Internet Engineering Task Force                                 J. Bound
INTERNET DRAFT                                                    Compaq
DHC Working Group                                              M. Carney
Obsoletes:  draft-ietf-dhc-dhcpv6-23.txt           Sun Microsystems, Inc
                                                              C. Perkins
                                                   Nokia Research Center
                                                               Ted Lemon
                                                                 Nominum
                                                             Bernie Volz
                                                                Ericsson
                                                           R. Droms(ed.)
                                                           Cisco Systems
                                                             22 Apr 2002


         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
                      draft-ietf-dhc-dhcpv6-24.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






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   separately or concurrently with the latter to obtain configuration
   parameters.

                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction and Overview                                          1
     1.1. Protocols and addressing  . . . . . . . . . . . . . . . .    2
     1.2. Protocol implementation . . . . . . . . . . . . . . . . .    2
     1.3. Client-server exchanges involving two messages  . . . . .    3
     1.4. 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. Anycast address . . . . . . . . . . . . . . . . . . . . .    8
     5.3. UDP ports . . . . . . . . . . . . . . . . . . . . . . . .    8
     5.4. DHCP message types  . . . . . . . . . . . . . . . . . . .    9
     5.5. Status Codes  . . . . . . . . . . . . . . . . . . . . . .   10
     5.6. Configuration Parameters  . . . . . . . . . . . . . . . .   11

 6. Message Formats                                                   11

 7. Relay agent messages                                              12
     7.1. Relay-forward message . . . . . . . . . . . . . . . . . .   13
     7.2. Relay-reply message . . . . . . . . . . . . . . . . . . .   14

 8. Representation and use of domain names                            14

 9. DHCP unique identifier (DUID)                                     14
     9.1. DUID contents . . . . . . . . . . . . . . . . . . . . . .   15
     9.2. DUID based on link-layer address plus time  . . . . . . .   15
     9.3. Vendor-assigned unique ID based on Domain Name (VUID-DN)    16
     9.4. Vendor-assigned unique ID based on Enterprise Number
             (VUID-EN)  . . . . . . . . . . . . . . . . . . . . . .   17
     9.5. Link-layer address  . . . . . . . . . . . . . . . . . . .   18

10. Identity association                                              19

11. Selecting addresses for assignment to an IA                       20

12. Management of temporary addresses                                 21



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13. Transmission of messages by a client                              21

14. Reliability of Client Initiated Message Exchanges                 21

15. Message validation                                                23
    15.1. Use of Transaction-ID field . . . . . . . . . . . . . . .   23
    15.2. Solicit message . . . . . . . . . . . . . . . . . . . . .   23
    15.3. Advertise message . . . . . . . . . . . . . . . . . . . .   24
    15.4. Request message . . . . . . . . . . . . . . . . . . . . .   24
    15.5. Confirm message . . . . . . . . . . . . . . . . . . . . .   24
    15.6. Renew message . . . . . . . . . . . . . . . . . . . . . .   24
    15.7. Rebind message  . . . . . . . . . . . . . . . . . . . . .   25
    15.8. Decline messages  . . . . . . . . . . . . . . . . . . . .   25
    15.9. Release message . . . . . . . . . . . . . . . . . . . . .   25
   15.10. Reply message . . . . . . . . . . . . . . . . . . . . . .   25
   15.11. Reconfigure message . . . . . . . . . . . . . . . . . . .   26
   15.12. Information-request message . . . . . . . . . . . . . . .   26
   15.13. Relay-forward message . . . . . . . . . . . . . . . . . .   26
   15.14. Relay-reply message . . . . . . . . . . . . . . . . . . .   26

16. Client Source Address and Interface Selection                     26

17. DHCP Server Solicitation                                          27
    17.1. Client Behavior . . . . . . . . . . . . . . . . . . . . .   27
          17.1.1. Creation of Solicit messages  . . . . . . . . . .   27
          17.1.2. Transmission of Solicit Messages  . . . . . . . .   28
          17.1.3. Receipt of Advertise messages . . . . . . . . . .   29
          17.1.4. Receipt of Reply message  . . . . . . . . . . . .   30
    17.2. Server Behavior . . . . . . . . . . . . . . . . . . . . .   30
          17.2.1. Receipt of Solicit messages . . . . . . . . . . .   30
          17.2.2. Creation and transmission of Advertise messages .   30
          17.2.3. Creation and Transmission of Reply messages . . .   31

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



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          18.2.3. Receipt of Renew messages . . . . . . . . . . . .   43
          18.2.4. Receipt of Rebind messages  . . . . . . . . . . .   44
          18.2.5. Receipt of Information-request messages . . . . .   44
          18.2.6. Receipt of Release messages . . . . . . . . . . .   45
          18.2.7. Receipt of Decline messages . . . . . . . . . . .   45
          18.2.8. Transmission of Reply messages  . . . . . . . . .   46

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

20. Relay Agent Behavior                                              50
    20.1. Relaying of client messages . . . . . . . . . . . . . . .   50
    20.2. Relaying of server messages . . . . . . . . . . . . . . .   50

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

22. DHCP options                                                      57
    22.1. Format of DHCP options  . . . . . . . . . . . . . . . . .   58
    22.2. Client Identifier option  . . . . . . . . . . . . . . . .   58
    22.3. Server Identifier option  . . . . . . . . . . . . . . . .   59
    22.4. Identity Association option . . . . . . . . . . . . . . .   59
    22.5. Identity Association for Temporary Addresses option . . .   61
    22.6. IA Address option . . . . . . . . . . . . . . . . . . . .   63
    22.7. Option Request option . . . . . . . . . . . . . . . . . .   64
    22.8. Preference option . . . . . . . . . . . . . . . . . . . .   64



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    22.9. Elapsed Time  . . . . . . . . . . . . . . . . . . . . . .   65
   22.10. Client message option . . . . . . . . . . . . . . . . . .   66
   22.11. Server message option . . . . . . . . . . . . . . . . . .   66
   22.12. Authentication option . . . . . . . . . . . . . . . . . .   67
   22.13. Server unicast option . . . . . . . . . . . . . . . . . .   68
   22.14. Status Code Option  . . . . . . . . . . . . . . . . . . .   68
   22.15. Rapid Commit option . . . . . . . . . . . . . . . . . . .   69
   22.16. User Class Option . . . . . . . . . . . . . . . . . . . .   69
   22.17. Vendor Class Option . . . . . . . . . . . . . . . . . . .   70
   22.18. Vendor-specific Information option  . . . . . . . . . . .   71
   22.19. Interface-Id Option . . . . . . . . . . . . . . . . . . .   72
   22.20. Reconfigure Message option  . . . . . . . . . . . . . . .   73

23. Security Considerations                                           73

24. Year 2000 considerations                                          73

25. IANA Considerations                                               73
    25.1. Multicast addresses . . . . . . . . . . . . . . . . . . .   74
    25.2. Anycast addresses . . . . . . . . . . . . . . . . . . . .   74
    25.3. DHCPv6 message types  . . . . . . . . . . . . . . . . . .   74
    25.4. DUID  . . . . . . . . . . . . . . . . . . . . . . . . . .   75
    25.5. DHCPv6 options  . . . . . . . . . . . . . . . . . . . . .   75
    25.6. Status codes  . . . . . . . . . . . . . . . . . . . . . .   76
    25.7. Authentication option . . . . . . . . . . . . . . . . . .   76

26. Acknowledgments                                                   77

References                                                            77

Chair's Address                                                       79

Authors' Addresses                                                    79

 A. Appearance of Options in Message Types                            81

 B. Appearance of Options in the Options Field of DHCP Options        81

 C. Full Copyright Statement                                          82


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 are carried in options.  DHCP can be extended through
   the definition of new options to carry configuration information not
   specified in this document.





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   DHCP is the "stateful address autoconfiguration protocol" and the
   "stateful autoconfiguration protocol" referred to in RFC2462, "IPv6
   Stateless Address Autoconfiguration".

   The remainder of this introduction summarizes DHCP, explaining
   the message exchange mechanisms and example message flows.  The
   message flows in sections 1.3 and 1.4 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 [17].  The
   client uses its link-local address determined through stateless
   autoconfiguration 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 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, 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 not
   prepared to process may simply discard that message.  For example, 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.







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1.3. Client-server exchanges involving two messages

   A DHCP client can obtain configuration information such as a list
   of available DNS servers or NTP servers 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. 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.





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   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], the IPv6 Addressing
   Architecture [7], IPv6 Stateless Address Autoconfiguration [19], IPv6
   Neighbor Discovery Processing [15], and Dynamic Updates to DNS [20].
   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 [7] 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 [19] specifies procedures
   by which a node may autoconfigure addresses based on router
   advertisements [15], 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.





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   IPv6 Neighbor Discovery [15] is the node discovery protocol in IPv6
   which replaces and enhances functions of ARP [16].  To understand
   IPv6 and stateless address autoconfiguration it is strongly
   recommended that implementors understand IPv6 Neighbor Discovery.

   Dynamic Updates to DNS [20] 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], IPv6 Addressing Architecture [7], and IPv6 Stateless
   Address Autoconfiguration [19] is included below.

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

      anycast address           An anycast address identifies a group
                                of nodes; message sent to an anycast
                                address is delivered to one node out of
                                the group of nodes associated with the
                                address

      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.




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      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::0000/64), 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 address 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.


      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
                                configuration information assigned to
                                the client.  A binding is indexed by
                                the tuple <DUID, IA-type, IAID> (where




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                                IA-type is the type of address in the
                                IA; for example, temporary)

      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 server is 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.




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      message                   A unit of data carried in a packet,
                                exchanged among DHCP servers, relay
                                agents and clients.

      transaction-ID            An unsigned integer 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. Anycast address

   DHCP proposes to use the following reserved anycast address:

      DHCP_Anycast (FEC0:0:0:0:FFFF::4) This document proposes the
                  assignment of the DHCP_Anycast address for use
                  by clients attached to links that do not support
                  multicast.


5.3. UDP ports

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






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5.4. 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 from a server.

      CONFIRM (4)        A client sends a Confirm message to servers to
                         request that the server validate and confirm
                         that the addresses and current configuration
                         parameters assigned by the server to the client
                         are still valid.

      RENEW (5)          A client sends a Renew message to the server
                         that originally provided the client's addresses
                         and configuration addresses to update the
                         addresses assigned to the client and the
                         lifetimes for those addresses, as well as the
                         current configuration parameters assigned by
                         the server to the client.

      REBIND (6)         A client sends a Rebind message to update
                         the addresses assigned to the client and the
                         lifetimes for those addresses, as well as the
                         current configuration parameters assigned by
                         the server to the client; 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
                         confirming or denying the validity of the
                         client's addresses and configuration parameters
                         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 sends a Release message to the server
                         that assigned addresses to the client to



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                         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.  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 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.


5.5. 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 25.6.














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5.6. Configuration Parameters

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

      Parameter     Default  Description
   -------------------------------------
   MIN_SOL_DELAY     1 sec   Min delay of first Solicit
   MAX_SOL_DELAY     5 secs  Max delay of first Solicit
   SOL_TIMEOUT     500 msecs Initial Solicit timeout
   SOL_MAX_RT       30 secs  Max Solicit timeout value
   REQ_TIMEOUT     250 msecs Initial Request timeout
   REQ_MAX_RT       30 secs  Max Request timeout value
   REQ_MAX_RC       10       Max Request retry attempts
   CNF_TIMEOUT     250 msecs Initial Confirm timeout
   CNF_MAX_RT        1 sec   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    Initial Information-request timeout
   INF_MAX_RT       30 secs  Max Information-request timeout value
   REL_TIMEOUT     250 msecs Initial Release timeout
   REL_MAX_RT        1 sec   Max Release timeout
   REL_MAX_RC        5       MAX Release attempts
   DEC_TIMEOUT     250 msecs Initial Decline timeout
   DEC_MAX_RT        1 sec   Max Decline timeout
   DEC_MAX_RC        5       Max Decline attempts



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 order.
















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   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                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                            options                            .
     .                          (variable)                           .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      msg-type             Identifies the DHCP message type; the
                           available message types are listed in
                           section 5.4.

      transaction-id       An unsigned integer used by a client or
                           server to match a response message to a
                           request message.

      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.
























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   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   |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                         link-address                          |
     |                                                               |
     |                                                               |
     |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |               |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                        client-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

      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   The address of the client from which the message
                       to be forwarded was received

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










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7.2. Relay-reply message

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

      msg-type         RELAY-REPL

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

      client-address   The client's address, copied from the
                       relay-forward message

      options          MUST include a "Server message option"; see
                       section 22.11; MAY include other options


8. Representation and use of domain names

   So that domain names may be encoded uniformly and compactly, a
   domain name or a list of domain names is encoded using the technique
   described in sections 3.1 and 4.1.4 of RFC1035 [12].  Section 4.1.4
   of RFC1035 describes how more than one domain name can be represented
   in a list of domain names.  For use in this specification, in a
   list of domain names, the compression pointer (see section 4.1.4 of
   RFC1035) refers to the offset within the list.


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 for any specific client or server - that is, the DUID
   used by a client or server SHOULD NOT change over time; for example,
   a device's DUID should not change as a result of a change in the
   device's network hardware.




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   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
   order, followed by a variable number of octets that make up the
   actual identifier.  A DUID can be no more than 256 octets long.  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        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

   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 order.

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

    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)              .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The choice of network interface can be completely arbitrary, as long
   as that interface provides a unique link-layer address, and 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.

   Clients and servers using this type of DUID MUST store the DUID
   in stable storage, and MUST continue to use this DUID even if the
   network interface used to generate the DUID 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 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. A DUID collision 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 using this mechanism MUST provide
   an administrative interface that replaces the existing DUID with a
   newly-generated DUID of this type.


9.3. Vendor-assigned unique ID based on Domain Name (VUID-DN)

   The vendor-assigned unique ID based on the domain name consists of a
   two-octet value giving the length of the identifier, the value of the
   identifier and the vendor's registered domain name.
















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   The following diagram summarizes the structure of a VUID-DN:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               2               |       identifier length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                           identifier                          .
   .                       (variable length)                       .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                  domain name (variable length)                .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The source of the identifier is left up to the vendor defining it,
   but each identifier part of each VUID-DN 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-DN SHOULD be recorded in non-erasable storage.  The domain name
   is simply any domain name that has been legally registered by the
   vendor in the domain name system [11], stored in the form described
   in section 8.

   If a domain name is being used by a vendor as a vendor identifier,
   then the vendor MUST ensure that the domain name has not previously
   been used by a different vendor.

   An example DUID of this type might look like this:

   +---+---+---+---+---+---+---+---+
   | 0 | 2 | 0 | 8 | 12|192|132|221|
   +---+---+---+---+---+---+---+---+
   | 3 | 0 | 9 | 18|101|120| 97|109|
   +---+---+---+---+---+---+---+---+
   |112|108|101| 46| 99|111|109|
   +---+---+---+---+---+---+---+


   This example includes the two-octet type of 2, the two-octet length
   of 8, eight octets of identifier data, followed by "example.com"
   represented in ASCII.


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

   The vendor-assigned unique ID based on Enterprise Number consists
   of the vendor's registered Enterprise Number as maintained by IANA
   followed by the value of the identifier.




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   The following diagram summarizes the structure of a VUID-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               |       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 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 SHOULD be recorded in non-erasable storage.  The Enterprise
   Number is the vendor's Enterprise code from the list "SMI Network
   Management Private Enterprise Codes", as maintained by IANA in file
   ftp://ftp.isi.edu/in-notes/iana/assignments/enterprise-numbers.  The
   Enterprise Number is stored as an unsigned 32 bit number.

   An example DUID of this type might look like this:

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


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


9.5. Link-layer address

   This type of DUID consists of two octets containing the DUID type 4,
   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.  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
   order.







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

    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               |    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 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 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
   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 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



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   options within the IA, and are associated with the addresses in the
   IA and the interface to which the 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.

   Each address in an IA has a preferred lifetime and a valid lifetime,
   as defined in RFC2462 [19].  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

   A server MUST generate link identifiers for unicast addresses with
   the "u" (universal/local) and "g" (individual/group) bits of the
   EUI-64 identifier set to 0 (see RFC 2373).




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   A server MUST NOT assign an address that is otherwise reserved by
   IANA.


12. Management of temporary addresses

   A client may be assigned temporary addresses (temporary addresses
   are defined in RFC 3041 [14]).  Clients and servers simply identify
   addresses as "temporary".  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, and MUST NOT update the DNS in any other way
   for a temporary address.


13. Transmission of messages by a client

   Unless otherwise specified, a client sends DHCP messages to the
   All_DHCP_Relay_Agents_and_Servers or the DHCP_Anycast address.

   If the client is attached to a link that supports multicast
   transmission, the client sends DHCP messages to the
   All_DHCP_Relay_Agents_and_Servers address.  If the client is
   attached to a link that does not support multicast transmission, the
   client uses the DHCP_Anycast address.

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


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,



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   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 for use with the Solicit message (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 numbers from each invocation of the DHCP client.

   RT for the first message transmission is based on IRT:

      RT = 2*IRT + RAND*IRT


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

      RT = 2*RTprev + RAND*RTprev






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   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.  If MRC has a value of 0, the client MUST
   continue to retransmit the original message until a response is
   received.  Otherwise, 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.  If MRD has a value of 0, the client MUST
   continue to retransmit the original message until a response is
   received.  Otherwise, the message exchange fails once the client has
   transmitted the message MRD seconds.

   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.


15. Message validation

   Servers MUST discard any received messages that include
   authentication information and fail the authentication check by the
   server.

   Clients MUST discard any received messages that include
   authentication information and fail the authentication check by the
   client, except as noted in section 21.5.5.2.


15.1. Use of Transaction-ID field

   The "transaction-ID" field holds a value used by clients and servers
   to synchronize server responses to client messages.  A client SHOULD
   choose a different transaction-ID for each new message it sends.  A
   client MUST leave the 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.






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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" 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

    -  the message does not include a Client Identifier option


15.5. Confirm message

   Clients MUST discard any received Confirm messages.

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


15.6. Renew message

   Clients MUST discard any received Renew messages.

   Servers MUST discard any received Renew message that meets 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



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    -  the message does not include a Client Identifier option


15.7. Rebind message

   Clients MUST discard any received Rebind messages.

   Servers MUST discard any received Rebind message that does not
   include a Client Identifier option.


15.8. Decline messages

   Clients MUST discard any received Decline messages.

   Servers MUST discard any received Decline message that meets 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

    -  the message does not include a Client Identifier option


15.9. Release message

   Clients MUST discard any received Release messages.

   Servers MUST discard any received Decline message that meets 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

    -  the message does not include a Client Identifier option


15.10. Reply message

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

    -  the message does not include a Server Identifier option

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






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

    -  the message includes a Client Identifier option and the contents
       of the Client Identifier option does not match the DUID of the
       client

   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 meet any of the
   following conditions:

    -  the message does not include a Server Identifier option

    -  the message does not contain an authentication option

    -  the message fails the authentication validation performed by the
       client


15.12. Information-request message

   Clients MUST discard any received Information-request messages.

   Servers MAY discard any received Information-request messages that do
   not include a Client Identifier option.

   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 multicast address, it MUST



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   use the IPv6 link-local address assigned to the interface for which
   the client is interested in obtaining configuration as the source
   address in the header of the IP datagram.

   When a client sends a DHCP message to the DHCP_Anycast address, it
   MUST use an address assigned to the interface for which the client
   is interested in obtaining configuration, which is suitable for use
   by the server in responding to the client, as the source address in
   the header of the IP datagram.  See "Default Address Selection for
   IPv6" [4] for more details.

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

   The client MUST transmit the message on the link that the interface
   for which configuration information is being obtained is attached
   to.  The client SHOULD send the message through that interface.  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.


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.


17.1. Client Behavior

   A client uses the Solicit message to discover DHCP servers configured
   to serve addresses 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"
   field.




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   The client MUST include a Client Identifier option to identify itself
   to the server.  The client MUST include one or more 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 except those specifically allowed as
   defined by specific options.

   The client may send just IA options for the assignment of
   non-temporary addresses, just IA_TA options for the assignment of
   only temporary options, or a mix of both IA and IA_TA options.

   The client MAY request specific options from the server by including
   an Option Request option (see section 22.7) indicating the options
   the client is interested in receiving.  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) 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 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 the ManagedFlag changes from FALSE to TRUE (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 message, the client terminates the retransmission process as
   soon as a Reply message is received.

   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 IRT has elapsed.  Rather, the client



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   collects Advertise messages until IRT 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 IRT 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 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 IRT
   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, 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.




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   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.  If the client receives a Reply message, it processes
   the message as described in section 18.1.6.  If the client does not
   receive a Reply message, the client restarts the server solicitation
   process by sending a Solicit message that does not include a Rapid
   Commit option.


17.2. Server Behavior

   A server sends an Advertise message in response to 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.

   Otherwise, the server generates and sends an Advertise message to the
   client.


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 field from the Solicit message
   received from the client to the Advertise message.  The server
   includes its server identifier in a Server Identifier option.

   The server MAY add a Preference option to carry the preference value
   for the Advertise message.  The server implementation SHOULD allow



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   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 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 to IAs in a subsequent
   Request from the client, the server MUST send an Advertise message to
   the client that includes only a status code option with the status
   code set to AddrUnavail and a status message for the user.

   The server MAY include other options the server will return to the
   client in a subsequent Reply message.  The information in these
   options will be used by the client in the selection of a server if
   the client receives more than one Advertise message.  The server
   SHOULD include options specifying values for options requested by the
   client in an Option Request Option included in the Solicit message.

   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 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.  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 message exchange, a server
      need not commit 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.




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      When using the Solicit-Reply message exchange, the server
      commits the assignment of 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 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 and Renew messages).


18.1. Client Behavior

   A client will use Request, Confirm, Renew, Rebind and
   Information-request messages to acquire and confirm the
   validity of configuration information.  The client uses the server
   identifier information and information about IAs from previous
   Advertise messages for use in constructing Request messages.


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.





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   The client generates a transaction ID and inserts this value in the
   "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).

   If the client chooses to request specific options from the server,
   it does so by including an Option Request option (see section 22.7),
   which MUST include all of the options the client is requesting.  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 (section 22.13) from the server, the client
   SHOULD unicast the Request message to the server.

   DISCUSSION:

      Use of multicast or anycast 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 of the
   following actions:

    -  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







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18.1.2. Creation and transmission of Confirm messages

   Whenever a client may have moved to a new link, its IPv6 addresses
   and other configuration information may no longer be valid.  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 the
   acceptability of the addresses with the status in the Reply message
   it returns to the client.

   The client sets the "msg-type" field to CONFIRM. The client generates
   a transaction ID and inserts this value in the "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 addresses the
   client currently has associated with those IAs.

   If the client chooses to request specific options from the server,
   it does so by including an Option Request option (see section 22.7,
   which MUST include all of the options the client is requesting.  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   When the client sends the Confirm message, it MUST use an IPv6
   address that the client has confirmed to be valid on the link to
   which it is currently attached and that is assigned to the interface
   for which the client is interested in obtaining configuration
   information as the source address in the IP header of the datagram
   carrying the Confirm message.

   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



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   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 associated with
   addresses, the client sends a Renew message to the server containing
   an IA option for the IA and its associated addresses.  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" 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.

   If the client chooses to request specific options from the server,
   it does so by including an Option Request option (see section 22.7),
   which MUST include all of the options the client is requesting.  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.




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   DISCUSSION:

      Use of multicast or anycast 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

      MRC   0

      MRD   0

   The mechanism in section 14 is modified as follows for use in the
   transmission of Renew messages.  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.  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"
   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.

   If the client chooses to request specific options from the server,
   it does so by including an Option Request option (see section 22.7),
   which MUST include all of the options the client is requesting.  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



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      MRT   REB_MAX_RT

      MRC   0

      MRD   0

   The mechanism in section 14 is modified as follows for use in the
   transmission of Rebind messages.  The message exchange is terminated
   when the 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" 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.

   If the client chooses to request specific options from the server,
   it does so by including an Option Request option (see section 22.7),
   which MUST include all of the options the client is requesting.  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 MUST NOT include any IA options.

   If the client has an IPv6 address of sufficient scope, the
   client MAY choose to send the Information-request message to the
   All_DHCP_Servers multicast address.

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

      IRT   INF_TIMEOUT

      MRT   INF_MAX_RT



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      MRC   0

      MRD   0


18.1.6. Receipt of Reply message in response to a Request, Confirm,
   Renew, Rebind or Information-request message

   Upon the receipt of a valid Reply message in response to a Request,
   Confirm, Renew, Rebind or Information-request message, the client
   extracts the configuration information contained in the Reply.  The
   client MAY choose to report any status code or message from the
   status code option in the Reply message.

   The client SHOULD perform duplicate address detection [19] on each
   of the addresses in any IAs it receives in the Reply message before
   using that address for traffic.  If any of the addresses are found
   to be in use on the link, the client sends a Decline message to the
   server as described in section 18.1.9.

   The client records the T1 and T2 times for each IA in the Reply
   message.  The client records any addresses included with IAs in
   the Reply message.  The client updates the preferred and valid
   lifetimes for the addresses in the IA from the lifetime information
   in the IA option.  The client leaves any addresses that the client
   has associated with the IA that are not included in the IA option
   unchanged.

   If the Reply was received in response to a Renew or Rebind message,
   the client must update the information in any IA option contained in
   the Reply 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 the IA option.

   Management of the specific configuration information is detailed in
   the definition of each option, in section 22.

   When the client receives a NotOnLink status in an IA from the server
   in response to a Confirm message, the client can assume it needs to
   send a Request to the server to obtain appropriate addresses for the
   IA. If the client receives any Reply messages that do not indicate
   a NotOnLink status, the client can use the addresses in the IA and
   ignore any messages that do indicate a NotOnLink status.

   When the client receives an AddrUnavail status in an IA from the
   server for a Request message the client will have to find a new
   server to create an IA.





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   When the client receives a NoBinding status in an IA from the server
   for a Renew message the client can assume it needs to send a Request
   to reestablish an IA with the server.

   When the client receives an AddrUnavail status in an IA from the
   server for a Renew message the client can assume it needs to send a
   Request to reestablish an IA with the server.

   When 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
   to reestablish an IA with the server or try another server.

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


18.1.7. Creation and transmission of Release messages

   To release one or more addresses, a client sends a Release message to
   the server.

   The client sets the "msg-type" field to RELEASE. The client generates
   a transaction ID and places this value in the "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 in the "options" field.  The addresses
   to be released 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 is releasing as
   the source address in the Release message or in any subsequently
   transmitted message.

   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 Release message to the server.

   DISCUSSION:

      Use of multicast or anycast 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.





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

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

      IRT   REL_TIMEOUT

      MRT   0

      MRC   REL_MAX_MRC

      MRD   0

   The client MUST abandon the attempt to release addresses if the
   Release message exchange fails.

   The client MUST stop using all of the addresses being released as
   soon as the client begins the Release message exchange process.
   If addresses are released but the Reply from a DHCP server is
   lost, the client will retransmit the Release message, and the
   server may respond with a Reply indicating a status of "Nobinding".
   Therefore, the client does not treat a Reply message with a status
   of "Nobinding" in a Release message exchange as if it indicates an
   error.

   Note that if the client fails to release the addresses, the addresses
   assigned to the IA will be reclaimed by the server when the lifetime
   of the address expires.


18.1.8. Receipt of Reply message 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 and transmission of Decline messages

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



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

   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 Decline message to the server.

   DISCUSSION:

      Use of multicast or anycast 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   DEC_TIMEOUT

      MRT   DEC_MAX_RT

      MRC   DEC_MAX_RC

      MRD   0

   The client MUST abandon the attempt to decline addresses if the
   Decline message exchange fails.


18.1.10. Receipt of Reply message in response to a Decline message

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


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.


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 option with value UseMulticast and no other
   options.




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   When the server receives a Request 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 the
   client.

   The server contructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Request message into
   the 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 the
   link to which the client is connected, the server MUST return the IA
   to the client with the status field set to 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 set to AddrUnavail 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.

   The server adds options to the Reply message for any other
   configuration information to be assigned to the client.  If the
   Request message contained an Option Request option, the server MUST
   include options in the Reply message for any options in the Option
   Request option the server is configured to return to the client.  The
   server MAY choose to return other options not specified in the Option
   Request option.


18.2.2. Receipt of Confirm messages

   When the server receives a Confirm message, the client is requesting
   confirmation that the configuration information it will use is valid.
   The server locates the binding for that client and compares the
   information in the Confirm message from the client to the information
   associated with that client.

   If the server finds that the information for the client does not
   match what is in the binding for that client or the configuration
   information is not valid, the server sends a Reply message containing
   a Status Code option with the value ConfNoMatch.

   If the server finds that the information for the client does match
   the information in the binding for that client, and the configuration




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   information is still valid, the server sends a Reply message
   containing a Status Code option with the value Success.

   If the server cannot determine if the information in the Confirm
   message is valid or invalid, 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 the Confirm message
   appears valid, the server does not reply.

   The server contructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Confirm message into
   the 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 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, the server discards
   the Renew message and responds with a Reply message containing a
   status code option with value UseMulticast and no other options.

   When the server receives a Renew and IA option from a 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 set to NoBinding
   in the Renew message.

   If the server finds that any of the addresses are no longer valid
   for the client, 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 contructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Renew message into the
   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.




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18.2.4. Receipt of Rebind messages

   When the server receives a Rebind and IA option from a 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 set to NoBinding
   in the Rebind message.

   If the server finds that the any of the addresses are no longer valid
   for the client, 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 if the default is not being used.

   The server contructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Rebind message into the
   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 adds options to the Reply message for any other
   configuration information to be assigned to the client.  If the
   Rebind message contained an Option Request option, the server MUST
   include options in the Reply message for any options in the Option
   Request option the server is configured to return to the client.  The
   server MAY choose to return other options not specified in the Option
   Request option.


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 server contructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Rebind message into the
   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 adds options to the Reply message for all of the
   configuration parameters to be returned to the client.  If the



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   Information-request message contained an Option Request option, the
   server MUST include options in the Reply message for any options in
   the Option Request option the server is configured to return to the
   client.  The server MAY choose to return other options not specified
   in the Option Request option.

   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 option with value UseMulticast 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 invalid addresses
   (though it may choose to log an error if it finds an invalid
   address).

   After all the addresses have been processed, the server generates a
   Reply message and includes a Status Code option with value Success
   and a Server Identifier option with the server's DUID. The server
   MUST NOT include any other options in the Reply message.

   If the server cannot find a binding for the client, the server
   sends a Reply message that includes a Status Code option with value
   NoBinding.

   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 option with value UseMulticast and no other
   options.




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   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 invalid addresses (though it may
   choose to log an error if it finds an invalid address).

   After all the address have been processed, the server generates a
   Reply message and includes a Status Code option with value Success
   and a Server Identifier option with the server's DUID. The server
   MUST NOT include any other options in the Reply message.


18.2.8. Transmission of Reply messages

   If the Request, Confirm, Renew, Rebind, Release, Decline or
   Information-request message from the client was originally received
   in a Relay-forward message from a relay, the server places the Reply
   message in the options field of a Relay-response message and copies
   the link-address and client-address fields from the Relay-forward
   message into the Relay-response message.

   The server then unicasts the Reply or Relay-reply to the source
   address from the IP datagram in which the original 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 a
   Server Identifier option.



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   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.

   The server MUST include an authentication option in the Reconfigure
   message.  The server MUST include a Reconfigure Message option
   (defined in section 22.20) 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 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 RECREP_MSG_TIMEOUT milliseconds, the server
   retransmits the Reconfigure message, doubles the RECREP_MSG_TIMEOUT
   value and waits again.  The server continues this process until
   REC_MSG_ATTEMPTS unsuccessful attempts have been made, at which point
   the server SHOULD abort the reconfigure process for that client.

   Default and initial values for RECREP_MSG_TIMEOUT and
   REC_MSG_ATTEMPTS are documented in section 5.6.


19.1.3. Receipt of Renew messages

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



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


19.2. Receipt of Information-request messages

   The server generates and sends Reply message(s) 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 other parameters to
   be reconfigured, even if those parameters were not specified in the
   Information-request message from the client.


19.3. Client Behavior

   A client MUST accept Reconfigure messages sent to UDP port 546on
   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. Receipt of Reconfigure messages

   Upon receipt of a valid Reconfigure message, the client initiates a
   transaction with the server by sending a Reply or Information-request
   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).

   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



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      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. 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 Option
   Request option specifying the IA option, 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. 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. 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. Receipt of Reply messages

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




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20. Relay Agent Behavior

   For this discussion, 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 client messages

   When a relay agent receives a valid client message, it constructs
   a Relay-forward message.  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.

   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) in the Relay-forward message.  The server
   will include the Interface-id option in its Relay-reply message.
   The relay agent puts the client's address in the link-address field
   regardless of whether the relay agent includes an Interface-id option
   in the Relay-forward message.

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

   The relay agent constructs a Client Message option (see
   section 22.10) that contains the entire message from the client in
   the data field of the option.  The relay agent places the Client
   Message option along with any "relay agent-specific" options in the
   options field of the Relay-forward message.  The relay agent sends
   the Relay-forward message to the list of server destination addresses
   with which 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 in the
   Relay-reply message as appropriate to those options.  The relay
   agents then discards those options.

   If the Relay-reply message includes a 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, the relay agent



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   forwards the message on the link identified by the link-address
   field.

   In either case, the relay agent extracts the server message from the
   Server Message option (see section 22.11) and forwards the message to
   the address in the client-address field in the Relay-reply message.


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.

   Because of the risk of denial of service attacks against DHCP
   clients, the use of authentication is mandated in Reconfigure
   messages.  A DHCP server MUST include an authentication option in
   Reconfigure messages sent to clients.

   The DHCP authentication mechanism is based on the design of
   authentication for DHCP for IPv4 [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.





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   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 addresses, or the exhaustion of CPU or
   network bandwidth, and are present anytime there is a shared
   resource.


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 agents and servers is not specified in this
   document.


21.3. Summary of DHCP authentication

   Authentication of DHCP messages is accomplished through the use of
   the Authentication option (see section 22.12).  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 protocols 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]) can reduce the danger of replay attacks.
   This method MUST be supported by all protocols.








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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]
   using the MD5 hash [18] 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, RDM and Replay detection fields, but no 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 Authentication
   option should appear as close to the beginning of the options area
   in the DHCP message to facilitate processing of the authentication
   information.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                            |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   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]
   and the MD5 hash function [18].  The entire DHCP message (except
   the MAC field of the authentication option itself), 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. Message validation

   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].  The entire DHCP
   message (except the MAC field of the authentication option itself),
   is used as input to the HMAC-MDS 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. Key utilization

   Each DHCP client has a key, K. The client uses its key to encode
   any messages it sends to the server and to authenticate and verify



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   any messages it receives from the server.  The client's key SHOULD
   be initially distributed to the client through some out-of-band
   mechanism, and SHOULD be stored locally on the client for use in all
   authenticated DHCP messages.  Once the client has been given its key,
   it SHOULD use 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 MUST know, or be able to obtain in a secure 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.


21.5.5. Client considerations for delayed authentication protocol

21.5.5.1. 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, RDM and replay detection field as described
   in section 21.5.  The client does not include any authentication
   information in the Authentication option.


21.5.5.2. 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.

   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



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   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. 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. 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, 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. 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.  The client MAY choose to remember which
   server replied with a Reply message that failed to pass validation
   and discard subsequent messages from that server.

   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. Receiving Reconfigure messages

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







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21.5.6. Server considerations for delayed authentication protocol

21.5.6.1. 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. 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.  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. 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 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




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   data areas of the options MUST NOT be concatenated or otherwise
   combined.


22.1. Format of DHCP options

      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 Client Identifier option
   MUST appear before any IA options in the DHCP message.  The format of
   the Client Identifier option is:


















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      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


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.




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   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.





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   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 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)                        |



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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                           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 lifetimes of all of the addresses in an IA have
   expired, the IA can be considered as having expired.

   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.

   An identity association for temporary addresses option MUST NOT
   appear in a Renew or Rebind message.  This option MAY appear in a
   Confirm message if the lifetimes on the temporary addresses in the
   associated IA have not expired.








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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 (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.



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   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.


22.8. Preference 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_PREFERENCE       |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  pref-value   |
     +-+-+-+-+-+-+-+-+



      option-code   OPTION_PREFERENCE (7)



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      option-len    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

      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 SHOULD include an Elapsed Time option in messages to
   indicate how long the client has been trying to complete a DHCP
   transaction.  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.




















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22.10. Client 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 Message option in a Relay-forward message.


22.11. Server 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_SERVER_MSG       |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                       DHCP-server-message                     .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option-code   OPTION_SERVER_MSG (10)

      option-len    Length of DHCP server message.

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

   A server sends a DHCP message to be forwarded to a client by a relay
   agent as the contents of a Server Message option in a Relay-reply
   message.



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22.12. 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.  If
   the Authentication option appears in a DHCP message, it should be
   included as close to the beginning of the options field as possible
   for improved efficiency of authentication processing.

   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 detect.|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Replay Detection (64 bits)                 |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Replay cont.                  | Auth. Info    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                   Authentication Information                  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      option-code                  OPTION_AUTH (11)

      option-len                   12 + 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






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22.13. 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 of the option.

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

    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. Status Code Option

   This option returns a status indication related to the DHCP message
   or option in which it appears.

    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)



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      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 5.5.

      status-message       A UTF-8 encoded text string, which MUST NOT
                           be null-terminated.

   A Status Code option may appear in a DHCP message option, or in the
   options area of another option.


22.15. Rapid Commit option

   A client MAY include this option in 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.

    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


22.16. User Class Option

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



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      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 order.

   Servers can interpret the meanings of multiple class specifications
   in an implementation dependent or configuration dependent manner, and
   so the use of multiple classes by a DHCP client should be based on
   the specific server implementation and configuration which will be
   used to process that User class option.


22.17. 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.

      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.



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

   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 order.

   A DHCP message MUST NOT contain more than one Vendor Class option.


22.18. Vendor-specific Information option

   This option is used by clients and servers to exchange
   vendor-specific information.  The definition 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.

      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 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.



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      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. 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.

    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



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   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.


22.20. 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.

    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 for Renew message, 2 for
                           Information-request message


23. Security Considerations

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


24. Year 2000 considerations

   Since all times are relative to the current time of the transaction,
   there is no problem within the DHCPv6 protocol related to any
   hardcoded dates or two-digit representation of the current year.


25. IANA Considerations

   This document defines several new name spaces associated with DHCPv6
   and DHCPv6 options.  IANA is requested to manage the allocation of
   values from these name spaces, which are described in the remainder



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

   New values in each of these name spaces should be approved by the
   process of IETF consensus [13].


25.1. 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

   IANA is requested to manage definition of additional multicast
   addresses in the future.


25.2. Anycast addresses

   Section 5.2 defines the following anycast address, which is requested
   for assignment to DHCP by IANA:

      DHCP_Anycast:  FEC0:0:0:0:FFFF::4

   IANA is requested to manage definition of additional anycast
   addresses in the future.


25.3. DHCPv6 message types

   IANA is requested to record the following message types (defined in
   section 5.4).  IANA is requested to manage definition of additional
   message types in the future.

      SOLICIT               1

      ADVERTISE             2

      REQUEST               3

      CONFIRM               4

      RENEW                 5

      REBIND                6

      REPLY                 7

      RELEASE               8




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      DECLINE               9

      RECONFIGURE           10

      INFORMATION-REQUEST   11

      RELAY-FORW            12

      RELAY-REPL            13


25.4. 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-DN                        2

      VUID-EN                        3

      Link-layer address             4


25.5. DHCPv6 options

   IANA is requested to record the following option-codes (as defined
   in section 22).  IANA is requested to manage the definition of
   additional DHCPv6 option-codes in the future.

      OPTION_CLIENTID       1

      OPTION_SERVERID       2

      OPTION_IA             3

      OPTION_IA_TMP         4

      OPTION_IADDR          5

      OPTION_ORO            6

      OPTION_PREFERENCE     7

      OPTION_ELAPSED_TIME   8

      OPTION_CLIENT_MSG     9

      OPTION_SERVER_MSG     10

      OPTION_AUTH           11



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      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


25.6. 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 unavailable
   NoBinding       4 Client record (binding) unavailable
   ConfNoMatch     5 Client record Confirm doesn't match IA
   NotOnLink       6 One or more prefixes of the addresses
                     in the IA is not valid for the link
                     from which the client message was received
   UseMulticast    7 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



25.7. Authentication option

   Section 21 defines three new name spaces associated with the
   Authentication Option (section 22.12), which are to be created and
   maintained by IANA: Protocol, Algorithm and RDM.






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   Initial values assigned from the Protocol name space are 0 (reserved)
   and 1 (for the delayed authentication Protocol in section 21.5).
   Additional protocols may be defined in the future.

   The Algorithm name space is specific to individual Protocols.  That
   is, each Protocol has its own Algorithm name space.  The guidelines
   for assigning Algorithm name space values for a particular protocol
   should be specified along with the definition of a new Protocol.

   For the delayed authentication Protocol, the Algorithm value 1
   is assigned to the HMAC-MD5 generating function as defined in
   section 21.5.  Additional algorithms for the delayed authentication
   protocol may be defined in the future.

   The initial value of 0 from the RDM name space is assigned to the
   use of a monotonically increasing value as defined in section 21.4.
   Additional replay detection methods may be defined in the future.


26. Acknowledgments

   Thanks to the DHC Working Group for their time and input into the
   specification.  In particular, thanks also for the consistent input,
   ideas, and review by (in alphabetical order) Thirumalesh Bhat,
   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, 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.

   Bill Arbaugh reviewed the authentication mechanism described in
   section 21.

   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.


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





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    [4] R. Draves.  Default Address Selection for IPv6, September 2001.
        work in progress.

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

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

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

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

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

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

   [11] P.V. Mockapetris.  Domain names - concepts and facilities,
        November 1987.  RFC 1034.

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

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

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

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

   [16] 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.

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

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

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

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





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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 can be directed to:






































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

        Mike Carney
        Sun Microsystems, Inc
        Mail Stop:  UMPK17-202
        901 San Antonio Road
        Palo Alto, CA 94303-4900
        USA
        Voice:  +1-650-786-4171
        E-mail:  mwc@eng.sun.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, 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
        USA
        Voice:  +1 978 479 4733
        E-mail:  rdroms@cisco.com






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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).


         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:



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             Option   IA/   IAADDR Client Server
             Field   IA_TA          Msg.   Msg.
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).  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
   of developing Internet standards in which case the procedures
   for copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "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
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.





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