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Internet Engineering Task Force                                 J. Bound
INTERNET DRAFT                                                    Compaq
DHC Working Group                                              M. Carney
Obsoletes:  draft-ietf-dhc-dhcpv6-19.txt           Sun Microsystems, Inc
                                                              C. Perkins
                                                   Nokia Research Center
                                                           R. Droms(ed.)
                                                           Cisco Systems
                                                             15 Oct 2001


         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
                      draft-ietf-dhc-dhcpv6-20.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" [20], 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                                                       1

 2. Requirements                                                       1

 3. Background                                                         1

 4. Design Goals                                                       2

 5. Non-Goals                                                          3

 6. Terminology                                                        3
     6.1. IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .    3
     6.2. DHCP Terminology  . . . . . . . . . . . . . . . . . . . .    5

 7. DHCP Constants                                                     6
     7.1. Multicast Addresses . . . . . . . . . . . . . . . . . . .    6
     7.2. UDP ports . . . . . . . . . . . . . . . . . . . . . . . .    6
     7.3. DHCP message types  . . . . . . . . . . . . . . . . . . .    7
     7.4. Status Codes  . . . . . . . . . . . . . . . . . . . . . .    8
           7.4.1. Generic Status Codes  . . . . . . . . . . . . . .    9
           7.4.2. Server-specific Status Codes  . . . . . . . . . .    9
     7.5. Configuration Variables . . . . . . . . . . . . . . . . .   10

 8. Message Formats                                                   10
     8.1. DHCP Solicit Message Format . . . . . . . . . . . . . . .   11
     8.2. DHCP Advertise Message Format . . . . . . . . . . . . . .   11
     8.3. DHCP Request Message Format . . . . . . . . . . . . . . .   12
     8.4. DHCP Confirm Message Format . . . . . . . . . . . . . . .   12
     8.5. DHCP Renew Message Format . . . . . . . . . . . . . . . .   12
     8.6. DHCP Rebind Message Format  . . . . . . . . . . . . . . .   12
     8.7. DHCP Reply Message Format . . . . . . . . . . . . . . . .   13
     8.8. DHCP Release Message Format . . . . . . . . . . . . . . .   13
     8.9. DHCP Decline Message Format . . . . . . . . . . . . . . .   13
    8.10. DHCP Reconfigure-init Message Format  . . . . . . . . . .   13

 9. Relay messages                                                    14
     9.1. Relay-forward message . . . . . . . . . . . . . . . . . .   14
     9.2. Relay-reply message . . . . . . . . . . . . . . . . . . .   15

10. DHCP unique identifier (DUID)                                     15
    10.1. DUID contents . . . . . . . . . . . . . . . . . . . . . .   15
    10.2. DUID based on link-layer address plus time  . . . . . . .   16
    10.3. Vendor-assigned unique ID.  . . . . . . . . . . . . . . .   17
    10.4. Link-layer address  . . . . . . . . . . . . . . . . . . .   17



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11. Identity association                                              18

12. Selecting addresses for assignment to an IA                       18

13. Reliability of Client Initiated Message Exchanges                 19

14. Message validation                                                20
    14.1. Use of Transaction-ID field . . . . . . . . . . . . . . .   21
    14.2. Solicit message . . . . . . . . . . . . . . . . . . . . .   21
    14.3. Advertise message . . . . . . . . . . . . . . . . . . . .   21
    14.4. Request message . . . . . . . . . . . . . . . . . . . . .   21
    14.5. Confirm message . . . . . . . . . . . . . . . . . . . . .   21
    14.6. Renew message . . . . . . . . . . . . . . . . . . . . . .   21
    14.7. Rebind message  . . . . . . . . . . . . . . . . . . . . .   22
    14.8. Decline messages  . . . . . . . . . . . . . . . . . . . .   22
    14.9. Release message . . . . . . . . . . . . . . . . . . . . .   22
   14.10. Reply message . . . . . . . . . . . . . . . . . . . . . .   22
   14.11. Reconfigure-init message  . . . . . . . . . . . . . . . .   22
   14.12. Relay-forward message . . . . . . . . . . . . . . . . . .   23
   14.13. Relay-reply message . . . . . . . . . . . . . . . . . . .   23

15. DHCP Server Solicitation                                          23
    15.1. Client Behavior . . . . . . . . . . . . . . . . . . . . .   23
          15.1.1. Creation of Solicit messages  . . . . . . . . . .   23
          15.1.2. Transmission of Solicit Messages  . . . . . . . .   23
          15.1.3. Receipt of Advertise messages . . . . . . . . . .   25
    15.2. Server Behavior . . . . . . . . . . . . . . . . . . . . .   25
          15.2.1. Receipt of Solicit messages . . . . . . . . . . .   25
          15.2.2. Creation and transmission of Advertise messages .   26

16. DHCP Client-Initiated Configuration Exchange                      26
    16.1. Client Behavior . . . . . . . . . . . . . . . . . . . . .   27
          16.1.1. Creation and transmission of Request messages . .   27
          16.1.2. Creation and transmission of Confirm messages . .   28
          16.1.3. Creation and transmission of Renew messages . . .   29
          16.1.4. Creation and transmission of Rebind messages  . .   31
          16.1.5. Receipt of Reply message in response to a Request,
                          Confirm, Renew or Rebind message . . . . .  32
          16.1.6. Creation and transmission of Release messages . .   33
          16.1.7. Receipt of Reply message in response to a Release
                          message  . . . . . . . . . . . . . . . . .  35
          16.1.8. Creation and transmission of Decline messages . .   35
          16.1.9. Receipt of Reply message in response to a Decline
                          message  . . . . . . . . . . . . . . . . .  36
    16.2. Server Behavior . . . . . . . . . . . . . . . . . . . . .   36
          16.2.1. Receipt of Request messages . . . . . . . . . . .   36
          16.2.2. Receipt of Confirm messages . . . . . . . . . . .   37
          16.2.3. Receipt of Renew messages . . . . . . . . . . . .   38
          16.2.4. Receipt of Rebind messages  . . . . . . . . . . .   39
          16.2.5. Receipt of Release messages . . . . . . . . . . .   40
          16.2.6. Receipt of Decline messages . . . . . . . . . . .   40
          16.2.7. Sending of Reply messages . . . . . . . . . . . .   41




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17. DHCP Server-Initiated Configuration Exchange                      41
    17.1. Server Behavior . . . . . . . . . . . . . . . . . . . . .   41
          17.1.1. Creation and transmission of Reconfigure-init
                          messages . . . . . . . . . . . . . . . . .  41
          17.1.2. Time out and retransmission of Reconfigure-init
                          messages . . . . . . . . . . . . . . . . .  42
          17.1.3. Receipt of Request messages . . . . . . . . . . .   42
    17.2. Client Behavior . . . . . . . . . . . . . . . . . . . . .   43
          17.2.1. Receipt of Reconfigure-init messages  . . . . . .   43
          17.2.2. Creation and sending of Request messages  . . . .   44
          17.2.3. Time out and retransmission of Request messages .   44
          17.2.4. Receipt of Reply messages . . . . . . . . . . . .   44

18. Relay Behavior                                                    44
    18.1. Relaying of client messages . . . . . . . . . . . . . . .   45
    18.2. Relaying of server messages . . . . . . . . . . . . . . .   45

19. Authentication of DHCP messages                                   45
    19.1. DHCP threat model . . . . . . . . . . . . . . . . . . . .   46
    19.2. Security of messages sent between servers and relay agents  46
    19.3. Summary of DHCP authentication  . . . . . . . . . . . . .   46
    19.4. Replay detection  . . . . . . . . . . . . . . . . . . . .   47
    19.5. Configuration token protocol  . . . . . . . . . . . . . .   47
    19.6. Delayed authentication protocol . . . . . . . . . . . . .   48
          19.6.1. Management issues in the delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  48
          19.6.2. Use of the Authentication option in the delayed
                          authentication protocol  . . . . . . . . .  48
          19.6.3. Message validation  . . . . . . . . . . . . . . .   49
          19.6.4. Key utilization . . . . . . . . . . . . . . . . .   49
          19.6.5. Client considerations for delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  50
          19.6.6. Server considerations for delayed authentication
                          protocol . . . . . . . . . . . . . . . . .  51

20. DHCP options                                                      52
    20.1. Format of DHCP options  . . . . . . . . . . . . . . . . .   52
    20.2. DHCP unique identifier option . . . . . . . . . . . . . .   53
    20.3. Identity association option . . . . . . . . . . . . . . .   53
    20.4. Option request option . . . . . . . . . . . . . . . . . .   56
    20.5. Preference option . . . . . . . . . . . . . . . . . . . .   56
    20.6. Elapsed Time  . . . . . . . . . . . . . . . . . . . . . .   57
    20.7. Client message option . . . . . . . . . . . . . . . . . .   57
    20.8. Server message option . . . . . . . . . . . . . . . . . .   58
    20.9. DSTM Global IPv4 Address Option . . . . . . . . . . . . .   58
   20.10. Authentication option . . . . . . . . . . . . . . . . . .   59
   20.11. Server unicast option . . . . . . . . . . . . . . . . . .   60
   20.12. Domain Search Option  . . . . . . . . . . . . . . . . . .   60
   20.13. Domain Name Server Option . . . . . . . . . . . . . . . .   61
   20.14. Status Code Option  . . . . . . . . . . . . . . . . . . .   61
   20.15. Circuit-ID Option . . . . . . . . . . . . . . . . . . . .   62
   20.16. User Class Option . . . . . . . . . . . . . . . . . . . .   63
   20.17. Vendor Class Option . . . . . . . . . . . . . . . . . . .   63



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21. Security Considerations                                           65

22. Year 2000 considerations                                          65

23. IANA Considerations                                               65
    23.1. Multicast addresses . . . . . . . . . . . . . . . . . . .   65
    23.2. DHCPv6 message types  . . . . . . . . . . . . . . . . . .   65
    23.3. DUID  . . . . . . . . . . . . . . . . . . . . . . . . . .   65
    23.4. DHCPv6 options  . . . . . . . . . . . . . . . . . . . . .   66
    23.5. Status codes  . . . . . . . . . . . . . . . . . . . . . .   66
    23.6. Authentication option . . . . . . . . . . . . . . . . . .   66

24. Acknowledgments                                                   66

 A. Comparison between DHCPv4 and DHCPv6                              67

 B. Full Copyright Statement                                          69

References                                                            69

Chair's Address                                                       71

Authors' Addresses                                                    71

































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1. Introduction

   This document describes DHCP for IPv6 (DHCP), a UDP [18]
   client/server protocol designed to reduce the cost of management
   of IPv6 nodes in environments where network managers require more
   control over the allocation of IPv6 addresses and configuration
   of network stack parameters than that offered by "IPv6 Stateless
   Address Autoconfiguration" [20].  DHCP is a stateful counterpart to
   stateless autoconfiguration.  Note that both stateful and stateless
   autoconfiguration can be used concurrently in the same environment,
   leveraging the strengths of both mechanisms in order to reduce the
   cost of ownership and management of network nodes.

   DHCP reduces the cost of ownership by centralizing the management
   of network resources such as IP addresses, routing information, OS
   installation information, directory service information, and other
   such information on a few DHCP servers, rather than distributing such
   information in local configuration files among each network node.
   DHCP is designed to be easily extended to carry new configuration
   parameters through the addition of new DHCP "options" defined to
   carry this information.

   Those readers familiar with DHCP for IPv4 [7] will find DHCP for IPv6
   provides a superset of features, and benefits from the additional
   features of IPv6 and freedom from the constraints of backward
   compatibility with BOOTP [5].  For more information about the
   differences between DHCP for IPv6 and DHCP for IPv4, see Appendix A.


2. Requirements

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

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


3. Background

   The IPv6 Specification provides the base architecture and design of
   IPv6.  Related work in IPv6 that would best serve an implementor
   to study is the IPv6 Specification [6], the IPv6 Addressing
   Architecture [9], 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



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   provide both robust stateful autoconfiguration and autoregistration
   of DNS Host Names.

   The IPv6 Addressing Architecture specification [9] defines the
   address scope that can be used in an IPv6 implementation, and the
   various configuration architecture guidelines for network designers
   of the IPv6 address space.  Two advantages of IPv6 are that support
   for multicast is required, and nodes can create link-local addresses
   during initialization.  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.

   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 (see Section 4).

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

    -  DHCP is a mechanism rather than a policy.  Network administrators
       set their administrative policies through the configuration
       parameters they place upon the DHCP servers in the DHCP domain
       they're managing.  DHCP is simply used to deliver parameters
       according to that policy to each of the DHCP clients within the
       domain.

    -  DHCP is compatible with IPv6 stateless address
       autoconfiguration [20], statically configured, non-participating
       nodes and with existing network protocol implementations.

    -  DHCP does not require manual configuration of network parameters
       on DHCP clients, except in cases where such configuration is
       needed for security reasons.  A node configuring itself using
       DHCP should require no user intervention.





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    -  DHCP does not require a server on each link.  To allow for scale
       and economy, DHCP must work across DHCP relays.

    -  DHCP clients can operate on a link without IPv6 routers present.

    -  DHCP will provide the ability to renumber network(s) when
       required by network administrators [4].

    -  A DHCP client can make multiple, different requests for
       configuration parameters when necessary from one or more DHCP
       servers at any time.

    -  DHCP will contain the appropriate time out and retransmission
       mechanisms to efficiently operate in environments with high
       latency and low bandwidth characteristics.


5. Non-Goals

   This specification explicitly does not cover the following:

    -  Specification of a DHCP server to server protocol.

    -  How a DHCP server stores its DHCP data.

    -  How to manage a DHCP domain or DHCP server.

    -  How a DHCP relay is configured or what sort of information it may
       log.


6. Terminology

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


6.1. IPv6 Terminology

   IPv6 terminology relevant to this specification from the IPv6
   Protocol [6], IPv6 Addressing Architecture [9], and IPv6 Stateless
   Address Autoconfiguration [20] is included below.

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

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

      multicast address       An identifier for a set of interfaces
                              (typically belonging to different nodes).



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                              A packet sent to a multicast address is
                              delivered to all interfaces identified by
                              that address.

      host                    Any node that is not a router.

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

      interface               A node's attachment to a link.

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

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

      link-local address      An IPv6 address having link-only
                              scope, indicated by having the prefix
                              (FE80::0000/64), that can be used to reach
                              neighboring nodes attached to the same
                              link.  Every interface has a link-local
                              address.

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

      neighbor                A node attached to the same link.

      node                    A device that implements IP.

      packet                  An IP header plus payload.

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

      prefix length           The number of bits in a prefix.

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





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6.2. DHCP Terminology

   Terminology specific to DHCP can be found below.


      agent address             The address of a neighboring DHCP Agent
                                on the same link as the DHCP client.

      binding                   A binding (or, client binding) is a
                                group of server data records containing
                                the information the server has about
                                the addresses in an IA and any other
                                configuration information assigned to
                                the client.  A binding is indexed by the
                                tuple <DUID, IAID>.

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

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

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

      DHCP agent (or agent)     Either a DHCP server on the same link as
                                a client, or a DHCP relay.

      DUID                      A DHCP Unique IDentifier for a client.




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      Identity association (IA) A collection of addresses assigned to
                                a client.  Each IA has an associated
                                IAID. An IA may have 0 or more addresses
                                associated with it.

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

      transaction-ID            An unsigned integer to match responses
                                with replies initiated either by a
                                client or server.


7. DHCP Constants

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


7.1. Multicast Addresses

   DHCP makes use of the following multicast addresses:

      All_DHCP_Agents address:  FF02::1:2 This link-scoped multicast
                 address is used by clients to communicate with the
                 on-link agent(s) when they do not know the link-local
                 address(es) for those agents.  All agents (servers and
                 relays) are members of this multicast group.

      All_DHCP_Servers address:  FF05::1:3 This site-scoped multicast
                 address is used by clients or relays to communicate
                 with server(s), either because they want to send
                 messages to all servers or because they do not know
                 the server(s) unicast address(es).  Note that in order
                 for a client to use this address, it must have an
                 address of sufficient scope to be reachable by the
                 server(s).  All servers within the site are members of
                 this multicast group.


7.2. UDP ports

   DHCP uses the following destination UDP [18] port numbers.  While
   source ports MAY be arbitrary, client implementations SHOULD permit
   their specification through a local configuration parameter to
   facilitate the use of DHCP through firewalls.

      546        Client port.  Used by servers as the destination port
                 for messages sent to clients and relays.  Used by relay




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                 agents as the destination port for messages sent to
                 clients.

      547        Agent port.  Used as the destination port by clients
                 for messages sent to agents.  Used as the destination
                 port by relays for messages sent to servers.


7.3. DHCP message types

   DHCP defines the following message types.  More detail on these
   message types can be found in Section 8.  Message types 0 and 13-255
   are reserved for future use.  The message code for each message type
   is shown with the message name.

      SOLICIT (1)          The Solicit message is used by clients to
                           locate servers.

      ADVERTISE (2)        The Advertise message is used by servers
                           responding to Solicits.

      REQUEST (3)          The Request message is used by clients
                           to request configuration parameters from
                           servers.

      CONFIRM (4)          The Confirm message is used by clients to
                           confirm that the addresses assigned to an IA
                           and the lifetimes for those addresses, as
                           well as the current configuration parameters
                           assigned by the server to the client are
                           still valid.

      RENEW (5)            The Renew message is used by clients to
                           obtain the addresses assigned to an IA and
                           the lifetimes for those addresses, as well as
                           the current configuration parameters assigned
                           by the server to the client.  A client sends
                           a Renew message to the server that originally
                           assigned the IA when the lease on an IA is
                           about to expire.

      REBIND (6)           The Rebind message is used by clients to
                           obtain the addresses assigned to an IA and
                           the lifetimes for those addresses, as well as
                           the current configuration parameters assigned
                           by the server to the client.  A clients
                           sends a Rebind message to all available DHCP
                           servers when the lease on an IA is about to
                           expire.

      REPLY (7)            The Reply message is used by servers
                           responding to Request, Confirm, Renew,
                           Rebind, Release and Decline messages.  In the



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                           case of responding to a Request, Confirm,
                           Renew or Rebind message, the Reply contains
                           configuration parameters destined for the
                           client.

      RELEASE (8)          The Release message is used by clients to
                           return one or more IP addresses to servers.

      DECLINE (9)          The Decline message is used by clients to
                           indicate that the client has determined that
                           one or more addresses in an IA are already
                           in use on the link to which the client is
                           connected.

      RECONFIG-INIT (10)   The Reconfigure-init message is sent by
                           server(s) to inform client(s) that the
                           server(s) has new or updated configuration
                           parameters, and that the client(s) are to
                           initiate a Request/Reply transaction with the
                           server(s) in order to receive the updated
                           information.

      RELAY-FORW (11)      The Relay-forward message is used by relays
                           to forward client messages to servers.  The
                           client message is encapsulated in an option
                           in the Relay-forward message.

      RELAY-REPL (12)      The Relay-reply message is used by servers
                           to send messages to clients through a relay.
                           The server encapsulates the client message
                           as an option in the Relay-reply message,
                           which the relay extracts and forwards to the
                           client.


7.4. Status Codes

   This section describes status codes exchanged between DHCP
   implementations.  These status codes may appear in the Status Code
   option or in the status field of an IA.
















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7.4.1. Generic Status Codes

   The status codes in this section are used between clients and servers
   to convey status conditions.  The following table contains the status
   codes, the name for each code (as used in this document) and a brief
   description.  Note that the numeric values do not start at 1, nor are
   they consecutive.  The status codes are organized in logical groups.

   Name         Code Description
   ----------   ---- -----------
   Success         0 Success
   UnspecFail     16 Failure, reason unspecified
   AuthFailed     17 Authentication failed or nonexistent
   PoorlyFormed   18 Poorly formed message
   AddrUnavail    19 Addresses unavailable
   OptionUnavail  20 Requested options unavailable



7.4.2. Server-specific Status Codes

   The status codes in this section are used by servers to convey status
   conditions to clients.  The following table contains the status
   codes, the name for each code (as used in this document) and a brief
   description.  Note that the numeric values do not start at 1, nor are
   they consecutive.  The status codes are organized in logical groups.

   Name         Code Description
   ----         ---- -----------
   NoBinding      32 Client record (binding) unavailable
   ConfNoMatch    33 Client record Confirm not match IA
   RenwNoMatch    34 Client record Renew not match IA
   RebdNoMatch    35 Client record Rebind not match IA
   InvalidSource  36 Invalid Client IP address
   NoServer       37 Relay cannot find Server Address
   NoPrefixMatch  38 One or more prefixes of the addresses
                     in the IA is not valid for the link
                     from which the client message was received
   ICMPError      64 Server unreachable (ICMP error)

















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7.5. Configuration Variables

   This section presents a table of client and server configuration
   variables and the default or initial values for these variables.

      Parameter     Default  Description
   -------------------------------------
   MIN_SOL_DELAY     1 sec   Min delay of first Solicit
   MAX_SOL_DELAY     5 secs  Max delay of first Solicit
   SOL_TIMEOUT     500 msecs Initial Solicit timeout
   SOL_MAX_RT       30 secs  Max Solicit timeout value
   REQ_TIMEOUT     250 msecs Initial Request timeout
   REQ_MAX_RT       30 secs  Max Request timeout value
   REQ_MAX_RC       10       Max Request retry attempts
   CNF_TIMEOUT     250 msecs Initial Confirm timeout
   CNF_MAX_RT        1 sec   Max Confirm timeout
   CNF_MAX_RD       10 secs  Max Confirm duration
   REN_TIMEOUT      10 sec   Initial Renew timeout
   REN_MAX_RT      600 secs  Max Renew timeout value
   REB_TIMEOUT      10 sec   Initial Rebind timeout
   REB_MAX_RT      600 secs  Max Rebind timeout value
   REL_TIMEOUT     250 msecs Initial Release timeout
   REL_MAX_RT        1 sec   Max Release timeout
   REL_MAX_RC        5       MAX Release/Decline attempts
   DEC_TIMEOUT     250 msecs Initial Release timeout
   DEC_MAX_RT        1 sec   Max Release timeout
   DEC_MAX_RC        5       MAX Release/Decline attempts



8. Message Formats

   All DHCP messages sent between clients and servers share an identical
   fixed format header and a variable format area for options.  Not all
   fields in the header are used in every message.

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


















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   The following diagram illustrates the DHCP message header:

      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                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         server-address                        |
     |                          (16 octets)                          |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                            options                            .
     .                          (variable)                           .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   The following sections describe the use of the fields in the DHCP
   message header in each of the DHCP messages.  In these descriptions,
   fields that are not used in a message are marked as "unused".  All
   unused fields in a message MUST be transmitted as zeroes and ignored
   by the receiver of the message.


8.1. DHCP Solicit Message Format

      msg-type         SOLICIT

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Solicit message.

      server-address   (unused) MUST be 0

      options          See section 20.


8.2. DHCP Advertise Message Format

      msg-type         ADVERTISE

      transaction-ID   An unsigned integer used to identify this
                       Advertise message.  Copied from the Solicit
                       message received from the client.

      server-address   The IP address of the server that generated this
                       message.  The address must have sufficient scope
                       to be reachable from the client.

      options          See section 20.




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8.3. DHCP Request Message Format

      msg-type         REQUEST

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Request message.

      server-address   The IP address of the server to which the this
                       message is directed, copied from an Advertise
                       message.

      options          See section 20.


8.4. DHCP Confirm Message Format

      msg-type         CONFIRM

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Confirm message.

      server-address   MUST be zero.

      options          See section 20.


8.5. DHCP Renew Message Format

      msg-type         RENEW

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Renew message.

      server-address   The IP address of the server to which this Renew
                       message is directed, which MUST be the address
                       of the server from which the IAs in this message
                       were originally assigned.

      options          See section 20.


8.6. DHCP Rebind Message Format

      msg-type         REBIND

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Rebind message.

      server-address   MUST be zero.

      options          See section 20.





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8.7. DHCP Reply Message Format

      msg-type         REPLY

      transaction-ID   An unsigned integer used to identify this
                       Reply message.  Copied from the client Request,
                       Confirm, Renew or Rebind message received from
                       the client.

      server-address   The IP address of the server.  The address must
                       have sufficient scope to be reachable from the
                       client.

      options          See section 20.


8.8. DHCP Release Message Format

      msg-type         RELEASE

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Release message.

      server-address   The IP address of the server that assigned the
                       addresses.

      options          See section 20.


8.9. DHCP Decline Message Format

      msg-type         DECLINE

      transaction-ID   An unsigned integer generated by the client used
                       to identify this Decline message.

      server-address   The IP address of the server that assigned the
                       addresses.

      options          See section 20.


8.10. DHCP Reconfigure-init Message Format

      msg-type         RECONFIG-INIT

      transaction-ID   An unsigned integer generated by the server used
                       to identify this Reconfigure-init message.

      server-address   The IP address of the DHCP server issuing the
                       Reconfigure-init message.  The address must have
                       sufficient scope to be reachable from the client.




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      options          See section 20.


9. Relay 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 messages, which share the following format:

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


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


9.1. Relay-forward message

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

      msg-type                RELAY-FORW

      link-prefix             An address with a prefix that is assigned
                              to the link from which the client should
                              be assigned an address.

      client-return-address   The source address from the IP datagram
                              in which the message from the client was
                              received by the relay agent

      options                 MUST include a "Client message option";
                              see section 20.7; MAY include other
                              options added by the relay agent



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9.2. Relay-reply message

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

      msg-type                RELAY-REPL

      link-prefix             An address with a prefix that is assigned
                              to the link from which the client should
                              be assigned an address.

      client-return-address   The source address from the IP datagram
                              in which the message from the client was
                              received by the relay agent

      options                 MUST include a "Server message option";
                              see section 20.8; MAY include other
                              options


10. DHCP unique identifier (DUID)

   Each DHCP client has a DUID. DHCP servers use DUIDs to identify
   clients for the selection of configuration parameters and in
   the association of IAs with clients.  See section 20.2 for the
   representation of a DUID in a DHCP message.

   Servers MUST treat DUIDs as opaque values and must only compare DUIDs
   for equality.  Servers MUST NOT in any other way interpret DUIDs.
   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 options (e.g., messages
   sent by servers need not include a DUID). The DUID must be unique
   across all DHCP clients, and it must also be consistent for the same
   client - that is, the DUID used by a client SHOULD NOT change over
   time; for example, as a result of network hardware reconfiguration.

   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.


10.1. DUID contents

   A DUID consists of a sixteen-bit type code represented in network
   order, followed by a variable number of octets that make up the
   actual identifier.  A DUID can be no more than 256 octets long.  The
   following types are currently defined:



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       1        Link-layer address plus time
       2        Vendor-assigned unique ID
       3        Link-layer address


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


10.2. DUID based on link-layer address plus time

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

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Time (32 bits)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    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 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.

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

   DHCP clients 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 (e.g., a real-time clock) in generating the
   DUID, even if that time source is not configured by the user prior
   to generating the DUID. The use of a time source makes it unlikely
   that if the network interface is removed from the client and another
   client then uses the same network interface to generate a DUID,
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   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.


10.3. Vendor-assigned unique ID.

   The vendor-assigned unique ID consists of an eight-octet
   vendor-unique identifier, followed by the vendor's registered domain
   name.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        VUID (64 bits)                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                  domain name (variable length)                .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The structure of the VUID is left up to the vendor defining it, but
   each device containing such a VUID MUST be unique to each device
   that is using it, and MUST be assigned to the device at the time of
   manufacture and stored in some form of non-volatile storage.  The
   VUID SHOULD be recorded in non-erasable storage.  The domain name is
   simply any domain name that has been legally registered by the vendor
   in the domain name system, stored in canonical form.  An example DUID
   of this type might look like this:

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


   This is eight octets of VUID data, followed by "example.com"
   represented in ASCII.


10.4. Link-layer address

   This type of DUID consists of a two octet network hardware type code,
   followed by the link-layer address of any one network interface that
   is permanently connected to the DHCP client device.  The hardware
   type MUST be a valid hardware type assigned by the IANA as described
   in the section on ARP in RFC 826.  The hardware type is stored in
   network order.



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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Hardware type (16 bits)    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   .                                                               .
   .             link-layer address (variable length)              .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

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


11. Identity association

   An "identity-association" (IA) is a construct through which a server
   and a client can identify, group and manage IPv6 addresses.  Each IA
   consists of an IAID and a list of associated IPv6 addresses (the list
   may be empty).  A client associates an IA with one of its interfaces
   and uses the IA to obtain IPv6 addresses for that interface from a
   server.

   See section 20.3 for the representation of an IA in a DHCP message.


12. 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 the following sources:

    -  The link to which the client 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 a link-local address, then the client
           is on the same link to which the interface over which the
           message was received is attached




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        *  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

        *  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-prefix field in
           the message from the relay is attached

    -  The DUID supplied by the client

    -  Other information in options supplied by the client

    -  Other information in options supplied by the relay agent


13. Reliability of Client Initiated Message Exchanges

   DHCP clients are responsible for reliable delivery of messages in the
   client-initiated message exchanges described in sections 15 and 16.
   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.

   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 describe by five
   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.




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

   RT for the first message transmission is based on IRT:

      RT = 2*IRT + RAND*IRT


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

      RT = 2*RTprev + RAND*RTprev


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

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


   MRC specifies an upper bound on the number of times a client may
   retransmit a message.  If MRC has a value of 0, the client MUST
   continue to retransmit the original message until a response is
   received.  Otherwise, the message exchange fails if the client
   attempts to transmit the original message more than MRC times.

   MRD specifies an upper bound on the length of time a client may
   retransmit a message.  If MRD has a value of 0, the client MUST
   continue to retransmit the original message until a response is
   received.  Otherwise, the message exchange fails if the client
   attempts to transmit the original message more than MRD seconds.

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


14. Message validation

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

   Clients MUST discard any received messages that include
   authentication information and fail the authentication check by the
   client, except as noted in section 19.6.5.2.





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14.1. Use of Transaction-ID field

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


14.2. Solicit message

   Clients MUST discard any received Solicit messages.

   Relay agents MUST discard any Solicit messages received through port
   546.


14.3. Advertise message

   Clients MUST discard any received Advertise messages in which 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.


14.4. Request message

   Clients MUST discard any received Request messages.

   Relay agents MUST discard any Request messages received through port
   546.

   Servers MUST discard any received Request message in which the value
   in the ``server-address'' field does not match any of the addresses
   used by the server.


14.5. Confirm message

   Clients MUST discard any received Confirm messages.

   Relay agents MUST discard any Confirm messages received through port
   546.


14.6. Renew message

   Clients MUST discard any received Renew messages.

   Relay agents MUST discard any Renew messages received through port
   546.



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   Servers MUST discard any received Renew message in which the value in
   the ``server-address'' field does not match any of the addresses used
   by the server.


14.7. Rebind message

   Clients MUST discard any received Rebind messages.

   Relay agents MUST discard any Rebind messages received through port
   546.


14.8. Decline messages

   Clients MUST discard any received Decline messages.

   Relay agents MUST discard any Decline messages received through port
   546.

   Servers MUST discard any received Decline message in which the value
   in the ``server-address'' field does not match any of the addresses
   used by the server.


14.9. Release message

   Clients MUST discard any received Release messages.

   Relay agents MUST discard any Release messages received through port
   546.

   Servers MUST discard any received Release message in which the value
   in the ``server-address'' field does not match any of the addresses
   used by the server.


14.10. Reply message

   Clients MUST discard any received Reply messages in which the
   ``transaction-ID'' field in the message does not match the value used
   in the original message.

   Servers and relay agents MUST discard any received Reply messages.


14.11. Reconfigure-init message

   Servers and relay agents MUST discard any received Reconfigure-init
   messages.






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   Clients MUST discard any Reconfigure-init messages that do not
   contain an authentication option or that fail the authentication
   performed by the client.


14.12. Relay-forward message

   Clients MUST discard any received Relay-forward messages.


14.13. Relay-reply message

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


15. DHCP Server Solicitation

   This section describes how a client locates servers.


15.1. Client Behavior

   A client uses the Solicit message to discover DHCP servers configured
   to serve addresses on the link to which the client is attached.


15.1.1. 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 DUID option to identify itself to the
   server.  The client MUST include options for any IAs to which it
   wants the server to assign addresses.  The client MAY choose not to
   include any IAs in the Solicit message if it does not need to request
   that any addresses be assigned.  The client MAY include addresses in
   the IAs as a hint to the server about addresses for which the client
   may have a preference.  The client MAY include an Option Request
   Option in the Solicit message.  The client MUST NOT include any other
   options except those specifically allowed as defined by specific
   options.


15.1.2. Transmission of Solicit Messages

   The client sends the Solicit message to the All_DHCP_Agents
   multicast address.  The client MUST use an IPv6 address assigned
   to the interface for which the client is interested in obtaining
   configuration information as the source address in the IP header of
   the datagram carrying the Solicit message.





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   The Solicit message MUST be transmitted on the link that the
   interface for which configuration information is being obtained
   is attached to.  The client SHOULD send the message through that
   interface.  The client MAY send the message through another interface
   attached to the same link if and only if the client is certain the
   the two interface are attached to the same link.

   The first Solicit message from the client on the interface MUST
   be delayed by a random amount of time between MIN_SOL_DELAY and
   MAX_SOL_DELAY. This random delay desynchronizes clients which start
   at the same time (e.g., after a power outage).

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   SOL_TIMEOUT

      MRT   SOL_MAX_RT

      MRC   0

      MRD   0

   The mechanism in section 13 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 SOL_TIMEOUT has
   elapsed.  Rather, the client collects Advertise messages until
   SOL_TIMEOUT has elapsed.  The first RT MUST be selected to be
   strictly greater than SOL_TIMEOUT by choosing RAND to be strictly
   greater than 0.

   A client MUST collect Advertise messages for SOL_TIMEOUT seconds,
   unless it receives an Advertise message with a preference value
   of 255.  The preference value is carried in the Preference option
   (section  20.5).  Any Solicit that does not include a Preference
   option is considered to have a preference value of 0.  If the client
   receives an Advertise message with a preference value of 255, then
   the client MAY act immediately on that Advertise message without
   waiting for any more 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 MAY choose to restart the reconfiguration process after
   some external event, such as user input, system restart, or when the
   client is attached to a new link.









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15.1.3. Receipt of Advertise messages

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

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

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

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

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

   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.  Depending on the requirements of the user that
   invoked the DHCP client, the client MAY initiate a configuration
   exchange with the server(s) immediately, or MAY defer this exchange
   until later.

   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.


15.2. Server Behavior

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


15.2.1. Receipt of Solicit messages

   The server determines the information about the client and its
   location as described in section 12.  If administrative policy
   permits the server to respond to the client, the server will generate
   and send an Advertise message to the client.







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15.2.2. 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 places
   one of its IP addresses (determined through administrator setting)
   in the "server-address" field of the Advertise message.  The server
   MAY add a Preference option to carry the preference value for the
   Advertise message.

   The server implementation SHOULD allow the setting of a server
   preference value by the administrator.  The server preference value
   MUST default to zero unless otherwise configured by the server
   administrator.

   The server MUST include IA options in the Advertise message
   containing any addresses that would be assigned to IAs contained in
   the Solicit message from the client.  If the Solicit message from the
   client included no IAs, the server MUST not include any IAs in the
   Advertise message.  If the server will not assign any addresses to
   IAs in a subsequent Request from the client, the server MAY choose to
   send an Advertise message to the client that includes only a status
   code option with the status code set to AddrUnavail and a status
   message for the user.

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

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

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


16. 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




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   system configuration process or when requested to do so by the
   application layer.


16.1. Client Behavior

   A client will use Request, Confirm, Renew and Rebind messages to
   acquire and confirm the validity of configuration information.  The
   client uses the server address information from previous Advertise
   message(s) for use in constructing Request and Renew message(s).
   Note that a client may request configuration information from one or
   more servers at any time.


16.1.1. Creation and transmission of Request messages

   If the client is using stateful address configuration and needs
   either an initial set of addresses or additional addresses, it
   MUST send a Request message to obtain new addresses and other
   configuration information.  The client includes one or more IAs in
   the Request message, to which the server assigns new addresses.  The
   server then returns IA(s) to the client in a Reply message.

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

   The client places the address of the destination server in the
   "server-address" field.

   The client MUST include a DUID 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 list of addresses in each
   included IA MUST be empty.  If the client is not requesting that the
   server assign it any addresses, the client omits the IA option.

   If the client has a source address that can be used by the server
   as a return address and the client has received a Client Unicast
   option (section 20.11) from the server, the client SHOULD unicast
   the Request message to the server.  Otherwise, the client MUST send
   the Request message to the All_DHCP_Agents multicast address.  The
   client MUST use an address assigned to the interface for which the
   client is interested in obtaining configuration information as the
   source address in the IP header of the datagram carrying the Request
   message.

   DISCUSSION:

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




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   If the client multicasts the Request message, the message MUST be
   transmitted on the link that the interface for which configuration
   information is being obtained is attached to.  The client SHOULD send
   the message through that interface.  The client MAY send the message
   through another interface attached to the same link if and only if
   the client is certain the the two interface are attached to the same
   link.

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   REQ_TIMEOUT

      MRT   REQ_MAX_RT

      MRC   REQ_MAX_RC

      MRD   0

   If the message exchange fails, the client MAY choose one of the
   following actions:

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

    -  Initiate the server discovery process described in section 15

    -  Terminate the configuration process and report failure


16.1.2. Creation and transmission of Confirm messages

   Whenever a client may have moved to a new link, its IPv6 addresses
   and other configuration information may no longer be valid.  Examples
   of times when a client may have moved to a new link include:

     o The client reboots

     o The client is physically disconnected from a wired connection

     o The client returns from sleep mode

     o The client using a wireless technology changes cells

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






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   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 sets the "server-address" field to 0.

   The client MUST include a DUID option to identify itself to the
   server.  The client adds any appropriate options, including one or
   more IA options (if the client is requesting that the server confirm
   the validity of some IPv6 addresses).  If the client does include
   any IA options, it MUST include the list of addresses the client
   currently has associated with that IA.

   The client sends the Confirm message to the All_DHCP_Agents
   multicast address.  The client MUST use an IPv6 address assigned
   to the interface for which the client is interested in obtaining
   configuration information as the source address in the IP header of
   the datagram carrying the Confirm message.

   The Confirm message MUST be transmitted on the link that the
   interface for which configuration information is being obtained
   is attached to.  The client SHOULD send the message through that
   interface.  The client MAY send the message through another interface
   attached to the same link if and only if the client is certain the
   the two interface are attached to the same link.

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   CNF_TIMEOUT

      MRT   CNF_MAX_RT

      MRC   0

      MRD   CNF_MAX_RD

   If the client receives no responses before the message transmission
   process as described in section 13 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.


16.1.3. Creation and transmission of Renew messages

   IPv6 addresses assigned to a client through an IA use the same
   preferred and valid lifetimes as IPv6 addresses obtained through
   stateless address autoconfiguration.  The server assigns preferred
   and valid lifetimes to the IPv6 addresses it assigns to an IA. To
   extend those lifetimes, the client sends a Renew message to the
   server containing an "IA option" for the IA and its associated
   addresses.  The server determines new lifetimes for the addresses in



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   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 the server does not assign an
   explicit value to T1 or T2 for an IA, T1 defaults to 0.5 times the
   shortest preferred lifetime of any address assigned to the IA and
   T2 defaults to 0.875 times the shortest preferred lifetime of any
   address assigned to 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 address of the destination server in the
   "server-address" field.

   The client MUST include a DUID option to identify itself to the
   server.  The client adds any appropriate options, including one or
   more IA options (if the client is requesting that the server extend
   the lease on some IAs; note that the client may check the status of
   other configuration parameters without asking for lease extensions).
   If the client does include any IA options, it MUST include the list
   of addresses the client currently has associated with that IA.

   If the client has a source address that can be used by the server as
   a return address and the client has received a Client Unicast option
   (section 20.11) from the server, the client SHOULD unicast the Renew
   message to the server.  Otherwise, the client sends the Renew message
   to the All_DHCP_Agents multicast address.  The client MUST use an
   address assigned to the interface for which the client is interested
   in obtaining configuration information as the source address in the
   IP header of the datagram carrying the Renew message.

   If the Renew message is multicast, it MUST be transmitted on the
   link that the interface for which configuration information is being
   obtained is attached to.  The client SHOULD send the message through
   that interface.  The client MAY send the message through another
   interface attached to the same link if and only if the client is
   certain the the two interface are attached to the same link.

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   REN_TIMEOUT




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      MRT   REP_MAX_RT

      MRC   0

      MRD   0

   The mechanism in section 13 is modified as follows for use in the
   transmission of Renew messages.  The message exchange is terminated
   when time T2 is reached (see section 16.1.4), at which time the
   client begins a Rebind message exchange.


16.1.4. Creation and transmission of Rebind messages

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

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

   The client sets the "server-address" field to 0.

   The client MUST include a DUID option to identify itself to the
   server.  The client adds any appropriate options, including one or
   more IA options.  If the client does include any IA options (if the
   client is requesting that the server extend the lease on some IAs;
   note that the client may check the status of other configuration
   parameters without asking for lease extensions), it MUST include the
   list of addresses the client currently has associated with that IA.

   The client sends the Rebind message to the All_DHCP_Agents
   multicast address.  The client MUST use an IPv6 address assigned
   to the interface for which the client is interested in obtaining
   configuration information as the source address in the IP header of
   the datagram carrying the Rebind message.

   The Rebind message MUST be transmitted on the link that the interface
   for which configuration information is being obtained is attached
   to.  The client SHOULD send the message through that interface.  The
   client MAY send the message through another interface attached to the
   same link if and only if the client is certain the the two interface
   are attached to the same link.

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   REB_TIMEOUT

      MRT   REB_MAX_RT



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      MRC   0

      MRD   0

   The mechanism in section 13 is modified as follows for use in the
   transmission of Rebind messages.  The message exchange is terminated
   when the lease for the IA expires (see section 11), at which time the
   client has several alternative actions to choose from:

    -  When the lease on the IA expires, 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

    -  Some addresses in the IA may have lifetimes that extend beyond
       the lease of the IA, so the client may choose to continue to use
       those addresses; once all of the addresses have expired, the
       client may choose to locate a new DHCP 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


16.1.5. Receipt of Reply message in response to a Request, Confirm,
   Renew or Rebind message

   Upon the receipt of a valid Reply message in response to a
   Request, Confirm, Renew or Rebind 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.  If any of
   the addresses are found to be in use on the link, the client sends a
   Decline message to the server as described in section 16.1.8.

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

   Management of the specific configuration information is detailed in
   the definition of each option, in section 20.

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




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   client can use the addresses in the IA and ignore any messages that
   do indicate a NoPrefixMatch status.

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

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

   When the client receives a ConfNoMatch status in an IA from the
   server for a Confirm message the client can send a Renew message to
   the server to extend the lease for the addresses.

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

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

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

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

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

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


16.1.6. Creation and transmission of Release messages

   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 IP address of the server that allocated the
   address(es) in the "server-address" field.

   The client MUST include a DUID option to identify itself to the
   server.  The client includes options containing the IAs it is
   releasing in the "options" field.  The addresses to be released
   MUST be included in the IAs.  The appropriate "status" field in the
   options MUST be set to indicate the reason for the release.



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

   If the client has a source address that can be used by the server
   as a return address and the client has received a Client Unicast
   option (section 20.11) from the server, the client SHOULD unicast the
   Release message to the server.  Otherwise, the client MUST send the
   Release message to the All_DHCP_Agents multicast address.  The client
   MUST use an address for the interface to which the IAs in the Release
   message are assigned as the source address for the Release message.

   DISCUSSION:

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

   If the Release message is multicast, it MUST be transmitted on the
   link that the interface for which configuration information is being
   obtained is attached to.  The client SHOULD send the message through
   that interface.  The client MAY send the message through another
   interface attached to the same link if and only if the client is
   certain the the two interface are attached to the same link.

   A client MAY choose to wait for a Reply message from the server in
   response to the Release message.  If the client does wait for a
   Reply, the client MAY choose to retransmit the Release message.

   The client transmits the message according to section 13, using the
   following parameters:

      IRT   REL_TIMEOUT

      MRT   0

      MRC   REL_MAX_MRC

      MRD   0

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

   The client MUST stop using all of the addresses in the IA(s) being
   released as soon as the client begins the Release message exchange
   process.  If an IA is 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.




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   Note that if the client fails to release the IA, the addresses
   assigned to the IA will be reclaimed by the server when the lease
   associated with it expires.


16.1.7. Receipt of Reply message in response to a Release message

   Upon receipt of a valid Reply message, the client can consider the
   Release event successful, and SHOULD return the successful status to
   the application layer, if an application initiated the release.


16.1.8. Creation and transmission of Decline messages

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

   The client places the IP address of the server that allocated the
   address(es) in the "server-address" field.

   The client MUST include a DUID option to identify itself to the
   server.  The client includes options containing the IAs it is
   declining in the "options" field.  The addresses to be released
   MUST be included in the IAs.  The appropriate "status" field in the
   options MUST be set to indicate the reason for declining the address.

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

   If the client has a source address that can be used by the server
   as a return address and the client has received a Client Unicast
   option (section 20.11) from the server, the client SHOULD unicast the
   Decline message to the server.  Otherwise, the client MUST send the
   Decline message to the All_DHCP_Agents multicast address.  The client
   MUST use an IPv6 address for the interface to which the IAs in the
   Release message are assigned as the source address for the Decline
   message.

   DISCUSSION:

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

   If the Decline message is multicast, it MUST be transmitted on the
   link that the interface for which configuration information is being
   obtained is attached to.  The client SHOULD send the message through
   that interface.  The client MAY send the message through another
   interface attached to the same link if and only if the client is
   certain the the two interface are attached to the same link.




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   The client transmits the message according to section 13, using the
   following parameters:

      IRT   DEC_TIMEOUT

      MRT   DEC_MAX_RT

      MRC   DEC_MAX_RC

      MRD   0

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


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

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


16.2. Server Behavior

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


16.2.1. Receipt of Request messages

   The server MAY choose to discard Request messages received via
   unicast from a client to which the server has not sent a unicast
   option.

   Upon the receipt of a valid Request message from a client the server
   can respond to, (implementation-specific administrative policy
   satisfied) the server scans the options field.

   The server then constructs a Reply message and sends it to the
   client.

   The server SHOULD process each option for the client in an
   implementation-specific manner.  The server MUST construct a Reply
   message containing the following values:

      msg-type         REPLY

      transaction-ID   The transaction-ID from the Request message.

      server address   One of the IP addresses assigned to the interface
                       through which the server received the message
                       from the client.




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   When the server receives a Request and IA option is included the
   client is requesting the configuration of a new IA by the server.
   The server MUST take the IA from the client and associate a binding
   for that client in an implementation-specific manner within the
   configuration parameter database for DHCP clients managed by the
   server.

   If the server finds that the prefix on one or more IP addresses in
   any IA in the message fro the client is not a valid prefix for the
   link to which the client is connected, the server MUST return the IA
   to the client with the status field set to NoPrefixMatch.

   If the server cannot provide addresses to the client it SHOULD
   send back an empty IA to the client with the status field set to
   AddrUnavail.

   If the server can provide addresses to the client it MUST send back
   the IA to the client with all fields entered and a status of Success,
   and add the IA as a new client binding.

   The server adds options to the Reply message for any other
   configuration information to be assigned to the client.


16.2.2. Receipt of Confirm messages

   Upon the receipt of a valid Confirm message from a client the server
   can respond to, (implementation-specific administrative policy
   satisfied) the server scans the options field.

   The server then constructs a Reply message and sends it to the
   client.

   The server SHOULD process each option for the client in an
   implementation-specific manner.  The server MUST construct a Reply
   message containing the following values:

      msg-type         REPLY

      transaction-ID   The transaction-ID from the Confirm message.

      server address   One of the IP addresses assigned to the interface
                       through which the server received the message
                       from the client.

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

   If the server cannot determine if the information in the Confirm
   message is valid or invalid, the server MUST NOT send a reply to the



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   client.  For example, if the server does not have a binding for the
   client, but the configuration information in the Confirm message
   appears valid, the server does not reply.

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

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

   The Reply message from the server MUST contain a Status Code option
   and MUST NOT include any other options.


16.2.3. Receipt of Renew messages

   The server MAY choose to discard Renew messages received via unicast
   from a client to which the server has not sent a unicast option.

   Upon the receipt of a valid Renew message from a client the server
   can respond to, (implementation-specific administrative policy
   satisfied) the server scans the options field.

   The server then constructs a Reply message and sends it to the
   client.

   The server SHOULD process each option for the client in an
   implementation-specific manner.  The server MUST construct a Reply
   message containing the following values:

      msg-type         REPLY

      transaction-ID   The transaction-ID from the Confirm message.

      server address   One of the IP addresses assigned to the interface
                       through which the server received the message
                       from the client.

   When the server receives a Renew and IA option from a client it
   SHOULD locate the clients binding and verify the information in the
   IA from the client matches the information stored for that client.

   If the server cannot find a client entry for this IA the server
   SHOULD return an empty IA with status set to NoBinding.

   If the server finds that the addresses in the IA for the client do
   not match the clients binding the server should return an empty IA
   with status set to RenwNoMatch.




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   If the server cannot Renew addresses for the client it SHOULD
   send back an empty IA to the client with the status field set to
   AddrUnavail.

   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 lease times
   and T1/T2 times if the default is not being used, and set status to
   Success.


16.2.4. Receipt of Rebind messages

   Upon the receipt of a valid Rebind message from a client the server
   can respond to, (implementation-specific administrative policy
   satisfied) the server scans the options field.

   The server then constructs a Reply message and sends it to the
   client.

   The server SHOULD process each option for the client in an
   implementation-specific manner.  The server MUST construct a Reply
   message containing the following values:

      msg-type         REPLY

      transaction-ID   The transaction-ID from the Confirm message.

      server address   One of the IP addresses assigned to the interface
                       through which the server received the message
                       from the client.

   When the server receives a Rebind and IA option from a client it
   SHOULD locate the clients binding and verify the information in the
   IA from the client matches the information stored for that client.

   If the server cannot find a client entry for this IA the server
   SHOULD return an empty IA with status set to NoBinding.

   If the server finds that the addresses in the IA for the client do
   not match the clients binding the server should return an empty IA
   with status set to RebdNoMatch.

   If the server cannot Rebind addresses for the client it SHOULD
   send back an empty IA to the client with the status field set to
   AddrUnavail.

   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 lease times
   and T1/T2 times if the default is not being used, and set status to
   Success.

   There is a significant difference between Renew and Rebind messages:
   Because the Renew message is processed by a single server, the



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   responding server can actually change the addresses in the IA.
   However, because multiple servers may respond to a Rebind, all they
   can safely do is update T1, T2 (for the IA) and lifetimes (for
   individual addresses).


16.2.5. Receipt of Release messages

   The server MAY choose to discard Release messages received via
   unicast from a client to which the server has not sent a unicast
   option.

   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 then generates a Reply message.  If all of the IAs were
   valid and the addresses successfully released, the server includes
   a Status Code option with value Success.  If any of the IAs were
   invalid or if any of the addresses were not successfully released,
   the server leaves all of the IAs in the message unchanged (the server
   releases none of the addresses in any of the IAs in the message) and
   includes a Status Code option with value NoBinding.  The server MUST
   NOT include any other options in the Reply message.

   A client can send an option containing an IA with no listed addresses
   to release implicitly all of the addresses in the IA.

   A server is not required to (but may choose to as an implementation
   strategy) retain any record of an IA from which all of the addresses
   have been released.


16.2.6. Receipt of Decline messages

   The server MAY choose to discard Decline messages received via
   unicast from a client to which the server has not sent a unicast
   option.

   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 then generates a Reply message.  If all of the IAs were
   valid and the addresses successfully declined,, the server includes



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   a Status Code option with value Success.  If any of the IAs were
   invalid or if any of the addresses were not successfully declined,
   the server leaves all of the IAs in the message unchanged (the server
   releases none of the addresses in any of the IAs in the message) and
   includes a Status Code option with value NoBinding.  The server MUST
   NOT include any other options in the Reply message.

   A client can send an option containing an IA with no listed addresses
   to decline implicitly all of the addresses in the IA.


16.2.7. Sending of Reply messages

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

   The server then unicasts the Reply or Relay-reply to the source
   address from the IP datagram in which the original message was
   received.


17. 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 (system or application).


17.1. Server Behavior

   A server sends a Reconfigure-init message to cause a client to
   initiate immediately a Request/Reply message exchange with the
   server.


17.1.1. Creation and transmission of Reconfigure-init messages

   The server sets the "msg-type" field to RECONFIG-INIT. The server
   generates a transaction-ID and inserts it in the "transaction-ID"
   field.  The server places its address (of appropriate scope) in the
   "server-address" field.

   The server MAY include an ORO option to inform the client of what
   information has been changed or new information that has been added.




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

   The server MUST include an authentication option with the appropriate
   settings and add that option as the last option in the "options"
   field of the Reconfigure-init message.

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

   A server sends each Reconfigure-init 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-init message to additional clients while previous
   reconfiguration message exchanges are still in progress.

   The Reconfigure-init message causes the client to initiate a
   Request/Reply message exchange with the server.  The server
   interprets the receipt of a Request message from the client as
   satisfying the Reconfigure-init message request.


17.1.2. Time out and retransmission of Reconfigure-init messages

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

   Default and initial values for RECREP_MSG_TIMEOUT and
   REC_MSG_ATTEMPTS are documented in section 7.5.


17.1.3. Receipt of Request messages

   The server generates and sends Reply message(s) to the client as
   described in section 16.2.7, including in the "options" field new
   values for configuration parameters.

   It is possible that the client may send a Request message after the
   server has sent a Reconfigure-init but before the Reconfigure-init
   is received by the client.  In this case, the Request message from
   the client may not include all of the IAs and requests for parameters



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   to be reconfigured by the server.  To accommodate this scenario, the
   server MAY choose to send a Reply with the IAs and other parameters
   to be reconfigured, even if those IAs and parameters were not in the
   Request message from the client.


17.2. Client Behavior

   A client MUST always monitor UDP port 546 for Reconfigure-init
   messages on interfaces upon which it has acquired DHCP parameters.
   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.


17.2.1. Receipt of Reconfigure-init messages

   Upon receipt of a valid Reconfigure-init message, the client
   initiates a Request/Reply transaction with the server.  While
   the Request/Reply transaction is in progress, the client silently
   discards any Reconfigure-init messages it receives.

   DISCUSSION:

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

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

      It might be possible for a duplicate or retransmitted
      Reconfigure-init to be sufficiently delayed (and
      delivered out of order) to arrive at the client after
      the Request/Reply exchange (initiated by the original
      Reconfigure-init) has been completed.  In this case, the



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      client would initiate a redundant Request/Reply 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.


17.2.2. Creation and sending of Request messages

   When responding to a Reconfigure-init, the client creates and
   sends the Request message in exactly the same manner as outlined in
   section 16.1.1 with the following difference:

      IAs   The client includes IA options containing the addresses the
            client currently has assigned to those IAs for the interface
            through which the Reconfigure-init message was received.


17.2.3. Time out and retransmission of Request messages

   The client uses the same variables and retransmission algorithm as it
   does with Request messages generated as part of a client-initiated
   configuration exchange.  See section 16.1.1 for details.


17.2.4. Receipt of Reply messages

   Upon the receipt of a valid Reply message, the client extracts the
   contents of the "options" field, and sets (or resets) configuration
   parameters appropriately.  The client records and updates the
   lifetimes for any addresses specified in IAs in the Reply message.
   If the configuration parameters changed were requested by the
   application layer, the client notifies the application layer of the
   changes using an implementation-specific interface.

   As discussed in section 17.1.3, the Reply from the server may include
   IAs and parameters that were not included in the Request message from
   the client.  The client MUST configure itself with all of the IAs and
   parameters in the Reply from the server.


18. Relay Behavior

   For this discussion, the Relay may be configured to use a list of
   server destination addresses, which may include unicast addresses,
   the All_DHCP_Servers multicast address, or other multicast addresses
   selected by the network administrator.  If the Relay has not been
   explicitly configured, it MUST use the All_DHCP_Servers multicast
   address as the default.








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18.1. Relaying of client messages

   When a Relay receives a valid client message, it constructs a
   Relay-forward message.  The relay places an address with a prefix
   assigned to the link on which the client should be assigned an
   address in the link-prefix field.  This address will be used by the
   server to determine the link from which the client should be assigned
   an address and other configuration information.

   If the relay cannot use the address in the link-prefix field to
   identify the interface through which the response to the client
   will be forwarded, the relay MUST include a circuit-id option (see
   section 20.15)in the Relay-forward message.  The server will include
   the circuit-id option in its Relay-reply message.

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

   The relay constructs a "client-message" option 20.7 that contains
   the entire message from the client in the data field of the
   option.  The relay places the "relay-message" option along with any
   "relay-specific" options in the options field of the Relay-forward
   message.  The Relay then sends the Relay-forward message to the list
   of server destination addresses that it has been configured with.


18.2. Relaying of server messages

   When the relay receives a Relay-reply message, it extracts the server
   message from the "server-message" option.  If the Relay-reply message
   includes a circuit-id option, the relay forwards the message from the
   server to the client on the link identified by the circuit-id option.
   Otherwise, the relay forwards the message on the link identified
   by the link-prefix option.  In either case, the relay forwards the
   message to the address in the client-return-address field in the
   Relay-reply message.


19. Authentication of DHCP messages

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

   Because of the risk of denial of service attacks against DHCP
   clients, the use of authentication is mandated in Reconfigure-init



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   messages.  A DHCP server MUST include an authentication option in
   Reconfigure-init messages sent to clients.

   The DHCP authentication mechanism is based on the design of
   authentication for DHCP for IPv4 [8].


19.1. DHCP threat model

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

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

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

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

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


19.2. 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].  The way in which IPsec
   is employed by relay agents and servers is not specified in this
   document.


19.3. Summary of DHCP authentication

   Authentication of DHCP messages is accomplished through the use of
   the Authentication option.  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.  Two such protocols are 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.


19.4. 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 (e.g., an NTP-format
   timestamp [12]) can reduce the danger of replay attacks.  This method
   MUST be supported by all protocols.


19.5. Configuration token protocol

   If the protocol field is 0, the authentication information field
   holds a simple configuration token.  The configuration token is an
   opaque, unencoded value known to both the sender and receiver.  The
   sender inserts the configuration token in the DHCP message and the
   receiver matches the token from the message to the shared token.  If
   the configuration option is present and the token from the message
   does not match the shared token, the receiver MUST discard the
   message.

   Configuration token may be used to pass a plain-text configuration
   token and provides only weak entity authentication and no message
   authentication.  This protocol is only useful for rudimentary
   protection against inadvertently instantiated DHCP servers.

   DISCUSSION:

      The intent here is to pass a constant, non-computed token
      such as a plain-text password.  Other types of entity
      authentication using computed tokens such as Kerberos
      tickets or one-time passwords will be defined as separate
      protocols.






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19.6. 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.


19.6.1. Management issues in the delayed authentication protocol

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


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

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

   In an Advertise, Request, Renew, Rebind or Confirm message, the
   Authentication option carries the Protocol, Algorithm, RDM and Replay
   detection fields and Authentication information.  The format of the
   Authentication information is:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Secret ID (32 bits)                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                     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 secret value shared between the source and
                       destination of the message; each secret has a



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                       unique identifier (secret ID)
   secret ID         - the unique identifier for the secret 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 (except
   the MAC field of the authentication option itself), including the
   DHCP message header and the options field, is used as input to the
   HMAC-MD5 computation function.  The 'secret ID' field MUST be set to
   the identifier of the secret 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 secret key has a unique
      identifier that can be used by a receiver to determine which
      secret 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.


19.6.3. Message validation

   To validate an incoming message, the receiver first checks that
   the value in the replay detection field is acceptable according
   to the replay detection method specified by the RDM field.  Next,
   the receiver computes the MAC as described in [11].  The receiver
   MUST set the 'MAC' field of the authentication option to all 0s for
   computation of the MAC. If the MAC computed by the receiver does not
   match the MAC contained in the authentication option, the receiver
   MUST discard the DHCP message.


19.6.4. Key utilization

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




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


19.6.5. Client considerations for delayed authentication protocol

19.6.5.1. Sending Solicit messages

   When the client sends a Solicit message and wishes to use
   authentication, it includes an Authentication option with the desired
   protocol, algorithm, RDM and replay detection field as described
   in section 19.6.  The client does not include any authentication
   information in the Authentication option.


19.6.5.2. Receiving Advertise messages

   The client validates any Advertise messages containing an
   Authentication option specifying the delayed authentication protocol
   using the validation test described in section 19.6.3.

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

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

   A client MUST be configurable to discard unauthenticated messages,
   and SHOULD be configured by default to discard unauthenticated
   messages.  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.







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19.6.5.3. Sending Request, Confirm, Renew, Rebind 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 19.6.  When the
   client sends a subsequent message, it MUST use the same secret used
   by the server to generate the authentication information.


19.6.5.4. Receiving Reply messages

   If the client authenticated the Advertise it accepted, the client
   MUST validate the associated Reply message from the server.  The
   client MUST discard the Reply if the message fails to pass validation
   and MAY log the validation failure.  If the Reply fails to pass
   validation, the client MUST restart the DHCP configuration process by
   sending a Solicit message.  The client MAY choose to remember which
   server replied with a Reply message that failed to pass validation
   and discard subsequent messages from that server.

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


19.6.6. Server considerations for delayed authentication protocol

19.6.6.1. Receiving Solicit messages and Sending Advertise messages

   The server selects a secret for the client and includes
   authentication information in the Advertise message returned to the
   client as specified in section 19.6.  The server MUST record the
   identifier of the secret selected for the client and use that same
   secret for validating subsequent messages with the client.


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

   The server uses the secret identified in the message and validates
   the message as specified in section 19.6.3.  If the message fails to
   pass validation or the server does not know the secret identified by
   the 'secret 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 16.2.  The server
   MUST include authentication information generated using the secret
   identified in the received message as specified in section 19.6.






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19.6.6.3. Sending Reconfigure-Init messages

   The server MUST include authentication information in a
   Reconfigure-Init message, generated as specified in section 19.6
   using the secret the server initially selected for the client to
   which the Reconfigure-Init message is to be sent.


20. DHCP options

   Options are used to carry additional information and parameters
   in DHCP messages.  Every option shares a common base format, as
   described in section 20.1.

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


20.1. Format of DHCP options

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



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

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

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
















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20.2. DHCP unique identifier option

   The DHCP unique identifier option is used to carry a DUID. The format
   for the DUID is keyed to mark the type of identifier and is of
   variable length.  The format of the DUID 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 DUID          |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           DUID type           |             DUID              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     .                         DUID (cont.)                          .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




20.3. Identity association option

   The identity association option is used to carry an identity
   association, the parameters associated with the IA and the addresses
   assigned to the IA.





























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   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 status   |   num-addrs   |T| addr status | prefix length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 address                          |
     |                          (16 octets)                          |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      preferred lifetime                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        valid lifetime                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |T| addr status | prefix length |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                         IPv6 address                          |
     |                          (16 octets)                          |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |      preferred lifetime       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | pref. lifetime (cont.)        |        valid lifetime         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | valid lifetime (cont.)        |T| addr status | prefix length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 address                          |
     |                          (16 octets)                          |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option-code          OPTION_IA (TBD)

      option-len           Variable; equal to 24 + num-addrs*26

      IA ID                The unique identifier for this IA; chosen by
                           the client





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      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.

      T2                   The time at which the client contacts any
                           available server to extend the lifetimes of
                           the addresses assigned to the IA.

      T                    When set to 1, indicates that this address is
                           a "temporary address" [15]; when set to 0,
                           the address is not a temporary address.

      IA status            Status of the IA in this option.

      num-addrs            An unsigned integer giving the number of
                           addresses carried in this IA option (MAY be
                           zero).

      addr status          Status of the addresses in this IA.

      prefix length        Prefix length for this address.

      IPv6 address         An IPv6 address assigned to this IA.

      preferred lifetime   The preferred lifetime for the associated
                           IPv6 address.

      valid lifetime       The valid lifetime for the associated IPv6
                           address.

   The "IPv6 address", "preferred lifetime" and "valid lifetime" fields
   are repeated for each address in the IA option (as determined by the
   "num-addrs" field).

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

   The 'T' bit identifies the associated address as a temporary address.
   If the server is configured to assign temporary addresses to the
   client, the server marks those temporary addresses with the 'T'
   bit.  The number of temporary addresses assigned to the client and
   the lifetimes of those addresses is determined by the administrative
   configuration of the server.  The 'T' bit only identifies an address
   as a temporary address; identification of an address as "temporary"
   has no implication on the lifetime of the extensibility of the
   lifetime of the address.






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20.4. Option request 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_ORO          |           option-len          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    requested-option-code-1    |    requested-option-code-2    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option-code   OPTION_ORO (TBD)

      option-len    Variable; equal to twice the number of option codes
                    carried in this option.

      option-data   A list of the option codes for the options requested
                    in this option.

   A client MAY include an Option Request option in a Solicit, Request,
   Renew, Rebind or Confirm message to inform the server about options
   the client wants the server to send to the client.


20.5. Preference option

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



      option-code   OPTION_PREFERENCE (TBD)

      option-len    MUST be 1

      option-data   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 15.1.3
   for the use of the Preference option by the client and the
   interpretation of Preference option data value.







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20.6. Elapsed Time

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


      option-code   OPTION_ELAPSED_TIME (TBD)

      option-len    MUST be 2

      option-data   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 MAY include an Elapsed Time option in messages to indicate
   how long the client has been trying to complete a DHCP transaction.
   Servers MAY use the data value in this option as input to policy
   controlling how a server responds to a client message.


20.7. Client message option

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



      option-code   OPTION_CLIENT_MSG (TBD)

      option-len    Variable; equal to the length of the forwarded DHCP
                    client message.

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











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20.8. Server message option

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



      option-code   OPTION_SERVER_MSG (TBD)

      option-len    Variable; equal to the length of the forwarded DHCP
                    server message.

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


20.9. DSTM Global IPv4 Address Option

   The DSTM Global IPv4 Address Option informs a client or server that
   the Identity Association Option (IA) following this option will
   contain an IPv4-Mapped IPv6 Address [9] in the case of a Client
   receiving the option, or is a Request for an IPv4-Mapped IPv6 Address
   from a client in the case of a DHCPv6 Server receiving the option.
   The option can also provide a set of IPv6 addresses to be used as the
   Tunnel Endpoint (TEP) to encapsulate an IPv6 packet within IPv6.

   This option can be used with the Request, Reply, and Reconfigure-Init
   Messages for cases where a server wants to assign to clients
   IPv4-Mapped IPv6 Addresses, thru the Option Request Option (ORO).

    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_DSTM          |             option-length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Tunnel End Point (TEP)               |
   |                           (If Present)                        |
   |                            (16 octets)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option code        OPTION_DSTM (TBD)

      option length      Variable:  0 or multiple of 16

      tunnel end point   IPv6 Address or addresses if Present



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   A DSTM IPv4 Global Address Option MUST only apply to the IA following
   this option.


20.10. 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 19.

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


      option-code                  OPTION_AUTH (TBD)

      option-length                Variable

      protocol                     The authentication protocol used in
                                   this authentication option

      algorithm                    The algorithm used in the
                                   authentication protocol

      RDM                          The replay detection method used in
                                   this authentication option

      Replay detection             The replay detection information for
                                   the RDM

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







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20.11. Server unicast option

   This option is used by a server to send to a client to inform
   the client it MAY send a Request, Renew, Release, and Decline by
   unicasting directly to the server instead of the All_DHCPv6_Agents
   Multicast address as an optimization, when the client as an address
   of sufficient scope to reach the server.

    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-length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      option-code     OPTION_UNICAST (TBD)

      option-length   0

   This option only applies to the server address that sends this to the
   client.


20.12. Domain Search Option

   This option provides a list of domain names a client can use to
   resolve DNS names.

    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_DOMAIN_SEARCH_LIST   |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Domain Search List                       |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      option-code                 OPTION_DOMAIN_SEARCH_LIST (TBD)

      option-length               variable

      Domain Search List          The DNS domain search list the client
                                  should use to resolve names.

   So that the search list may be encoded compactly and uniformly,
   search strings in the search list are concatenated and encoded using
   the technique described in section 4.1 of [13].

   For use in this specification, the compression pointer (see section
   4.1.4 of [13]) refers to the offset within the SearchString portion
   of the option.




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20.13. Domain Name Server Option

   This option provides a list of Domain Name System [13] that a client
   name resolver can use to access DNS services.  There must be at least
   1 server listed in this 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_DNS_SERVERS       |         option_length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                   DNS server (IP address)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                   DNS server (IP address)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option-code          OPTION_DNS_SERVERS (11)

      option-length        variable

      DNS server           IPv6 address of a DNS name server for the
                           client to use.  The DNS servers are listed in
                           the order of preference for use by the client
                           resolver.


20.14. Status Code Option

   This option returns indications of status not related to a specific
   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_STATUS_CODE      |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          status-code          |         status-message        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      option-code          OPTION_STATUS_CODE (TBD)




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      option-length        variable

      status-code          The numeric code for the status encoded in
                           this option.  The status codes are defined in
                           section 7.4.

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


20.15. Circuit-ID Option

   This option provides a mechanism through which a relay agent can
   identify the network attachment point through which a message was
   received from a DHCP client.

    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_CIRCUIT_ID       |         option_length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                          Circuit-ID                           |
   .                                                               .
   .                                                               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      option-code          OPTION_CIRCUIT_ID (TBD)

      option-length        variable

      Circuit-ID           An opaque value of arbitrary length; this
                           value must uniquely identify one of the
                           network attachments used by the relay agent


















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20.16. User Class Option

   This option is used by a client to identify the type or category of
   user or applications it represents.  The information contained in
   this option is an opaque field that represents the user class of
   which the client is a member.  Based on this class, a DHCP server
   selects the appropriate address pool to assign an address to the
   client and the appropriate configuration parameters.

      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          TBD

      option-len           Variable; If n user classes are carried
                           by the option, the length of the option
                           option-len = sum of each of the user class
                           lengths + 2*n.

      option-data          The user classes carried by the client.

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

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
     |       user class1 len         |           user1 class data    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+


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

   Servers may interpret the meanings of multiple class specifications
   in an implementation dependent or configuration dependent manner,
   and so the use of multiple classes by a DHCP client should be based
   on the specific server implementation and configuration which will
   be used to process that User class option.  Servers not equipped to
   interpret the user class information sent by a client MUST ignore it
   (although it may be reported).


20.17. Vendor Class Option

   This option is used by clients and servers to exchange vendor-
   specific information.  The definition of this information is vendor
   specific.  The vendor is indicated in the vendor class identifier
   option.  Servers not equipped to interpret the vendor-specific



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   information sent by a client MUST ignore it (although it may be
   reported).  Clients which do not receive desired vendor-specific
   information SHOULD make an attempt to operate without it, although
   they may do so(and announce they are doing so) in a degraded mode.

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


      option-code          TBD

      option-len           Variable

      option-data          The information is an opaque object of
                           option-len octets, presumably interpreted
                           by vendor-specific code on the clients and
                           servers

   If a vendor potentially encodes more than one item of information
   in this option, then the vendor SHOULD encode the option using
   "Encapsulated vendor-specific options".

   The Encapsulated vendor-specific options field SHOULD be encoded as a
   sequence of code/length/value fields of identical syntax to the DHCP
   options field.

   When encapsulated vendor-specific extensions are used, 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             |             opt_len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          option-data                          |
     |                             . . .                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      opt_code             The code for the encapsulated option

      opt_len              The length of the encapsulated option

      option-data          The data area for the encapsulated option








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21. Security Considerations

   Section 19 describes a threat model and an option that provides an
   authentication framework to defend against that threat model.


22. Year 2000 considerations

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


23. IANA Considerations

   This document defines several new name spaces associated with DHCPv6
   and DHCPv6 options.  IANA is requested to manage the allocation of
   values from these name spaces, which are described in the remainder
   of this section.  These name spaces are all to be managed separately
   from the name spaces defined for DHCPv4 [7, 2].

   New values in each of these name spaces should be approved by the
   process of IETF consensus [14].


23.1. Multicast addresses

   Section 7.1 defines the following multicast addresses, which have
   been assigned by IANA for use by DHCPv6:

      All_DHCP_Agents address:    FF02::1:2

      All_DHCP_Servers address:   FF05::1:3

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


23.2. DHCPv6 message types

   IANA is requested to record the message types defined in section 7.3.
   IANA is requested to manage definition of additional message types in
   the future.


23.3. DUID

   IANA is requested to record the DUID types defined in section 10.1.
   IANA is requested to manage definition of additional DUID types in
   the future.






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23.4. DHCPv6 options

   IANA is requested to assign option-codes to the options defined
   in section 20.1.  IANA is requested to manage the definition of
   additional DHCPv6 option-codes in the future.


23.5. Status codes

   IANA is requested to record the status codes defined in section 7.4.
   IANA is requested to manage the definition of additional status codes
   in the future.


23.6. Authentication option

   Section 19 defines three new name spaces associated with the
   Authentication Option (section 20.10), which are to be created and
   maintained by IANA: Protocol, Algorithm and RDM.

   Initial values assigned from the Protocol name space are 0 (for the
   configuration token Protocol in section 19.5) and 1 (for the delayed
   authentication Protocol in section 19.6).  Additional protocols may
   be defined in the future.

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

   For the configuration token Protocol, the Algorithm field MUST be
   0, as described in section 19.5.  For the delayed authentication
   Protocol, the Algorithm value 1 is assigned to the HMAC-MD5
   generating function as defined in section 19.6.  Additional
   algorithms for the delayed authentication protocol may be defined in
   the future.

   The initial value of 0 from the RDM name space is assigned to the
   use of a monotonically increasing value as defined in section 19.4.
   Additional replay detection methods may be defined in the future.


24. Acknowledgments

   Thanks to the DHC Working Group for their time and input into the
   specification.  Ralph Droms and Thomas Narten have had a major
   role in shaping the continued improvement of the protocol by their
   careful reviews.  Many thanks to Matt Crawford, Erik Nordmark, Gerald
   Maguire, and Mike Carney for their studied review as part of the
   Last Call process.  Thanks also for the consistent input, ideas, and
   review by (in alphabetical order) Brian Carpenter, Francis DuPont,
   Ted Lemon, Jack McCann, Yakov Rekhter, Matt Thomas, Sue Thomson,
   Bernie Volz and Phil Wells.



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

   Bill Arbaugh reviewed the authentication mechanism described in
   section 19.

   The Domain Search option described in section 20.12 is based on the
   DHCPv4 domain search option, [1], and was reviewed by Bernard Aboba.


A. Comparison between DHCPv4 and DHCPv6

   This appendix is provided for readers who will find it useful to see
   a model and architecture comparison between DHCPv4 [7, 2] and DHCPv6.
   There are three key reasons for the differences:

     o IPv6 inherently supports a new model and architecture for
       communications and autoconfiguration of addresses.

     o DHCPv6 benefits from the new IPv6 features.

     o New features were added to support the expected evolution and
       the existence of more complicated Internet network service
       requirements.

   IPv6 Architecture/Model Changes:

     o The link-local address permits a node to have an address
       immediately when the node boots, which means all clients have a
       source IP address at all times to locate an on-link server or
       relay.

     o The need for BOOTP compatibility and the broadcast flag have been
       removed.

     o Multicast and address scoping in IPv6 permit the design of
       discovery packets that would inherently define their range by the
       multicast address for the function required.

     o Stateful autoconfiguration has to coexist and integrate with
       stateless address autoconfiguration supporting duplicate address
       detection [20] and the two IPv6 address lifetimes, to facilitate
       the dynamic renumbering of addresses and the management of those
       addresses.

     o Multiple addresses per interface are inherently supported in
       IPv6.

     o Some DHCPv4 options are unnecessary now because the configuration
       parameters are either obtained through IPv6 Neighbor Discovery or
       the Service Location protocol [21].




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   DHCPv6 Architecture/Model Changes:

     o The message type is the first octet in the packet.

     o IPv6 Address allocations are now handled in a message option as
       opposed to the message header.

     o Client/Server bindings are now mandatory and take advantage
       of the link-local address of the client to always permit
       communications either directly from an on-link server, or from a
       off-link server through an on-link relay.

     o Servers are discovered by a client Solicit, followed by a server
       Advertise message

     o The client will know if the server is on-link or off-link.

     o The on-link relay may locate off-link server addresses from
       system configuration or by the use of a site-wide multicast
       packet.

     o ACKs and NAKs are not used.

     o The server assumes the client receives its responses unless it
       receives a retransmission of the same client request.  This
       permits recovery in the case where the network has faulted.

     o Clients can issue multiple, unrelated Request messages to the
       same or different servers.

     o The function of DHCPINFORM is inherent in the new packet design;
       a client can request configuration parameters other than IPv6
       addresses in the optional option headers.

     o Clients MUST listen to their UDP port for the new
       Reconfigure-init message from servers.

     o New options have been defined.

   With the changes just enumerated, we can support new user features,
   including

     o Configuration of Dynamic Updates to DNS

     o Address deprecation, for dynamic renumbering.

     o Relays can be preconfigured with server addresses, or use of
       multicast.

     o Authentication

     o Clients can ask for multiple IP addresses.




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     o Addresses can be reclaimed using the Reconfigure-init message.

     o Integration between stateless and stateful address
       autoconfiguration.

     o Enabling relays to locate off-link servers.


B. Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However,
   this document itself may not be modified in any way, such as by
   removing the copyright notice or references to the Internet Society
   or other Internet organizations, except as needed for the purpose
   of developing Internet standards in which case the procedures
   for copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

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

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


References

    [1] B. Aboba.  DHCP Domain Search Option.  Internet Draft, Internet
        Engineering Task Force, December 2000.  Work in progress.

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

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

    [4] S. Bradner and A. Mankin.  The Recommendation for the IP Next
        Generation Protocol, January 1995.  RFC 1752.

    [5] W.J. Croft and J. Gilmore.  Bootstrap Protocol, September 1985.
        RFC 951.



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

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

    [8] R. Droms and W. Arbaugh.  Authentication for DHCP Messages.
        Internet Draft, Internet Engineering Task Force, January 2001.
        Work in progress.

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

   [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.

   [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] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan.  Service
        Location Protocol, June 1997.  RFC 2165.

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




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Chair's Address

   The working group can be contacted via the current chair:


         Ralph Droms
         Cisco Systems
         300 Apollo Drive
         Chelmsford, MA 01824

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



Authors' Addresses

   Questions about this memo can be directed to:






































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

        Mike Carney
        Sun Microsystems, Inc
        Mail Stop:  UMPK17-202
        901 San Antonio Road
        Palo Alto, CA 94303-4900
        USA
        Phone:  +1-650-786-4171
        Email:  mwc@eng.sun.com

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

        Ralph Droms
        Cisco Systems
        300 Apollo Drive
        Chelmsford, MA 01824
        USA
        Phone:  +1 978 244 4733
        Email:  rdroms@cisco.com




















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