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Internet Engineering Task Force                                 J. Bound
INTERNET DRAFT                                     Compaq Computer Corp.
DHC Working Group                                              M. Carney
Obsoletes:  draft-ietf-dhc-dhcpv6-14.txt           Sun Microsystems, Inc
                                                              C. Perkins
                                                   Nokia Research Center
                                                              5 May 2000
          Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
                      draft-ietf-dhc-dhcpv6-15.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 dhcp-v6@bucknell.edu 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 using extensions 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'' [15], 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. Terminology                                                        2
     2.1. IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .    2
     2.2. DHCP Terminology  . . . . . . . . . . . . . . . . . . . .    3


 3. DHCP Constants                                                     5
     3.1. Multicast Addresses . . . . . . . . . . . . . . . . . . .    5
     3.2. UDP ports . . . . . . . . . . . . . . . . . . . . . . . .    5
     3.3. DHCP message types  . . . . . . . . . . . . . . . . . . .    6
     3.4. Error Values  . . . . . . . . . . . . . . . . . . . . . .    8
           3.4.1. Generic Error Values  . . . . . . . . . . . . . .    8
           3.4.2. Server-specific Error Values  . . . . . . . . . .    8
     3.5. Configuration Variables . . . . . . . . . . . . . . . . .    8


 4. Requirements                                                       9


 5. Background                                                         9


 6. Design Goals                                                      11


 7. Non-Goals                                                         11


 8. Overview                                                          12
     8.1. How does a node know to use DHCP? . . . . . . . . . . . .   12
     8.2. How does a client find out about DHCP agents? . . . . . .   12
     8.3. What if the client and server(s) are on different links?    12
     8.4. How does a client request configuration parameters from
             servers? . . . . . . . . . . . . . . . . . . . . . . .   13
     8.5. What are releasable resources, and when are they used?  .   13
     8.6. Can a client release its releasable resources before the lease
             expires? . . . . . . . . . . . . . . . . . . . . . . .   14
     8.7. What if the client determines its releasable resource is
             already being used by another client?  . . . . . . . .   14
     8.8. How are clients notified of server configuration changes?   14


 9. Message Formats                                                   15
     9.1. DHCP Solicit Message Format . . . . . . . . . . . . . . .   15
     9.2. DHCP Advertise Message Format . . . . . . . . . . . . . .   16
     9.3. DHCP Request Message Format . . . . . . . . . . . . . . .   18
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     9.4. DHCP Reply Message Format . . . . . . . . . . . . . . . .   19
     9.5. DHCP Release Message Format . . . . . . . . . . . . . . .   20
     9.6. DHCP Reconfigure Message Format . . . . . . . . . . . . .   22
     9.7. DHCP Reconfigure-reply Message Format . . . . . . . . . .   23
     9.8. DHCP Reconfigure-init Message Format  . . . . . . . . . .   24


10. DHCP Server Solicitation and Subnet Prefix Discovery              25
    10.1. Solicit Message Validation  . . . . . . . . . . . . . . .   25
    10.2. Advertise Message Validation  . . . . . . . . . . . . . .   25
    10.3. Client Behavior . . . . . . . . . . . . . . . . . . . . .   26
          10.3.1. Creation and sending of the Solicit message . . .   26
          10.3.2. Time out and retransmission of Solicit Messages .   27
          10.3.3. Receipt of Advertise messages . . . . . . . . . .   27
    10.4. Relay Behavior  . . . . . . . . . . . . . . . . . . . . .   28
          10.4.1. Relaying of Solicit messages  . . . . . . . . . .   28
          10.4.2. Relaying of Advertise messages  . . . . . . . . .   28
    10.5. Server Behavior . . . . . . . . . . . . . . . . . . . . .   28
          10.5.1. Receipt of Solicit messages . . . . . . . . . . .   28
          10.5.2. Creation and sending of Advertise messages  . . .   29


11. DHCP Client-Initiated Configuration Exchange                      29
    11.1. Request Message Validation  . . . . . . . . . . . . . . .   29
    11.2. Reply Message Validation  . . . . . . . . . . . . . . . .   30
    11.3. Release Message Validation  . . . . . . . . . . . . . . .   31
    11.4. Client Behavior . . . . . . . . . . . . . . . . . . . . .   31
          11.4.1. Creation and sending of Request messages  . . . .   32
          11.4.2. Time out and retransmission of Request Messages .   33
          11.4.3. Receipt of Reply message in response to a Request   33
          11.4.4. Creation and sending of Release messages  . . . .   33
          11.4.5. Time out and retransmission of Release Messages .   34
          11.4.6. Receipt of Reply message in response to a Release   35
    11.5. Relay Behavior  . . . . . . . . . . . . . . . . . . . . .   35
          11.5.1. Relaying of Request or Release messages . . . . .   35
    11.6. Server Behavior . . . . . . . . . . . . . . . . . . . . .   35
          11.6.1. Receipt of Request messages . . . . . . . . . . .   35
          11.6.2. Receipt of Release messages . . . . . . . . . . .   36
          11.6.3. Creation and sending of Reply messages  . . . . .   36


12. DHCP Server-Initiated Configuration Exchange                      37
    12.1. Reconfigure Message Validation  . . . . . . . . . . . . .   37
    12.2. Reconfigure-reply Message Validation  . . . . . . . . . .   38
    12.3. Reconfigure-init Message Validation . . . . . . . . . . .   38
    12.4. Server Behavior . . . . . . . . . . . . . . . . . . . . .   38
          12.4.1. Creation and sending of Reconfigure messages  . .   39
          12.4.2. Time out and retransmission of Reconfigure
                          messages . . . . . . . . . . . . . . . . .  40
          12.4.3. Receipt of Reconfigure-reply messages . . . . . .   40
          12.4.4. Creation and sending of Reconfigure-init messages   40


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          12.4.5. Time out and retransmission of Reconfigure-init
                          messages . . . . . . . . . . . . . . . . .  41
          12.4.6. Receipt of Request messages . . . . . . . . . . .   41
    12.5. Client Behavior . . . . . . . . . . . . . . . . . . . . .   41
          12.5.1. Receipt of Reconfigure messages . . . . . . . . .   42
          12.5.2. Creation and sending of Reconfigure-reply messages  42
          12.5.3. Receipt of Reconfigure-init messages  . . . . . .   43
          12.5.4. Creation and sending of Request messages  . . . .   43
          12.5.5. Time out and retransmission of Request messages .   43
          12.5.6. Receipt of Reply messages . . . . . . . . . . . .   43


13. Using DHCP for network renumbering                                43
    13.1. Passive Renumbering . . . . . . . . . . . . . . . . . . .   44
    13.2. Active Renumbering  . . . . . . . . . . . . . . . . . . .   44


14. DHCP Client Implementator Notes                                   44
    14.1. Primary Interface . . . . . . . . . . . . . . . . . . . .   45
    14.2. Advertise Message and Configuration Parameter Caching . .   45
    14.3. Time out and retransmission variables . . . . . . . . . .   45
    14.4. Server Preference . . . . . . . . . . . . . . . . . . . .   45


15. DHCP Server Implementator Notes                                   46
    15.1. Client Bindings . . . . . . . . . . . . . . . . . . . . .   46
    15.2. Reconfigure Considerations  . . . . . . . . . . . . . . .   46
    15.3. Server Preference . . . . . . . . . . . . . . . . . . . .   46
    15.4. Request Message Transaction-ID Cache  . . . . . . . . . .   47


16. DHCP Relay Implementator Notes                                    47


17. Open Issues for Working Group Discussion                          47
    17.1. Trade-offs:  Optional fields in DHCP messages . . . . . .   47
    17.2. Use DHCPv4 authentication or the current DHCPv6 method? .   48
    17.3. The Reconfigure Message and Subnet Prefix Extensions  . .   48
    17.4. ``R'' bit in Request message not needed?  . . . . . . . .   48


18. Security Considerations                                           48


19. Year 2000 considerations                                          49


20. IANA Considerations                                               49


21. Acknowledgements                                                  50


 A. Comparison between DHCPv4 and DHCPv6                              50


 B. Full Copyright Statement                                          52


Chair's Address                                                       55


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Author's Address                                                      55
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1. Introduction


   This document describes DHCP for IPv6 (DHCP), a UDP [14] 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 network resources more varied than that
   offered by ``IPv6 Stateless Autoconfiguration'' [15].  The 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.


   The 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.
   The DHCP is designed to be easily extended to carry new configuration
   parameters through the addition of new DHCP ``extensions'' defined to
   carry this information.  See this document's companion specification,
   ``Extensions for the Dynamic Host Configuration Protocol for
   IPv6'' [2] for specifications of existing extensions as well as
   information on the process by which an interested party might specify
   new extensions.


   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 BOOTP [5]-backward compatibility
   constraints.  For more information about the differences between DHCP
   for IPv6 and DHCP for IPv4, see Appendix A.


   This document is organized as follows.  Section 2 defines terminology
   used throughout this document.  Section 3 defines constant values
   used by DHCP. Section 4 briefly discusses requirement levels.
   Section 5 points the reader to helpful background specifications
   covering related IPv6 protocols.  Section 6 discusses the design
   goals that influenced DHCP. Section 7 identifies some of the
   non-goals of this specification.  Section 8 gives a high level
   overview of DHCP, its message types, and identifies DHCP functional
   entities (client, relay, server).  Section 9 describes in detail the
   format of each DHCP message type.  Section 10 discusses DHCP server
   solicitation and subnet prefix discovery.  Section 11 discusses DHCP
   client-initiated configuration information exchange.  Section 12
   discusses DHCP server-initiated configuration information exchange.
   Section 13 describes how DHCP can be used to renumber networks.
   Section 14 presents helpful notes for DHCP client implementators.
   Section 15 presents helpful notes for DHCP server implementors.


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   Section 16 presents helpful notes for DHCP relay implementors.
   Section 18 discusses security considerations for DHCP.
2. Terminology


2.1. IPv6 Terminology


   IPv6 terminology relevant to this specification from the IPv6
   Protocol [6], IPv6 Addressing Architecture [8], and IPv6 Stateless
   Address Autoconfiguration [15] 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).  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 IP address having link-only scope, indicated by
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                 having the subnet 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     A bit string that consists of some number of initial
                 bits of an address.


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


   Terminology specific to DHCP can be found below.
      abort status
                 A status value returned to the application that has
                 invoked a DHCP client operation, indicating anything
                 other than success.


      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 indexed by <client's link-local address,
                 subnet prefix> containing the releasable resource data
                 which a DHCP server has assigned to a client.


                 Note that the transaction-ID from the Request message
                 that produced the assignment of the releasable resource
                 is also stored in the server data record including the
                 releasable resource identifier.


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


      Releasable resource
                 Any configuration resource allocated by a server for
                 a finite period of time.  As of this writing, the
                 only example of such a resource is the IP address.
                 Releasable resources are carried in extensions
                 allocated out of the 1--8192 range.


      solicit-ID
                 An unsigned integer generated by the client and
                 inserted into its DHCP Solicit messages, and echoed
                 back to the client by the server in its resultant DHCP
                 Advertise message(s).  The client uses the solicit-ID
                 to match received Advertise messages to Solicit
                 messages it has generated.


      transaction-ID
                 An unsigned integer to match responses with replies
                 initiated either by a client or server.  Servers
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                 allocate their transaction-IDs from the range of
                 0--1023, and clients allocate their transaction-IDs
                 from the range of 1024--65535.  Limiting clients and
                 servers to different ranges prevents transaction-ID
                 collisions (e.g.  client and server happen to use the
                 same transaction-ID for unrelated transactions (e.g.
                 client Request, server Reconfigure-init).
3. DHCP Constants


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


   The DHCP makes use of the following multicast addresses:


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


      All DHCP Servers address:  FF05::1:3
                 This site-local 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.
3.2. UDP ports


   The DHCP uses the following destination UDP [14] 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 agents to send messages to
                 clients.  Also used by servers to send messages to
                 relays.
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      547        Agent port.  Used by clients to send messages to
                 agents.  Also used by relays to send messages to
                 servers.
3.3. DHCP message types


   The DHCP defines the following message types.  More detail on these
   message types can be found in Section 9.  Message types 0 and 9--255
   are reserved and MUST be silently ignored.


      01 DHCP Solicit


         The DHCP Solicit (or Solicit) message is used by clients to
         locate servers and (optionally) learn about the subnet prefixes
         on the client's link for networks that are managed by DHCP.
         This message is multicast using the All-DHCP-Agents address.
         Relay(s) forward Solicits as necessary to off-link servers.


         Section 9.1 contains more details about the Solicit message.


      02 DHCP Advertise


         The DHCP Advertise (or Advertise) message is used by servers
         responding to Solicits.  This message is unicast to the
         client's link-local address (if the server and client are
         on the same link) or unicast to the relay through which the
         Solicit was sent for final delivery to the client.


         Section 9.2 contains more details about the Advertise message.


      03 DHCP Request


         The DHCP Request (or Request) message is used by clients to
         request configuration parameters from servers.  This message
         is unicast to the server if the client has an address with
         sufficient scope to be reachable by the server, otherwise it
         is unicast to the on-link relay through which the Advertise
         message was relayed.


         Section 9.3 contains more details about the Request message.


      04 DHCP Reply


         The DHCP Reply (or Reply) message is used by servers responding
         to Request and Release messages.  In the case of responding to
         a Request message, the Reply contains configuration parameters
         destined for the client.  This message is unicast to the client
         if the client has an address of sufficient scope that is
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         reachable by the server.  Otherwise, it is unicast to the relay
         through which the Request or Release message was sent for final
         delivery to the client.


         Section 9.4 contains more details about the Reply message.


      05 DHCP Release


         The DHCP Release (or Release) message is used by clients to
         return one or more instances of releasable resources (e.g.  IP
         addresses) to servers.  This message is unicast to the server
         if the client will have an address of sufficient scope after
         the Release operation to receive a Reply message.  Otherwise,
         the Release message is sent through the relay.  The server will
         acknowledge the receipt of the Release message by sending the
         client a Reply message.


         Section 9.5 contains more details about the Release message.


      06 DHCP Reconfigure


         The DHCP Reconfigure (or Reconfigure) message is sent by
         servers to client(s).  It contains new or updated configuration
         parameters for use by the client(s).  This message may be
         unicast or multicast to the client(s).


         Section 9.6 contains more details about the Reconfigure
         message.


      07 DHCP Reconfigure-reply


         The DHCP Reconfigure-reply (or Reconfigure-reply) message is
         unicast by client(s) to the server to acknowledge the receipt
         of a Reconfigure message.


         Section 9.7 contains more details about the Reconfigure-reply
         message.


      08 DHCP Reconfigure-init


         The DHCP Reconfigure-init (or Reconfigure-init) message is set
         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.


         Section 9.8 contains more details about the Reconfigure-init
         message.


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3.4. Error Values


   This section describes error values exchanged between DHCP
   implementations.
3.4.1. Generic Error Values


   The following symbolic names are used between client and server
   implementations to convey error conditions.  The following table
   contains the actual numeric values for each name.  Note that the
   numeric values do not start at 1, nor are they consecutive.  The
   errors are organized in logical groups.

   _______________________________________________________________
   |_Error_Name___|Error_ID_|Description__________________________|
   |_Success______|00_______|Success______________________________|
   |_UnspecFail___|16_______|Failure,_reason_unspecified__________|
   |_AuthFailed___|17_______|Authentication_failed_or_nonexistent_|
   |_PoorlyFormed_|18_______|Poorly_formed_message________________|
   |_Unavail______|19_______|Resources_unavailable________________|



3.4.2. Server-specific Error Values


   The following symbolic names are used by server implementations to
   convey error conditions to clients.  The following table contains the
   actual numeric values for each name.
   _______________________________________________________________
   |_Error_Name____|Error_ID_|Description_________________________|
   |_NoBinding_____|20_______|Client_record_(binding)_unavailable_|
   |_InvalidSource_|21_______|Invalid_Client_IP_address___________|
   |_NoServer______|23_______|Relay_cannot_find_Server_Address____|
   |_ICMPError_____|64_______|Server_unreachable_(ICMP_error)_____|



3.5. Configuration Variables


   This section presents a table of client and server configuration
   variables and the default or initial values for these variables.  The
   client-specific variables MAY be configured on the server and MAY be
   delivered to the client through the ``DHCP Retransmission Parameter
   Extension''carried in a Reply message.  This extension is documented
   in the ``extensions document'' [2].



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   ______________________________________________________________
   |_Parameter__________|Default_|Description____________________|
   |_MIN_SOL_DELAY______|1_______|MIN_(secs)_to_delay_1st_mesg___|
   |_MAX_SOL_DELAY______|5_______|MAX_(secs)_to_delay_1st_mesg___|
   |_ADV_MSG_TIMEOUT____|500_____|SOL_Retrans_timer_(msecs)______|
   |_ADV_MSG_MAX________|30______|MAX_timer_value_(secs)_________|
   |_SOL_MAX_ATTEMPTS___|-1______|MAX_attempts_(-1_=_infinite)___|
   |_REP_MSG_TIMEOUT____|250_____|REQ_Retrans_timer_(msecs)______|
   |_REQ_MSG_ATTEMPTS___|10______|MAX_Request_attempts___________|
   |_REL_MSG_ATTEMPTS___|5_______|MAX_Release_attempts___________|
   |_RECREP_MSG_TIMEOUT_|2000____|Retrans_timer_(msecs)__________|
   |_REC_MSG_ATTEMPTS___|10______|Reconfigure_attempts___________|
   |_REC_REP_MIN________|5_______|Minimum_pause_interval_(secs)__|
   |_REC_REP_MAX________|7200____|Maximum_pause_interval_(secs)__|
   |_REC_THRESHOLD______|100_____|%_of_required_clients__________|
   |_SRVR_PREF_WAIT_____|2_______|Advertise_Collect_timer_(secs)_|



4. 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 demostrate
   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.
5. Background


   Related work in IPv6 that would best serve an implementor to study
   is the IPv6 Specification [6], the IPv6 Addressing Architecture [8],
   IPv6 Stateless Address Autoconfiguration [15], IPv6 Neighbor
   Discovery Processing [12], and Dynamic Updates to DNS [17].  These
   specifications enable DHCP to build upon the IPv6 work to provide
   both robust stateful autoconfiguration and autoregistration of DNS
   Host Names.


   The IPv6 Specification provides the base architecture and design of
   IPv6.  A key point for DHCP implementors to understand is that IPv6
   requires that every link in the Internet have an MTU of 1280 octets
   or greater (in IPv4 the requirement is 68 octets).  This means that
   a UDP packet of 536 octets will always pass through an internetwork

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   (less 40 octets for the IPv6 header), as long as there are no IP
   options prior to the UDP header in the packet.  But, IPv6 does not
   support fragmentation at routers, so that fragmentation takes place
   end-to-end between hosts.  If a DHCP implementation needs to send a
   packet greater than 1500 octets it can either fragment the UDP packet
   into fragments of 1500 octets or less, or use Path MTU Discovery [10]
   to determine the size of the packet that will traverse a network
   path.


   DHCP clients use Path MTU discovery when they have an address of
   sufficient scope to reach the DHCP server.  If a DHCP client does not
   have such an address, that client MUST fragment its packets if the
   resultant message size is greater than the minimum 1280 octets.


   Path MTU Discovery for IPv6 is supported for both UDP and TCP and
   can cause end-to-end fragmentation when the PMTU changes for a
   destination.


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


   IPv6 Stateless Address Autoconfiguration [15] (Addrconf) specifies
   procedures by which a node may autoconfigure addresses based on
   router advertisements [12], 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.  The DHCP is one vehicle to perform
   stateful autoconfiguration.  Compatibility with addrconf is a design
   requirement of DHCP (see Section 6).


   IPv6 Neighbor Discovery [12] is the node discovery protocol in IPv6
   which replaces and enhances functions of ARP [13].  To understand
   IPv6 and Addrconf it is strongly recommended that implementors
   understand IPv6 Neighbor Discovery.


   Dynamic Updates to DNS [17] 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.  The security model to be used with DHCPv6 should conform
   as closely as possible to the authentication model outlined in
   RFC2402 [9].
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6. 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 autoconf [15].


    -  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.


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


    -  DHCP coexists with statically configured, non-participating nodes
       and with existing network protocol implementations.


    -  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 provide enough information
       to enable a DHCP server to keep track of a DHCP client's
       configuration state.


    -  DHCP will contain the appropriate time out and retransmission
       mechanisms to efficiently operate in environments with high
       latency and low bandwidth characteristics.
7. 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.
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    -  How a DHCP relay is configured or what sort of information it may
       log.
8. Overview


   This section provides a general overview of the interaction
   between the functional entities of DHCP. The overview is organized
   as a series of questions and answers.  Details of DHCP such
   as message formats and retransmissions are left to sections 9,
   10, 11, 12, 14, 15, and  16.
8.1. How does a node know to use DHCP?


   An unconfigured node determines that it is to use DHCP for
   configuration of an interface by detecting the presence (or absence)
   of routers on the link.  If router(s) are present, the node examines
   router advertisements to determine if DHCP should be used to
   configure the interface.  If there are no routers present, then
   the node MUST use DHCP to configure the interface.  Detail on
   this process can be found in neighbor discovery [12] and stateless
   autoconfiguration [15].
8.2. How does a client find out about DHCP agents?


   The client forms a Solicit message, and multicasts it to the
   FF02::1:2(All DHCP Agents) address.  Server(s) receiving the Solicit
   respond with Advertise message(s).  If requested in the client's
   Solicit message, the Advertise message(s) can include one or more
   subnet prefix extensions [2], informing the client of subnet prefixes
   for the networks(s) managed by the server(s) on the client's link.
   Now that the client knows the IP address(es) of agents(s) on the
   link, it can request configuration parameters from servers.
8.3. What if the client and server(s) are on different links?


   Use of DHCP in such environments requires one or more DHCP relays
   be set up on the client's link, because a client may only have a
   link-local address.  Relays pick up the Solicit and Request messages
   from the client and forward them to some set of servers within the
   DHCP domain.  A relay will include one of its own addresses (of
   sufficient scope) of the interface on the same link as the client.
   The relay also includes the subnet prefix length of that address
   in the client's messages.  Servers receiving the forwarded traffic
   use this information to aid in selecting configuration parameters
   appropriate to the client's link.  The servers also use the relay's
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   address as the destination to forward client-destined messages
   for final delivery by the relay.  Relays forward client messages
   to servers using some combination of the FF05::1:3(All Servers)
   site-local multicast address, some other (perhaps a combination)
   of site-local multicast addresses set up within the DHCP domain to
   include the servers in that domain, or a list of unicast addresses
   for servers.  The network administrator makes relay configuration
   decisions based upon the topological requirements (scope) of the
   DHCP domain they are managing.  Note that if the DHCP domain spans
   more than the site-local scope, then the relays MUST be configured
   with global addresses for the client's link so as to be reachable by
   servers outside the relays' site-local environment.
8.4. How does a client request configuration parameters from servers?


   To request configuration parameters, the client forms a Request
   message, and sends it to the server either directly (client has an
   address of sufficient scope) or indirectly (through the on-link
   relay).  The client MAY include a Extension Request Extension [2]
   along with other extensions to request specific information from the
   server.  Note that the client MAY form multiple Request messages
   and send each of them to different servers to request potentially
   different information (perhaps based upon what was advertised) in
   order to satisfy its needs.  As a client's needs may change over time
   (perhaps based upon an application's requirements), the client may
   form additional Request messages to request additional information as
   it is needed.


   The server(s) respond with Reply messages containing the requested
   configuration parameters, which can include status information
   regarding the information requested by the client.  The Reply MAY
   also include additional information, such as a reconfiguration event
   multicast group for the client to join to monitor reconfiguration
   events, as described in section 8.8.


   The receipt of a Reply from a server concludes the basic
   request/reply transaction of the protocol.
8.5. What are releasable resources, and when are they used?


   A releasable resource is configuration information leased to a client
   by a server for some finite period of time.  When negotiating for a
   releasable resource, the client and server agree upon a finite period
   of time the client may use the resource.  The client MAY request a
   renewal of the lease on the resource at any time.  The length of time
   of the lease (and whether it is renewable) are server-based policy
   tunables.  The client MUST stop using the resource when the lease on
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   the resource expires.  The server MUST NOT reallocate an assigned
   resource before either its lease expires or the client releases the
   resource.


   See the ``extensions document'' [2] for more information about
   releasable resources.
8.6. Can a client release its releasable resources before the lease
   expires?


   A client forms a Release message, including extensions carrying the
   resource(s) to be released.  The client sends the Release to the
   server which leased the resource(s) to the client initially.  If that
   server cannot be reached after a certain number of attempts (see
   section 3.5), the client can abandon the Release attempt.  In this
   case, the resource(s) will be reclaimed by the server(s) when the
   client's lease(s) expire.
8.7. What if the client determines its releasable resource is already
   being used by another client?


   If the client determines through a releasable resource-specific
   manner that the resource it was assigned by the server is already
   in use by another client, the client will form a Release message,
   including the extension carrying the in-use resource.  The
   extension's status field MUST be set to the extension-specific value
   reflecting the ``in use'' status of the resource.


   For example, if the releasable resource is an IP address, the client
   uses Duplicate Address Detection (DAD) to verify that the IP address
   is not in use.  If the client determines that the IP address is
   already in use, it forms a Release message including the IP address
   extension containing the appropriate status value and sends it to the
   server.  See the ``extensions document''for details on the IP address
   extension. [2].
8.8. How are clients notified of server configuration changes?


   There are two possibilities.  Either the clients discover the new
   information when they revisit the server(s) to request additional
   configuration information / renew the lease on a releasable resource,
   or through a server-initiated event known as a reconfigure event.


   The reconfiguration feature of DHCP offers network administrators
   the opportunity to update configuration information on DHCP clients
   whenever necessary.  If the information to be updated is not
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   client-specific, the server will form a Reconfigure message and add
   the new or changed configuration information to it.  The Reconfigure
   may be unicast or multicast (to a preassigned multicast address for
   this purpose) to one or more client(s) to which the new or updated
   information needs to be directed.  The client(s) will acknowledge the
   receipt of the Reconfigure message by forming a Reconfigure-reply
   message and unicasting it to the server.  If the configuration
   information change is different for each client (e.g.  a change in
   subnet prefix, perhaps, which would affect the IP address releasable
   resource(s)), the server will form a Reconfigure-init message and
   unicast / multicast as needed to the client(s).  A Reconfigure-init
   is a trigger which will cause the client(s) to initiate a standard
   Request/Reply exchange with the server in order to acquire the new or
   updated resources.
9. Message Formats


   All reserved fields in a message MUST be transmitted as zeroes and
   ignored by the receiver of the message.
9.1. DHCP Solicit Message Format


   A client multicasts a DHCP Solicit message to the FF02::1:2(All DHCP
   agents) address over the interface to be configured to locate one or
   more servers which are configured to provide configuration parameters
   to nodes on the client's link.


   Unless otherwise noted, the value of all fields are set by the
   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  msg-type = 1 |C|P|  reserved |  prefix-len |   solicit-ID    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         relay-address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      C          If set, the client requests that all servers receiving
                 the message deallocate the releasable resources (e.g.
                 IP addresses) associated with the client's binding.


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      P          If set, the client requests that all servers receiving
                 the message SHOULD return a list of subnet prefix
                 extensions identifying the networks on the client's
                 link that the server(s) are configured to manage.


      reserved   0


      prefix-len
                 An unsigned 7 bit number (0-127) non-zero prefix-len is
                 the number of leftmost bits of the agent's IPv6 address
                 which make up the subnet prefix.  The prefix-len field
                 is set by the relay if the relay receives the Solicit
                 message and forwards it to one or more servers.


      solicit-ID
                 An unsigned 9 bit number (0-511) generated by the
                 client used to identify this Solicit message.


      client's link-local address
                 The IP link-local address of the client interface
                 through which the client will issue the Solicit
                 message.


      relay-address
                 Set by the client to be zero.  If received by a relay,
                 set by the relay to the site-local IP address of the
                 interface on which the relay received the client's
                 Solicit message.  Note that if the DHCP domain crosses
                 site boundaries, the relay MUST place a globally-scoped
                 address in this field.


   A client MUST send the Solicit message to the All-DHCP-Agents
   multicast group (see section 3.1), setting the relay-address to zero.
9.2. DHCP Advertise Message Format


   A server sends an Advertise message in response to a client's
   Solicit message.  The Advertise message notifies the client of the
   server's IP address.  If the server is so configured by the network
   administrator and the client requests it through the ``P'' bit in
   its Solicit message, the server SHOULD add a list of subnet prefix
   extensions to the Advertise message to notify the client of the
   networks it manages on the client's link.


   When the client and server are on different links, the server sends
   the Advertise message back through the relay whence the corresponding
   Solicit came.  The solicit-ID is copied from the client's Solicit


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   Message.  The value of all fields in the Advertise message are filled
   in by the server and not changed in any way by any intervening relay.


      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 = 2 |  reserved   |   solicit-ID    |  preference   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         relay-address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server-address                        |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              extensions (variable number and length) ...      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      reserved     0


      solicit-ID   An unsigned 9 bit number (0-511) used to identify
                   this Advertise message.  Copied from the client's
                   Solicit message.


      preference   An octet (unsigned) indicating a server's willingness
                   to provide service to the client.


      client's link-local address
                   The IP link-local address of the client interface
                   from which the client issued the Solicit message.


      relay-address
                   The IP address of the relay interface on the same
                   link as the client.  Copied from the client's
                   Solicit.  If the server is on the same link as the
                   client, then this field MUST be zero.


      server-address
                   The site-local IP address of the server.  If the DHCP
                   domain crosses site boundaries, then this address
                   MUST be globally-scoped.


      extensions   See the ``extensions document'' for details [2].


   See Sections 14.4 and 15.3 for information about how clients and
   servers handle the preference field.
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9.3. DHCP Request Message Format


   A client sends a Request message to request configuration parameters
   from a server.  It MAY append appropriate extensions [2].


   When a client reboots, it often does not have a valid IP address of
   sufficient scope for the server to communicate with the client.  In
   such cases, the client MUST NOT unicast the message to the server
   because the server could not return a response to the client.  The
   client MUST send the message to the server indirectly, by using the
   on-link relay.  The client MUST fill in the relay address field with
   the on-link relay's IP address.


   If the Request message is being formed in response to a
   Reconfigure-init message from the server, then the transaction ID
   used must be copied from the Reconfigure-init.


   All fields in the DHCP Request message are entered by the 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  msg-type = 3 |C|R|  reserved |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         relay-address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server-address                        |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         extensions (variable number and length)   ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      C          If set, the client requests the server to remove
                 all releasable resources associated with the client
                 binding, except those releasable resources provided as
                 extensions.


      R          If set, the client has rebooted and requests that the
                 server clear any transaction-ID cache entries for the
                 client.


      reserved   0
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      transaction-ID
                 An unsigned integer identifier used to identify this
                 request.


      client's link-local address
                 The link-local address of the client interface from
                 which the client will issue the Request message.


      relay-address
                 The IP address of a relay's interface, copied from an
                 Advertise message.  If the server is on the same link
                 as the client, then this field MUST BE zero.


      server-address
                 The IP address of the server to which the the client's
                 Request message is directed, copied from an Advertise
                 message.


      extensions
                 See the ``extensions document'' [2].


   A DHCP client selects the transaction-ID from the range of
   1024--65535 used to identify its Request.  In contrast, a
   transaction-ID from the range of 0--1023is selected by a DHCP server
   to identify a Reconfigure-init.  In the latter case, the transaction
   ID from the Reconfigure-init is copied by the client into its Request
   message.


   When the client sets the `C' bit and adds extensions documenting
   the releasable resources the client wishes to keep, the server is
   expected to deallocate all other releasable resources not listed.
   The server SHOULD examine the included extensions to check whether
   the client is still authorized to use them.
9.4. DHCP Reply Message Format


   A server sends a Reply message in response to a client's Request
   message or Release message.


   If a Request message is received which contains a non-zero relay
   address field, then the client could not unicast the Request message
   to the server and thus had to use a on-link relay.  In that case, the
   server unicasts the Reply message to the relay address found in the
   Request message.


   If a Release message is received which contains a non-zero relay
   address field, then the client will not have an IP address of
   sufficient scope after the Release to receive the Reply message.  In
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   this case, the server unicasts the Reply message to the relay address
   found in the Release message.


   All the fields in the DHCP Reply message are set by the DHCP 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  msg-type = 4 |R|  status     |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                           (16 octets)                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   relay-address (if present)                  |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         extensions (variable number and length)   ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      R          If set, the ``relay-address'' field is present.


      status
                 This 7-bit field contains one of the values in the
                 errors table in section 3.4.


      transaction-ID
                 Copied from the client's Request or Release.


      client's link-local address
                 Copied from the client's Request or Release message.


      relay-address
                 The IP address of a relay's interface, copied from the
                 Request or Release message.  If the server is on the
                 same link as the client, then the ``R'' bit is not set
                 and this field is not present.


      extensions
                 See the ``extensions document'' [2].
9.5. DHCP Release Message Format


   A client sends a Release message to a server when it wishes to return
   one or more releasable resources to the server which allocated
   them.  This can occur either because the client no longer needs the
   resource(s) or the client has determined through a resource-specific
   manner that the resource(s) are already in use by different
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   client(s).  The client communicates the reason for the premature
   release of the resource in the status field of the resource's
   extension.  See ``extensions document'' [2] for more details.


   When a client sends a Release message, it needs to have a valid IP
   address with sufficient scope to allow access by the target server.
   If such an address is not available, a relay is used.  Only those
   releasable resources identified by extensions are released.  If no
   extensions are included in the Release message, then all releasable
   resources associated with the client's binding are to be released.


   The values of all fields of the Release message are set by the
   client.  The DHCP server acknowledges the Release message by sending
   a Reply message.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  msg-type = 5 |R|  reserved   |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server-address                        |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           X-address                           |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         extensions (variable number and length)   ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      R          If set, the ``X-address'' field contains the address of
                 relay.  If not set, the ``X-address'' field contains a
                 non-local scope client address.


      reserved   0


      transaction-ID
                 An unsigned integer identifier used to identify this
                 Release message.


      client's link-local address
                 The client's link-local address for the interface
                 from which the client issued the Release message (and
                 to which the releasable resources are bound at the
                 server).


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      server-address
                 The IP address of the server which allocated the
                 resource.


      X-address
                 If the ``R'' bit is set, the ``X-address'' field
                 contains the IP address of the relay interface on the
                 same link as the client.  If the ``R'' bit is not set,
                 this field contains a non-link-local IP address of the
                 client interface from which the the client issued the
                 Release message.


      extensions See the ``extensions document'' [2].


   A client selects the transaction-ID from the range of
   1024--65535 used to identify the Release message.


   A client MUST NOT specify an IP address in the client-address field
   that it is releasing in the extensions field.
9.6. DHCP Reconfigure Message Format


   A server sends a Reconfigure message when it wishes to inform one or
   more clients of new or updated values for configuration parameters.
   The new configuration parameters are carried in the extensions
   portion of the Reconfigure message.  Note that a Reconfigure message
   MUST NOT carry releasable resource extensions.


   Reconfigure messages can ONLY be sent to clients which have
   established an IP address of sufficient scope as to be directly
   reachable by the server.


   Clients acknowledge Reconfigure messages with Reconfigure-reply
   messages.


      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 = 6 |   reserved    |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        server-address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         extensions (variable number and length)   ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      reserved   0
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      transaction-ID
                 An unsigned integer identifier in the range of
                 0--1023 chosen by the server to identify this
                 Reconfigure message.


      server-address
                 The IP address of the DHCP server issuing the
                 Reconfigure message.  MUST be of sufficient scope to be
                 reachable by all clients.


      extensions
                 See the ``extensions document'' [2].
9.7. DHCP Reconfigure-reply Message Format


   A client sends a Reconfigure-reply message to acknowledge receipt of
   a Reconfigure message from a server.


   A Reconfigure-reply message can only be sent if the client has an IP
   address of sufficient scope to contact the server.  No interaction
   with a relay is possible.


   All fields in the DHCP Reconfigure-reply message are entered by the
   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  msg-type = 7 |r|  status     |      transaction-ID           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server-address                        |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      r          reserved (0)


      status
                 This 7-bit field contains one of the values from the
                 errors table in section 3.4.


      transaction-ID
                 An unsigned integer identifier copied from the server's
                 Reconfigure message.


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      client's link-local address
                 The client's link-local address for the interface from
                 which the client issued the Reconfigure-reply message.


      server-address
                 Copied from the Reconfigure message.
9.8. DHCP Reconfigure-init Message Format


   A server sends a Reconfigure-init message when it wishes to notify
   one or more clients of new or updated values for configuration
   parameters available on the server.


   Reconfigure-init messages can ONLY be sent to clients which have
   established an IP address of sufficient scope as to be directly
   reachable by the server.


   A ``Reconfigure-init'' serves as a trigger which will cause the
   clients to initiate a Request/Reply exchange with the server in order
   to receive the new information.

      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 = 8 |   reserved    |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        server-address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         extensions (variable number and length)   ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      reserved   0


      transaction-ID
                 An unsigned integer identifier in the range of
                 0--1023 chosen by the server to identify this
                 Reconfigure-init message.


      server-address
                 The IP address of the DHCP server issuing the
                 Reconfigure-init message.  MUST be of sufficient scope
                 to be reachable by all clients.


      extensions SHOULD only include an ERE and/or authentication
                 extensions.  No configuration information SHOULD be


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                 included.  See the ``extensions document'' [2] for more
                 information about extensions.
10. DHCP Server Solicitation and Subnet Prefix Discovery


   This section describes how a client locates agents (relays and
   servers) and how it can learn about the networks on its link that are
   managed by these servers.  The behavior of client, server, and relay
   implementations is discussed, along with the messages they use.
10.1. Solicit Message Validation


   Clients MUST silently discard any received Solicit messages.


   Agents MUST discard any received Solicit messages if the ``client's
   link-local address'' field does not contain a valid link-local
   address.


   Servers MUST discard each received Solicit message which meet the
   following criteria:


     o The ``relay-address'' field does not contain an address of
       sufficient scope that is reachable by the server.


     o The ``relay-address'' field is non-zero, but prefix-len is zero.


   An error message MAY be logged by the agent.  The logging of
   such messages SHOULD be controlled by an agent implementation
   configuration flag.
10.2. Advertise Message Validation


   Servers MUST silently discard any received Advertise messages.


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


     o The ``Solicit-ID'' field value does not match the value the
       client used in its Solicit message.


     o The ``client's link-local address'' field value does not match
       the link-local address of the interface upon which the client
       sent the Solicit message.


   Relays MUST discard any Advertise messages that meet any of the
   following criteria:
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     o The ``relay-address'' field does not contain the relay's address
       on the same link as the client.


     o The ``client's link-local address'' field does not contain a
       valid link-local address.
10.3. Client Behavior


   Clients use the Solicit message primarily to discover DHCP servers
   configured to serve networks on the link containing the client.
   Optionally, the client MAY set the ``P'' bit which has the effect
   of requesting that the server return subnet prefix extensions
   identifying the networks on the client's link the server is
   configured to manage.
10.3.1. Creation and sending of the Solicit message


   When creating a Solicit message, the client SHOULD start out with
   a buffer initialized with zeroed octets.  The client sets the
   ``msg-type'' field to 1, and places the link-local address of the
   interface it wishes to configure in the link-local address field.


   If the client is prepared to process multiple Advertise messages
   in response to its Solicit message, the client will set the
   Solicit-ID field to 1.  Every time the client initiates a new server
   solicitation attempt (not a retransmission), the client increments
   the Solicit-ID by one.  If the 9-bit field rolls over to 0, then the
   client sets the Solicit-ID to 1.  A client which will only accept
   the first Advertise message it receives leaves the Solicit-ID field
   initialized to zero.


   The ``C'' bit of the Solicit message is set by the client when the
   client has no cached knowledge of previous DHCP configuration for the
   interface.  Setting this bit requests that the server release any
   information assigned to the client for the networks on the client's
   link.


   If the client desires to learn of the networks managed by DHCP on
   the link its interface is attached to, it sets the ``P'' bit in the
   Solicit message.


   The client transmits the Solicit message to the FF02::1:2  (All DHCP
   Agents) multicast address, destination port 547.  The source port
   selection can be arbitrary, although it SHOULD be possible using a
   client configuration facility to set a specific source port value.
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10.3.2. Time out and retransmission of Solicit Messages


   The client's first Solicit message on the interface MUST be delayed
   by a random amount of time between the interval of 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 waits ADV_MSG_TIMEOUT, collecting Advertise messages.
   If no Advertise messages are received, the client retransmits
   the Solicit, and doubles the ADV_MSG_TIMEOUT value.  This process
   continues until either one or more Advertise messages are received or
   ADV_MSG_TIMEOUT reaches the ADV_MSG_MAX value.  Thereafter, Solicits
   are retransmitted every ADV_MSG_MAX until SOL_MAX_ATTEMPTS have been
   made, at which time the client stops trying to DHCP configure the
   interface.  An event external to DHCP is required to restart the DHCP
   configuration process.


   Default and initial values for MIN_SOL_DELAY, MAX_SOL_DELAY,
   ADV_MSG_TIMEOUT, AND ADV_MSG_MAX are documented in section 3.5.
10.3.3. Receipt of Advertise messages


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


    -  Those Advertise messages with the highest server preference
       value (see section 14.4) 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, one server may be advertising the
       availability of IP addresses on networks which have an address
       scope of interest to the client.


   Once a client has selected Advertise message(s), the client will
   typically store information about each server, such as relay address
   and prefix length, server preference value, networks advertised,
   when the advertisement was received, and so on.  Depending on the
   requirements of the client's invoking user, the client MAY initiate a
   configuration exchange with the server(s) immediately, or MAY defer
   this exchange until later.



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10.4. Relay Behavior


   For this discussion, the Relay is assumed to have been configured
   with some list of server destination addresses, which may be unicast,
   the FF05::1:3 (All DHCP Servers) multicast address, or some other
   multicast address selected by the network administrator.
10.4.1. Relaying of Solicit messages


   When a Relay receives a valid Solicit message, it places the IP
   address of the interface upon which it received the Solicit message
   in the ``relay-address'' field of the Solicit.  The Relay also places
   the number of bits of that make up the subnet prefix for this address
   in the ``prefix-len'' field of the Solicit.


   The Relay then forwards this Solicit to the list of server
   destination addresses that it has been configured with.
10.4.2. Relaying of Advertise messages


   When a Relay receives a valid Advertise message, it unicasts the
   message to the link-local address found in the ``client's link-local
   address'' field by way of the appropriate network interface.
10.5. Server Behavior


   For this discussion, the Server is assumed to have been configured in
   an implementation specific manner.  This configuration is assumed to
   contain all network topology information for the DHCP domain, as well
   as any necessary authentication information.
10.5.1. Receipt of Solicit messages


   Upon the receipt of a valid Solicit message, the server first
   identifies the client's location within the DHCP domain.  If the
   ``relay-address'' and / or ``prefix-len'' fields of the Solicit are
   zeroed, then the client is attached to the same link as the server.
   If these fields are non-zero, then the client exists on the same link
   as the network identified by these two fields.


   If administrative policy permits the server to respond to a client on
   that link, the server will generate and send an Advertise message to
   the client.
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10.5.2. Creation and sending of Advertise messages


   When creating an Advertise message, the server SHOULD start out
   with a buffer initialized with zeroed octets.  The server sets the
   ``msg-type'' field to 2 and copies the values of the following fields
   from the client's Solicit to the Advertise message:


     o solicit-ID


     o client's link-local address


     o relay-address


   The server places one of its IP addresses (determined through
   administrator setting) in the ``server-address'' field of the
   Advertise message.  The server initializes the ``preference''
   field from its configuration information.  See section 15.3 for a
   description of server preference.


   If the client requests subnet prefix extensions (by setting the ``P''
   bit in its Solicit) and the server implements and is configured to
   provide prefix extensions, the server will generate and insert a
   subnet prefix extension for each network on the client's link it is
   configured to manage.


   If the ``relay-address'' field of the Advertise message is zero, then
   the server unicasts the Advertise message directly to the client
   using the ``client's link-local address'' field value as destination
   address.  If the ``relay-address'' field is non-zero, then the server
   unicasts the Advertise message directly to the relay using the
   ``relay-address'' field value as the destination address.
11. DHCP Client-Initiated Configuration Exchange


   A client initiates a configuration exchange with one or more servers
   it has found through DHCP server solicitation whenever requested to
   do so by the application layer in order to acquire configuration
   information of interest.
11.1. Request Message Validation


   Clients MUST silently discard any received Request messages.


   Agents MUST discard any Request messages in which the ``client's
   link-local address'' field does not contain a valid link-local
   address.


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   Relays MUST discard any received Request messages in which the
   ``relay-address'' field value does not match any of the relay's
   addresses.


   Servers MUST discard any received Request message which meets any of
   the following criteria:


     o The ``server-address'' field value does not match any of the
       server's addresses.


     o If the ``relay-address'' field is set, and that field's value
       does not contain an address of sufficient scope as to be
       reachable by the server.


     o The ``extensions'' field contains an authentication extension,
       and the server cannot successfully authenticate the client.
11.2. Reply Message Validation


   Servers MUST silently discard any received Reply messages.


   Clients MUST discard any Reply message that meets any of the
   following criteria:


     o The ``transaction-ID'' field value does not match the value the
       client used in its Request or Release message.


     o The ``client's link-local address'' field value does not match
       the link-local address of the interface upon which the client
       sent in its Request or Release message.


     o The Reply message contains an authentication extension, and the
       client's attempt to authenticate the message fails.


   Relays MUST discard any Reply message that meets any of the following
   criteria:


     o The ``R'' bit isn't set.


     o The ``relay-address'' field value does not contain the relay's
       address on the same link as the client.


     o The ``client's link-local address'' field value does not contain
       a valid link-local address.



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11.3. Release Message Validation


   Clients MUST silently discard any received Release messages.


   Agents MUST discard any Release message that meets any of the
   following criteria:


     o The ``transaction-ID'' field contains a value not in the
       1024--65535 range.


     o The ``client's link-local address'' field does not contain a
       valid link-local address.


   Relays MUST discard any received Release message that meets any of
   the following criteria:


     o The ``R'' bit is not set.


     o The ``X-address'' field value does not match any of the relay's
       addresses.


   Servers MUST discard any received Release message which meets any of
   the following criteria:


     o The ``X-address'' field does not contain an address of sufficient
       scope as to be reachable by the server.


     o The ``extensions'' field contains an authentication extension,
       and the server cannot successfully authenticate the client.
11.4. Client Behavior


   A client will generate one or more Request messages when prompted by
   the application layer in order to acquire configuration information.
   A client may initiate such an exchange automatically in order to
   acquire the necessary network parameters to communicate with nodes
   off-link.  The client uses the server and relay address information
   from previous Advertise message(s) for use in delivering Request
   message(s).  Note that a client may request configuration information
   from one or more servers at any time.


   A client uses the Release message in the management of releasable
   resources when:


     o The client has determined through a resource-specific manner
       that the resource assigned by the server is already in use by a
       different client.


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     o The client has been instructed to release the resource prior to
       the lease expiration time since it is no longer needed.
11.4.1. Creation and sending of Request messages


   When creating a Request message, the client SHOULD start out with
   a buffer initialized with zeroed octets.  The client sets the
   ``msg-type'' field to 3, and places the link-local address of the
   interface it wishes to associate with the configuration information
   with in the ``client's link-local address'' field.


   Unless the Request message is created in response to a
   Reconfigure-init message, the client generates a transaction
   ID in the range of 1024--65535 and inserts this value in the
   ``transaction-ID'' field.


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


   If the client is not on the same link as the destination
   server, the client places the appropriate relay's address in the
   ``relay-address'' field.


   If the client is acquiring configuration information on the interface
   for the first time, the client SHOULD set the ``C'' bit in the
   header.  How the client determines if this is the first configuration
   attempt on the interface is implementation-specific.  A client may
   implement a cache of configuration information on a per-interface
   basis; if that cache does not exist, that client would set the
   ``C'' bit.  Clients which do not implement caching of per-interface
   configuration information MUST always set the ``C'', and include
   any extensions carrying releasable resources received from earlier
   configuration exchanges in the extensions field of the Request.


   If the client has determined through an implementation-specific
   manner that the client implementation itself has restarted, it MUST
   set the ``R'' bit in the header.  After the first successful exchange
   with the server, the client MUST NOT set the ``R'' bit in subsequent
   Request messages.


   Client considerations for extensions are now considered (see the
   ``extensions document'', [2] for more details).


   If the client already has an IP address of sufficient scope to
   directly reach the server, then the client SHOULD unicast the Request
   to the server.  Otherwise, if the server is off-link, the client
   unicasts the Request message to the appropriate relay.


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11.4.2. Time out and retransmission of Request Messages


   The client waits REP_MSG_TIMEOUT milliseconds, collecting
   Reply messages.  If no Reply messages are received, the client
   retransmits the Request with the same transaction-ID, and doubles
   the REP_MSG_TIMEOUT value, and waits again.  The client continues
   this process until a Reply is received or REQUEST_MSG_ATTEMPTS
   unsuccessful attempts have been made, at which time the client MUST
   abort the configuration attempt.  The client SHOULD report the abort
   status to the application layer.


   Default and initial values for REP_MSG_TIMEOUT and REQ_MSG_ATTEMPTS
   are documented in section 3.5.
11.4.3. Receipt of Reply message in response to a Request


   Upon the receipt of a valid Reply message, the client extracts the
   configuration information contained in the Reply.  If the ``status''
   field contains a non-zero value, the client reports the error status
   to the application layer.


   If the extensions field contains one or more ``Reconfigure Multicast
   Address'' extensions (see ``extensions document'', ``Reconfigure
   Multicast Address Extension'' section [2]), the client MUST join
   these multicast groups, and MUST monitor the UDP 546 port for
   Reconfigure or Reconfigure-init messages on the networks configured
   by DHCP.


   If the configuration information returned in the Reply contains
   releasable resources, then the client MUST take over lease management
   of the resource.  A client MUST NOT request releasable resources
   unless it is prepared to appropriately manage the resource lease.
11.4.4. Creation and sending of Release messages


   When creating a Release message, the client SHOULD start out with
   a buffer initialized with zeroed octets.  The client sets the
   ``msg-type'' field to 5, and places the link-local address of the
   interface the configuration information it wishes to release is
   associated with in the ``client's link-local address'' field.


   The client generates a transaction ID in the range of
   1024--65535  and inserts this value in the ``transaction-ID''
   field.


   The client includes extensions containing the releasable resources it
   is releasing in the ``extensions'' field.  The appropriate ``status''
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   field in the extensions MUST be set to indicate the reason for the
   release.


   The client places the IP address of the server who allocated the
   resource(s) in the ``server-address'' field.


   If the client will have an appropriately scoped IP address after the
   release transaction is completed, the client clears the ``R'' bit
   and places this address in the ``X-address'' field.  If the client
   will not have an appropriately scoped IP address after the release
   transaction is completed, the client sets the ``R'' bit and places
   the address of the appropriate relay in the ``X-address'' field.


   If the client is configured to use authentication, the client
   generates the appropriate authentication extension, and adds this
   extension to the ``extensions'' field.  Note that the authentication
   extension MUST be the last extension in the ``extensions''
   field.  See the ``extension document'' for more details about the
   authentication extension [2].


   If the ``R'' bit is set, then the client MUST unicast the Release
   to the relay indicated in the ``X-address'' field.  Otherwise, the
   client unicasts the Release message directly to the server indicated
   in the ``server-address'' field.
11.4.5. Time out and retransmission of Release Messages


   The client waits REP_MSG_TIMEOUT milliseconds, collecting Reply
   messages.  If no Reply messages are received, the client retransmits
   the Release, and doubles the REP_MSG_TIMEOUT value, and waits again.
   The client continues this process until a Reply is received or
   REL_MSG_ATTEMPTS unsuccessful attempts have been made, at which
   time the client SHOULD abort the release attempt.  The client
   SHOULD return the abort status to the application, if an application
   initiated the release.


   Default and initial values for REP_MSG_TIMEOUT and REL_MSG_ATTEMPTS
   are documented in section 3.5.


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



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11.4.6. Receipt of Reply message in response to a Release


   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.
11.5. Relay Behavior


11.5.1. Relaying of Request or Release messages


   When a Relay receives a valid Request or Release message, it forwards
   it to the IP address found in the ``server-address'' field of the
   message.
11.6. Server Behavior


   For this discussion, the Server is assumed to have been configured
   in an implementation specific manner with configuration of interest
   to clients.  Such configuration information MAY contain releasable
   resources such as IP addresses.
11.6.1. Receipt of Request messages


   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 extensions field.


   If the client has set the ``C'' bit, the server MUST release all
   releasable resources currently associated with the client's binding
   that do not appear in the ``extensions'' field.


   If the client has set the ``R'' bit, the server MUST delete any
   transaction-ID cache entries it is maintaining for this client, if
   the server implements such a cache.


   Server considerations for extensions are now evaluated (see the
   ``extensions document'', [2] for more details).


   If the configuration information to be returned to the client
   includes releasable resources, the server checks if a binding
   already exists for the client.  If so, the server examines the
   data records within the binding to determine if the client's
   Request is a retransmission of an earlier Request or a new Request.
   Releasable resource identifiers are stored within the binding with
   the transaction-ID used by the client to request the resource's
   assignment.  If the transaction-ID's match, this is a retransmission
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   and the server simply return the contents of the client's binding
   which satisfy its request.  If the transaction-ID's do not match,
   the server records the additional resources it is assigning in the
   existing binding with the new Request's transaction-ID.


   If the client does not have an existing binding, the server creates a
   binding for the client and records the resources it is assigning in
   this binding along with the transaction-ID from the client's Request.


   The server then constructs a Reply message and sends it to the
   client.
11.6.2. Receipt of Release messages


   Upon the receipt of a valid Release message, the server performs a
   lookup to find the client's binding.  If the binding is found, the
   server examines the binding to see if the resource(s) identified by
   the client in the Release message's extensions field are in fact
   assigned to the client.  If they are, the server deletes these
   resources from the client's binding, making them available to other
   clients.


   The server then generates a Reply message.  If a binding was
   found and the resources presented to the server were deleted from
   the client's binding, the server sets the ``status'' field to
   ``Success''.  If no binding is found, the server sets the ``status''
   field to ``NoBinding''(section 3.4).
11.6.3. Creation and sending of Reply messages


   When creating a Reply message, the server SHOULD start out with
   a buffer initialized with zeroed octets.  The server sets the
   ``msg-type'' field to 4 and copies the values of the following fields
   from the client's Request or Release to the Reply message:


     o transaction-ID


     o client's link-local address


     o If the client's message is a Request with a non-zero
       ``relay-address'' field value, the server sets the ``R'' bit in
       the Reply and copies the ``relay-address'' field value from the
       Request to the Reply.  If the client's message is a Release with
       the ``R'' bit set, the server sets the ``R'' bit in the Reply and
       sets the ``relay-agent'' field to the contents of the Release's
       X-address field.


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   The server sets the ``status'' field appropriately (see the table
   in section 3.4) based upon the results of processing the client's
   request.


   If configured to do so, a server will include ``Reconfigure Multicast
   Address'' extensions (see ``extensions document'', ``Reconfigure
   Multicast Address Extension'' [2]), in Reply messages sent in
   response to a Request, informing the client of one or more multicast
   groups it should join to facilitate the receipt of Reconfigure or
   Reconfigure-init messages.


   If the DHCP domain is using authentication, the server will generate
   an authentication extension with the appropriate settings and add
   that extension as the last extension in the ``extensions'' field of
   the Reply message.


   If the ``relay-address'' field of the Reply message is zero, then the
   server unicasts the Reply directly to the client using the ``client's
   link-local address'' field value as destination address.  If the
   ``relay-address'' field is non-zero, then the server unicasts the
   Reply directly to the relay using the ``relay-address'' field value
   as the destination address.


   If the server implements a transaction-ID cache, the server would add
   an entry for the client to this cache.
12. DHCP Server-Initiated Configuration Exchange


   A server initiates a configuration exchange on behalf of the
   administrator of the DHCP domain.  An administrator may initiate such
   an exchange when new networks are added to the domain or existing
   networks 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).
12.1. Reconfigure Message Validation


   Agents MUST silently discard any received Reconfigure messages.


   Clients MUST discard any Reconfigure message that meets any of the
   following criteria:


     o The ``transaction-ID'' field value is not within
       the 0--1023 range.


     o The Reconfigure message contains an authentication extension, and
       the client's attempt to authenticate the message fails.
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12.2. Reconfigure-reply Message Validation


   Clients and Relays MUST silently discard any received
   Reconfigure-reply messages.


   Servers MUST discard any Reconfigure-reply message that meets any of
   the following criteria:


     o The ``transaction-ID'' field value is not that same value the
       server used in its Reconfigure message.


     o The ``server-address'' field value does not match the value the
       server placed in its Reconfigure message.
12.3. Reconfigure-init Message Validation


   Agents MUST silently discard any received Reconfigure-init messages.


   Clients MUST discard any Reconfigure-init messages that meets any of
   the following criteria:


     o The ``transaction-ID'' field value is not within
       the 0--1023 range.


     o The Reconfigure-init message contains an authentication
       extension, and the client's attempt to authenticate the message
       fails.
12.4. Server Behavior


   For this discussion, the server is assumed to have a
   implementation-specific interface by which an administrator
   may initiate a reconfiguration event with some set of clients.


   There are two methods of initiating a reconfiguration event.  Each
   has its advantages:


      Reconfigure with payload
                   This method uses the Reconfigure message.  Items
                   to be changed are included as extensions in the
                   ``extensions'' field.  This method MUST NOT be used
                   to reconfigure releasable resources.  Examples of
                   information which can be reconfigured using this
                   method are DNS domain and servers, NTP servers, other
                   name service parameters.  The server generates and
                   sends the Reconfigure message; clients respond with
                   Reconfigure-reply messages.
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      Reconfigure Trigger
                   This method uses the Reconfigure-init message.  When
                   a client receives a Reconfigure-init message, it
                   initiates a Request/Reply exchange with the server.
                   Any kind of resource can be reconfigured using this
                   method, including releasable resources.  An example
                   of an releasable resource is an IP address.


   A server can send Reconfigure and Reconfigure-init messages only to
   those clients who have an address of sufficient scope to be reachable
   by the server.  Thus, those clients who have not requested an IP
   address and are off-link cannot be reconfigured by the server.


   Before initiating a reconfigure process, the server SHOULD be
   configured with a REC_THRESHOLD threshold value which represents
   the percentage of clients successfully reconfigured before the
   reconfigure process is considered a success.  See section 3.5 for the
   default setting of REC_THRESHOLD. Note that the server MUST be able
   to determine the set of clients that should receive the reconfigure,
   in order to determine when the reconfigure process is complete.
12.4.1. Creation and sending of Reconfigure messages


   When creating a Reconfigure message, the server SHOULD start out
   with a buffer initialized with zeroed octets.  The server sets the
   ``msg-type'' field to 6.  The server generates a transaction-ID
   from the 0--1023 range and inserts it in the ``transaction-ID''
   field.  The server places its address (of appropriate scope) in the
   ``server-address'' field.


   The server then generates extensions for the non-releasable resources
   to be changed and places them in the ``extensions'' field.


   If the DHCP domain is using authentication, the server will generate
   an authentication extension with the appropriate settings and add
   that extension as the last extension in the ``extensions'' field of
   the Reconfigure message.


   The server multicasts the Reconfigure message to one or more
   Reconfigure Multicast Addresses previously sent as extensions to the
   clients.  Note that a server MAY unicast Reconfigure message(s) to
   specific clients by walking its list of bindings to determine the
   unicast address(es) of the clients.  Whether or not the Reconfigure
   is multicast or unicast is an implementation detail.


   A server waits for Reconfigure-reply messages from clients confirming
   that they have received the Reconfigure.


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12.4.2. Time out and retransmission of Reconfigure messages


   The server waits RECREP_MSG_TIMEOUT milliseconds, collecting
   Reconfigure-reply messages.  If all the expected Reconfigure-reply
   messages are received, then the reconfigure process is successful.
   If some or all of the expected Reconfigure-reply messages are not
   received, then the server retransmits the Reconfigure, and doubles
   the RECREP_MSG_TIMEOUT value, and waits again.  The server continues
   this process until all Reconfigure-reply messages are received or
   REC_MSG_ATTEMPTS unsuccessful attempts have been made, at which time
   the server SHOULD abort the reconfigure process.  The server SHOULD
   log the result of the reconfigure process.


   Default and initial values for RECREP_MSG_TIMEOUT and
   REC_MSG_ATTEMPTS are documented in section 3.5.
12.4.3. Receipt of Reconfigure-reply messages


   Upon receipt of a valid Reconfigure-reply message, the server
   removes that client from the list of clients it is expecting a
   Reconfigure-reply message from.
12.4.4. Creation and sending of Reconfigure-init messages


   When creating a Reconfigure-init message, the server SHOULD start
   out with a buffer initialized with zeroed octets.  The server sets
   the ``msg-type'' field to 8.  The server generates a transaction-ID
   from the 0--1023 range and inserts it in the ``transaction-ID''
   field.  The server places its address (of appropriate scope) in the
   ``server-address'' field.


   The server MAY generate an ERE extension to inform the client of what
   information has been changed or new information that has been added.


   If the DHCP domain is using authentication, the server will generate
   an authentication extension with the appropriate settings and add
   that extension as the last extension in the ``extensions'' field of
   the Reconfigure-init message.


   Typically, the server will not provide more than an ERE and / or
   Authentication extension, since it will provide the new configuration
   information as part of the Request/Reply transaction triggered by the
   Reconfigure-init message.


   The server multicasts the Reconfigure-init message to one or more
   Reconfigure Multicast Addresses previously sent as extensions
   to the clients.  Note that a server MAY unicast Reconfigure-init
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   message(s) to specific clients by walking its list of bindings to
   determine the unicast address(es) of the clients.  Whether or not the
   Reconfigure-init is multicast or unicast is an implementation detail.


   A server waits for a Request message from each client confirming that
   they have received the Reconfigure-init and are thus initiating a
   Request/Reply transaction with the server.  The server can determine
   that a Request message is in response to a Reconfigure-init because
   the transaction-ID in the Request will be the same value as was used
   in the Reconfigure-init message.
12.4.5. Time out and retransmission of Reconfigure-init messages


   The server uses the same algorithm and configuration values for
   sending Reconfigure-init messages as it does with Reconfigure
   messages.  See Section 12.4.2 for this algorithm.
12.4.6. Receipt of Request messages


   Upon receipt of a valid Request message with the same transaction-ID
   as the Reconfigure-init messages it sent, the server removes that
   client from the list of clients it is expecting to initiate a
   Request/Reply transaction.


   The server generates and sends Reply message(s) to the client as
   described in section 11.6.3, including in the ``extension'' field
   new values for configuration parameters.  If the extensions include
   releasable resources, the server will include two extensions for each
   resource - one with the original values with the lease times set to
   zero, and another with new values and lease times.  Note that the
   server can terminate the client's ability to use a resource simply by
   including only the first extension value.
12.5. Client Behavior


   A client MUST always monitor UDP port 546 for Reconfigure and
   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.

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12.5.1. Receipt of Reconfigure messages


   Upon receipt of a valid Reconfigure message, the client extracts
   the configuration parameters contained in the ``extensions''
   field, and notifies the application layer that new values for these
   parameters are available.  The client then generates and sends a
   Reconfigure-reply message to the server.
12.5.2. Creation and sending of Reconfigure-reply messages


   When creating a Reconfigure-reply message, the client SHOULD start
   out with a buffer initialized with zeroed octets.  The client sets
   the ``msg-type'' field to 7, and places the link-local address of
   the interface upon which it received the Reconfigure message in
   the ``client's link-local address'' field.  The client copies the
   values of the following fields from the Reconfigure message to the
   Reconfigure-reply message:


     o transaction-ID


     o server-address


   The client sets the ``status'' field appropriately (see the table
   in section 3.4) based upon the results of processing the server's
   reconfigure-reply.


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


   If the client is configured to use authentication, the client
   generates the appropriate authentication extension, and adds this
   extension to the ``extensions'' field.  Note that the authentication
   extension MUST be the last extension in the ``extensions'' field.


   The client delays the sending of the Reconfigure-reply by some
   random value selected in the range of REC_REP_MIN and REC_REP_MAX
   seconds.  This delay helps reduce the load on the server generated by
   processing large numbers of Reconfigure-reply messages.


   Default and initial values for REC_REP_MIN and REC_REP_MAX are
   documented in section 3.5.


   The client unicasts the Reconfigure-reply to the address identified
   in the ``server-address'' field.  Sending the Reconfigure-reply
   completes the reconfiguration process for the client.

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12.5.3. Receipt of Reconfigure-init messages


   Upon receipt of a valid Reconfigure-init message, the client
   initiates a Request/Reply transaction with the server.
12.5.4. 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 11.4.1 with the following differences:


      transaction-ID
                   The client copies the transaction-ID from the
                   Reconfigure-init message into the Request message.


      Pause before sending Request
                   The client pauses before sending the Request for
                   a random value within the range REC_REP_MIN and
                   REC_REP_MAX seconds, as outlined in section 12.5.2.
12.5.5. 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 11.4.2 for details.
12.5.6. Receipt of Reply messages


   Upon the receipt of a valid Reply message, the client extracts
   the contents of the ``extension'' field, and sets (or resets)
   configuration parameters appropriately.  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.  If the resources changed are
   releasable, the client makes the appropriate adjustments to its
   management of the leases of these resources.
13. Using DHCP for network renumbering


   An administrator can use DHCP to renumber links within her DHCP
   domain through two techniques, passive renumbering and active
   renumbering.

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13.1. Passive Renumbering


   The administrator can configure her servers to return relatively
   short preferred and valid lifetimes for the IP addresses she
   makes available to clients.  When she determines that she'd like
   to renumber a network, she configures her servers through an
   implementation-specific manner to disallow the extension of the IP
   address lifetimes on the original network, and adds the new network
   configuration data to the server's database.


   The clients on the original network will fail to acquire lifetime
   extensions on their IP addresses, and will request and acquire
   IP addresses from the new network when the valid lifetime of the
   original IP addresses approaches expiration.


   When the lifetimes for all of the IP addresses on the original
   network expire, the network can be considered renumbered.
13.2. Active Renumbering


   The administrator can force the renumbering of networks in her DHCP
   domain by using the reconfigure feature of DHCP. She instructs her
   servers of the network renumbering through an implementation-specific
   interface.  The servers in the domain will generate Reconfigure-init
   messages, which will cause the clients to initiate a Request/Reply
   transaction with the server.  The servers will include two IP address
   extensions for each IP address being changed.  The first will contain
   the original IP address, with the preferred and valid lifetimes set
   to zero.  The second will contain the new IP address, with non-zero
   preferred and valid lifetimes.


   A server implementation MAY permit the administrator to set the
   original IP address lifetimes to some small value greater than zero,
   to allow applications running on the client to orderly transfer to
   the new network over time.
14. DHCP Client Implementator Notes


   This section provides helpful information for the client implementor
   regarding their implementations.  The text described here is not part
   of the protocol, but rather a discussion of implementation features
   we feel the implementor should consider during implementation.

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14.1. Primary Interface


   Since configuration parameters acquired through DHCP can be
   interface-specific or more general, the client implementor SHOULD
   provide a mechanism by which the client implementation can be
   configured to specify which interface is the primary interface.  The
   client SHOULD always query the DHCP data associated with the primary
   interface for non-interface specific configuration parameters.  An
   implementation MAY implement a list of interfaces which would be
   scanned in order to satisfy the general request.  In either case, the
   first interface scanned is considered the primary interface.


   By allowing the specification of a primary interface, the client
   implementor identifies which interface is authoritative for
   non-interface specific parameters, which prevents configuration
   information ambiguity within the client implementation.
14.2. Advertise Message and Configuration Parameter Caching


   If the hardware the client is running on permits it, the implementor
   SHOULD provide a cache for Advertise messages and a cache of
   configuration parameters received through DHCP. Providing these
   caches prevents unnecessary DHCP traffic and the subsequent load
   this generates on the servers.  The implementor SHOULD provide a
   configuration knob for setting the amount of time the cache(s) are
   valid.
14.3. Time out and retransmission variables


   Note that the client time out and retransmission variables outlined
   in section 3.5 can be configured on the server and sent to the client
   through the use of the ``DHCP Retransmission Parameter Extension'',
   which is documented in the ``extensions document'' [2].  A client
   implementation SHOULD be able to reset these variables using the
   values from this extension.
14.4. Server Preference


   A client MUST wait for SRVR_PREF_WAIT seconds after sending a DHCP
   Solicit message to collect Advertise messages and compare their
   preferences (see section 15.3), unless it receives an Advertise
   message with a preference of 255.  If the client receives an
   Advertise message with a preference of 255, then the client MAY act
   immediately on that Advertise without waiting for any more additional
   Advertise messages.


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15. DHCP Server Implementator Notes


   This section provides helpful information for the server implementor.
15.1. Client Bindings


   A server implementation can use the client's link-local address
   and the subnet prefix specification from which the client sent its
   Request message(s) as an index for finding configuration parameters
   assigned to the client.  While it isn't critical to keep track
   of which clients were given information (resources) that isn't
   releasable, it IS critical for the server to keep track of which
   client it has assigned releasable resources.  The server MUST
   include the transaction-ID from the client's Request along with
   the releasable resource identifier(s) within the binding.  This is
   done so that the server can detect whether a client Request is a
   retransmission of an earlier Request or an entirely new Request.


   The server should periodically scan its bindings for releasable
   resources whose leases have expired.  When the server finds expired
   resource assignments, it MUST delete these assignments, thereby
   making these resources available to other clients.


   The client bindings MUST be stored in non-volatile storage.


   The server implementation should provide policy knobs to control
   whether or not the lease on a releasable resource is renewable, and
   by how long.
15.2. Reconfigure Considerations


   A server implementation MUST provide an interface to the
   administrator for initiating reconfigure events.


   A server implementation may provide a mechanism for allowing the
   specification of how many clients comprise a reconfigure multicast
   group.  This enables the administrator to control the hit a server
   takes when a reconfigure event occurs.
15.3. Server Preference


   The server implementation SHOULD allow the setting of a server
   preference value by the administrator.  The server preference
   variable is an unsigned single octet value (0--255), with the lowest
   preference being 0 and the highest 255.  Clients will choose higher
   preference servers over those with lower preference values.  If you
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   don't choose to implement this feature in your server, you MUST set
   the server preference field to 0 in the Advertise messages generated
   by your server.
15.4. Request Message Transaction-ID Cache


   In order to improve performance, a server implementation MAY include
   an in memory transaction-ID cache.  This cache is indexed by client
   binding and transaction-ID, and enables the server to quickly
   determine whether a Request is a retransmission or a new Request
   without the cost of a database lookup.  If an implementor chooses to
   implement this cache, then they SHOULD provide a configuration knob
   to tune the lifetime of the cache entries.
16. DHCP Relay Implementator Notes


   A relay implementation SHOULD allow the specification of a list of
   destination addresses for Solicit messages.  This list MAY contain
   any mixture of unicast addresses and multicast addresses.


   If a relay receives an ICMP message in response to a DHCP message it
   has forwarded, it SHOULD log this event.
17. Open Issues for Working Group Discussion


   This section contains some items for discussion by the working group.
17.1. Trade-offs:  Optional fields in DHCP messages


   You'll notice that the message formats have changed.  In particular,
   some of the optional fields are now required.  This will increase the
   size of DHCP messages in some cases, consuming network bandwidth and
   memory on the DHCP client (an issue for small devices such as PDAs).


   The changes were made for the following reasons:


     o Fields that were used most of the time were made required.


     o Some fields that were optional were either made required or added
       to messages which previously didn't have them.  This was done for
       robustness reasons (receivers can validate that the message is
       for them, and in the case of clients, know which interface the
       message is intended for).


     o Simplicity.
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   Please look at the messages as they are now defined, and let us know
   your opinion.
17.2. Use DHCPv4 authentication or the current DHCPv6 method?


   Now that the DHCPv4 authentication draft is in last call, should
   we use the technique described in that document to provide
   authentication for DHCPv6, or should we continue with the
   authentication technique currently documented in the extensions
   draft?
17.3. The Reconfigure Message and Subnet Prefix Extensions


   The drafts currently specify that Releasable resources (such as an IP
   address) can only be reconfigured using the Reconfigure-init trigger
   message.  This was done for simplicity (enables clients to perform
   DAD on the new address and return the appropriate result to the
   server) using the same mechanism as a standard Request/Reply/Release
   exchange.  This method also makes no assumptions about the
   charactistics of the releasable resource.


   However, for IP addresses with interface IDs, one could send out
   two IP address extensions, one for the old prefix and one for the
   new, and cause clients to change the prefix and thus renumber over
   time.  This scheme avoids the added DHCP Request traffic - clients
   acknowledge with a Reconfigure-reply message.
17.4. ``R'' bit in Request message not needed?


   Now that the transaction-ID is stored along with the releasable
   resource identifier in a client's binding, the transaction-ID cache
   becomes an optional feature of the DHCP server implementation, not a
   requirement of the protocol.  Should we do away with the ``R'' bit?
18. Security Considerations


   Clients and servers often have to authenticate the messages they
   exchange.  For instance, a server may wish to be certain that a
   Request originated from the client identified by the <link-local
   address, subnet-prefix> fields included within the Request message
   header.  Conversely, it is quite often essential for a client to
   be certain that the configuration parameters and addresses it has
   received were sent to it by an authoritative server.  Similarly, a
   server should only accept a Release message which seems to be from
   one of its clients, if it has some assurance that the client actually
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   did transmit the Release message.  Again, a client might wish to only
   accept Reconfigure or Reconfigure-init messages that are certain to
   have originated from a server with authority to issue them.


   The IPv6 Authentication Header can provide security for DHCPv6
   messages when both endpoints have a suitable IP address.  However,
   a client often has only a link-local address, and such an address
   is not sufficient for a server which is off-link.  In those
   circumstances the relay is involved, so that the DHCP message MUST
   have the relay's address in the IP destination address field, even
   though the client aims to deliver the message to the server.  The
   DHCP Client-Server Authentication Extension [2] is intended to be
   used in these circumstances.


   Note that, if a client receives a DHCP message which fails
   authentication, it should continue to wait for another message which
   might be correctly authenticated just as if the failed message had
   never arrived; however, receiving such failed messages SHOULD be
   logged.
19. 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.
20. IANA Considerations


   This document defines message types 1--8 to be received by UDP at
   port numbers 546 and 547.  Additional message types may be defined in
   the future.


   Section 3.1 lists several multicast addresses used by DHCP.


   This document also defines several status codes that are to
   be returned with the Reply and Reconfigure-reply messages (see
   sections 9.4 and 9.7).  The non-zero values for these status codes
   which are currently specified are shown in the table in section 3.4.


   There is a DHCPv6 extension [2] which allows clients and servers to
   exchange values for some of the timing and retransmission parameters
   defined in section 3.5.  Adding new parameters in the future would
   require extending the values by which the parameters are indicated in
   the DHCP extension.  Since there needs to be a list kept, the default
   values for each parameter should also be stored as part of the list.
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   All of these protocol elements may be specified to assume new values
   at some point in the future.  New values should be approved by the
   process of IETF Consensus [11].
21. Acknowledgements


   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, Jack McCann, Yakov
   Rekhter, Matt Thomas, Sue Thomson, and Phil Wells.


   Thanks to Steve Deering and Bob Hinden, who have consistently
   taken the time to discuss the more complex parts of the IPv6
   specifications.
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, 1] 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.
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     o Stateful autoconfiguration has to coexist and integrate with
       stateless autoconfiguration supporting Duplicate Address
       Detection and the two IPv6 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 [16].


   DHCPv6 Architecture/Model Changes:


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


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


     o Client/Server bindings are now mandatory and take advantage of
       the client's link-local address 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 extension headers.


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


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     o New extensions 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.


     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 (2000).  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
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
References


    [1] S. Alexander and R. Droms.  DHCP Options and BOOTP Vendor
        Extensions.  Request for Comments (Draft Standard) 2132,
        Internet Engineering Task Force, March 1997.


    [2] J. Bound, M. Carney, and C. Perkins.  Extensions for the Dynamic
        Host Configuration Protocol for IPv6.
        draft-ietf-dhc-dhcpv6ext-12.txt, May 2000.  (work in progress).


    [3] S. Bradner.  Key words for use in RFCs to Indicate Requirement
        Levels.  Request for Comments (Best Current Practice) 2119,
        Internet Engineering Task Force, March 1997.


    [4] S. Bradner and A. Mankin.  The Recommendation for the IP Next
        Generation Protocol.  Request for Comments (Proposed Standard)
        1752, Internet Engineering Task Force, January 1995.


    [5] W. J. Croft and J. Gilmore.  Bootstrap Protocol.  Request for
        Comments 951, Internet Engineering Task Force, September 1985.


    [6] S. Deering and R. Hinden.  Internet Protocol, Version 6 (IPv6)
        Specification.  Request for Comments (Draft Standard) 2460,
        Internet Engineering Task Force, December 1998.


    [7] R. Droms.  Dynamic Host Configuration Protocol.  Request for
        Comments (Draft Standard) 2131, Internet Engineering Task Force,
        March 1997.


    [8] R. Hinden and S. Deering.  IP Version 6 Addressing Architecture.
        Request for Comments (Proposed Standard) 2373, Internet
        Engineering Task Force, July 1998.


    [9] S. Kent and R. Atkinson.  IP Authentication Header.  Request for
        Comments (Proposed Standard) 2402, Internet Engineering Task
        Force, November 1998.


   [10] J. McCann, S. Deering, and J. Mogul.  Path MTU Discovery for
        IP version 6.  Request for Comments (Proposed Standard) 1981,
        Internet Engineering Task Force, August 1996.


   [11] T. Narten and H. Alvestrand.  Guidelines for Writing an IANA
        Considerations Section in RFCs.  Request for Comments (Best
        Current Practice) 2434, Internet Engineering Task Force, October
        1998.
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   [12] T. Narten, E. Nordmark, and W. Simpson.  Neighbor Discovery for
        IP Version 6 (IPv6).  Request for Comments (Draft Standard)
        2461, Internet Engineering Task Force, December 1998.


   [13] D. C. Plummer.  Ethernet Address Resolution Protocol:  Or
        converting network protocol addresses to 48.bit Ethernet address
        for transmission on Ethernet hardware.  Request for Comments
        (Standard) 826, Internet Engineering Task Force, November 1982.


   [14] J. Postel.  User Datagram Protocol.  Request for Comments
        (Standard) 768, Internet Engineering Task Force, August 1980.


   [15] S. Thomson and T. Narten.  IPv6 Stateless Address
        Autoconfiguration.  Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.


   [16] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan.  Service
        Location Protocol.  Request for Comments (Proposed Standard)
        2165, Internet Engineering Task Force, June 1997.


   [17] P. Vixie, Ed., S. Thomson, Y. Rekhter, and J. Bound.  Dynamic
        Updates in the Domain Name System (DNS UPDATE).  Request for
        Comments (Proposed Standard) 2136, Internet Engineering Task
        Force, April 1997.
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Chair's Address


   The working group can be contacted via the current chair:


      Ralph Droms
      Computer Science Department
      323 Dana Engineering
      Bucknell University
      Lewisburg, PA 17837


      Phone:  (570) 577-1145
      E-mail:  droms@bucknell.edu

Author's Address


   Questions about this memo can be directed to:


        Jim Bound
        Compaq Computer Corporation
        Mail Stop:  ZK03-3/U14
        110 Spitbrook Road
        Nashua, NH 03062
        USA
        Phone:  +1-603-884-0400
        Email:  bound@zk3.dec.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


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