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


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


Status of This Memo

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

   Distribution of this memo is unlimited.

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

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

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



Abstract

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










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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

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

 2. Requirements                                                       4

 3. Background                                                         4

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

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

 6. Message Formats                                                   11

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

 8. Representation and use of domain names                            13

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

10. Identity association                                              17

11. Selecting addresses for assignment to an IA                       18

12. Management of temporary addresses                                 19

13. Transmission of messages by a client                              19

14. Reliability of Client Initiated Message Exchanges                 20

15. Message validation                                                21



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

16. Client Source Address and Interface Selection                     25

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

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

19. DHCP Server-Initiated Configuration Exchange                      45
    19.1. Server Behavior . . . . . . . . . . . . . . . . . . . . .   45
          19.1.1. Creation and transmission of Reconfigure messages   45




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          19.1.2. Time out and retransmission of Reconfigure
                          messages . . . . . . . . . . . . . . . . .  46
    19.2. Receipt of Renew messages . . . . . . . . . . . . . . . .   47
    19.3. Receipt of Information-request messages . . . . . . . . .   47
    19.4. Client Behavior . . . . . . . . . . . . . . . . . . . . .   47
          19.4.1. Receipt of Reconfigure messages . . . . . . . . .   47
          19.4.2. Creation and transmission of Renew messages . . .   48
          19.4.3. Creation and transmission of Information-request
                          messages . . . . . . . . . . . . . . . . .  48
          19.4.4. Time out and retransmission of Renew or
                          Information-request messages . . . . . . .  49
          19.4.5. Receipt of Reply messages . . . . . . . . . . . .   49

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

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  . . . . . . . . . . . . . . . . . . . .   53
    21.5. Delayed authentication protocol . . . . . . . . . . . . .   53
          21.5.1. Use of the Authentication option in the delayed
                          authentication protocol  . . . . . . . . .  53
          21.5.2. Message validation  . . . . . . . . . . . . . . .   55
          21.5.3. Key utilization . . . . . . . . . . . . . . . . .   55
          21.5.4. Client considerations for delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  55
          21.5.5. 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 . . . . . . . . . . . . . . .   60
    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 . . . . . . . . . . . . . . . . . . . .   65
    22.9. Elapsed Time option . . . . . . . . . . . . . . . . . . .   65
   22.10. Relay Message option  . . . . . . . . . . . . . . . . . .   66
   22.11. Authentication option . . . . . . . . . . . . . . . . . .   66
   22.12. Server unicast option . . . . . . . . . . . . . . . . . .   67
   22.13. Status Code Option  . . . . . . . . . . . . . . . . . . .   68
   22.14. Rapid Commit option . . . . . . . . . . . . . . . . . . .   69
   22.15. User Class option . . . . . . . . . . . . . . . . . . . .   70
   22.16. Vendor Class Option . . . . . . . . . . . . . . . . . . .   71
   22.17. Vendor-specific Information option  . . . . . . . . . . .   72



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   22.18. Interface-Id Option . . . . . . . . . . . . . . . . . . .   73
   22.19. Reconfigure Message option  . . . . . . . . . . . . . . .   74
   22.20. Reconfigure Nonce option  . . . . . . . . . . . . . . . .   74

23. Security Considerations                                           75

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

25. Acknowledgments                                                   79

26. Changes in draft-ietf-dhc-dhcpv6-25.txt                           80

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

References                                                            83

Chair's Address                                                       84

Authors' Addresses                                                    85

 A. Appearance of Options in Message Types                            86

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

 C. Full Copyright Statement                                          87


























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1. Introduction and Overview

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

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

   DHCP is the "stateful address autoconfiguration protocol" and the
   "stateful autoconfiguration protocol" referred to in "IPv6 Stateless
   Address Autoconfiguration" [20].

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

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


1.1. Protocols and addressing

   Clients and servers exchange DHCP messages using UDP [18].  The
   client uses a link-local address or addresses determined through
   other mechanisms for transmitting and receiving DHCP messages.

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

   To allow a DHCP client to send a message to a DHCP server that is not
   attached to the same link, a DHCP relay agent on the client's link
   will 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.






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1.2. Client-server exchanges involving two messages

   When a DHCP client does not need to have a DHCP server assign
   it IP addresses, the client can obtain configuration information
   such as a list of available DNS servers [7] or NTP servers [21]
   through a single message and reply exchanged with a DHCP server.
   To obtain configuration information the client first sends an
   Information-Request message to the All_DHCP_Relay_Agents_and_Servers
   multicast address.  The server responds with a Reply message
   containing the configuration information for the client.

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

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

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


1.3. Client-server exchanges involving four messages

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

   As described in the previous section, the client sends a Renew
   messages to the server to extend the lifetimes associated with its




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   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 [4], the IPv6 Addressing
   Architecture [8], IPv6 Stateless Address Autoconfiguration [20], IPv6
   Neighbor Discovery Processing [16], and Dynamic Updates to DNS [22].
   These specifications enable DHCP to build upon the IPv6 work to
   provide both robust stateful autoconfiguration and autoregistration
   of DNS Host Names.

   The IPv6 Addressing Architecture specification [8] defines the
   address scope that can be used in an IPv6 implementation, and the
   various configuration architecture guidelines for network designers
   of the IPv6 address space.  Two advantages of IPv6 are that support
   for multicast is required, and nodes can create link-local addresses
   during initialization.  This means that a client can immediately use
   its link-local address and a well-known multicast address to begin
   communications to discover neighbors on the link.  For instance, a
   client can send a Solicit message and locate a server or relay agent.

   IPv6 Stateless Address Autoconfiguration [20] specifies procedures
   by which a node may autoconfigure addresses based on router
   advertisements [16], and the use of a valid lifetime to support
   renumbering of addresses on the Internet.  In addition the
   protocol interaction by which a node begins stateless or stateful
   autoconfiguration is specified.  DHCP is one vehicle to perform
   stateful autoconfiguration.  Compatibility with stateless address
   autoconfiguration is a design requirement of DHCP.

   IPv6 Neighbor Discovery [16] is the node discovery protocol in IPv6
   which replaces and enhances functions of ARP [17].  To understand




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

   Dynamic Updates to DNS [22] is a specification that supports the
   dynamic update of DNS records for both IPv4 and IPv6.  DHCP can use
   the dynamic updates to DNS to integrate addresses and name space to
   not only support autoconfiguration, but also autoregistration in
   IPv6.


4. Terminology

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


4.1. IPv6 Terminology

   IPv6 terminology relevant to this specification from the IPv6
   Protocol [4], IPv6 Addressing Architecture [8], and IPv6 Stateless
   Address Autoconfiguration [20] is included below.

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

      host                      Any node that is not a router.

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

      interface                 A node's attachment to a link.

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

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

      link-local address        An IPv6 address having link-only
                                scope, indicated by having the prefix
                                (FE80::0000/10), that can be used to



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

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

      neighbor                  A node attached to the same link.

      node                      A device that implements IP.

      packet                    An IP header plus payload.

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

      prefix length             The number of bits in a prefix.

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

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


4.2. DHCP Terminology

   Terminology specific to DHCP can be found below.


      appropriate to the link   an address is "appropriate to the link"
                                when the address is consistent with the
                                DHCP server's knowledge of the network
                                topology, prefix assignment and address
                                assignment policies

      binding                   A binding (or, client binding) is a
                                group of server data records containing
                                the information the server has about
                                the addresses in an IA or configuration
                                information explicitly assigned to the
                                client.  Configuration information that
                                has been returned to a client through a
                                policy - for example, the information
                                returned to all clients on the same
                                link - does not require a binding.  A
                                binding containing information about



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                                an IA is indexed by the tuple <DUID,
                                IA-type, IAID> (where IA-type is the
                                type of address in the IA; for example,
                                temporary).  A binding containing
                                configuration information for a client
                                is indexed by <DUID>.

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

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

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

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

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

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

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

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

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



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                                all IAIDs for IAs belonging to that
                                client.

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

      reconfiguration nonce     An opaque value used to provide security
                                for Reconfigure messages.

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


5. DHCP Constants

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


5.1. Multicast Addresses

   DHCP makes use of the following multicast addresses:

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

      All_DHCP_Servers (FF05::1:3) A site-scoped multicast address used
                  by a relay agent to communicate with servers, either
                  because the 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 relay agent to use this address, it must have an
                  address of sufficient scope to be reachable by the
                  servers.  All servers within the site are members of
                  this multicast group.


5.2. UDP ports

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


5.3. DHCP message types

   DHCP defines the following message types.  More detail on these
   message types can be found in Section 6.  Message types not listed




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   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, including IP
                         addresses, from a specific server.

      CONFIRM (4)        A client sends a Confirm message to any
                         available server to determine whether the
                         addresses it was assigned are still appropriate
                         to the link to which the client is connected.

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

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

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

                         A server sends a Reply message containing
                         assigned addresses and configuration parameters
                         in response to a Solicit, Request, Renew,
                         Rebind message received from a client.  A
                         server sends a Reply message containing
                         configuration parameters in response to an



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                         Information-request message.  A server sends a
                         Reply message in response to a Confirm message
                         confirming or denying that the addresses
                         assigned to the client are appropriate to the
                         link to which the client is connected.  A
                         server sends a Reply message to acknowledge
                         receipt of a Release or Decline message.

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

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

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

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

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

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


5.4. Status Codes

   DHCPv6 uses status codes to communicate the success or failure of
   operations requested in messages from clients and servers, and to
   provide additional information about the specific cause of the




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   failure of a message.  The specific status codes are defined in
   section 24.4.


5.5. Transmission and Retransmission Parameters

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

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



6. Message Formats

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

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

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

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




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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |               transaction-id                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                            options                            .
     .                           (variable)                          .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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

      transaction-id       The transaction ID for this message exchange.

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


7. Relay agent messages

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

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |   hop-count   |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     |                         link-address                          |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     |                         peer-address                          |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     .                                                               .
     .            options (variable number and length)   ....        .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The following sections describe the use of the Relay Agent message
   header.


7.1. Relay-forward message

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

      msg-type       RELAY-FORW

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

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

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

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


7.2. Relay-reply message

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

      msg-type       RELAY-REPL

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

      link-address   Copied from the Relay-forward message

      peer-address   The client or relay agent address to which the
                     message contained in the Relay Message option in
                     this Relay-reply message is to be forwarded

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


8. Representation and use of domain names

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



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

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


9.1. DUID contents

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

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


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


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

   This type of DUID consists of a two octet type field containing the
   value 1, a two octet hardware type code, four octets containing
   a time value, followed by link-layer address of any one network
   interface that is connected to the DHCP device at the time that the



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   DUID is generated.  The time value is the time that the DUID is
   generated represented in seconds since midnight (UTC), January 1,
   2000, modulo 2^32.  The hardware type MUST be a valid hardware type
   assigned by the IANA as described in the section on ARP in RFC 826.
   Both the time and the hardware type are stored in network byte order.
   The link-layer address is stored in canonical form, as described in
   RFC2464 [3].

   The following diagram illustrates the format of a DUID-LLT:

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


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

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

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

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

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



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


9.3. DUID assigned by vendor based on Enterprise number [DUID-EN]

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

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

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


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

   An example DUID of this type might look like this:

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


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








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

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

   The following diagram illustrates the format of a DUID-LL:

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


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

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


10. Identity association

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

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




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   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 along with the times T1 and T2 for the IA. See section 22.4
   for the representation of an IA in a DHCP message.

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


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




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    -  Other information in options supplied by the client

    -  Other information in options supplied by the relay agent

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

   A server MUST NOT assign an address that is otherwise reserved for
   some other purpose.  For example, a server MUST NOT assign reserved
   anycast addresses, as defined in RFC2526, from any subnet.


12. Management of temporary addresses

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

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

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

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

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


13. Transmission of messages by a client

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

   A client uses multicast to reach all servers or an individual server.
   An individual server is indicated by specifying that server's DUID in
   a Server Identifier option (see section 22.3) in the client's message
   (all servers will receive this message but only the indicated server



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   will respond).  All servers are indicated by not supplying this
   option.

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


14. Reliability of Client Initiated Message Exchanges

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

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

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

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

      RT     Retransmission timeout

      IRT    Initial retransmission time

      MRC    Maximum retransmission count

      MRT    Maximum retransmission time

      MRD    Maximum retransmission duration

      RAND   Randomization factor

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

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



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   RT for the first message transmission is based on IRT:

      RT = IRT + RAND*IRT


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

      RT = 2*RTprev + RAND*RTprev


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

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


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

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

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

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


15. Message validation

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

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


15.1. Use of Transaction IDs

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



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


15.2. Solicit message

   Clients MUST discard any received Solicit messages.

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


15.3. Advertise message

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

    -  the message does not include a Server Identifier option

    -  the message does not include a Client Identifier option

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

    -  the "transaction-id" 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 DUID

    -  the message does not include a Client Identifier option


15.5. Confirm message

   Clients MUST discard any received Confirm messages.




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


15.6. Renew message

   Clients MUST discard any received Renew messages.

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

    -  the message MUST include a Server Identifier option

    -  the contents of the Server Identifier option MUST match the
       server's identifier

    -  the message MUST include a Client Identifier option


15.7. Rebind message

   Clients MUST discard any received Rebind messages.

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


15.8. Decline messages

   Clients MUST discard any received Decline messages.

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

    -  the message MUST include a Server Identifier option

    -  the contents of the Server Identifier option MUST match the
       server's identifier

    -  the message MUST include a Client Identifier option


15.9. Release message

   Clients MUST discard any received Release messages.

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

    -  the message MUST include a Server Identifier option




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    -  the contents of the Server Identifier option MUST match the
       server's identifier

    -  the message MUST include a Client Identifier option


15.10. Reply message

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

    -  the message MUST include a Server Identifier option

    -  the "transaction-id" field in the message MUST match the value
       used in the original message

    -  the message MUST include a Client Identifier option and the
       original message from the client contained a Client Identifier
       option

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

   Servers and relay agents MUST discard any received Reply messages.


15.11. Reconfigure message

   Servers and relay agents MUST discard any received Reconfigure
   messages.

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

    -  the message MUST include a Server Identifier option

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

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

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

        *  the server uses DHCP authentication:

            +  the message MUST contain an authentication option




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


15.12. Information-request message

   Clients MUST discard any received Information-request messages.

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


15.13. Relay-forward message

   Clients MUST discard any received Relay-forward messages.


15.14. Relay-reply message

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


16. Client Source Address and Interface Selection

   When a client sends a DHCP message to the
   All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the
   message through the interface for which configuration information is
   being requested.  However, the client MAY send the message through
   another interface attached to the same link if and only if the
   client is certain the two interface are attached to the same link.
   In addition, the client SHOULD use an IP address assigned to the
   interface for which it is requesting configuration information as
   the source address in the header of the IP datagram.  If the client
   uses a different address as the source address, the address MUST
   be assigned to an interface that will be attached to the same link
   as the interface for which the client is requesting configuration
   information when the response is received from the server.

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


17. DHCP Server Solicitation

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





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

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


17.1. Client Behavior

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


17.1.1. Creation of Solicit messages

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

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

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

   The client MUST include an Option Request option (see section 22.7)
   to indicate 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 22.14) in the
   Solicit message.







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

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

      IRT   SOL_TIMEOUT

      MRT   SOL_MAX_RT

      MRC   0

      MRD   0

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

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

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




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

   A DHCP client SHOULD choose MRC and MRD to be 0.  If the DHCP client
   is configured with either MRC or MRD set to a value other than
   0, it MUST stop trying to configure the interface if the message
   exchange fails.  After the DHCP client stops trying to configure
   the interface, it SHOULD 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 NoAddrsAvail, with the exception
   that the client MAY display the associated status message to the
   user.

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

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

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

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

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

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







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17.1.4. Receipt of Reply message

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


17.2. Server Behavior

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


17.2.1. Receipt of Solicit messages

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

   If the client has included a Rapid Commit option in the Solicit
   message and the server has been configured to respond with committed
   address assignments and other resources, the server responds to
   the Solicit with a Reply message as described in section 17.2.3.
   Otherwise, the server ignores the Rapid Commit option and processes
   the remainder of the message as if no Rapid Commit option were
   present.


17.2.2. Creation and transmission of Advertise messages

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

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





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

   If the Solicit message from the client included one or more IA
   options, the server MUST include IA or IAID 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 any 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
   code NoAddrsAvail, a status message for the user, a Server Identifier
   option with the server's DUID and a Client Identifier option with the
   client's DUID.

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

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


17.2.3. Creation and Transmission of Reply messages

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

   DISCUSSION:

      When using the Solicit-Reply message exchange, the server
      commits the assignment of any addresses before sending the
      Reply message.  The client can assume it has been assigned




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      the addresses in the Reply message and does not need to send
      a Request message for those addresses.

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

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

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

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


18. DHCP Client-Initiated Configuration Exchange

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


18.1. Client Behavior

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

   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 22.12) from the server, the client
   SHOULD unicast any Request, Renew, Release and Decline messages to
   the server.

   DISCUSSION:

      Use of unicast may avoid delays due to forwarding of
      messages by relay agents as well as avoid overhead and
      duplicate responses by servers due to delivery of client
      messages to multiple servers.  Requiring the client to



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      relay all DHCP messages through a relay agent 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.


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.  The server then returns
   addresses and other information about the IAs to the client in IA
   options in a Reply message.

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

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

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

   The client MUST include an Option Request option (see section 22.7)
   to indicate 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.

   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 takes an action based on
   the client's local policy.  Examples of actions the client might take
   include:

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

    -  Initiate the server discovery process described in section 17

    -  Terminate the configuration process and report failure





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18.1.2. Creation and transmission of Confirm messages

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

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

     o The client reboots

     o The client is physically disconnected from a wired connection

     o The client returns from sleep mode

     o The client using a wireless technology changes access points

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

   In any situation when a client may have moved to a new link, the
   client MUST initiate a Confirm/Reply message exchange.  The client
   includes any IAs, along with the addresses associated with those IAs,
   in its Confirm message.  Any responding servers will indicate whether
   those addresses are appropriate to the link to which the client is
   attached 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 includes IA options for all of the IAs
   currently in use by the client.  The IA options include all of the
   addresses the client currently has associated with those IAs.  The
   client SHOULD set the T1 and T2 fields in the IA options and the
   preferred-lifetime and valid-lifetime fields in the IA Address
   options to 0, and the server will ignore these fields.

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

      IRT   CNF_TIMEOUT




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      MRT   CNF_MAX_RT

      MRC   0

      MRD   CNF_MAX_RD

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


18.1.3. Creation and transmission of Renew messages

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

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

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

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

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

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

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

   The client MUST include an Option Request option (see section 22.7)
   to indicate the options the client is interested in receiving.  The




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   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   REN_TIMEOUT

      MRT   REN_MAX_RT

      MRC   0

      MRD   Remaining time until T2

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


18.1.4. Creation and transmission of Rebind messages

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

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

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

   The client MUST include an Option Request option (see section 22.7)
   to indicate 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.

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

      IRT   REB_TIMEOUT

      MRT   REB_MAX_RT

      MRC   0




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      MRD   Remaining time until valid lifetimes of all addresses have
            expired

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

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

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


18.1.5. Creation and Transmission of Information-request messages

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

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

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

   The client MUST include an Option Request option (see section 22.7)
   to indicate 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.

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

      IRT   INF_TIMEOUT

      MRT   INF_MAX_RT

      MRC   0

      MRD   0








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18.1.6. 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 MUST NOT be in added to the
   IAs.

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

   Because Release messages may be lost, the client should retransmit
   the Release if no Reply is received.  However, there are scenarios
   where the client may not wish to wait for the normal retransmission
   timeout before giving up (e.g., on power down).  Implementations
   SHOULD retransmit one or more times, but MAY choose to terminate the
   retransmission procedure early.

   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 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 valid
   lifetime of the address expires.





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18.1.7. Creation and transmission of Decline messages

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

   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.

   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.

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

      IRT   DEC_TIMEOUT

      MRT   DEC_MAX_RT

      MRC   DEC_MAX_RC

      MRD   0

   If addresses are released but the Reply from a DHCP server is lost,
   the client will retransmit the Decline 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 Decline message exchange as if it indicates an error.


18.1.8. Receipt of Reply messages

   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 [20] 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




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   to be in use on the link, the client sends a Decline message to the
   server as described in section 18.1.7.

   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, and discards any addresses with valid lifetime set to 0
   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 Request, Renew or Rebind
   message, the client must update the information it has recorded about
   IAs from the IA options contained in the Reply message:

    -  Add any new addresses in the IA option to the IA as recorded by
       the client

    -  Update lifetimes for any addresses in the IA option that the
       client already has recorded in the IA

    -  Discard any addresses from the IA as recorded by the client that
       have a lifetime of 0 in the IA Address option

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

   When the client receives a Reply message with a Status Code option
   with value UseMulticast, the client records the receipt of the
   message and sends subsequent messages to the server through the
   interface on which the message was received using multicast.  The
   client resends the original message using multicast.

   When the client receives a NotOnLink status from the server in
   response to a Confirm message, the client performs DHCP server
   solicitation as described in section 17 and client-initiated
   configuration as described in section 18.  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 a status code with value NoBinding in an IA
   from the server in response to a Renew message or a Rebind message,
   the client sends a Request to reestablish an IA with the server.

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

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



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18.2. Server Behavior

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

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


18.2.1. Receipt of Request messages

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

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

   The server constructs a Reply message by setting the "msg-type" field
   to REPLY, copying the transaction ID from the Request message into
   the transaction-id 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 appropriate to the link
   to which the client is connected, the server MUST return the IA to
   the client with a Status Code option with value NotOnLink.

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

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

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




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


18.2.2. Receipt of Confirm messages

   When the server receives a Confirm message, the server determines
   if the addresses in the Confirm message are appropriate to the
   link to which the client is attached.  The server ignores the T1
   and T2 fields in the IA options and the preferred-lifetime and
   valid-lifetime fields in the IA Address options.

   If all of the addresses in the Confirm message pass this test, the
   server returns a status of Success.  If any of the addresses do not
   pass this test, the server returns a status of NotOnLink.

   If the server does not find any addresses that are not appropriate to
   the link to which the client is connected, but cannot determine if
   some of the addresses are appropriate to the link or not appropriate
   to the link, the server MUST NOT send a reply to the client.  For
   example, if the server does not have information about prefixes on
   the link to which the client is connected, the server does not reply.

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

   The server MUST include a Server Identifier option containing the
   server's DUID and the Client Identifier option from the Confirm
   message in the Reply message.  The server includes a Status Code
   option indicating the status of the Confirm message.


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, a Server Identifier
   option containing the server's DUID, the Client Identifier option
   from the client message and no other options.

   When the server receives a Renew message that contains an IA option
   from a client, it locates the client's binding and verifies that the




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   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 a Status Code option set
   to NoBinding in the Reply message.

   If the server finds that any of the addresses are not appropriate
   to the link to which the client is attached, the server returns the
   address to the client with lifetimes of 0.

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

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

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


18.2.4. Receipt of Rebind messages

   When the server receives a Rebind message that contains an 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 a Status Code option set
   to NoBinding in the Reply message.

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

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

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




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


18.2.5. Receipt of Information-request messages

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

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

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

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

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

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





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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, a Server
   Identifier option containing the server's DUID, the Client Identifier
   option from the client message and no other options.

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

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

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


18.2.7. Receipt of Decline messages

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

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

   After all the address have been processed, the server generates a
   Reply message and includes a Status Code option with value Success,



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


18.2.8. Transmission of Reply messages

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

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


19. DHCP Server-Initiated Configuration Exchange

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


19.1. Server Behavior

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


19.1.1. Creation and transmission of Reconfigure messages

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

   The server MAY include an Option Request option to inform the client
   of what information has been changed or new information that has been



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   added.  In particular, the server specifies the IA option in the
   Option Request option if the server wants the client to obtain new
   address information.

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

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

    -  The server includes an authentication option in the Reconfigure
       message

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

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

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

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

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


19.1.2. Time out and retransmission of Reconfigure messages

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





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


19.2. Receipt of Renew messages

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

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


19.3. Receipt of Information-request messages

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

   The server MAY include options containing new values for other
   configuration parameters in the Reply message, even if those
   parameters were not requested in the Information-request message from
   the client.


19.4. Client Behavior

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


19.4.1. Receipt of Reconfigure messages

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

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







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   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 until the
      exchange is complete.  Subsequent Reconfigure messages cause
      the client to initiate a new exchange.

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

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


19.4.2. Creation and transmission of Renew messages

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


19.4.3. Creation and transmission of Information-request messages

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







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

   The client uses the same variables and retransmission algorithm as
   it does with Renew or Information-request messages generated as part
   of a client-initiated configuration exchange.  See sections 18.1.3
   and 18.1.5 for details.  If the client does not receive a response
   from the server by the end of the retransmission process, the client
   ignores and discards the Reconfigure message.


19.4.5. Receipt of Reply messages

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


20. Relay Agent Behavior

   The relay agent MAY be configured to use a list of destination
   addresses, which MAY include unicast addresses, the All_DHCP_Servers
   multicast address, or other 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. Forwarding a client message or a Relay-forward message

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


20.1.1. Forwarding a message from a client

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




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   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 22.18) in the Relay-forward message.  The server
   will include the Interface-id option in its Relay-reply message.
   The relay agent fills in the link-address field as described in the
   previous paragraph regardless of whether the relay agent includes an
   Interface-id option in the Relay-forward message.


20.1.2. Forwarding a message from a relay agent

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

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

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


20.2. Forwarding a Relay-reply message

   The relay agent processes any options included in the Relay-reply
   message in addition to the Relay Message option and then discards
   those options.

   The relay agent extracts the message from the Relay Message option
   and forwards it to the address contained in the peer-address field of
   the Relay-reply message.

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

   If the Relay-reply message does not include an Interface-id option
   and the link-address field is zero, the address in the peer-address
   field must be a global address or a site-local address (and the
   device on which the relay agent is running belongs to only one
   site).  If the address in the peer-address field does not meet this
   condition, the relay agent discards the Relay-reply message and
   SHOULD communicate an error condition.








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20.3. Construction of Relay-reply messages

   A server uses a Relay-reply message to return a response to a client
   if the original message from the client was forwarded to the server
   in a Relay-forward message.  The response to the client MUST be
   forwarded through the same relay agents as the original client
   message.  The server causes this to happen by creating a Relay-reply
   message that includes a Relay Message option containing the message
   for the next relay agent in the return path to the client.  The
   contained Relay-reply message contains another Relay Message option
   to be sent to the next relay agent, and so on.  The server must
   record the contents of the peer-address fields in the received
   message so it can construct the appropriate Relay-reply message
   carrying the response from the server.

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

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



21. Authentication of DHCP messages

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

   The DHCP authentication mechanism is based on the design of
   authentication for DHCPv4 [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



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   configuration, however, the potential attacks by insiders and
   outsiders are the same.

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

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

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

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

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


21.2. Security of messages sent between servers and relay agents

   Relay agents and servers that choose to exchange messages securely
   use the IPsec mechanisms for IPv6 [10].  Relay agents and servers
   MUST support manual configuration and installation of static keys.
   If a client message is forwarded through multiple relay agents, each
   of the relay agents must have established independent, pairwise trust
   relationships.  That is, if messages from client C will be forwarded
   by relay agent A to relay agent B and then to the server, relay
   agents A and B must be configured to use IPSec for the messages they
   exchange, and relay agent B and the server must be configured to use
   IPSec for the messages they exchange.


21.3. Summary of DHCP authentication

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





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

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


21.4. Replay detection

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

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


21.5. Delayed authentication protocol

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

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


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

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

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




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

   The format of the Authentication information is:

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


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

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


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

   DISCUSSION:

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

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




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21.5.2. Message validation

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

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


21.5.3. Key utilization

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

   Each DHCP server stores or is able to obtain in a secure manner the
   keys for all authorized clients.  To authenticate the identity of
   individual clients, each client must be configured with a unique key
   and a key ID for that key.


21.5.4. Client considerations for delayed authentication protocol

21.5.4.1. Sending Solicit messages

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


21.5.4.2. Receiving Advertise messages

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

   Client behavior if no Advertise messages include authentication
   information or pass the validation test is controlled by local policy




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   on the client.  According to client policy, the client MAY choose to
   respond to a Advertise message that has not been authenticated.

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

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


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

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


21.5.4.4. Sending Information-request messages

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


21.5.4.5. Receiving Reply messages

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




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   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.


21.5.4.6. Receiving Reconfigure messages

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


21.5.5. Server considerations for delayed authentication protocol

21.5.5.1. Receiving Solicit messages and Sending Advertise messages

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


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

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

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


21.5.5.3. Sending Reconfigure messages

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

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


22. DHCP options

   Options are used to carry additional information and parameters
   in DHCP messages.  Every option shares a common base format, as



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   described in section 22.1.  All values in options are represented in
   network byte order.

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

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


22.1. Format of DHCP options

   The format of DHCP options is:

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



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

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

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

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


22.2. Client Identifier option

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








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   The format of the Client Identifier option is:

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



      option-code   OPTION_CLIENTID (1)

      option-len    Length of DUID in octets

      DUID          The DUID for the client


22.3. Server Identifier option

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

   The format of the Server Identifier option is:

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



      option-code   OPTION_SERVERID (2)

      option-len    Length of DUID in octets

      DUID          The DUID for the server

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






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22.4. Identity Association option

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

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

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



      option-code          OPTION_IA (3)

      option-len           12 + length of IA-options field

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

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

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

      IA-options           Options associated with this IA.





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

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

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

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

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

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

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

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


22.5. Identity Association for Temporary Addresses option

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








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   The format of the IA_TA option is:

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



      option-code          OPTION_IA_TA (4)

      option-len           4 + length of IA-options field

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

      IA-options           Options associated with this IA.

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

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

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

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

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

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




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

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

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


22.6. IA Address option

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

   The format of the IA Address option is:

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




      option-code   OPTION_IAADDR (5)

      option-len    24 + length of IAaddr-options field




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      IPv6 address  An IPv6 address

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

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

      IAaddr-options Options associated with this address

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

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

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


22.7. Option Request option

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

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



      option-code   OPTION_ORO (6)

      option-len    2 * number of requested options

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





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


22.8. Preference option

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

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



      option-code   OPTION_PREFERENCE (7)

      option-len    1

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

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


22.9. Elapsed Time option

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


      option-code   OPTION_ELAPSED_TIME (8)

      option-len    2.






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

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


22.10. Relay Message option

   The Relay Message option carries a DHCP message in a Relay-forward or
   Relay-reply message.

   The format of the Relay Message option is:

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



      option-code   OPTION_RELAY_MSG (9)

      option-len    Length of DHCP-relay-message

      DHCP-relay-message In a Relay-forward message, the received
                    message, forwarded verbatim to the next relay agent
                    or server; in a Relay-reply message, the message to
                    be copied and forwarded to the relay agent or client
                    whose address is in the peer-address field of the
                    Relay-reply message


22.11. Authentication option

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



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   The format of the Authentication option is:

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


      option-code                  OPTION_AUTH (11)

      option-len                   15 + length of Authentication
                                   Information field

      protocol                     The authentication protocol used in
                                   this authentication option

      algorithm                    The algorithm used in the
                                   authentication protocol

      RDM                          The replay detection method used in
                                   this authentication option

      Replay detection             The replay detection information for
                                   the RDM

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


22.12. Server unicast option

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










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   The format of the Server Unicast option is:

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



      option-code     OPTION_UNICAST (12)

      option-len      16

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

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

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


22.13. Status Code Option

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

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


      option-code          OPTION_STATUS_CODE (13)




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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 24.4.

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

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


22.14. Rapid Commit option

   The Rapid Commit option is used to signal the use of the two message
   exchange for address assignment.  The format of the Rapid Commit
   option is:

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



      option-code     OPTION_RAPID_COMMIT (14)

      option-len      0

   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.

   DISCUSSION:

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




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


22.15. User Class option

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

   The format of the User Class option is:

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


      option-code          OPTION_USER_CLASS (15)

      option-len           Length of user class data field

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

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

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

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


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

   A server interprets the classes identified in this option according
   to its configuration to select the appropriate configuration



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


22.16. Vendor Class Option

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

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


      option-code          OPTION_VENDOR_CLASS (16)

      option-len           4 + length of vendor class data field

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

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

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

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

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






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


22.17. Vendor-specific Information option

   This option is used by clients and servers to exchange
   vendor-specific information.  The format of this option
   is:

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


      option-code          OPTION_VENDOR_OPTS (17)

      option-len           4 + length of option-data field

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

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

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

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











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   Each of the encapsulated options is formatted as follows.

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


      opt-code             The code for the encapsulated option

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

      option-data          The data area for the encapsulated option

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


22.18. Interface-Id Option

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

   The format of the Interface ID option is:

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



      option-code          OPTION_INTERFACE_ID (18)

      option-len           Length of interface-id field



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

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

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


22.19. Reconfigure Message option

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

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



      option-code          OPTION_RECONF_MSG (19)

      option-len           1

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

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


22.20. Reconfigure Nonce option

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



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

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



      option-code          OPTION_RECONF_NONCE (20)

      option-len           8

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

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


23. Security Considerations

   Section 21 describes the DHCP threat model.  The primary threat
   to a DHCP client is the incorrect configuration of the client
   through a DHCP exchange with a malicious DHCP server.  The incorrect
   configuration may present a denial of service attack by causing
   communication with a service to fail, or a masquerade attack by
   causing the client to communicate with a malicious server instead of
   a valid server for some service such as DNS or NTP.

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

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



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   still be able to mount a Reconfigure attack on a client.  The use of
   transaction IDs that are cryptographically sound and cannot easily be
   predicted will also reduce the probability that such an attack will
   be successful.

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

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

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

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


24. IANA Considerations

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

    -  Message types

    -  Status codes

    -  DUID

    -  Option codes

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




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

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

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

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

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


24.1. Multicast addresses

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

      All_DHCP_Relay_Agents_and_Servers address:   FF02::1:2

      All_DHCP_Servers address:                    FF05::1:3


24.2. DHCP message types

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

      SOLICIT               1

      ADVERTISE             2

      REQUEST               3

      CONFIRM               4

      RENEW                 5

      REBIND                6

      REPLY                 7



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      RELEASE               8

      DECLINE               9

      RECONFIGURE           10

      INFORMATION-REQUEST   11

      RELAY-FORW            12

      RELAY-REPL            13


24.3. DHCP options

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

      OPTION_CLIENTID       1

      OPTION_SERVERID       2

      OPTION_IA             3

      OPTION_IA_TMP         4

      OPTION_IAADDR         5

      OPTION_ORO            6

      OPTION_PREFERENCE     7

      OPTION_ELAPSED_TIME   8

      OPTION_CLIENT_MSG     9

      OPTION_SERVER_MSG     10

      OPTION_AUTH           11

      OPTION_UNICAST        12

      OPTION_STATUS_CODE    13

      OPTION_RAPID_COMMIT   14

      OPTION_USER_CLASS     15

      OPTION_VENDOR_CLASS   16

      OPTION_VENDOR_OPTS    17




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      OPTION_INTERFACE_ID   18

      OPTION_RECONF_MSG     19

      OPTION_RECONF_NONCE   20


24.4. Status codes

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

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



24.5. DUID

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

      DUID-LLT                       1

      DUID-EN                        2

      DUID-LL                        3


25. Acknowledgments

   Thanks to the DHC Working Group and the members of the IETF for
   their time and input into the specification.  In particular, thanks



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   also for the consistent input, ideas, and review by (in alphabetical
   order) Bill Arbaugh, Thirumalesh Bhat, Steve Bellovin, A. K.
   Vijayabhaskar, Brian Carpenter, Matt Crawford, Francis Dupont, Tony
   Lindstrom, Josh Littlefield, Gerald Maguire, Jack McCann, Thomas
   Narten, Erik Nordmark, Yakov Rekhter, Mark Stapp, Matt Thomas, Sue
   Thomson, Tatuya Jinmei and Phil Wells.

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

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


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

    -  Eliminated definition of VUID-DN.

    -  Changed the second sentence in section 17.1.2 to:

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

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

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

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

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

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

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



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       that server checks addresses and returns current configuration
       information for the client.

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

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

    -  Clarified use of Status Code option in Release message.

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

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

    -  Edited section 16 to clarify interface selection.

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

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

    -  Clarified capabilities and constraints on relay agent forwarding.

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

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

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

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

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

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

    -  Edited text about Option Request option to clarify that the
       option is a hint from the client to the server about options the




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       client has a preference to receive; includes recommendation that
       client send Option Request option if it has options it requires.

    -  Added nonce value for security of Reconfigure messages.


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

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

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

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

    -  Reconfigure message now requires Client Identifier option with
       receiving client's DUID.

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

    -  Fixed definition of "binding" to allow a binding with no IAs to
       be indexed by just <DUID>.

    -  Clarified text in section 17.2.2 to explain that AddrUnavail is
       returned only if the Solicit message from the client included one
       or more IA options.

    -  Edited text about Option Request option to clarify that the
       client is required to include the option and to identify all
       options the client has a preference to receive; also clarified
       that a server may include additional options if the server is
       configured to do so.

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

    -  Changed use of Confirm so that server simply checks whether
       addresses are appropriate to link to which client is attached.

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

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

    -  Modified several retransmission parameters (in section 5.5) for
       consistency and anticipated operation.



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    -  Defined "appropriate to the link" and changed "valid on the link"
       to "appropriate to the link" throughout for clarity

    -  Added text requiring transaction IDs to be not predictable and
       described potential attack in security considerations


References

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

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

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

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

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

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

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

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

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

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

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

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





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

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

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

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

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

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

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

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


Chair's Address

   The working group can be contacted via the current chair:


         Ralph Droms
         Cisco Systems
         300 Apollo Drive
         Chelmsford, MA 01824

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

















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Authors' Addresses

   Questions about this document can be directed to:


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

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

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

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

        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










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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 19.4.3).


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




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

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



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





C. Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However,
   this document itself may not be modified in any way, such as by
   removing the copyright notice or references to the Internet Society
   or other Internet organizations, except as needed for the purpose
   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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