail, or a masquerade attack by
   causing the name
   spaces defined for DHCPv4 [4, 1].

   New values in each client to communicate with a malicious server instead of these name spaces except
   a valid server for some service such as DNS or NTP.

   A DHCP option codes
   should server may be approved subject to a theft of service attack by a
   malicious client representing itself as a valid client, or a denial
   of service attack in which a malicious client exhausts the supply of
   available addresses or consumes all of the computation resources or
   network bandwidth available to the DHCP server.

   A DHCP client may also be subject to attack through the receipt
   of a Reconfigure message from a malicious server that causes the
   client to obtain incorrect configuration information from that
   server.  Note that although a client sends its response (Renew or
   Information-request message) through a relay agent and, therefore,
   that response will only be received by servers to which DHCP messages
   are forwarded, a malicious server could send a Reconfigure message to
   a client, followed (after an appropriate delay) by a Reply message
   that would be accepted by the client.  Thus, a malicious server that
   is not on the network path between the client and the server may
   still be able to mount a Reconfigure attack on a client.  The use of
   transaction IDs that are cryptographically sound and cannot easily be
   predicted will also reduce the probability that such an attack will
   be successful.

   DHCP authentication provides for authentication of the identity of
   DHCP clients and servers, and for the integrity of messages delivered
   between DHCP clients and servers.  DHCP authentication does not
   provide any privacy for the contents of DHCP messages.

   The "delayed authentication" protocol described in section 21.5
   uses a secret key that is shared between a client and a server and
   does not attempt to address situations where a client may roam from
   one administrative domain to another, i.e.  interdomain roaming.
   The use of shared keys may not scale well and does not provide for
   repudiation of compromised keys.  This protocol is focused on solving
   the intradomain problem where the out-of-band exchange of a shared
   key is feasible.

   The Reconfigure Nonce option (see section 22.20) provides a way
   for a client to confirm the identity of the server from which it
   has received a Reconfigure message.  The Reconfigure Nonce option
   protects the client from attack by a malicious server that is not on
   the network path from the server to the client; a malicious server
   between the server and the client can read the nonce value sent
   from the server to the client and spoof the server's identity in a
   Reconfigure message.

   Communication between a server and a relay agent can be secured
   through the use of IPSec.  The use of manual configuration and
   installation of static keys are acceptable in this instance because
   the relay agent and server will belong to the same administrative
   domain and the relay agent will require other specific configuration
   (for example, configuration of the DHCP server address) as well as
   the IPSec configuration.

24. IANA Considerations

   This document defines several new name spaces associated with DHCPv6
   and DHCPv6 options:

    -  Message types

    -  Status codes

    -  DUID

    -  Option codes

   IANA is requested to manage a registry of values for each of these
   name spaces, which are described in the process remainder of IETF consensus [12]. this section.

   These name spaces are all to be managed separately from the name
   spaces defined for DHCPv4 [5, 1].

   New multicast addresses, message types, status codes and DUID types
   are assigned via Standards Action [14].

   New DHCP option codes should be approved through are tentatively assigned after the
   specification for the associated option, published as an Internet
   Draft, has received expert review by a designated expert [12]. [14].
   The final assignment of DHCP option codes is through Standards
   Action [14].

   This document also references three name spaces in section 21 that
   are associated with the Authentication Option (section 22.11).  These
   name spaces are defined by the authentication mechanism for DHCPv4 in
   RFC3118 [6].

   The authentication name spaces currently registered by IANA will
   apply to both DHCPv6 and DHCPv4.  In the future, specifications that
   define new Protocol, Algorithm and RDM mechanisms will explicitly
   define whether the new mechanisms are used with DHCPv4, DHCPv6 or
   both.

24.1. Multicast addresses

   Section 5.1 defines the following multicast addresses, which have
   been assigned by IANA for use by DHCPv6:

      All_DHCP_Relay_Agents_and_Servers address:   FF02::1:2

      All_DHCP_Servers address:                    FF05::1:3

24.2. DHCP message types

   IANA is requested to record the following message types (defined
   in section 5.3).  IANA is requested to maintain a registry of DHCP
   message types.

      SOLICIT               1

      ADVERTISE             2

      REQUEST               3

      CONFIRM               4

      RENEW                 5

      REBIND                6

      REPLY                 7
      RELEASE               8

      DECLINE               9

      RECONFIGURE           10

      INFORMATION-REQUEST   11

      RELAY-FORW            12

      RELAY-REPL            13

24.3. DHCP options

   IANA is requested to record the following option-codes (as defined in
   section 22).  IANA is requested to maintain a registry of DHCP option
   codes.

      OPTION_CLIENTID       1

      OPTION_SERVERID       2

      OPTION_IA             3

      OPTION_IA_TMP         4

      OPTION_IADDR

      OPTION_IAADDR         5

      OPTION_ORO            6

      OPTION_PREFERENCE     7

      OPTION_ELAPSED_TIME   8

      OPTION_CLIENT_MSG     9

      OPTION_SERVER_MSG     10

      OPTION_AUTH           11

      OPTION_UNICAST        12

      OPTION_STATUS_CODE    13

      OPTION_RAPID_COMMIT   14

      OPTION_USER_CLASS     15

      OPTION_VENDOR_CLASS   16

      OPTION_VENDOR_OPTS    17
      OPTION_INTERFACE_ID   18

      OPTION_RECONF_MSG     19

      OPTION_RECONF_NONCE   20

24.4. Status codes

   IANA is requested to record the status codes defined in the following
   table.  IANA is requested to manage the definition of additional
   status codes in the future.

   Name         Code Description
   ----------   ---- -----------
   Success         0 Success
   UnspecFail      1 Failure, reason unspecified; this
                     status code is sent by either a client
                     or a server to indicate a failure
                     not explicitly specified in this
                     document
   AuthFailed      2 Authentication failed or nonexistent
   AddrUnavail     3 Addresses Address unavailable
   NoBinding
   NoAddrsAvail    4 Server has no addresses available to assign to
                     the IA(s)
   NoBinding       5 Client record (binding) unavailable
   ConfNoMatch     5     6 Client record Confirm doesn't match IA
   NotOnLink       6 One or more prefixes of the addresses
                     in       7 The prefix for the IA address is not valid for appropriate to
                     the link
                     from to which the client message was received is attached
   UseMulticast    7    8 Sent by a server to a client to force the
                     client to send messages to the server
                     using the All_DHCP_Relay_Agents_and_Servers
                     address

24.5. DUID

   IANA is requested to record the following DUID types (as defined in
   section 9.1).  IANA is requested to manage definition of additional
   DUID types in the future.

      Link-layer address plus time   1

      VUID-EN                        2

      Link-layer address             3

24.6. Authentication option

   Section 21 references three name spaces associated with the
   Authentication Option (section 22.12), which are defined in the
   authentication mechanism for DHCPv4 [5].

   The authentication name spaces currently registered by IANA will
   apply to both DHCPv6 and DHCPv4.  In the future, specifications that
   define new Protocol, Algorithm and RDM mechanisms will explicitly
   define whether to manage definition of additional
   DUID types in the new mechanisms are used with DHCPv4, DHCPv6 or
   both. future.

      DUID-LLT                       1

      DUID-EN                        2

      DUID-LL                        3

25. Acknowledgments

   Thanks to the DHC Working Group and the members of the IETF for
   their time and input into the specification.  In particular, thanks
   also for the consistent input, ideas, and review by (in alphabetical
   order) Bill Arbaugh, Thirumalesh Bhat, Steve Bellovin, A. K.
   Vijayabhaskar, Brian Carpenter, Matt Crawford, Francis Dupont, Tony
   Lindstrom, Josh Littlefield, Gerald Maguire, Jack McCann, Thomas
   Narten, Erik Nordmark, Yakov Rekhter, Mark Stapp, Matt Thomas, Sue
   Thomson, Tatuya Jinmei and Phil Wells.

   Thanks to Steve Deering and Bob Hinden, who have consistently
   taken the time to discuss the more complex parts of the IPv6
   specifications.

   And, thanks to Steve Deering for pointing out at IETF 51 in London
   that the DHCPv6 specification has the highest revision number of any
   Internet Draft.

26. Changes in draft-ietf-dhc-dhcpv6-25.txt

    -  Eliminated definition of VUID-DN.

    -  Changed the second sentence in section 17.1.2 to:

       In the case of a Solicit message transmitted when DHCP is
       initiated by IPv6 Neighbor Discovery, the delay gives the amount
       of time to wait after IPv6 Neighbor Discovery causes the client
       to invoke the stateful address autoconfiguration protocol (see
       section 5.5.3 of RFC2462).

    -  Changed Rapid Commit to allow client to use Advertise messages
       received while waiting for Reply, rather than restarting with
       Solicit; if client receives Advertise with preference 255, client
       immediately sends Request to that server.

    -  Removed the use of All_DHCP_Servers multicast address as
       destination address in section 18.1.5.

    -  Added text improving summary description of Confirm, Renew,
       Rebind.

    -  Removed restriction on extension of lifetimes for temporary
       addresses; added text pointing to RFC 3041 for guidance on
       extending lifetimes for temporary addresses and when to request
       additional temporary addresses.

    -  Clarified text in section 20 to emphasize that the relay agent
       puts an address in the link-address field regardless of whether
       it includes an Interface-ID option; added text explaining why the
       interface-identifier for an interface should remain stable.

    -  Changed use of T1/T2 and lifetimes in Confirm message from
       client:  client uses those fields for preferred values or sets
       to 0; server checks only addresses for correctness and returns
       values chosen by server for T1/T2 and lifetimes.  Clarified
       that server checks addresses and returns current configuration
       information for the client.

    -  Description of User Class option extended with an example
       and clarified to indicate that use of User Class options is
       determined by configuration/policies on server.

    -  Clarified description of the use of Vendor-specific information
       to indicate that client need not receive all requested
       vendor-specific information before proceeding with normal
       operation.

    -  Clarified use of Status Code option in Release message.

    -  Edited section 14 to make clear that a client transmits until it
       receives a response only if both MRC and MRD are zero.

    -  Removed suggestions about ordering options (for example, for
       improved performance).

    -  Edited section 16 to clarify interface selection.

    -  Removed use of anycast; use of anycast over individual link
       technologies will be specified in separate separate documents.

    -  Removed replay detection information field from Solicit message
       to avoid potential DOS attack.

    -  Clarified capabilities and constraints on relay agent forwarding.

    -  Edited definition of vendor class data to clarify that instances
       of vendor-class-data are individual characteristics of the
       client.

    -  Added text in section 21.5.3 to specify that client key is
       identified by client DUID.

    -  Removed "Year 2000 Considerations" section; hope we don't need a
       "Year 3000 Consideration" section.

    -  Authentication mechanism now shares Protocol, Algorithm and RDM
       name spaces with DHCPv4.

    -  Added text to specify return of NoBinding if server cannot
       find binding for IA in Decline; added text allowing client to
       disregard NoBinding in Reply to Decline.

    -  Clarified that Solicit, Confirm and Rebind are invalid if Server
       Identifier option is included.

    -  Edited text about Option Request option to clarify that the
       option is a hint from the client to the server about options the
       client has a preference to receive; includes recommendation that
       client send Option Request option if it has options it requires.

    -  Added nonce value for security of Reconfigure messages.

27. Changes in draft-ietf-dhc-dhcpv6-26.txt

    -  Clarified section 18.1.1 to allow Request message to be sent at
       any time.

    -  Fixed Reconfigure Message option so that msg-type field is one
       octet and uses the DHCP message code to indicate which message
       the client sends back to the server.

    -  Added text anticipating description of how to emulate multicast
       in specific "IPv6 over X" documents.

    -  Removed replay detection information field from Solicit  Reconfigure message
       to avoid potential DOS attack. now requires Client Identifier option with
       receiving client's DUID.

    -  Clarified capabilities and constraints on  Added text to allow a relay agent forwarding. to forward a message from a
       client to another relay agent as well as to a server.  Added text
       in authentication section specifying use of IPSec between relay
       agents and servers for message security.

    -  Edited  Fixed definition of vendor class data "binding" to clarify that instances
       of vendor-class-data are individual characteristics of the
       client. allow a binding with no IAs to
       be indexed by just <DUID>.

    -  Added  Clarified text in section 21.5.4 17.2.2 to specify explain that client key AddrUnavail is
       identified by
       returned only if the Solicit message from the client DUID.

    -  Removed "Year 2000 Considerations" section; hope we don't need a
       "Year 3000 Consideration" section. included one
       or more IA options.

    -  Authentication mechanism now shares Protocol, Algorithm  Edited text about Option Request option to clarify that the
       client is required to include the option and RDM
       name spaces with DHCPv4. to identify all
       options the client has a preference to receive; also clarified
       that a server may include additional options if the server is
       configured to do so.

    -  Added text to specify return explaining why DHCPv6 and DHCPv4 are not integrated in
       this document.

    -  Changed use of NoBinding if Confirm so that server cannot
       find binding for IA in Decline; added text allowing simply checks whether
       addresses are appropriate to link to which client is attached.

    -  Expanded Security Considerations section to
       disregard NoBinding in Reply give more detail on
       threat model and limitations to authentication.

    -  Client now restarts with server solicitation when server returns
       NotOnLink to Decline.

    -  Clarify that Solicit, Confirm message.

    -  Modified several retransmission parameters (in section 5.5) for
       consistency and Rebind are invalid if Server
       Identifier option is included. anticipated operation.

    -  Edited text about Option Request option  Defined "appropriate to clarify that the
       option is a hint from link" and changed "valid on the client link"
       to the server about options the
       client has a preference "appropriate to receive; includes recommendation that
       client send Option Request option if it has options it requires. the link" throughout for clarity

    -  Added nonce value for text requiring transaction IDs to be not predictable and
       described potential attack in security of Reconfigure messages. considerations

References

    [1] S. Alexander and R. Droms.  DHCP Options and BOOTP Vendor
        Extensions, March 1997.  RFC 2132.

    [2] S. Bradner.  Key words for use in RFCs to Indicate Requirement
        Levels, March 1997.  RFC 2119.

    [3] M. Crawford.  Transmission of IPv6 Packets over Ethernet
        Networks, December 1998.  RFC 2464.

    [4] S. Deering and R. Hinden.  Internet Protocol, Version 6 (IPv6)
        Specification, December 1998.  RFC 2460.

    [4]

    [5] R. Droms.  Dynamic Host Configuration Protocol, March 1997.  RFC
        2131.

    [5]

    [6] R. Droms, Editor, W. Arbaugh, and Editor.  Authentication for
        DHCP Messages, June 2001.  RFC 3118.

    [6]

    [7] R. (ed.) Droms.  DNS Configuration options for DHCPv6.  Internet
        Draft, Internet Engineering Task Force, April 2002.  Work in
        progress.

    [8] R. Hinden and S. Deering.  IP Version 6 Addressing Architecture,
        July 1998.  RFC 2373.

    [7]

    [9] IANA.  Private Enterprise Numbers.
        http://www.iana.org/assignments/enterprise-numbers.

    [8]

   [10] S. Kent and R. Atkinson.  Security Architecture for the Internet
        Protocol, November 1998.  RFC 2401.

    [9]

   [11] H. Krawczyk, M. Bellare, and R. Canetti.  HMAC: Keyed-Hashing
        for Message Authentication, February 1997.  RFC 2104.

   [10]

   [12] David L. Mills.  Network Time Protocol (Version 3)
        Specification, Implementation, March 1992.  RFC 1305.

   [11]

   [13] P.V. Mockapetris.  Domain names - implementation and
        specification, November 1987.  RFC 1035.

   [12]

   [14] T. Narten and H. Alvestrand.  Guidelines for Writing an IANA
        Considerations Section in RFCs, October 1998.  RFC 2434.

   [13]

   [15] T. Narten and R. Draves.  Privacy Extensions for Stateless
        Address Autoconfiguration in IPv6, January 2001.  RFC 3041.

   [14]

   [16] T. Narten, E. Nordmark, and W. Simpson.  Neighbor Discovery for
        IP Version 6 (IPv6), December 1998.  RFC 2461.

   [15]

   [17] D.C. Plummer.  Ethernet Address Resolution Protocol:  Or
        converting network protocol addresses to 48.bit Ethernet address
        for transmission on Ethernet hardware, November 1982.  RFC 826.

   [16]

   [18] J. Postel.  User Datagram Protocol, August 1980.  RFC 768.

   [17]

   [19] R. Rivest.  The MD5 Message-Digest Algorithm, April 1992.  RFC
        1321.

   [18]

   [20] S. Thomson and T. Narten.  IPv6 Stateless Address
        Autoconfiguration, December 1998.  RFC 2462.

   [19]

   [21] A. K. Vijayabhaskar.  Time Configuration Options for DHCPv6.
        Internet Draft, Internet Engineering Task Force, May 2002.  Work
        in progress.

   [22] P. Vixie, Ed., S. Thomson, Y. Rekhter, and J. Bound.  Dynamic
        Updates in the Domain Name System (DNS UPDATE), April 1997.  RFC
        2136.

Chair's Address

   The working group can be contacted via the current chair:

         Ralph Droms
         Cisco Systems
         300 Apollo Drive
         Chelmsford, MA 01824

         Phone:  (978) 244-4733
         E-mail:  rdroms@cisco.com

Authors' Addresses

   Questions about this memo document can be directed to:

        Jim Bound
        Hewlett Packard Corporation
        ZK3-3/W20
        110 Spit Brook Road
        Nashua, NH 03062-2698
        USA
        Voice:  +1 603 884 0062
        E-mail:  Jim.Bound@hp.com

        Mike Carney
        Sun Microsystems, Inc
        Mail Stop:  UMPK17-202
        901 San Antonio Road
        Palo Alto, CA 94303-4900

        Bernie Volz
        Ericsson
        959 Concord St
        Framingham, MA 01701
        USA
        Voice:  +1-650-786-4171  +1-508-875-3162
        E-mail:  mwc@eng.sun.com  bernie.volz@ericsson.com

        Ted Lemon
        Nominum, Inc.
        950 Charter Street
        Redwood City, CA 94043
        USA
        E-mail:  Ted.Lemon@nominum.com

        Charles E. Perkins
        Communications Systems Lab
        Nokia Research Center
        313 Fairchild Drive
        Mountain View, California 94043
        USA
        Voice:  +1-650 625-2986
        E-mail:  charliep@iprg.nokia.com
        Fax:  +1 650 625-2502

        Ted Lemon
        Nominum, Inc.
        950 Charter Street
        Redwood City,

        Mike Carney
        Sun Microsystems, Inc
        Mail Stop:  UMPK17-202
        901 San Antonio Road
        Palo Alto, CA 94043
        E-mail:  Ted.Lemon@nominum.com

        Bernie Volz
        Ericsson
        959 Concord St
        Framingham, MA 01701
        Voice:  +1-508-875-3162
        Fax:  +1-508-875-3018
        E-mail:  bernie.volz@ericsson.com

        Ralph Droms
        Cisco Systems
        300 Apollo Drive
        Chelmsford, MA 01824 94303-4900
        USA
        Voice:  +1 978 479 4733  +1-650-786-4171
        E-mail:  rdroms@cisco.com  mwc@eng.sun.com

A. Appearance of Options in Message Types

   The following table indicates with a "*" the options are allowed in
   each DHCP message type:

        Client Server IA/  Option Pref  Time Client Server
          ID     ID  IA_TA Request            Msg.   Msg.
Solicit    *           *     *           *
Advert.    *      *    *           *     *
Request    *      *    *     *           *
Confirm    *           *     *           *
Renew      *      *    *     *           *
Rebind     *           *     *           *
Decline    *      *    *     *           *
Release    *      *    *     *           *
Reply      *      *    *           *     *
Reconf.    *      *          *
Inform.    * (see note)      *           *
R-forw.                                        *
R-repl.                                              *

   NOTE:

      Only included in Information-Request messages that are sent
      in response to a Reconfigure (see section 19.3.3). 19.4.3).

         Auth Server Status  Rap. User  Vendor Vendor Inter. Recon.
              Unica.  Code  Comm. Class Class  Spec.    ID    Msg.
Solicit    *                  *     *     *      *
Advert.    *           *            *     *      *
Request    *                        *     *      *
Confirm    *                        *     *      *
Renew      *                        *     *      *
Rebind     *                        *     *      *
Decline    *           *            *     *      *
Release    *           *            *     *      *
Reply      *    *      *            *     *      *
Reconf.    *                                                   *
Inform.    *                        *     *      *
R-forw.    *                        *     *      *      *
R-repl.    *                        *     *      *      *

B. Appearance of Options in the Options Field of DHCP Options

   The following table indicates with a "*" where options can appear in
   the options field of other options:

             Option   IA/   IAADDR Relay  Relay
             Field   IA_TA         Forw.  Reply
Client ID      *
Server ID      *
IA/IA_TA       *
IAADDR                 *
ORO            *
Pref           *
Time           *
Authentic.     *
Server Uni.    *
Status Code    *       *      *      *      *
Rapid Comm.    *
User Class     *
Vendor Class   *
Vendor Info.   *
Interf. ID                           *      *
Reconf. msg.   *

C. Full Copyright Statement

   Copyright (C) The Internet Society (2001). (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However,
   this document itself may not be modified in any way, such as by
   removing the copyright notice or references to the Internet Society
   or other Internet organizations, except as needed for the purpose
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   The limited permissions granted above are perpetual and will not be
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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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