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Versions: (draft-dhcwg-dhc-rfc3315bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 8415

Dynamic Host Configuration (DHC)                       T. Mrugalski, Ed.
Internet-Draft                                              M. Siodelski
Obsoletes: 3315,3633,3736,7083,7550 (if                              ISC
           approved)                                             B. Volz
Intended status: Standards Track                          A. Yourtchenko
Expires: April 21, 2016                                            Cisco
                                                           M. Richardson
                                                                     SSW
                                                                S. Jiang
                                                                  Huawei
                                                                T. Lemon
                                                                 Nominum
                                                        October 19, 2015


       Dynamic Host Configuration Protocol for IPv6 (DHCPv6) bis
                      draft-ietf-dhc-rfc3315bis-02

Abstract

   The Dynamic Host Configuration Protocol for IPv6 (DHCP) enables DHCP
   servers to pass configuration parameters such as IPv6 network
   addresses to IPv6 nodes.  It offers the capability of automatic
   allocation of reusable network addresses and additional configuration
   flexibility.  This protocol is a stateful counterpart to "IPv6
   Stateless Address Autoconfiguration" (RFC 4862), and can be used
   separately or concurrently with the latter to obtain configuration
   parameters.

   This document updates the original DHCPv6 specification (RFC 3315)
   and incorporates RFC 3633, RFC 3736, RFC 7083, and RFC 7550.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on April 21, 2016.



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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction and Overview . . . . . . . . . . . . . . . . . .   6
     1.1.  Relation to previous DHCPv6 standards . . . . . . . . . .   6
     1.2.  Relation to DHCP in IPv4  . . . . . . . . . . . . . . . .   7
     1.3.  Protocols and Addressing  . . . . . . . . . . . . . . . .   7
     1.4.  Client-server Exchanges Involving Two Messages  . . . . .   8
     1.5.  Client-server Exchanges Involving Four Messages . . . . .   8
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .  10
     4.2.  DHCP Terminology  . . . . . . . . . . . . . . . . . . . .  11
   5.  Operational Models  . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Stateless DHCP  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  DHCP for Non-Temporary Address Assignment . . . . . . . .  15
     5.3.  DHCP for Prefix Delegation  . . . . . . . . . . . . . . .  16
     5.4.  DHCP for Customer Edge Routers  . . . . . . . . . . . . .  18
     5.5.  DHCP for Temporary Addresses  . . . . . . . . . . . . . .  19
   6.  DHCP Constants  . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Multicast Addresses . . . . . . . . . . . . . . . . . . .  19



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     6.2.  UDP Ports . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.3.  DHCP Message Types  . . . . . . . . . . . . . . . . . . .  19
     6.4.  Status Codes  . . . . . . . . . . . . . . . . . . . . . .  21
     6.5.  Transmission and Retransmission Parameters  . . . . . . .  22
     6.6.  Representation of time values and "Infinity" as a time
           value . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Client/Server Message Formats . . . . . . . . . . . . . . . .  22
   8.  Relay Agent/Server Message Formats  . . . . . . . . . . . . .  23
     8.1.  Relay-forward Message . . . . . . . . . . . . . . . . . .  24
     8.2.  Relay-reply Message . . . . . . . . . . . . . . . . . . .  25
   9.  Representation and Use of Domain Names  . . . . . . . . . . .  25
   10. DHCP Unique Identifier (DUID) . . . . . . . . . . . . . . . .  25
     10.1.  DUID Contents  . . . . . . . . . . . . . . . . . . . . .  26
     10.2.  DUID Based on Link-layer Address Plus Time, DUID-LLT . .  26
     10.3.  DUID Assigned by Vendor Based on Enterprise Number,
            DUID-EN  . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.4.  DUID Based on Link-layer Address, DUID-LL  . . . . . . .  29
   11. Identity Association  . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Identity Associations for Address Assignment . . . . . .  30
     11.2.  Identity Associations for Prefix Delegation  . . . . . .  30
   12. Selecting Addresses for Assignment to an IA . . . . . . . . .  31
   13. Management of Temporary Addresses . . . . . . . . . . . . . .  32
   14. Transmission of Messages by a Client  . . . . . . . . . . . .  33
     14.1.  Rate Limiting  . . . . . . . . . . . . . . . . . . . . .  33
     14.2.  Choosing transmission time when T1,T2 is 0 . . . . . . .  34
   15. Reliability of Client Initiated Message Exchanges . . . . . .  34
   16. Message Validation  . . . . . . . . . . . . . . . . . . . . .  36
     16.1.  Use of Transaction IDs . . . . . . . . . . . . . . . . .  36
     16.2.  Solicit Message  . . . . . . . . . . . . . . . . . . . .  37
     16.3.  Advertise Message  . . . . . . . . . . . . . . . . . . .  37
     16.4.  Request Message  . . . . . . . . . . . . . . . . . . . .  37
     16.5.  Confirm Message  . . . . . . . . . . . . . . . . . . . .  37
     16.6.  Renew Message  . . . . . . . . . . . . . . . . . . . . .  38
     16.7.  Rebind Message . . . . . . . . . . . . . . . . . . . . .  38
     16.8.  Decline Messages . . . . . . . . . . . . . . . . . . . .  38
     16.9.  Release Message  . . . . . . . . . . . . . . . . . . . .  38
     16.10. Reply Message  . . . . . . . . . . . . . . . . . . . . .  39
     16.11. Reconfigure Message  . . . . . . . . . . . . . . . . . .  39
     16.12. Information-request Message  . . . . . . . . . . . . . .  40
     16.13. Relay-forward Message  . . . . . . . . . . . . . . . . .  40
     16.14. Relay-reply Message  . . . . . . . . . . . . . . . . . .  40
   17. Client Source Address and Interface Selection . . . . . . . .  40
     17.1.  Address Assignment . . . . . . . . . . . . . . . . . . .  40
     17.2.  Prefix Delegation  . . . . . . . . . . . . . . . . . . .  41
   18. DHCP Server Solicitation  . . . . . . . . . . . . . . . . . .  41
     18.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . .  42
       18.1.1.  Creation of Solicit Messages . . . . . . . . . . . .  42
       18.1.2.  Transmission of Solicit Messages . . . . . . . . . .  43



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       18.1.3.  Receipt of Advertise Messages  . . . . . . . . . . .  44
       18.1.4.  Receipt of Reply Message . . . . . . . . . . . . . .  45
     18.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . .  46
       18.2.1.  Receipt of Solicit Messages  . . . . . . . . . . . .  46
       18.2.2.  Creation and Transmission of Advertise Messages  . .  46
       18.2.3.  Creation and Transmission of Reply Messages  . . . .  48
   19. DHCP Client-Initiated Configuration Exchange  . . . . . . . .  48
     19.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . .  49
       19.1.1.  Creation and Transmission of Request Messages  . . .  49
       19.1.2.  Creation and Transmission of Confirm Messages  . . .  50
       19.1.3.  Creation and Transmission of Renew Messages  . . . .  52
       19.1.4.  Creation and Transmission of Rebind Messages . . . .  53
       19.1.5.  Creation and Transmission of Information-request
                Messages . . . . . . . . . . . . . . . . . . . . . .  54
       19.1.6.  Creation and Transmission of Release Messages  . . .  55
       19.1.7.  Creation and Transmission of Decline Messages  . . .  56
       19.1.8.  Receipt of Reply Messages  . . . . . . . . . . . . .  57
       19.1.9.  Bindings Verification by Requesting Router . . . . .  60
     19.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . .  60
       19.2.1.  Receipt of Request Messages  . . . . . . . . . . . .  61
       19.2.2.  Receipt of Confirm Messages  . . . . . . . . . . . .  62
       19.2.3.  Receipt of Renew Messages  . . . . . . . . . . . . .  62
       19.2.4.  Receipt of Rebind Messages . . . . . . . . . . . . .  64
       19.2.5.  Receipt of Information-request Messages  . . . . . .  66
       19.2.6.  Receipt of Release Messages  . . . . . . . . . . . .  67
       19.2.7.  Receipt of Decline Messages  . . . . . . . . . . . .  67
       19.2.8.  Transmission of Reply Messages . . . . . . . . . . .  68
   20. DHCP Server-Initiated Configuration Exchange  . . . . . . . .  68
     20.1.  Server Behavior  . . . . . . . . . . . . . . . . . . . .  69
       20.1.1.  Creation and Transmission of Reconfigure Messages  .  69
       20.1.2.  Time Out and Retransmission of Reconfigure Messages   70
     20.2.  Receipt of Renew or Rebind Messages  . . . . . . . . . .  70
     20.3.  Receipt of Information-request Messages  . . . . . . . .  70
     20.4.  Client Behavior  . . . . . . . . . . . . . . . . . . . .  71
       20.4.1.  Receipt of Reconfigure Messages  . . . . . . . . . .  71
       20.4.2.  Creation and Transmission of Renew or Rebind
                Messages . . . . . . . . . . . . . . . . . . . . . .  72
       20.4.3.  Creation and Transmission of Information-request
                Messages . . . . . . . . . . . . . . . . . . . . . .  72
       20.4.4.  Time Out and Retransmission of Renew, Rebind or
                Information-request Messages . . . . . . . . . . . .  72
       20.4.5.  Receipt of Reply Messages  . . . . . . . . . . . . .  72
     20.5.  Prefix Delegation Reconfiguration  . . . . . . . . . . .  73
       20.5.1.  Delegating Router Behavior . . . . . . . . . . . . .  73
       20.5.2.  Requesting Router Behavior . . . . . . . . . . . . .  73
   21. Relay Agent Behavior  . . . . . . . . . . . . . . . . . . . .  73
     21.1.  Relaying a Client Message or a Relay-forward Message . .  73
       21.1.1.  Relaying a Message from a Client . . . . . . . . . .  74



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       21.1.2.  Relaying a Message from a Relay Agent  . . . . . . .  74
       21.1.3.  Relay Agent Behavior with Prefix Delegation  . . . .  75
     21.2.  Relaying a Relay-reply Message . . . . . . . . . . . . .  75
     21.3.  Construction of Relay-reply Messages . . . . . . . . . .  75
   22. Authentication of DHCP Messages . . . . . . . . . . . . . . .  76
     22.1.  Security of Messages Sent Between Servers and Relay
            Agents . . . . . . . . . . . . . . . . . . . . . . . . .  77
     22.2.  Summary of DHCP Authentication . . . . . . . . . . . . .  78
     22.3.  Replay Detection . . . . . . . . . . . . . . . . . . . .  79
     22.4.  Reconfigure Key Authentication Protocol  . . . . . . . .  79
       22.4.1.  Use of the Authentication Option in the Reconfigure
                Key Authentication Protocol  . . . . . . . . . . . .  79
       22.4.2.  Server considerations for Reconfigure Key protocol .  80
       22.4.3.  Client considerations for Reconfigure Key protocol .  81
   23. DHCP Options  . . . . . . . . . . . . . . . . . . . . . . . .  81
     23.1.  Format of DHCP Options . . . . . . . . . . . . . . . . .  81
     23.2.  Client Identifier Option . . . . . . . . . . . . . . . .  82
     23.3.  Server Identifier Option . . . . . . . . . . . . . . . .  83
     23.4.  Identity Association for Non-temporary Addresses Option   83
     23.5.  Identity Association for Temporary Addresses Option  . .  86
     23.6.  IA Address Option  . . . . . . . . . . . . . . . . . . .  87
     23.7.  Option Request Option  . . . . . . . . . . . . . . . . .  89
     23.8.  Preference Option  . . . . . . . . . . . . . . . . . . .  90
     23.9.  Elapsed Time Option  . . . . . . . . . . . . . . . . . .  90
     23.10. Relay Message Option . . . . . . . . . . . . . . . . . .  91
     23.11. Authentication Option  . . . . . . . . . . . . . . . . .  92
     23.12. Server Unicast Option  . . . . . . . . . . . . . . . . .  93
     23.13. Status Code Option . . . . . . . . . . . . . . . . . . .  94
     23.14. Rapid Commit Option  . . . . . . . . . . . . . . . . . .  96
     23.15. User Class Option  . . . . . . . . . . . . . . . . . . .  97
     23.16. Vendor Class Option  . . . . . . . . . . . . . . . . . .  98
     23.17. Vendor-specific Information Option . . . . . . . . . . . 100
     23.18. Interface-Id Option  . . . . . . . . . . . . . . . . . . 102
     23.19. Reconfigure Message Option . . . . . . . . . . . . . . . 103
     23.20. Reconfigure Accept Option  . . . . . . . . . . . . . . . 103
     23.21. Identity Association for Prefix Delegation Option  . . . 104
     23.22. IA Prefix Option . . . . . . . . . . . . . . . . . . . . 106
     23.23. SOL_MAX_RT Option  . . . . . . . . . . . . . . . . . . . 107
     23.24. INF_MAX_RT Option  . . . . . . . . . . . . . . . . . . . 108
   24. Security Considerations . . . . . . . . . . . . . . . . . . . 110
   25. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 112
   26. Obsoleted mechanisms  . . . . . . . . . . . . . . . . . . . . 112
   27. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 113
   28. References  . . . . . . . . . . . . . . . . . . . . . . . . . 114
     28.1.  Normative References . . . . . . . . . . . . . . . . . . 114
     28.2.  Informative References . . . . . . . . . . . . . . . . . 116
   Appendix A.  Changes since RFC3315  . . . . . . . . . . . . . . . 118
   Appendix B.  Changes since RFC3633  . . . . . . . . . . . . . . . 122



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   Appendix C.  Appearance of Options in Message Types . . . . . . . 122
   Appendix D.  Appearance of Options in the Options Field of DHCP
                Options  . . . . . . . . . . . . . . . . . . . . . . 123
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 124

1.  Introduction and Overview

   This document describes DHCP for IPv6 (DHCPv6), a client/server
   protocol that provides managed configuration of devices.  Relay agent
   functionality is also defined for enabling communication between
   clients and servers that are not on the same link.

   DHCPv6 can provide a device with addresses assigned by a DHCPv6
   server and other configuration information, which are carried in
   options.  DHCPv6 can be extended through the definition of new
   options to carry configuration information not specified in this
   document.

   DHCPv6 is the "stateful address autoconfiguration protocol" and the
   "stateful autoconfiguration protocol" referred to in "IPv6 Stateless
   Address Autoconfiguration" [RFC4862].

   This document also provides a mechanism for automated delegation of
   IPv6 prefixes using DHCPv6.  Through this mechanism, a delegating
   router can delegate prefixes to requesting routers.

   DHCPv6 can also operate in mode, where only configuration options
   (but no addresses or prefixes) are provided.  That implies that the
   server doesn't have to track any state, and thus the mode is called
   stateless DHCPv6.  Mechanisms necessary to support stateless DHCPv6
   are much smaller than to support stateful DHCPv6.

   The remainder of this introduction summarizes relation to the
   previous DHCPv6 standards Section 1.1, clarifies the stance with
   regards to DHCPv4 Section 1.2.  More detailed description of the
   message exchange mechanisms and example message flows in Section 1.4
   and Section 1.5 are intended as illustrations of DHCP operation
   rather than an exhaustive list of all possible client-server
   interactions.  Section 5 provides an overview of common operational
   models.  Section 18, Section 19, and Section 20 explain client and
   server operation in detail.

1.1.  Relation to previous DHCPv6 standards

   The initial specification of DHCPv6 was defined in [RFC3315] and a
   number of follow up extensions published over the years.  Several
   notable extensions were published: prefix delegation [RFC3633],
   stateless [RFC3736], update to SOL_MAX_RT and INF_MAX_RT option



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   values [RFC7083] and harmonization between addresses and prefixes
   support [RFC7550].  Understanding a protocol which definition is
   spread between large number of documents may be cumbersome.
   Furthermore, a significant operational experience has been gained
   over the years and certain small elements of the protocol have been
   reworked.  This document provides a unified, corrected and cleaned up
   definition of the DHCPv6 that also covers all erratas filled against
   older RFCs.  As such, it obsoletes a number of aforementioned RFCs.
   There is a small number of mechanisms that were obsoleted.  They are
   listed in Section 26.

1.2.  Relation to DHCP in IPv4

   The operational models and relevant configuration information for
   DHCPv4 [RFC2132][RFC2131] and DHCPv6 are sufficiently different that
   integration between the two services is not included in this
   document.  [RFC3315] suggested that future work might be to extend
   DHCPv6 to carry IPv4 address and configuration information.  However,
   the current consensus of the IETF is that DHCPv4 should be used
   rather than DHCPv6 when conveying IPv4 configuration information to
   nodes.  [RFC7341] describes a transport mechanism to carry DHCPv4
   messages using the DHCPv6 protocol for the dynamic provisioning of
   IPv4 address and configuration information across IPv6-only networks.

1.3.  Protocols and Addressing

   Clients and servers exchange DHCP messages using UDP [RFC0768].  The
   client uses a link-local address or addresses determined through
   other mechanisms for transmitting and receiving DHCP messages.

   A DHCP client sends most messages using a reserved, link-scoped
   multicast destination address so that the client need not be
   configured with the address or addresses of DHCP servers.

   To allow a DHCP client to send a message to a DHCP server that is not
   attached to the same link, a DHCP relay agent on the client's link
   will relay messages between the client and server.  The operation of
   the relay agent is transparent to the client and the discussion of
   message exchanges in the remainder of this section will omit the
   description of message relaying by relay agents.

   Once the client has determined the address of a server, it may under
   some circumstances send messages directly to the server using
   unicast.







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1.4.  Client-server Exchanges Involving Two Messages

   When a DHCP client does not need to have a DHCP server assign it IP
   addresses, the client can obtain configuration information such as a
   list of available DNS servers [RFC3646] or NTP servers [RFC4075]
   through a single message and reply exchanged with a DHCP server.  To
   obtain configuration information the client first sends an
   Information-request message to the All_DHCP_Relay_Agents_and_Servers
   multicast address.  Servers respond with a Reply message containing
   the configuration information for the client.

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

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

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

1.5.  Client-server Exchanges Involving Four Messages

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



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   As described in the previous section, the client sends a Renew
   message to the server to extend the lifetimes associated with its
   addresses, allowing the client to continue to use those addresses
   without interruption.

2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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

3.  Background

   The IPv6 Specification provides the base architecture and design of
   IPv6.  Related work in IPv6 that would best serve an implementor to
   study includes the IPv6 Specification [RFC2460], the IPv6 Addressing
   Architecture [RFC4291], IPv6 Stateless Address Autoconfiguration
   [RFC4862], IPv6 Neighbor Discovery Processing [RFC4861], and Dynamic
   Updates to DNS [RFC2136].  These specifications enable DHCP to build
   upon the IPv6 work to provide both robust stateful autoconfiguration
   and autoregistration of DNS Host Names.

   The IPv6 Addressing Architecture specification [RFC4291] 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.  The availability of these features means that
   a client can use its link-local address and a well-known multicast
   address to discover and communicate with DHCP servers or relay agents
   on its link.

   IPv6 Stateless Address Autoconfiguration [RFC4862] specifies
   procedures by which a node may autoconfigure addresses based on
   router advertisements [RFC4861], and the use of a valid lifetime to
   support renumbering of addresses on the Internet.  In addition, the
   protocol interaction by which a node begins stateless or stateful
   autoconfiguration is specified.  DHCP is one vehicle to perform



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   stateful autoconfiguration.  Compatibility with stateless address
   autoconfiguration is a design requirement of DHCP.

   IPv6 Neighbor Discovery [RFC4861] is the node discovery protocol in
   IPv6 which replaces and enhances functions of ARP [RFC0826].  To
   understand IPv6 and stateless address autoconfiguration, it is
   strongly recommended that implementors understand IPv6 Neighbor
   Discovery.

   Dynamic Updates to DNS [RFC2136] is a specification that supports the
   dynamic update of DNS records for both IPv4 and IPv6.  DHCP can use
   the dynamic updates to DNS to integrate addresses and name space to
   not only support autoconfiguration, but also autoregistration in
   IPv6.

4.  Terminology

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

4.1.  IPv6 Terminology

   IPv6 terminology relevant to this specification from the IPv6
   Protocol [RFC2460], IPv6 Addressing Architecture [RFC4291], and IPv6
   Stateless Address Autoconfiguration [RFC4862] is included below.

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

   host                      Any node that is not a router.

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

   interface                 A node's attachment to a link.

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





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

   link-local address        An IPv6 address having a link-only scope,
                             indicated by having the prefix (FE80::/10),
                             that can be used to reach neighboring nodes
                             attached to the same link.  Every interface
                             has a link-local address.

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

   neighbor                  A node attached to the same link.

   node                      A device that implements IP.

   packet                    An IP header plus payload.

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

   prefix length             The number of bits in a prefix.

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

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

4.2.  DHCP Terminology

   Terminology specific to DHCP can be found below.

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





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

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

   delegating router         The router that acts as a DHCP server, and
                             is responding to the prefix request.

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

   DHCP client (or client)   A node that initiates requests on a link to
                             obtain configuration parameters from one or
                             more DHCP servers.  Depending on the
                             purpose of the client, it may feature the
                             requesting router functionality, if it
                             supports prefix delegation.

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

   DHCP realm                A name used to identify the DHCP
                             administrative domain from which a DHCP
                             authentication key was selected.





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   DHCP relay agent (or relay agent)  A node that acts as an
                             intermediary to deliver DHCP messages
                             between clients and servers.  In certain
                             configurations there may be more than one
                             relay agent between clients and servers, so
                             a relay agent may send DHCP messages to
                             another relay agent.

   DHCP server (or server)   A node that responds to requests from
                             clients, and may or may not be on the same
                             link as the client(s).  Depending on its
                             capabilities, it may also feature the
                             functionality of delegating router, if it
                             supports prefix delegation.

   DUID                      A DHCP Unique IDentifier for a DHCP
                             participant; each DHCP client and server
                             has exactly one DUID.  See Section 10 for
                             details of the ways in which a DUID may be
                             constructed.

   IA                        Identity Association: A collection of
                             leases assigned to a client.  Each IA has
                             an associated IAID.  A client may have more
                             than one IA assigned to it; for example,
                             one for each of its interfaces.  Each IA
                             holds one type of address; for example, an
                             identity association for temporary
                             addresses (IA_TA) holds temporary addresses
                             (see "identity association for temporary
                             addresses") and identity association for
                             prefix delegation (IA_PD) holds delegated
                             prefixes.  Throughout this document, "IA"
                             is used to refer to an identity association
                             without identifying the type of a lease in
                             the IA.  At the time of writing this
                             document, there are 3 IA types defined:
                             IA_NA, IA_TA and IA_PD.  New IA types may
                             be defined in the future.

   IAID                      Identity Association IDentifier: An
                             identifier for an IA, chosen by the client.
                             Each IA has an IAID, which is chosen to be
                             unique among IAIDs for IAs of a specific
                             type, belonging to that client.

   IA_NA                     Identity association for Non-temporary
                             Addresses: An IA that carries assigned



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                             addresses that are not temporary addresses
                             (see "identity association for temporary
                             addresses")

   IA_TA                     Identity Association for Temporary
                             Addresses: An IA that carries temporary
                             addresses (see [RFC4941]).

   IA_PD                     Identity Association for Prefix Delegation:
                             A collection of prefixes assigned to the
                             requesting router.  Each IA_PD has an
                             associated IAID.  A requesting router may
                             have more than one IA_PD assigned to it;
                             for example, one for each of its
                             interfaces.

   lease                     It is an address assigned by the DHCP
                             server to the client or a delegated prefix
                             assigned by the delegating router to the
                             requesting router.  Leases are carried in
                             the IA Address or IA Prefix options within
                             the IA_NA/IA_TA and IA_PD options
                             respectively.

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

   Reconfigure key           A key supplied to a client by a server used
                             to provide security for Reconfigure
                             messages.

   requesting router         The router that acts as a DHCP client and
                             is requesting prefix(es) to be assigned.

   singleton option          An option that is allowed to appear only
                             once.  Most options are singletons.

   relaying                  A DHCP relay agent relays DHCP messages
                             between DHCP participants.

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







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5.  Operational Models

   This section describes some of the current most common DHCP
   operational models.  The described models are not mutually exclusive
   and are sometimes used together.  For example, a device may start in
   stateful mode to obtain an address, and at a later time when an
   application is started, request additional parameters using stateless
   mode.

   This document assumes that the DHCP servers and the client,
   communicating with the servers via specific interface, belong to a
   single provisioning domain.

5.1.  Stateless DHCP

   Stateless DHCP [RFC3736] is used when DHCP is not used for obtaining
   a lease, but a node (DHCP client) desires one or more DHCP "other
   configuration" parameters, such as a list of DNS recursive name
   servers or DNS domain search lists [RFC3646].  Stateless may be used
   when a node initially boots or at any time the software on the node
   requires some missing or expired configuration information that is
   available via DHCP.

   This is the simplest and most basic operation for DHCP and requires a
   client (and a server) to support only two messages - Information-
   request and Reply.  Note that DHCP servers and relay agents typically
   also need to support the Relay-Forw and Relay-Reply messages to
   accommodate operation when clients and servers are not on the same
   link.

5.2.  DHCP for Non-Temporary Address Assignment

   This model of operation was the original motivation for DHCP and is
   the "stateful address autoconfiguration protocol" for IPv6 [RFC2462].
   It is appropriate for situations where stateless address
   autoconfiguration is not desired, because of network policy,
   additional requirements (such as updating the DNS with forward or
   reverse resource records), or client specific requirements (i.e.,
   some prefixes are only available to some clients) which are not
   possible using stateless address autoconfiguration.

   The model of operation for non-temporary address assignment is as
   follows.  The server is provided with IPv6 prefixes from which it may
   allocate addresses to clients, as well as any related network
   topology information as to which prefixes are present on which links.
   A client requests a non-temporary address to be assigned by the
   server.  The server allocates an address or addresses appropriate for




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   the link on which the client is connected.  The server returns the
   allocated address or addresses to the client.

   Each address has an associated preferred and valid lifetime, which
   constitutes an agreement about the length of time over which the
   client is allowed to use the address.  A client can request an
   extension of the lifetimes on an address and is required to terminate
   the use of an address if the valid lifetime of the address expires.

   Typically clients request other configuration parameters, such as the
   domain server addresses and search lists, when requesting addresses.

5.3.  DHCP for Prefix Delegation

   The prefix delegation mechanism, originally described in [RFC3633],
   is another stateful mode of operation and intended for simple
   delegation of prefixes from a delegating router (DHCP server) to
   requesting routers (DHCP clients).  It is appropriate for situations
   in which the delegating router does not have knowledge about the
   topology of the networks to which the requesting router is attached,
   and the delegating router does not require other information aside
   from the identity of the requesting router to choose a prefix for
   delegation.  For example, these options would be used by a service
   provider to assign a prefix to a Customer Edge Router device acting
   as a router between the subscriber's internal network and the service
   provider's core network.

   The design of this prefix delegation mechanism meets the requirements
   for prefix delegation in [RFC3769].

   The model of operation for prefix delegation is as follows.  A
   delegating router is provided IPv6 prefixes to be delegated to
   requesting routers.  A requesting router requests prefix(es) from the
   delegating router, as described in Section 19.  The delegating router
   chooses prefix(es) for delegation, and responds with prefix(es) to
   the requesting router.  The requesting router is then responsible for
   the delegated prefix(es).  For example, the requesting router might
   assign a subnet from a delegated prefix to one of its interfaces, and
   begin sending router advertisements for the prefix on that link.

   Each prefix has an associated valid and preferred lifetime, which
   constitutes an agreement about the length of time over which the
   requesting router is allowed to use the prefix.  A requesting router
   can request an extension of the lifetimes on a delegated prefix and
   is required to terminate the use of a delegated prefix if the valid
   lifetime of the prefix expires.





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   The mechanism through which the delegating router selects prefix(es)
   for delegation is not specified in this document.  Examples of ways
   in which the server might select prefix(es) for a client include:
   static assignment based on subscription to an ISP; dynamic assignment
   from a pool of available prefixes; selection based on an external
   authority such as a RADIUS server using the Framed-IPv6-Prefix option
   as described in [RFC3162].

   This prefix delegation mechanism would be appropriate for use by an
   ISP to delegate a prefix to a subscriber, where the delegated prefix
   would possibly be subnetted and assigned to the links within the
   subscriber's network.

   Figure 1 illustrates a network architecture in which prefix
   delegation could be used.


                      ______________________        \
                     /                      \        \
                    |    ISP core network    |        \
                     \__________ ___________/          |
                                |                      |
                        +-------+-------+              |
                        |  Aggregation  |              | ISP
                        |    device     |              | network
                        |  (delegating  |              |
                        |    router)    |              |
                        +-------+-------+              |
                                |                     /
                                |DSL to subscriber   /
                                |premises           /
                                |
                         +------+------+            \
                         |     CPE     |             \
                         | (requesting |              \
                         |   router)   |               |
                         +----+---+----+               |
                              |   |                    | Subscriber
       ---+-------------+-----+   +-----+------        | Network
          |             |               |              |
     +----+-----+ +-----+----+     +----+-----+        |
     |Subscriber| |Subscriber|     |Subscriber|       /
     |    PC    | |    PC    |     |    PC    |      /
     +----------+ +----------+     +----------+     /


                    Figure 1: Prefix Delegation Network




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   In this example, the delegating router is configured with a set of
   prefixes to be used for assignment to customers at the time of each
   customer's first connection to the ISP service.  The prefix
   delegation process begins when the requesting router requests
   configuration information through DHCP.  The DHCP messages from the
   requesting router are received by the delegating router in the
   aggregation device.  When the delegating router receives the request,
   it selects an available prefix or prefixes for delegation to the
   requesting router.  The delegating router then returns the prefix or
   prefixes to the requesting router.

   The requesting router subnets the delegated prefix and assigns the
   longer prefixes to links in the subscriber's network.  In a typical
   scenario based on the network shown in Figure 1, the requesting
   router subnets a single delegated /48 prefix into /64 prefixes and
   assigns one /64 prefix to each of the links in the subscriber
   network.

   The prefix delegation options can be used in conjunction with other
   DHCP options carrying other configuration information to the
   requesting router.  The requesting router may, in turn, provide DHCP
   service to hosts attached to the internal network.  For example, the
   requesting router may obtain the addresses of DNS and NTP servers
   from the ISP delegating router, and then pass that configuration
   information on to the subscriber hosts through a DHCP server in the
   requesting router.

   If the requesting router assigns a delegated prefix to a link to
   which the router is attached, and begins to send router
   advertisements for the prefix on the link, the requesting router MUST
   set the valid lifetime in those advertisements to be no later than
   the valid lifetime specified in the IA_PD Prefix option.  A
   requesting router MAY use the preferred lifetime specified in the
   IA_PD Prefix option.

5.4.  DHCP for Customer Edge Routers

   The DHCP requirements and network architecture for Customer Edge
   Routers are described in [RFC7084].  This model of operation combines
   address assignment (see Section 5.2) and prefix delegation (see
   Section 5.3).  In general, this model assumes that a single set of
   transactions between the client and server will assign or extend the
   client's non-temporary addresses and delegated prefixes.








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5.5.  DHCP for Temporary Addresses

   Temporary addresses were originally introduced to avoid privacy
   concerns with stateless address autoconfiguration, which based
   64-bits of the address on the EUI-64 (see [RFC3041] and [RFC4941]).
   They were added to DHCP to provide complementary support when
   stateful address assignment is used.

   Temporary address assignment works mostly like non-temporary address
   assignment (see Section 5.2), however these addresses are generally
   intended to be used for a short period of time and not to have their
   lifetimes extended, though they can be if required.

6.  DHCP Constants

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

6.1.  Multicast Addresses

   DHCP makes use of the following multicast addresses:

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

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

6.2.  UDP Ports

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

6.3.  DHCP Message Types

   DHCP defines the following message types.  More detail on these
   message types can be found in Section 7 and Section 8.  Message types




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   not listed here are reserved for future use.  The numeric encoding
   for each message type is shown in parentheses.

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

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

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

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

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

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

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

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





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

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

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

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

   RELAY-REPL (13) A server sends a Relay-reply message to a relay agent
                   containing a message that the relay agent delivers to
                   a client.  The Relay-reply message may be relayed by
                   other relay agents for delivery to the destination
                   relay agent.

                   The server encapsulates the client message as an
                   option in the Relay-reply message, which the relay
                   agent extracts and relays to the client.

6.4.  Status Codes

   DHCPv6 uses status codes to communicate the success or failure of
   operations requested in messages from clients and servers, and to
   provide additional information about the specific cause of the
   failure of a message.  The specific status codes are defined in
   Section 23.12.

   If the Status Code option does not appear in a message in which the
   option could appear, the status of the message is assumed to be
   Success.






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6.5.  Transmission and Retransmission Parameters

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

   +-----------------+-----------+-------------------------------------+
   | Parameter       | Default   | Description                         |
   +-----------------+-----------+-------------------------------------+
   | SOL_MAX_DELAY   | 1 sec     | Max delay of first Solicit          |
   | SOL_TIMEOUT     | 1 sec     | Initial Solicit timeout             |
   | SOL_MAX_RT      | 3600 secs | Max Solicit timeout value           |
   | REQ_TIMEOUT     | 1 sec     | Initial Request timeout             |
   | REQ_MAX_RT      | 30 secs   | Max Request timeout value           |
   | REQ_MAX_RC      | 10        | Max Request retry attempts          |
   | CNF_MAX_DELAY   | 1 sec     | Max delay of first Confirm          |
   | CNF_TIMEOUT     | 1 sec     | Initial Confirm timeout             |
   | CNF_MAX_RT      | 4 secs    | Max Confirm timeout                 |
   | CNF_MAX_RD      | 10 secs   | Max Confirm duration                |
   | REN_TIMEOUT     | 10 secs   | Initial Renew timeout               |
   | REN_MAX_RT      | 600 secs  | Max Renew timeout value             |
   | REB_TIMEOUT     | 10 secs   | Initial Rebind timeout              |
   | REB_MAX_RT      | 600 secs  | Max Rebind timeout value            |
   | INF_MAX_DELAY   | 1 sec     | Max delay of first Information-     |
   |                 |           | request                             |
   | INF_TIMEOUT     | 1 sec     | Initial Information-request timeout |
   | INF_MAX_RT      | 3600 secs | Max Information-request timeout     |
   |                 |           | value                               |
   | REL_TIMEOUT     | 1 sec     | Initial Release timeout             |
   | REL_MAX_RC      | 4         | MAX Release retry attempts          |
   | DEC_TIMEOUT     | 1 sec     | Initial Decline timeout             |
   | DEC_MAX_RC      | 4         | Max Decline retry attempts          |
   | REC_TIMEOUT     | 2 secs    | Initial Reconfigure timeout         |
   | REC_MAX_RC      | 8         | Max Reconfigure attempts            |
   | HOP_COUNT_LIMIT | 32        | Max hop count in a Relay-forward    |
   |                 |           | message                             |
   +-----------------+-----------+-------------------------------------+

6.6.  Representation of time values and "Infinity" as a time value

   All time values for lifetimes, T1 and T2 are unsigned integers.  The
   value 0xffffffff is taken to mean "infinity" when used as a lifetime
   (as in [RFC4861]) or a value for T1 or T2.

7.  Client/Server Message Formats

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




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

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

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

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


                  Figure 2: Client/Server message format

      msg-type             Identifies the DHCP message type; the
                           available message types are listed in
                           Section 6.3.

      transaction-id       The transaction ID for this message exchange.

      options              Options carried in this message; options are
                           described in Section 23.

8.  Relay Agent/Server Message Formats

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

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

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

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





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


                Figure 3: Relay Agent/Server message format

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

8.1.  Relay-forward Message

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

      msg-type             RELAY-FORW

      hop-count            Number of relay agents that have relayed this
                           message.

      link-address         An address that will be used by the server to
                           identify the link on which the client is
                           located.  This is typically global, site-
                           scoped or ULA [RFC4193], but see discussion
                           in Section 21.1.1.

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





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      options              MUST include a "Relay Message option" (see
                           Section 23.10); MAY include other options
                           added by the relay agent.

8.2.  Relay-reply Message

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

      msg-type             RELAY-REPL

      hop-count            Copied from the Relay-forward message

      link-address         Copied from the Relay-forward message

      peer-address         Copied from the Relay-forward message

      options              MUST include a "Relay Message option"; see
                           Section 23.10; MAY include other options

9.  Representation and Use of Domain Names

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

10.  DHCP Unique Identifier (DUID)

   Each DHCP client and server has a DUID.  DHCP servers use DUIDs to
   identify clients for the selection of configuration parameters and in
   the association of IAs with clients.  DHCP clients use DUIDs to
   identify a server in messages where a server needs to be identified.
   See Section 23.2 and Section 23.3 for the representation of a DUID in
   a DHCP message.

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

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



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   example, a device's DUID should not change as a result of a change in
   the device's network hardware.

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

10.1.  DUID Contents

   A DUID consists of a two-octet type code represented in network byte
   order, followed by a variable number of octets that make up the
   actual identifier.  The length of the DUID (not including the type
   code) is at least 1 octet and at most 128 octets.  The following
   types are currently defined:

      +------+------------------------------------------------------+
      | Type | Description                                          |
      +------+------------------------------------------------------+
      | 1    | Link-layer address plus time                         |
      | 2    | Vendor-assigned unique ID based on Enterprise Number |
      | 3    | Link-layer address                                   |
      | 4    | Universally Unique IDentifier (UUID) - see [RFC6355] |
      +------+------------------------------------------------------+

   Formats for the variable field of the DUID for the first 3 of the
   above types are shown below.  The fourth type, DUID-UUID [RFC6355],
   can be used in situations where there is a UUID stored in a device's
   firmware settings.

10.2.  DUID Based on Link-layer Address Plus Time, DUID-LLT

   This type of DUID consists of a two octet type field containing the
   value 1, a two octet hardware type code, four octets containing a
   time value, followed by link-layer address of any one network
   interface that is connected to the DHCP device at the time that the
   DUID is generated.  The time value is the time that the DUID is
   generated represented in seconds since midnight (UTC), January 1,
   2000, modulo 2^32.  The hardware type MUST be a valid hardware type
   assigned by the IANA as described in [RFC0826].  Both the time and
   the hardware type are stored in network byte order.  The link-layer
   address is stored in canonical form, as described in [RFC2464].

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





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


                         Figure 4: DUID-LLT format

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

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

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

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

   Despite our best efforts, it is possible that this algorithm for
   generating a DUID could result in a client identifier collision.  A
   DHCP client that generates a DUID-LLT using this mechanism MUST
   provide an administrative interface that replaces the existing DUID
   with a newly-generated DUID-LLT.



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10.3.  DUID Assigned by Vendor Based on Enterprise Number, DUID-EN

   This form of DUID is assigned by the vendor to the device.  It
   consists of the vendor's registered Private Enterprise Number as
   maintained by IANA [IANA-PEN] followed by a unique identifier
   assigned by the vendor.  The following diagram summarizes the
   structure of a DUID-EN:

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


                         Figure 5: DUID-EN format

   The source of the identifier is left up to the vendor defining it,
   but each identifier part of each DUID-EN MUST be unique to the device
   that is using it, and MUST be assigned to the device no later than at
   the first usage and stored in some form of non-volatile storage.
   This typically means being assigned during manufacture process in
   case of physical devices or when the image is created or booted for
   the first time in case of virtual machines.  The generated DUID
   SHOULD be recorded in non-erasable storage.  The enterprise-number is
   the vendor's registered Private Enterprise Number as maintained by
   IANA [IANA-PEN].  The enterprise-number is stored as an unsigned 32
   bit number.

   An example DUID of this type might look like this:

      +---+---+---+---+---+---+---+---+
      | 0 | 2 | 0 | 0 | 0 |  9| 12|192|
      +---+---+---+---+---+---+---+---+
      |132|211| 3 | 0 | 9 | 18|
      +---+---+---+---+---+---+


                         Figure 6: DUID-EN example






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

10.4.  DUID Based on Link-layer Address, DUID-LL

   This type of DUID consists of two octets containing the DUID type 3,
   a two octet network hardware type code, followed by the link-layer
   address of any one network interface that is permanently connected to
   the client or server device.  For example, a host that has a network
   interface implemented in a chip that is unlikely to be removed and
   used elsewhere could use a DUID-LL.  The hardware type MUST be a
   valid hardware type assigned by the IANA, as described in [RFC0826].
   The hardware type is stored in network byte order.  The link-layer
   address is stored in canonical form, as described in [RFC2464].  The
   following diagram illustrates the format of a DUID-LL:

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


                         Figure 7: DUID-LL format

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

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








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11.  Identity Association

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

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

11.1.  Identity Associations for Address Assignment

   A client must associate at least one distinct IA with each of its
   network interfaces for which it is to request the assignment of IPv6
   addresses from a DHCP server.  The client uses the IAs assigned to an
   interface to obtain configuration information from a server for that
   interface.  Each IA must be associated with exactly one interface.

   The configuration information in an IA consists of one or more IPv6
   addresses along with the times T1 and T2 for the IA.  See
   Section 23.4 for the representation of an IA in a DHCP message.

   Each address in an IA has a preferred lifetime and a valid lifetime,
   as defined in [RFC4862].  The lifetimes are transmitted from the DHCP
   server to the client in the IA option.  The lifetimes apply to the
   use of IPv6 addresses, as described in section 5.5.4 of [RFC4862].

11.2.  Identity Associations for Prefix Delegation

   An IA_PD is different from an IA for address assignment, in that it
   does not need to be associated with exactly one interface.  One IA_PD
   can be associated with the requesting router, with a set of
   interfaces or with exactly one interface.  A requesting router must
   create at least one distinct IA_PD.  It may associate a distinct
   IA_PD with each of its downstream network interfaces and use that
   IA_PD to obtain a prefix for that interface from the delegating
   router.





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   The configuration information in an IA_PD consists of one or more
   IPv6 prefixes along with the times T1 and T2 for the IA_PD.  See
   Section 23.21 for the representation of an IA_PD in a DHCP message.

12.  Selecting Addresses for Assignment to an IA

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

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

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

      *  If the server receives the message from a forwarding relay
         agent, then the client is on the same link as the one to which
         the interface, identified by the link-address field in the
         message from the relay agent, is attached.  According to
         [RFC6221], the server MUST ignore any link-address field whose
         value is zero.  The link address field referes to the link-
         address field of the Relay-Forward message, and the link-
         address fields in any Relay-Forward messages that may be nested
         within the Relay-Forward message.

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

   -  The DUID supplied by the client.

   -  Other information in options supplied by the client, e.g.  IA
      Address options that include the client's requests for specific
      addresses.

   -  Other information in options supplied by the relay agent.





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   Any address assigned by a server that is based on an EUI-64
   identifier MUST include an interface identifier with the "u"
   (universal/local) and "g" (individual/group) bits of the interface
   identifier set appropriately, as indicated in section 2.5.1 of
   [RFC4291].

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

13.  Management of Temporary Addresses

   A client may request the assignment of temporary addresses (see
   [RFC4941] for the definition of temporary addresses).  DHCPv6
   handling of address assignment is no different for temporary
   addresses.

   Clients ask for temporary addresses and servers assign them.
   Temporary addresses are carried in the Identity Association for
   Temporary Addresses (IA_TA) option (see Section 23.5).  Each IA_TA
   option contains at most one temporary address for each of the
   prefixes on the link to which the client is attached.

   The lifetime of the assigned temporary address is set in the IA
   Address Option (see Section 23.6) with in the IA_TA option.  It is
   RECOMMENDED to set short lifetimes, typically shorter than
   TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME (see Section 5,
   [RFC4941].

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

   A DHCPv6 server implementation MAY generate temporary addresses
   referring to the algorithm defined in Section 3.2.1, [RFC4941], with
   additional condition that the new address is not duplicated with any
   assigned addresses.

   The server MAY update the DNS for a temporary address, as described
   in section 4 of [RFC4941].

   On the clients, by default, temporary addresses are preferred in
   source address selection, according to Rule 7, [RFC6724].  However,
   this policy is overridable.

   One of the most important properties of temporary address is
   unlinkability of different actions over time.  So, it is NOT
   RECOMMENDED for a client to renew expired temporary addresses, though
   DHCPv6 provides such possibility (see Section 23.5).



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14.  Transmission of Messages by a Client

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

   A client uses multicast to reach all servers or an individual server.
   An individual server is indicated by specifying that server's DUID in
   a Server Identifier option (see Section 23.3) in the client's message
   (all servers will receive this message but only the indicated server
   will respond).  All servers are indicated by not supplying this
   option.

   A client may send some messages directly to a server using unicast,
   as described in Section 23.12.

14.1.  Rate Limiting

   In order to avoid prolonged message bursts that may be caused by
   possible logic loops, a DHCPv6 client MUST limit the rate of DHCPv6
   messages it transmits.  One example is that a client obtains an
   address, but does not like the response; it reverts back to Solicit
   procedure, discovers the same (sole) server, requests an address and
   gets the same address as before (the server still has the lease that
   was requested just previously).  This loops can repeat infinitely if
   there is not a quit/stop mechanism.  Therefore, a client must not
   initiate transmissions too frequently.

   A recommended method for implementing the rate limiting function is a
   token bucket, limiting the average rate of transmission to a certain
   number in a certain time.  This method of bounding burstiness also
   guarantees that the long-term transmission rate will not exceed.

      TRT     Transmission Rate Limit

   The Transmission Rate Limit parameter (TRT) SHOULD be configurable.
   A possible default could be 20 packets in 20 seconds.

   For a device that has multiple interfaces, the limit MUST be enforced
   on a per interface basis.

   Rate limiting of forwarded DHCPv6 messages and server-side messages
   are out of scope of this specification.







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14.2.  Choosing transmission time when T1,T2 is 0

   In certain cases, T1 and/or T2 timers may be set to zero.  Currently
   there are three such cases: 1. a client received IA_NA option with
   zeroed values; 2. a client received IA_PD option with zeroed values;
   3. a client received IA_TA option (which does not contain T1 or T2
   fields).  Additional cases may appear in the future.  This is an
   indication that the transmission times are left at client's
   discretion.  They are not completely discretionary, though.

   When T1 and/or T2 timers are set to zero, client MUST choose
   transmission time to avoid packet storms.  In particular, it MUST NOT
   transmit immediately.  If the client received multiple IA containers,
   it SHOULD pick renew and/or rebind transmission time so all IA
   containers are received in one exchange, if possible.  Client MUST
   choose the transmission times to not violate rate limiting
   restrictions, defined in Section 14.1.

15.  Reliability of Client Initiated Message Exchanges

   DHCP clients are responsible for reliable delivery of messages in the
   client-initiated message exchanges described in Section 18 and
   Section 19.  If a DHCP client fails to receive an expected response
   from a server, the client must retransmit its message.  This section
   describes the retransmission strategy to be used by clients in
   client-initiated message exchanges.

   Note that the procedure described in this section is slightly
   modified when used with the Solicit message.  The modified procedure
   is described in Section 18.1.2.

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

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

      RT      Retransmission timeout

      IRT     Initial retransmission time

      MRC     Maximum retransmission count

      MRT     Maximum retransmission time




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      MRD     Maximum retransmission duration

      RAND    Randomization factor

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

   Each of the computations of a new RT include a randomization factor
   (RAND), which is a random number chosen with a uniform distribution
   between -0.1 and +0.1.  The randomization factor is included to
   minimize synchronization of messages transmitted by DHCP clients.

   The algorithm for choosing a random number does not need to be
   cryptographically sound.  The algorithm SHOULD produce a different
   sequence of random numbers from each invocation of the DHCP client.

   RT for the first message transmission is based on IRT:

      RT = IRT + RAND*IRT

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

      RT = 2*RTprev + RAND*RTprev

   MRT specifies an upper bound on the value of RT (disregarding the
   randomization added by the use of RAND).  If MRT has a value of 0,
   there is no upper limit on the value of RT.  Otherwise:

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

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

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

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





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

   A client is not expected to listen for a response during the entire
   period between transmission of Solicit or Information-request
   messages.

16.  Message Validation

   Clients and servers might get messages that contain options not
   allowed to appear in the received message.  For example, an IA option
   is not allowed to appear in an Information-request message.  Clients
   and servers MAY choose either to extract information from such a
   message if the information is of use to the recipient, or to ignore
   such message completely and just drop it.

   If a server receives a message that contains options it should not
   contain (such as an Information-request message with an IA option),
   is missing options that it should contain, or is otherwise not valid,
   it MAY send a Reply (or Advertise as appropriate) with a Server
   Identifier option, a Client Identifier option if one was included in
   the message and a Status Code option with status UnSpecFail.

   Clients, relay agents and servers MUST NOT discard messages that
   contain unknown options (or instances of vendor options with unknown
   enterprise-numbers).  These should be ignored as if they weren't
   present.

   A server MUST discard any Solicit, Confirm, Rebind or Information-
   request messages it receives with a unicast destination address.

   A client or server MUST silently discard any received DHCPv6 messages
   with an unknown message type.

16.1.  Use of Transaction IDs

   The "transaction-id" field holds a value used by clients and servers
   to synchronize server responses to client messages.  A client SHOULD
   generate a random number that cannot easily be guessed or predicted
   to use as the transaction ID for each new message it sends.  Note
   that if a client generates easily predictable transaction
   identifiers, it may become more vulnerable to certain kinds of
   attacks from off-path intruders.  A client MUST leave the transaction
   ID unchanged in retransmissions of a message.







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16.2.  Solicit Message

   Clients MUST discard any received Solicit messages.

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

16.3.  Advertise Message

   Clients MUST discard any received Advertise message that meets any of
   the following conditions:

   -  the message does not include a Server Identifier option.

   -  the message does not include a Client Identifier option.

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

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

   Servers and relay agents MUST discard any received Advertise
   messages.

16.4.  Request Message

   Clients MUST discard any received Request messages.

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

   -  the message does not include a Server Identifier option.

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

   -  the message does not include a Client Identifier option.

16.5.  Confirm Message

   Clients MUST discard any received Confirm messages.

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




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16.6.  Renew Message

   Clients MUST discard any received Renew messages.

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

   -  the message does not include a Server Identifier option.

   -  the contents of the Server Identifier option does not match the
      server's identifier.

   -  the message does not include a Client Identifier option.

16.7.  Rebind Message

   Clients MUST discard any received Rebind messages.

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

16.8.  Decline Messages

   Clients MUST discard any received Decline messages.

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

   -  the message does not include a Server Identifier option.

   -  the contents of the Server Identifier option does not match the
      server's identifier.

   -  the message does not include a Client Identifier option.

16.9.  Release Message

   Clients MUST discard any received Release messages.

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

   -  the message does not include a Server Identifier option.

   -  the contents of the Server Identifier option does not match the
      server's identifier.




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   -  the message does not include a Client Identifier option.

16.10.  Reply Message

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

   -  the message does not include a Server Identifier option.

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

   If the client included a Client Identifier option in the original
   message, the Reply message MUST include a Client Identifier option
   and the contents of the Client Identifier option MUST match the DUID
   of the client; OR, if the client did not include a Client Identifier
   option in the original message, the Reply message MUST NOT include a
   Client Identifier option.

   Servers and relay agents MUST discard any received Reply messages.

16.11.  Reconfigure Message

   Servers and relay agents MUST discard any received Reconfigure
   messages.

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

   -  the message was not unicast to the client.

   -  the message does not include a Server Identifier option.

   -  the message does not include a Client Identifier option that
      contains the client's DUID.

   -  the message does not contain a Reconfigure Message option.

   -  the Reconfigure Message option msg-type is not a valid value.

   -  the message includes any IA options and the msg-type in the
      Reconfigure Message option is INFORMATION-REQUEST.

   -  the message does not include DHCP authentication:

      *  the message does not contain an authentication option.





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      *  the message does not pass the authentication validation
         performed by the client.

16.12.  Information-request Message

   Clients MUST discard any received Information-request messages.

   Servers MUST discard any received Information-request message that
   meets any of the following conditions:

   -  The message includes a Server Identifier option and the DUID in
      the option does not match the server's DUID.

   -  The message includes an IA option.

16.13.  Relay-forward Message

   Clients MUST discard any received Relay-forward messages.

16.14.  Relay-reply Message

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

17.  Client Source Address and Interface Selection

   Client's behavior is different depending on the purpose of the
   configuration.

17.1.  Address Assignment

   When a client sends a DHCP message to the
   All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the message
   through the interface for which configuration information is being
   requested.  However, the client MAY send the message through another
   interface if the interface is a logical interface without direct link
   attachment or the client is certain that two interfaces are attached
   to the same link.

   When a client sends a DHCP message directly to a server using unicast
   (after receiving the Server Unicast option from that server), the
   source address in the header of the IPv6 datagram MUST be an address
   assigned to the interface for which the client is interested in
   obtaining configuration and which is suitable for use by the server
   in responding to the client.







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17.2.  Prefix Delegation

   Delegated prefixes are not associated with a particular interface in
   the same way as addresses are for address assignment, and mentioned
   above.

   When a client (acting as requesting router) sends a DHCP message for
   the purpose of prefix delegation, it SHOULD be sent on the interface
   associated with the upstream router (ISP network).  The upstream
   interface is typically determined by configuration.  This rule
   applies even in the case where a separate IA_PD is used for each
   downstream interface.

   When a requesting router sends a DHCP message directly to a
   delegating router using unicast (after receiving the Server Unicast
   option from that delegating router), the source address SHOULD be an
   address from the upstream interface and which is suitable for use by
   the delegating router in responding to the requesting router.

18.  DHCP Server Solicitation

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

   The client is responsible for creating IAs and requesting that a
   server assign IPv6 addresses and/or delegated prefixes to the IAs.
   The client first creates the IAs and assigns IAIDs to them.  The
   client then transmits a Solicit message containing the IA options
   describing the IAs.  The client MUST NOT be using any of the
   addresses or delegated prefixes for which it tries to obtain the
   bindings by sending the Solicit message.  In particular, if the
   client had some valid bindings and has chosen to start the server
   solicitation process to obtain the bindings from a different server,
   the client MUST stop using the addresses and delegated prefixes for
   the bindings it had obtained from the previous server, and which it
   is now trying to obtain from a new server.

   Servers that can assign addresses or delegated prefixes to the IAs
   respond to the client with an Advertise message.  The client then
   initiates a configuration exchange as described in Section 19.

   If the client will accept a Reply message with committed leases
   assignments and other resources in response to the Solicit message,
   the client includes a Rapid Commit option (see Section 23.14) in the
   Solicit message.






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18.1.  Client Behavior

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

18.1.1.  Creation of Solicit Messages

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

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client includes IA options for any IAs to which
   it wants the server to assign leases.

   The client MAY include addresses in the IA_NA and IA_TA options as
   hints to the server about the addresses for which the client has a
   preference.

   The client MAY include values in the IA Prefix option encapsulated
   within IA_PD option as hints for the delegated prefix and/or prefix
   length for which the client has a preference.

   The client MUST NOT include any other options in the Solicit message,
   except as specifically allowed in the definition of individual
   options.

   The client uses IA_NA options to request the assignment of non-
   temporary addresses, IA_TA options to request the assignment of
   temporary addresses and IA_PD options to request prefix delegation.
   Either IA_NA, IA_TA or IA_PD options, or a combination of all, can be
   included in DHCP messages.  In addition, multiple instances of any IA
   option type can be included.

   The client MUST include an Option Request option (see Section 23.7)
   to request the SOL_MAX_RT option (see Section 23.23) and any other
   options the client is interested in receiving.  The client MAY
   additionally include instances of those options that are identified
   in the Option Request option, with data values as hints to the server
   about parameter values the client would like to have returned.

   The client includes a Reconfigure Accept option (see Section 23.20)
   if the client is willing to accept Reconfigure messages from the
   server.






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18.1.2.  Transmission of Solicit Messages

   The first Solicit message from the client on the interface MUST be
   delayed by a random amount of time between 0 and SOL_MAX_DELAY.  In
   the case of a Solicit message transmitted when DHCP is initiated by
   IPv6 Neighbor Discovery, the delay gives the amount of time to wait
   after IPv6 Neighbor Discovery causes the client to invoke the
   stateful address autoconfiguration protocol (see section 5.5.3 of
   [RFC4862]).  This random delay desynchronizes clients which start at
   the same time (for example, after a power outage).

   The client transmits the message according to Section 15, using the
   following parameters:

      IRT     SOL_TIMEOUT

      MRT     SOL_MAX_RT

      MRC     0

      MRD     0

   If the client has included a Rapid Commit option in its Solicit
   message, the client terminates the waiting process as soon as a Reply
   message with a Rapid Commit option is received.

   If the client is waiting for an Advertise message, the mechanism in
   Section 15 is modified as follows for use in the transmission of
   Solicit messages.  The message exchange is not terminated by the
   receipt of an Advertise before the first RT has elapsed.  Rather, the
   client collects Advertise messages until the first RT has elapsed.
   Also, the first RT MUST be selected to be strictly greater than IRT
   by choosing RAND to be strictly greater than 0.

   A client MUST collect Advertise messages for the first RT seconds,
   unless it receives an Advertise message with a preference value of
   255.  The preference value is carried in the Preference option
   (Section 23.8).  Any Advertise that does not include a Preference
   option is considered to have a preference value of 0.  If the client
   receives an Advertise message that includes a Preference option with
   a preference value of 255, the client immediately begins a client-
   initiated message exchange (as described in Section 19) by sending a
   Request message to the server from which the Advertise message was
   received.  If the client receives an Advertise message that does not
   include a Preference option with a preference value of 255, the
   client continues to wait until the first RT elapses.  If the first RT
   elapses and the client has received an Advertise message, the client




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   SHOULD continue with a client-initiated message exchange by sending a
   Request message.

   If the client does not receive any Advertise messages before the
   first RT has elapsed, it begins the retransmission mechanism
   described in Section 15.  The client terminates the retransmission
   process as soon as it receives any Advertise message, and the client
   acts on the received Advertise message without waiting for any
   additional Advertise messages.

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

18.1.3.  Receipt of Advertise Messages

   The client MUST process SOL_MAX_RT and INF_MAX_RT options in an
   Advertise message, even if the message contains a Status Code option
   indicating a failure, and the Advertise message will be discarded by
   the client.

   The client MUST ignore any Advertise message that contains no
   addresses (IAADDR options encapsulated in IA_NA or IA_TA options) and
   no delegated prefixes (IAPREFIX options encapsulated in IA_PD
   options) with the exception that the client:

   -  MUST process an included SOL_MAX_RT option and

   -  MUST process an included INF_MAX_RT option.

   A client can display any associated status message(s) to the user or
   activity log.

   The client ignoring this Advertise message MUST NOT restart the
   Solicit retransmission timer.

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

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





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   -  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.

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

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

   In practice, this means that the client will maintain independent
   per-IA state machines per each selected server.

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

18.1.4.  Receipt of Reply Message

   If the client includes a Rapid Commit option in the Solicit message,
   it will expect a Reply message that includes a Rapid Commit option in
   response.  The client discards any Reply messages it receives that do
   not include a Rapid Commit option.  If the client receives a valid
   Reply message that includes a Rapid Commit option, it processes the
   message as described in Section 19.1.8.  If it does not receive such
   a Reply message and does receive a valid Advertise message, the
   client processes the Advertise message as described in
   Section 18.1.3.

   If the client subsequently receives a valid Reply message that
   includes a Rapid Commit option, it either:

   -  processes the Reply message as described in Section 19.1.8, and
      discards any Reply messages received in response to the Request
      message, or

   -  processes any Reply messages received in response to the Request
      message and discards the Reply message that includes the Rapid
      Commit option.








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18.2.  Server Behavior

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

18.2.1.  Receipt of Solicit Messages

   The server determines the information about the client and its
   location as described in Section 12 and checks its administrative
   policy about responding to the client.  If the server is not
   permitted to respond to the client, the server discards the Solicit
   message.  For example, if the administrative policy for the server is
   that it may only respond to a client that is willing to accept a
   Reconfigure message, if the client does not include a Reconfigure
   Accept option (see Section 23.20) in the Solicit message, the servers
   discard the Solicit message.

   If the client has included a Rapid Commit option in the Solicit
   message and the server has been configured to respond with committed
   lease assignments and other resources, the server responds to the
   Solicit with a Reply message as described in Section 18.2.3.
   Otherwise, the server ignores the Rapid Commit option and processes
   the remainder of the message as if no Rapid Commit option were
   present.

18.2.2.  Creation and Transmission of Advertise Messages

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

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

   The server includes a Reconfigure Accept option if the server wants
   to require that the client accept Reconfigure messages.

   The server includes options the server will return to the client in a
   subsequent Reply message.  The information in these options may be
   used by the client in the selection of a server if the client
   receives more than one Advertise message.  If the client has included



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   an Option Request option in the Solicit message, the server includes
   options in the Advertise message containing configuration parameters
   for all of the options identified in the Option Request option that
   the server has been configured to return to the client.  The server
   MAY return additional options to the client if it has been configured
   to do so.  The server must be aware of the recommendations on packet
   sizes and the use of fragmentation in section 5 of [RFC2460].

   If the Solicit message from the client included one or more IA
   options, the server MUST include IA options in the Advertise message
   containing any addresses and/or delegated prefixes that would be
   assigned to IAs contained in the Solicit message from the client.  If
   the client has included addresses in the IA in the Solicit message,
   the server MAY use those addresses as hints about the addresses that
   the client would like to receive.  If the client has included IA
   Prefix option in the IA_PD, the server MAY use the prefix contained
   in the IPv6 prefix field and/or the prefix length contained in the
   "prefix-length" field as a hints about the prefixes the client would
   like to receive.  If the server is not going to assign an address or
   delegated prefix received as a hint in the Solicit message, the
   server MUST NOT include this address or delegated prefix in the
   Advertise message

   If the server will not assign any addresses to an IA (IA_NA or IA_IA)
   in subsequent Request from the client, the server MUST include the IA
   in the Advertise message with no addresses in the IA and a Status
   Code option encapsulated in the IA containing status code
   NoAddrsAvail.

   If the server will not assign any prefixes to an IA_PD in subsequent
   Request from the client, the server MUST include the IA_PD in the
   Advertise message with no prefixes in the IA and a Status Code option
   encapsulated in the IA_PD containing status code NoPrefixAvail.

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

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



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18.2.3.  Creation and Transmission of Reply Messages

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

   DISCUSSION:

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

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

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

   The server includes a Reconfigure Accept option if the server wants
   to require that the client accept Reconfigure messages.

   The server produces the Reply message as though it had received a
   Request message, as described in Section 19.2.1.  The server
   transmits the Reply message as described in Section 19.2.8.

19.  DHCP Client-Initiated Configuration Exchange

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

   According to a terminology for the prefix delegation, a client
   requesting a delegation of a prefix is referred to as a requesting
   router and a server delegating the prefix is referred to as a
   delegating router.  The requesting router and the delegating router
   use the IA_PD Prefix option to exchange information about prefix(es)
   in much the same way as IA Address options are used for assigned



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   addresses.  Typically, a single DHCP session is used to exchange
   information about addresses and prefixes, i.e.  IA_NA and IA_PD
   options are carried in the same message.

19.1.  Client Behavior

   A client uses Request, Renew, Rebind, Release and Decline messages
   during the normal life cycle of addresses and delegated prefixes.
   When a client detects it may have moved to a new link, it uses
   Confirm if it only has addresses and Rebind if it has delegated
   prefixes (and addresses).  It uses Information-request messages when
   it needs configuration information but no addresses and no prefixes.

   If the client has a source address of sufficient scope that can be
   used by the server as a return address, and the client has received a
   Server Unicast option (Section 23.12) from the server, the client
   SHOULD unicast any Request, Renew, Release and Decline messages to
   the server.

   DISCUSSION:

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

19.1.1.  Creation and Transmission of Request Messages

   The client uses a Request message to populate IAs with leases and
   obtain other configuration information.  The client includes one or
   more IA options in the Request message.  The server then returns
   leases and other information about the IAs to the client in IA
   options in a Reply message.

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

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

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




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   The client MUST include an Option Request option (see Section 23.7)
   to indicate the options the client is interested in receiving.  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   The client includes a Reconfigure Accept option (see Section 23.20)
   indicating whether or not the client is willing to accept Reconfigure
   messages from the server.

   The client transmits the message according to Section 15, using the
   following parameters:

      IRT     REQ_TIMEOUT

      MRT     REQ_MAX_RT

      MRC     REQ_MAX_RC

      MRD     0

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

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

   -  Initiate the server discovery process described in Section 18.

   -  Terminate the configuration process and report failure.

19.1.2.  Creation and Transmission of Confirm Messages

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

   o  The client reboots (and has no stable storage or persisted DHCP
      state).

   o  The client is physically connected to a wired connection.

   o  The client returns from sleep mode.

   o  The client using a wireless technology changes access points.





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   In any situation when a client may have moved to a new link and the
   client does not have any delegated prefixes obtained from the DHCP
   server from which it has obtained the addresses, the client SHOULD
   initiate a Confirm/Reply message exchange.  The client includes any
   IAs assigned to the interface that may have moved to a new link,
   along with the addresses associated with those IAs, in its Confirm
   message.  Any responding servers will indicate whether those
   addresses are appropriate for the link to which the client is
   attached with the status in the Reply message it returns to the
   client.

   If the client has any valid delegated prefixes obtained from the DHCP
   server from which it has obtained the addresses, the client initiates
   Rebind/Reply exchange as described in Section 19.1.9 instead of
   sending the Confirm message.

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

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client includes IA options for all of the IAs
   assigned to the interface for which the Confirm message is being
   sent.  The IA options include all of the addresses the client
   currently has associated with those IAs.  The client SHOULD set the
   T1 and T2 fields in any IA_NA options and the preferred-lifetime and
   valid-lifetime fields in the IA Address options to 0, as the server
   will ignore these fields.

   The first Confirm message from the client on the interface MUST be
   delayed by a random amount of time between 0 and CNF_MAX_DELAY.  The
   client transmits the message according to Section 15, using the
   following parameters:

      IRT     CNF_TIMEOUT

      MRT     CNF_MAX_RT

      MRC     0

      MRD     CNF_MAX_RD

   If the client receives no responses before the message transmission
   process terminates, as described in Section 15, the client SHOULD
   continue to use any IP addresses, using the last known lifetimes for
   those addresses, and SHOULD continue to use any other previously
   obtained configuration parameters.




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19.1.3.  Creation and Transmission of Renew Messages

   To extend the valid and preferred lifetimes for the leases assigned
   to the IAs, the client sends a Renew message to the server from which
   the leases were obtained, which includes IA options for the IAs whose
   lease lifetimes are to be extended.  The client includes IA Address
   options within IA_NA and IA_TA options for the addresses assigned to
   the IAs.  The client includes IA Prefix options within IA_PD options
   for the delegated prefixes assigned to the IAs.  The server
   determines new lifetimes for the leases according to the
   administrative configuration of the server.  The server may also add
   leases to the IAs.  The server can remove leases from the IAs by
   returning IA Address options (for IA_NA and IA_TA) and IA Prefix
   options (for IA_PD) with preferred and valid lifetimes set to 0.

   The server controls the time at which the client contacts the server
   to extend the lifetimes on assigned leases through the T1 and T2
   parameters assigned to an IA.  However, as the client Renews/Rebinds
   all IAs from the server at the same time, the client MUST select a T1
   and T2 time from all IA options, which will guarantee that the client
   will send Renew/Rebind messages not later than at the T1/T2 times
   associated with any of the client's bindings.

   At time T1, the client initiates a Renew/Reply message exchange to
   extend the lifetimes on any leases in the IA.

   If T1 or T2 had been set to 0 by the server (for an IA_NA or IA_PD)
   or there are no T1 or T2 times (for an IA_TA) in a previous Reply,
   the client may send a Renew or Rebind message, respectively, at the
   client's discretion.  The client MUST follow the rules defined in
   Section 14.2.

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

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

   The client MUST include a Client Identifier option to identify itself
   to the server.  The client adds any appropriate options, including
   one or more IA options.

   For IAs to which leases have been assigned, the client includes a
   corresponding IA option containing an IA Address option for each
   address assigned to the IA and IA Prefix option for each prefix
   assigned to the IA.  The client MUST NOT include addresses and




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   prefixes in any IA option that the client did not obtain from the
   server or that are no longer valid (that have a valid lifetime of 0).

   The client MAY include an IA option for each binding it desires but
   has been unable to obtain.  In this case, if the client includes the
   IA_PD option to request prefix delegation, the client MAY include the
   IA Prefix option encapsulated within the IA_PD option, with the IPv6
   prefix field set to 0 and the "prefix-length" field set to the
   desired length of the prefix to be delegated.  The server MAY use
   this value as a hint for the prefix length.  The client SHOULD NOT
   include IA Prefix option with the IPv6 prefix field set to 0 unless
   it is supplying a hint for the prefix length.

   The client MUST include an Option Request option (see Section 23.7)
   to indicate the options the client is interested in receiving.  The
   client MAY include options with data values as hints to the server
   about parameter values the client would like to have returned.

   The client transmits the message according to Section 15, using the
   following parameters:

   IRT        REN_TIMEOUT

   MRT        REN_MAX_RT

   MRC        0

   MRD        Remaining time until T2

   The message exchange is terminated when time T2 is reached (see
   Section 19.1.4), at which time the client begins a Rebind message
   exchange.

19.1.4.  Creation and Transmission of Rebind Messages

   At time T2 (which will only be reached if the server to which the
   Renew message was sent at time T1 has not responded), the client
   initiates a Rebind/Reply message exchange with any available server.

   The client constructs the Rebind message as described in section
   18.1.3 with the following differences:

   -  The client sets the "msg-type" field to REBIND.

   -  The client does not include the Server Identifier option in the
      Rebind message.





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   The client transmits the message according to Section 15, using the
   following parameters:

      IRT     REB_TIMEOUT

      MRT     REB_MAX_RT

      MRC     0

      MRD     Remaining time until valid lifetimes of all addresses in
              all IAs have expired

   If all leases for an IA have expired, the client may choose to
   include this IA in subsequent Rebind messages to indicate that the
   client is interested in assignment of the leases to this IA.

   The message exchange is terminated when the valid lifetimes of all
   leases across all IAs have expired, at which time the client uses the
   Solicit message to locate a new DHCP server and sends a Request for
   the expired IAs to the new server.

19.1.5.  Creation and Transmission of Information-request Messages

   The client uses an Information-request message to obtain
   configuration information without having addresses and/or delegated
   prefixes assigned to it.

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

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

   The client MUST include an Option Request option (see Section 23.7)
   to request the INF_MAX_RT option (see Section 23.24) and any other
   options the client is interested in receiving.  The client MAY
   include options with data values as hints to the server about
   parameter values the client would like to have returned.

   The first Information-request message from the client on the
   interface MUST be delayed by a random amount of time between 0 and




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   INF_MAX_DELAY.  The client transmits the message according to
   Section 15, using the following parameters:

      IRT     INF_TIMEOUT

      MRT     INF_MAX_RT

      MRC     0

      MRD     0

19.1.6.  Creation and Transmission of Release Messages

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

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

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

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

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

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

   The client transmits the message according to Section 15, using the
   following parameters:

      IRT     REL_TIMEOUT

      MRT     0

      MRC     REL_MAX_RC



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      MRD     0

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

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

19.1.7.  Creation and Transmission of Decline Messages

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

   The Decline message is not used in prefix delegation and thus the
   client MUST NOT include IA_PD options in the Decline message.

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

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

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

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

   The client transmits the message according to Section 15, using the
   following parameters:

      IRT     DEC_TIMEOUT

      MRT     0




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      MRC     DEC_MAX_RC

      MRD     0

   If addresses are declined but the Reply from a DHCP server is lost,
   the client will retransmit the Decline message, and the server may
   respond with a Reply indicating a status of NoBinding.  Therefore,
   the client does not treat a Reply message with a status of NoBinding
   in a Decline message exchange as if it indicates an error.

   The client SHOULD NOT send a Release message for other bindings it
   may have received just because it sent a Decline message.  The client
   SHOULD retain the non-conflicting bindings.  The client SHOULD treat
   the failure to acquire a binding as a result of the conflict, to be
   equivalent to not having received the binding, insofar as it behaves
   when sending Renew and Rebind messages.

19.1.8.  Receipt of Reply Messages

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

   If the client receives a Reply message with a status code containing
   UnspecFail, the server is indicating that it was unable to process
   the message due to an unspecified failure condition.  If the client
   retransmits the original message to the same server to retry the
   desired operation, the client MUST limit the rate at which it
   retransmits the message and limit the duration of the time during
   which it retransmits the message (see Section 14.1).

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

   When the client receives a NotOnLink status from the server in
   response to a Confirm message, the client performs DHCP server
   solicitation, as described in Section 18, and client-initiated
   configuration, as described in Section 19.  If the client receives
   any Reply messages that do not indicate a NotOnLink status, the
   client can use the addresses in the IA and ignore any messages that
   indicate a NotOnLink status.




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   When the client receives a NotOnLink status from the server in
   response to a Solicit (with a Rapid Commit option) or a Request, the
   client can either re-issue the message without specifying any
   addresses or restart the DHCP server discovery process (see
   Section 18).

   The client SHOULD perform duplicate address detection [RFC4862] on
   each of the received addresses in any IAs, on which it has not
   performed duplicate address detection during processing of any of the
   previous Reply messages from the server.  The client performs the
   duplicate address detection before using the received addresses for
   the traffic.  If any of the addresses are found to be in use on the
   link, the client sends a Decline message to the server for those
   addresses as described in Section 19.1.7.

   If the Reply was received in response to a Solicit (with a Rapid
   Commit option), Request, Renew, or Rebind message, the client updates
   the information it has recorded about IAs from the IA options
   contained in the Reply message:

   -  Record T1 and T2 times.

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

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

   -  Discard any leases from the IA, as recorded by the client, that
      have a valid lifetime of 0 in the IA Address or IA Prefix option.

   -  Leave unchanged any information about leases the client has
      recorded in the IA but that were not included in the IA from the
      server.

   Management of the specific configuration information is detailed in
   the definition of each option in Section 23.

   The client examines the status code in each IA individually.  If the
   client receives a NoAddrsAvail and/or NoPrefixAvail status code, the
   client has received no usable addresses and/or delegated prefixes in
   the IA.

   If the client can operate with the addresses and prefixes obtained
   from the server, the client uses the addresses, delegated prefixes,
   and other information from any IAs that do not contain a Status Code
   option with the NoAddrsAvail or NoPrefixAvail status code.  The
   client MAY include the IAs for which it received the NoAddrsAvail or



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   NoPrefixAvail status code, with no addresses or prefixes, in
   subsequent Renew and Rebind messages sent to the server, to retry
   obtaining the addresses or prefixes for these IAs.

   If the client cannot operate without the addresses or prefixes for
   the IAs for which it received the NoAddrsAvail or NoPrefixAvail
   status code, the client may try another server (perhaps by restarting
   the DHCP server discovery process).

   If the client finds no usable addresses and prefixes in any of the
   IAs, it may either try another server (perhaps restarting the DHCP
   server discovery process) or use the Information-request message to
   obtain other configuration information only.

   Whenever a client restarts the DHCP server discovery process or
   selects an alternate server, as described in Section 18.1.3, the
   client SHOULD stop using all the addresses and delegated prefixes for
   which it has the bindings and try to obtain all required leases from
   the new server.  This facilitates the client using a single state
   machine for all bindings.

   When the client receives a Reply message in response to a Renew or
   Rebind message, the client:

   -  sends a Request message if any of the IAs in the Reply message
      contains the NoBinding status code.  The client places IA options
      in this message for only those IAs for which the server returned
      the NoBinding status code in the Reply message.  The client
      continues to use other bindings for which the server did not
      return an error.

   -  sends a Renew/Rebind if any of the IAs are not in the Reply
      message, but in this case the client MUST limit the rate at which
      it sends these messages, to avoid the Renew/Rebind storm.

   -  otherwise accepts the information in the IA.

   When the requesting router receives a valid Reply message, for each
   IA_PD the delegating router assigns a subnet from each of the
   delegated prefixes to each of the links to which the associated
   interfaces are attached.

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






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

19.1.9.  Bindings Verification by Requesting Router

   In some circumstances the requesting router may need verification
   that the delegating router still has a valid binding for the
   requesting router.  Examples of times when a requesting router may
   ask for such verification include:

   o  The requesting router reboots.

   o  The requesting router's upstream link flaps.

   o  The requesting router is physically disconnected from a wired
      connection.

   If such verification is needed the requesting router MUST initiate a
   Rebind/Reply message exchange as described in Section 19.1.4, with
   the exception that the retransmission parameters should be set as for
   the Confirm message, described in Section 19.1.2.  The requesting
   router includes any IA_PDs, along with prefixes associated with those
   IA_PDs in its Rebind message.

19.2.  Server Behavior

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

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

   In most Reply messages, the server includes options containing
   configuration information for the client.  The server must be aware
   of the recommendations on packet sizes and the use of fragmentation
   in section 5 of [RFC2460].  If the client included an Option Request
   option in its message, the server includes options in the Reply
   message containing configuration parameters for all of the options
   identified in the Option Request option that the server has been
   configured to return to the client.  The server MAY return additional
   options to the client if it has been configured to do so.






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19.2.1.  Receipt of Request Messages

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

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

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

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

   The server examines all IAs in the message from the client.

   For each IA_NA and IA_TA the server checks if the prefixes on
   included IP addresses are appropriate for the link to which the
   client is connected.  If any of the prefixes on the included IP
   addresses is not appropriate for the link to which the client is
   connected, the server MUST return the IA to the client with a Status
   Code option with the value NotOnLink.  If the server does not send
   the NotOnLink status code but it cannot assign any IP addresses to an
   IA, the server MUST return the IA in the Reply message with no
   addresses in the IA and a Status Code option containing status code
   NoAddrsAvail.

   For any IA_PD to which the server cannot assign any delegated
   prefixes, the server MUST return the IA_PD option in the Reply
   message with the Status Code option containing status code
   NoPrefixAvail.

   The server MAY assign different addresses and/or delegated prefixes
   to an IA than included in the IA within the Request message sent by
   the client.

   For all IAs to which the server can assign addresses or delegated
   prefixes, the server includes the IAs with addresses (for IA_NA and
   IA_TA), prefixes (for IA_PD) and other configuration parameters, and



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   records the IA as a new client binding.  The server MUST NOT include
   any addresses or delegated prefixes in the IA which the server does
   not assign to the client.

   The server includes a Reconfigure Accept option if the server wants
   to require that the client accept Reconfigure messages.

   The server includes other options containing configuration
   information to be returned to the client as described in
   Section 19.2.

   If the server finds that the client has included an IA in the Request
   message for which the server already has a binding that associates
   the IA with the client, the client has resent a Request message for
   which it did not receive a Reply message.  The server either resends
   a previously cached Reply message or sends a new Reply message.

19.2.2.  Receipt of Confirm Messages

   When the server receives a Confirm message, the server determines
   whether the addresses in the Confirm message are appropriate for the
   link to which the client is attached.  If all of the addresses in the
   Confirm message pass this test, the server returns a status of
   Success.  If any of the addresses do not pass this test, the server
   returns a status of NotOnLink.  If the server is unable to perform
   this test (for example, the server does not have information about
   prefixes on the link to which the client is connected), or there were
   no addresses in any of the IAs sent by the client, the server MUST
   NOT send a Reply to the client.

   The server ignores the T1 and T2 fields in the IA options and the
   preferred-lifetime and valid-lifetime fields in the IA Address
   options.

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

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

19.2.3.  Receipt of Renew Messages

   When the server receives a Renew message via unicast from a client to
   which the server has not sent a unicast option (or is not currently
   configured to send a unicast option to the client), the server



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   discards the Renew message and responds with a Reply message
   containing a Status Code option with the value UseMulticast, a Server
   Identifier option containing the server's DUID, the Client Identifier
   option from the client message, and no other options.

   For each IA in the Renew message from a client, the server locates
   the client's binding and verifies that the information in the IA from
   the client matches the information stored for that client.

   If the server finds the client entry for the IA, the server sends
   back the IA to the client with new lifetimes and, if applicable, T1/
   T2 times.  If the server is unable to extend the lifetimes of an
   address or delegated prefix in the IA, the server MAY choose not to
   include the IA Address or IA Prefix option for this address or
   delegated prefix.

   The server may choose to change the list of addresses or delegated
   prefixes and the lifetimes in IAs that are returned to the client.

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

   If the server finds that any of the delegated prefixes in the IA are
   not appropriate for the link to which the client is attached, the
   server returns the delegated prefix to the client with lifetimes of
   0.

   For each IA for which the server cannot find a client entry, the
   server has the following choices depending on the server's policy and
   configuration information:

   -  If the server is configured to create new bindings as a result of
      processing Renew messages, the server SHOULD create a binding and
      return the IA with assigned addresses or delegated prefixes with
      lifetimes and, if applicable, T1/T2 times and other information
      requested by the client.  If the client included the IA Prefix
      option within the IA_PD option with the non-zero value in the
      "prefix-length" field, the server MAY use this value as a hint for
      the length of the prefixes to be assigned (see
      [I-D.cui-dhc-dhcpv6-prefix-length-hint-issue] for further details
      on prefix length hints).

   -  If the server is configured to create new bindings as a result of
      processing Renew messages, but the server will not assign any
      leases to an IA, the server returns the IA option containing a
      Status Code option with the NoAddrsAvail or NoPrefixAvail status
      code and a status message for a user.



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   -  If the server does not support creation of new bindings for the
      client sending a Renew message, or if this behavior is disabled
      according to the server's policy or configuration information, the
      server returns the IA option containing a Status Code option with
      the NoBinding status code and a status message for a user.

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

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

   The server includes other options containing configuration
   information to be returned to the client as described in
   Section 19.2.

   The T1/T2 times set in each applicable IA option for a Reply MUST be
   the same values across all IAs.  The server MUST determine the T1/T2
   times across all of the applicable client's bindings in the Reply.
   This facilitates the client being able to renew all of the bindings
   at the same time.

19.2.4.  Receipt of Rebind Messages

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

   If the server finds the client entry for the IA and the server
   determines that the addresses or delegated prefixes in the IA are
   appropriate for the link to which the client's interface is attached
   according to the server's explicit configuration information, the
   server SHOULD send back the IA to the client with new lifetimes and,
   if applicable, T1/T2 times.  If the server is unable to extend the
   lifetimes of an address in the IA, the server MAY choose not to
   include the IA Address option for this address.  If the server is
   unable to extend the lifetimes of a delegated prefix in the IA, the
   server MAY choose not to include the IA Prefix option for this
   prefix.

   If the server finds that the client entry for the IA and any of the
   addresses or delegated prefixes are no longer appropriate for the
   link to which the client's interface is attached according to the
   server's explicit configuration information, the server returns the
   address or delegated prefix to the client with lifetimes of 0.



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   If the server cannot find a client entry for the IA, the server
   checks if the IA contains addresses (for IA_NA and IA_TA) or
   delegated prefixes (for IA_PD).  The server checks if the addresses
   and delegated prefixes are appropriate for the link to which the
   client's interface is attached according to the server's explicit
   configuration information.  For any address which is not appropriate
   for the link to which the client's interface is attached, the server
   MAY include the IA Address option with the lifetimes of 0.  For any
   delegated prefix which is not appropriate for the link to which the
   client's interface is attached, the server MAY include the IA Prefix
   option with the lifetimes of 0.  The Reply with lifetimes of 0
   constitutes an explicit notification to the client that the specific
   addresses and delegated prefixes are no longer valid and MUST NOT be
   used by the client.  In this situation, if the server does not send a
   Reply message, it silently discards the Rebind message.

   Otherwise, for each IA for which the server cannot find a client
   entry, the server has the following choices depending on the server's
   policy and configuration information:

   -  If the server is configured to create new bindings as a result of
      processing Rebind messages (also see the note about the Rapid
      Commit option below), the server SHOULD create a binding and
      return the IA with allocated leases with lifetimes and, if
      applicable, T1/T2 times and other information requested by the
      client.  The server MUST NOT return any addresses or delegated
      prefixes in the IA which the server does not assign to the client.

   -  If the server is configured to create new bindings as a result of
      processing Rebind messages, but the server will not assign any
      leases to an IA, the server returns the IA option containing a
      Status Code option with the NoAddrsAvail or NoPrefixAvail status
      code and a status message for a user.

   -  If the server does not support creation of new bindings for the
      client sending a Rebind message, or if this behavior is disabled
      according to the server's policy or configuration information, the
      server returns the IA option containing a Status Code option with
      the NoBinding status code and a status message for a user.

   When the server creates new bindings for the IA, it is possible that
   other servers also create bindings as a result of receiving the same
   Rebind message.  This is the same issue as in the Discussion under
   "Rapid Commit Option"; see Section 23.14.  Therefore, the server
   SHOULD only create new bindings during processing of a Rebind message
   if the server is configured to respond with a Reply message to a
   Solicit message containing the Rapid Commit option.




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

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

   The server includes other options containing configuration
   information to be returned to the client as described in
   Section 19.2.

   The T1/T2 times set in each applicable IA option for a Reply MUST be
   the same values across all IAs.  The server MUST determine the T1/T2
   times across all of the applicable client's bindings in the Reply.
   This facilitates the client being able to renew all of the bindings
   at the same time.

19.2.5.  Receipt of Information-request Messages

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

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

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

   The server includes options containing configuration information to
   be returned to the client as described in Section 19.2.

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







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19.2.6.  Receipt of Release Messages

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

   Upon the receipt of a valid Release message, the server examines the
   IAs and the leases in the IAs for validity.  If the IAs in the
   message are in a binding for the client, and the leases in the IAs
   have been assigned by the server to those IAs, the server deletes the
   leases from the IAs and makes the leases available for assignment to
   other clients.  The server ignores leases not assigned to the IA,
   although it may choose to log an error.

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

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

19.2.7.  Receipt of Decline Messages

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

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



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   The client has found any addresses in the Decline messages to be
   already in use on its link.  Therefore, the server SHOULD mark the
   addresses declined by the client so that those addresses are not
   assigned to other clients, and MAY choose to make a notification that
   addresses were declined.  Local policy on the server determines when
   the addresses identified in a Decline message may be made available
   for assignment.

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

19.2.8.  Transmission of Reply Messages

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

   If the original message was received in a Relay-forward message, the
   server constructs a Relay-reply message with the Reply message in the
   payload of a Relay Message option (see Section 23.10).  If the Relay-
   forward messages included an Interface-id option, the server copies
   that option to the Relay-reply message.  The server unicasts the
   Relay-reply message directly to the relay agent using the address in
   the source address field from the IP datagram in which the Relay-
   forward message was received.

20.  DHCP Server-Initiated Configuration Exchange

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








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20.1.  Server Behavior

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

20.1.1.  Creation and Transmission of Reconfigure Messages

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

   The server MAY include an Option Request option to inform the client
   of what information has been changed or new information that has been
   added.  In particular, the server specifies the IA option in the
   Option Request option if the server wants the client to obtain new
   address information.  If the server identifies the IA option in the
   Option Request option, the server MUST include an IA option to
   identify each IA that is to be reconfigured on the client.  The IA
   options included by the server MUST NOT contain any options.

   Because of the risk of denial of service attacks against DHCP
   clients, the use of a security mechanism is mandated in Reconfigure
   messages.  The server MUST use DHCP authentication in the Reconfigure
   message.

   The server MUST include a Reconfigure Message option (defined in
   Section 23.19) to select whether the client responds with a Renew
   message, a Rebind message, or an Information-request message.

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

   A server sends each Reconfigure message to a single DHCP client,
   using an IPv6 unicast address of sufficient scope belonging to the
   DHCP client.  If the server does not have an address to which it can
   send the Reconfigure message directly to the client, the server uses
   a Relay-reply message (as described in Section 21.3) to send the
   Reconfigure message to a relay agent that will relay the message to
   the client.  The server may obtain the address of the client (and the
   appropriate relay agent, if required) through the information the
   server has about clients that have been in contact with the server,
   or through some external agent.

   To reconfigure more than one client, the server unicasts a separate
   message to each client.  The server may initiate the reconfiguration



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   of multiple clients concurrently; for example, a server may send a
   Reconfigure message to additional clients while previous
   reconfiguration message exchanges are still in progress.

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

20.1.2.  Time Out and Retransmission of Reconfigure Messages

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

   Default and initial values for REC_TIMEOUT and REC_MAX_RC are
   documented in Section 6.5.

20.2.  Receipt of Renew or Rebind Messages

   In response to a Renew message, the server generates and sends a
   Reply message to the client as described in Section 19.2.3 and
   Section 19.2.8, including options for configuration parameters.

   In response to a Rebind message, the server generates and sends a
   Reply message to the client as described in Section 19.2.4 and
   Section 19.2.8, including options for configuration parameters.

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

20.3.  Receipt of Information-request Messages

   The server generates and sends a Reply message to the client as
   described in Section 19.2.5 and Section 19.2.8, including options for
   configuration parameters.

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



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20.4.  Client Behavior

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

20.4.1.  Receipt of Reconfigure Messages

   Upon receipt of a valid Reconfigure message, the client responds with
   either a Renew message, a Rebind message, or an Information-request
   message as indicated by the Reconfigure Message option (as defined in
   Section 23.19).  The client ignores the transaction-id field in the
   received Reconfigure message.  While the transaction is in progress,
   the client discards any Reconfigure messages it receives.

   DISCUSSION:

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

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

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





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20.4.2.  Creation and Transmission of Renew or Rebind Messages

   When responding to a Reconfigure, the client creates and sends the
   Renew message in exactly the same manner as outlined in
   Section 19.1.3, with the exception that the client copies the Option
   Request option and any IA options from the Reconfigure message into
   the Renew message.  The client MUST include a Server Identifier
   option in the Renew message, identifying the server with which the
   client most recently communicated.

   When responding to a Reconfigure, the client creates and sends the
   Rebind message in exactly the same manner as outlined in
   Section 19.1.4, with the exception that the client copies the Option
   Request option and any IA options from the Reconfigure message into
   the Rebind message.

   If a client is currently sending Rebind messages, as described in
   Section 19.1.3, the client ignores any received Reconfigure messages.

20.4.3.  Creation and Transmission of Information-request Messages

   When responding to a Reconfigure, the client creates and sends the
   Information-request message in exactly the same manner as outlined in
   Section 19.1.5, with the exception that the client includes a Server
   Identifier option with the identifier from the Reconfigure message to
   which the client is responding.

20.4.4.  Time Out and Retransmission of Renew, Rebind or Information-
         request Messages

   The client uses the same variables and retransmission algorithm as it
   does with Renew, Rebind, or Information-request messages generated as
   part of a client-initiated configuration exchange.  See
   Section 19.1.3, Section 19.1.4, and Section 19.1.5 for details.  If
   the client does not receive a response from the server by the end of
   the retransmission process, the client ignores and discards the
   Reconfigure message.

20.4.5.  Receipt of Reply Messages

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







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20.5.  Prefix Delegation Reconfiguration

   This section describes prefix delegation in Reconfigure message
   exchanges.

20.5.1.  Delegating Router Behavior

   The delegating router initiates a configuration message exchange with
   a requesting router, as described in Section 20, by sending a
   Reconfigure message (acting as a DHCP server) to the requesting
   router, as described in Section 20.1.  The delegating router
   specifies the IA_PD option in the Option Request option to cause the
   requesting router to include an IA_PD option to obtain new
   information about delegated prefix(es).

20.5.2.  Requesting Router Behavior

   The requesting router responds to a Reconfigure message, acting as a
   DHCP client, received from a delegating router as described in
   Section 20.4 The requesting router MUST include the IA_PD Prefix
   option(s) (in an IA_PD option) for prefix(es) that have been
   delegated to the requesting router by the delegating router from
   which the Reconfigure message was received.

21.  Relay Agent Behavior

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

   If the relay agent relays messages to the All_DHCP_Servers multicast
   address or other multicast addresses, it sets the Hop Limit field to
   32.

   If the relay agent receives a message other than Relay-forward and
   Relay-reply and the relay agent does not recognize its message type,
   it MUST forward them as described in Section 21.1.1.

21.1.  Relaying a Client Message or a Relay-forward Message

   A relay agent relays both messages from clients and Relay-forward
   messages from other relay agents.  When a relay agent receives a
   valid message (for a definition of a valid message, see Section 4.1
   of [RFC7283]) to be relayed, it constructs a new Relay-forward
   message.  The relay agent copies the source address from the header



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   of the IP datagram in which the message was received to the peer-
   address field of the Relay-forward message.  The relay agent copies
   the received DHCP message (excluding any IP or UDP headers) into a
   Relay Message option in the new message.  The relay agent adds to the
   Relay-forward message any other options it is configured to include.

   [RFC6221] defines a Lightweight DHCPv6 Relay Agent (LDRA) that allows
   Relay Agent Information to be inserted by an access node that
   performs a link- layer bridging (i.e., non-routing) function.

21.1.1.  Relaying a Message from a Client

   If the relay agent received the message to be relayed from a client,
   the relay agent places a global, ULA [RFC4193] or site-scoped address
   with a prefix assigned to the link on which the client should be
   assigned an address in the link-address field.  If not addresses of
   other scopes are available the relay agent may fill in the link-
   address field with a link-local address from the interface the
   original message was received on.  That is not recommended as it
   requires additional information to be provided in the server
   configuration.  See Section 3.2 of [I-D.ietf-dhc-topo-conf] for
   detailed discussion.

   This address will be used by the server to determine the link from
   which the client should be assigned an address and other
   configuration information.  The hop-count in the Relay-forward
   message is set to 0.

   If the relay agent cannot use the address in the link-address field
   to identify the interface through which the response to the client
   will be relayed, the relay agent MUST include an Interface-id option
   (see Section 23.18) in the Relay-forward message.  The server will
   include the Interface-id option in its Relay-reply message.  The
   relay agent fills in the link-address field as described in the
   previous paragraph regardless of whether the relay agent includes an
   Interface-id option in the Relay-forward message.

21.1.2.  Relaying a Message from a Relay Agent

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

   The relay agent copies the source address from the IP datagram in
   which the message was received from the relay agent into the peer-
   address field in the Relay-forward message and sets the hop-count
   field to the value of the hop-count field in the received message
   incremented by 1.



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   If the source address from the IP datagram header of the received
   message is a global or site-scoped address (and the device on which
   the relay agent is running belongs to only one site), the relay agent
   sets the link-address field to 0; otherwise the relay agent sets the
   link-address field to a global or site-scoped address assigned to the
   interface on which the message was received, or includes an
   Interface-ID option to identify the interface on which the message
   was received.

21.1.3.  Relay Agent Behavior with Prefix Delegation

   A relay agent forwards messages containing Prefix Delegation options
   in the same way as described earlier in this section.

   If a delegating router communicates with a requesting router through
   a relay agent, the delegating router may need a protocol or other
   out-of-band communication to configure routing information for
   delegated prefixes on any router through which the requesting router
   may forward traffic.

21.2.  Relaying a Relay-reply Message

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

   The relay agent extracts the message from the Relay Message option
   and relays it to the address contained in the peer-address field of
   the Relay-reply message.  Relay agents MUST NOT modify the message.

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

   If the relay agent receives a Relay-reply message, it MUST process
   the message as defined above, regardless of the type of message
   encapsulated in the Relay Message option.

21.3.  Construction of Relay-reply Messages

   A server uses a Relay-reply message to return a response to a client
   if the original message from the client was relayed to the server in
   a Relay-forward message or to send a Reconfigure message to a client
   if the server does not have an address it can use to send the message
   directly to the client.




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

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

      msg-type:       RELAY-REPLY
      hop-count:      1
      link-address:   0
      peer-address:   A
      Relay Message option, containing:
        msg-type:     RELAY-REPLY
        hop-count:    0
        link-address: address from link to which C is attached
        peer-address: C
        Relay Message option: <response from server>


                       Figure 8: Relay-reply Example

   When sending a Reconfigure message to a client through a relay agent,
   the server creates a Relay-reply message that includes a Relay
   Message option containing the Reconfigure message for the next relay
   agent in the return path to the client.  The server sets the peer-
   address field in the Relay-reply message header to the address of the
   client, and sets the link-address field as required by the relay
   agent to relay the Reconfigure message to the client.  The server
   obtains the addresses of the client and the relay agent through prior
   interaction with the client or through some external mechanism.

22.  Authentication of DHCP Messages

   Some network administrators may wish to provide authentication of the
   source and contents of DHCP messages.  For example, clients may be
   subject to denial of service attacks through the use of bogus DHCP
   servers, or may simply be misconfigured due to unintentionally
   instantiated DHCP servers.  Network administrators may wish to
   constrain the allocation of addresses to authorized hosts to avoid
   denial of service attacks in "hostile" environments where the network



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   medium is not physically secured, such as wireless networks or
   college residence halls.

   The DHCP authentication mechanism is based on the design of
   authentication for DHCPv4 [RFC3118].

22.1.  Security of Messages Sent Between Servers and Relay Agents

   Relay agents and servers that exchange messages securely use the
   IPsec mechanisms for IPv6 [RFC4301].  If a client message is relayed
   through multiple relay agents, each of the relay agents must have
   established independent, pairwise trust relationships.  That is, if
   messages from client C will be relayed by relay agent A to relay
   agent B and then to the server, relay agents A and B must be
   configured to use IPsec for the messages they exchange, and relay
   agent B and the server must be configured to use IPsec for the
   messages they exchange.

   Relay agents and servers that support secure relay agent to server or
   relay agent to relay agent communication use IPsec under the
   following conditions:

      Selectors            Relay agents are manually configured with the
                           addresses of the relay agent or server to
                           which DHCP messages are to be forwarded.
                           Each relay agent and server that will be
                           using IPsec for securing DHCP messages must
                           also be configured with a list of the relay
                           agents to which messages will be returned.
                           The selectors for the relay agents and
                           servers will be the pairs of addresses
                           defining relay agents and servers that
                           exchange DHCP messages on DHCPv6 UDP port
                           547.

      Mode                 Relay agents and servers use transport mode
                           and ESP.  The information in DHCP messages is
                           not generally considered confidential, so
                           encryption need not be used (i.e., NULL
                           encryption can be used).

      Key management       Because the relay agents and servers are used
                           within an organization, public key schemes
                           are not necessary.  Because the relay agents
                           and servers must be manually configured,
                           manually configured key management may
                           suffice, but does not provide defense against
                           replayed messages.  Accordingly, IKE with



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                           preshared secrets SHOULD be supported.  IKE
                           with public keys MAY be supported.

      Security policy      DHCP messages between relay agents and
                           servers should only be accepted from DHCP
                           peers as identified in the local
                           configuration.

      Authentication       Shared keys, indexed to the source IP address
                           of the received DHCP message, are adequate in
                           this application.

      Availability         Appropriate IPsec implementations are likely
                           to be available for servers and for relay
                           agents in more featureful devices used in
                           enterprise and core ISP networks.  IPsec is
                           less likely to be available for relay agents
                           in low end devices primarily used in the home
                           or small office markets.

22.2.  Summary of DHCP Authentication

   Authentication of DHCP messages is accomplished through the use of
   the Authentication option (see Section 23.11).  The authentication
   information carried in the Authentication option can be used to
   reliably identify the source of a DHCP message and to confirm that
   the contents of the DHCP message have not been tampered with.

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

   Any DHCP message MUST NOT include more than one Authentication
   option.

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







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22.3.  Replay Detection

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

   If the RDM field contains 0x00, the replay detection field MUST be
   set to the value of a strictly monotonically increasing counter.
   Using a counter value, such as the current time of day (for example,
   an NTP-format timestamp [RFC5905]), can reduce the danger of replay
   attacks.  This method MUST be supported by all protocols.

22.4.  Reconfigure Key Authentication Protocol

   The Reconfigure key authentication protocol provides protection
   against misconfiguration of a client caused by a Reconfigure message
   sent by a malicious DHCP server.  In this protocol, a DHCP server
   sends a Reconfigure Key to the client in the initial exchange of DHCP
   messages.  The client records the Reconfigure Key for use in
   authenticating subsequent Reconfigure messages from that server.  The
   server then includes an HMAC computed from the Reconfigure Key in
   subsequent Reconfigure messages.

   Both the Reconfigure Key sent from the server to the client and the
   HMAC in subsequent Reconfigure messages are carried as the
   Authentication information in an Authentication option.  The format
   of the Authentication information is defined in the following
   section.

   The Reconfigure Key protocol is used (initiated by the server) only
   if the client and server are not using any other authentication
   protocol and the client and server have negotiated to use Reconfigure
   messages.

22.4.1.  Use of the Authentication Option in the Reconfigure Key
         Authentication Protocol

   The following fields are set in an Authentication option for the
   Reconfigure Key Authentication Protocol:

      protocol   3

      algorithm  1

      RDM        0

   The format of the Authentication information for the Reconfigure Key
   Authentication Protocol is:




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |                 Value (128 bits)              |
      +-+-+-+-+-+-+-+-+                                               |
      .                                                               .
      .                                                               .
      .                                               +-+-+-+-+-+-+-+-+
      |                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 9: RKAP Authentication Information

      Type             Type of data in Value field carried in this
                       option:

                          1    Reconfigure Key value (used in Reply
                               message).

                          2    HMAC-MD5 digest of the message (used in
                               Reconfigure message).

      Value            Data as defined by the Type field.

22.4.2.  Server considerations for Reconfigure Key protocol

   The server selects a Reconfigure Key for a client during the Request/
   Reply, Solicit/Reply or Information-request/Reply message exchange.
   The server records the Reconfigure Key and transmits that key to the
   client in an Authentication option in the Reply message.

   The Reconfigure Key is 128 bits long, and MUST be a cryptographically
   strong random or pseudo-random number that cannot easily be
   predicted.

   To provide authentication for a Reconfigure message, the server
   selects a replay detection value according to the RDM selected by the
   server, and computes an HMAC-MD5 of the Reconfigure message using the
   Reconfigure Key for the client.  The server computes the HMAC-MD5
   over the entire DHCP Reconfigure message, including the
   Authentication option; the HMAC-MD5 field in the Authentication
   option is set to zero for the HMAC-MD5 computation.  The server
   includes the HMAC-MD5 in the authentication information field in an
   Authentication option included in the Reconfigure message sent to the
   client.





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22.4.3.  Client considerations for Reconfigure Key protocol

   The client will receive a Reconfigure Key from the server in the
   initial Reply message from the server.  The client records the
   Reconfigure Key for use in authenticating subsequent Reconfigure
   messages.

   To authenticate a Reconfigure message, the client computes an HMAC-
   MD5 over the DHCP Reconfigure message, using the Reconfigure Key
   received from the server.  If this computed HMAC-MD5 matches the
   value in the Authentication option, the client accepts the
   Reconfigure message.

23.  DHCP Options

   Options are used to carry additional information and parameters in
   DHCP messages.  Every option shares a common base format, as
   described in Section 23.1.  All values in options are represented in
   network byte order.

   This document describes the DHCP options defined as part of the base
   DHCP specification.  Other options may be defined in the future in
   separate documents.  See [RFC7227] for guidelines regarding new
   options definition.

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

   Options that are allowed to appear only once are called singleton
   options.  The only non-singleton options defined in this document are
   IA_NA (see Section 23.4), IA_TA (see Section 23.5), and IA_PD (see
   Section 23.21) options.  Also, IAAddress (see Section 23.6) and
   IAPrefix (see Section 23.22) may appear in their respective IA
   options more than once.

23.1.  Format of DHCP Options

   The format of DHCP options is:










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


                         Figure 10: Option Format

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

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

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

   DHCPv6 options are scoped by using encapsulation.  Some options apply
   generally to the client, some are specific to an IA, and some are
   specific to the addresses within an IA.  These latter two cases are
   discussed in Section 23.4 and Section 23.6.

23.2.  Client Identifier Option

   The Client Identifier option is used to carry a DUID (see Section 10)
   identifying a client between a client and a server.  The format of
   the Client Identifier option is:

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


                Figure 11: Client Identifier Option Format

      option-code          OPTION_CLIENTID (1).




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      option-len           Length of DUID in octets.

      DUID                 The DUID for the client.

23.3.  Server Identifier Option

   The Server Identifier option is used to carry a DUID (see Section 10)
   identifying a server between a client and a server.  The format of
   the Server Identifier option is:

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


                Figure 12: Server Identifier Option Format

      option-code          OPTION_SERVERID (2).

      option-len           Length of DUID in octets.

      DUID                 The DUID for the server.

23.4.  Identity Association for Non-temporary Addresses Option

   The Identity Association for Non-temporary Addresses option (IA_NA
   option) is used to carry an IA_NA, the parameters associated with the
   IA_NA, and the non-temporary addresses associated with the IA_NA.

   Addresses appearing in an IA_NA option are not temporary addresses
   (see Section 23.5).

   The format of the IA_NA option is:











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


    Figure 13: Identity Association for Non-temporary Addresses Option
                                  Format

      option-code          OPTION_IA_NA (3).

      option-len           12 + length of IA_NA-options field.

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

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

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

      IA_NA-options        Options associated with this IA_NA.

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




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   the addresses associated with this IA_NA are in the IA_NA-options
   field.

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

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

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

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

   In a message sent by a client to a server, the T1 and T2 fields
   SHOULD be set to 0.  The server MUST ignore any values in these
   fields in messages received from a client.

   In a message sent by a server to a client, the client MUST use the
   values in the T1 and T2 fields for the T1 and T2 parameters, unless
   those values in those fields are 0.  The values in the T1 and T2
   fields are the number of seconds until T1 and T2.

   The server selects the T1 and T2 times to allow the client to extend
   the lifetimes of any addresses in the IA_NA before the lifetimes
   expire, even if the server is unavailable for some short period of
   time.  Recommended values for T1 and T2 are .5 and .8 times the
   shortest preferred lifetime of the addresses in the IA that the
   server is willing to extend, respectively.  If the "shortest"
   preferred lifetime is 0xffffffff ("infinity"), the recommended T1 and
   T2 values are also 0xffffffff.  If the time at which the addresses in
   an IA_NA are to be renewed is to be left to the discretion of the
   client, the server sets T1 and T2 to 0.  The client MUST follow the
   rules defined in Section 14.2.

   If a server receives an IA_NA with T1 greater than T2, and both T1
   and T2 are greater than 0, the server ignores the invalid values of
   T1 and T2 and processes the IA_NA as though the client had set T1 and
   T2 to 0.

   If a client receives an IA_NA with T1 greater than T2, and both T1
   and T2 are greater than 0, the client discards the IA_NA option and




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   processes the remainder of the message as though the server had not
   included the invalid IA_NA option.

   Care should be taken in setting T1 or T2 to 0xffffffff ("infinity").
   A client will never attempt to extend the lifetimes of any addresses
   in an IA with T1 set to 0xffffffff.  A client will never attempt to
   use a Rebind message to locate a different server to extend the
   lifetimes of any addresses in an IA with T2 set to 0xffffffff.

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

23.5.  Identity Association for Temporary Addresses Option

   The Identity Association for the Temporary Addresses (IA_TA) option
   is used to carry an IA_TA, the parameters associated with the IA_TA
   and the addresses associated with the IA_TA.  All of the addresses in
   this option are used by the client as temporary addresses, as defined
   in [RFC4941].  The format of the IA_TA option is:

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


   Figure 14: Identity Association for Temporary Addresses Option Format

      option-code          OPTION_IA_TA (4).

      option-len           4 + length of IA_TA-options field.

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

      IA_TA-options        Options associated with this IA_TA.





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

   Each IA_TA carries one "set" of temporary addresses.

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

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

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

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

   The client obtains new temporary addresses by sending an IA_TA option
   with a new IAID to a server.  Requesting new temporary addresses from
   the server is the equivalent of generating new temporary addresses as
   described in [RFC4941].  The server will generate new temporary
   addresses and return them to the client.  The client should request
   new temporary addresses before the lifetimes on the previously
   assigned addresses expire.

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

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

23.6.  IA Address Option

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



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   The format of the IA Address option is:

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


                    Figure 15: IA Address Option Format

      option-code          OPTION_IAADDR (5).

      option-len           24 + length of IAaddr-options field.

      IPv6 address         An IPv6 address.

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

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

      IAaddr-options       Options associated with this address.

   In a message sent by a client to a server, the preferred and valid
   lifetime fields SHOULD be set to 0.  The server MUST ignore any
   received values.

   The client SHOULD NOT send the IA Address option with unspecified
   address (::).

   In a message sent by a server to a client, the client MUST use the
   values in the preferred and valid lifetime fields for the preferred




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   and valid lifetimes.  The values in the preferred and valid lifetimes
   are the number of seconds remaining in each lifetime.

   A client discards any addresses for which the preferred lifetime is
   greater than the valid lifetime.  A server ignores the lifetimes set
   by the client if the preferred lifetime is greater than the valid
   lifetime and ignores the values for T1 and T2 set by the client if
   those values are greater than the preferred lifetime.

   Care should be taken in setting the valid lifetime of an address to
   0xffffffff ("infinity"), which amounts to a permanent assignment of
   an address to a client.

   More than one IA Address Option can appear in an IA_NA option or an
   IA_TA option.

   The status of any operations involving this IA Address is indicated
   in a Status Code option in the IAaddr-options field, as specified in
   Section 23.13.

23.7.  Option Request Option

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

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


                  Figure 16: Option Request Option Format

      option-code          OPTION_ORO (6).

      option-len           2 * number of requested options.

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

   A client MAY include an Option Request option in a Solicit, Request,
   Renew, Rebind, Confirm or Information-request message to inform the



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   server about options the client wants the server to send to the
   client.  A server MAY include an Option Request option in a
   Reconfigure message to indicate which options the client should
   request from the server.  If there is a need to request encapsulated
   options, top-level Option Request option MUST be used for that
   purpose.  There is no need request IAADDR or IAPREFIX.

23.8.  Preference Option

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

   The format of the Preference option is:

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


                    Figure 17: Preference Option Format

      option-code          OPTION_PREFERENCE (7).

      option-len           1.

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

   A server MAY include a Preference option in an Advertise message to
   control the selection of a server by the client.  See Section 18.1.3
   for the use of the Preference option by the client and the
   interpretation of Preference option data value.

23.9.  Elapsed Time Option













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


                   Figure 18: Elapsed Time Option Format

      option-code          OPTION_ELAPSED_TIME (8).

      option-len           2.

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

   A client MUST include an Elapsed Time option in messages to indicate
   how long the client has been trying to complete a DHCP message
   exchange.  The elapsed time is measured from the time at which the
   client sent the first message in the message exchange, and the
   elapsed-time field is set to 0 in the first message in the message
   exchange.  Servers and Relay Agents use the data value in this option
   as input to policy controlling how a server responds to a client
   message.  For example, the elapsed time option allows a secondary
   DHCP server to respond to a request when a primary server has not
   answered in a reasonable time.  The elapsed time value is an
   unsigned, 16 bit integer.  The client uses the value 0xffff to
   represent any elapsed time values greater than the largest time value
   that can be represented in the Elapsed Time option.

23.10.  Relay Message Option

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

   The format of the Relay Message option is:











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


                  Figure 19: Relay Message Option Format

      option-code          OPTION_RELAY_MSG (9)

      option-len           Length of DHCP-relay-message

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

23.11.  Authentication Option

   The Authentication option carries authentication information to
   authenticate the identity and contents of DHCP messages.  The use of
   the Authentication option is described in Section 22.  The format of
   the Authentication option is:




















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          OPTION_AUTH          |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   protocol    |   algorithm   |      RDM      |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
      |                                                               |
      |          replay detection (64 bits)           +-+-+-+-+-+-+-+-+
      |                                               |   auth-info   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
      .                   authentication information                  .
      .                       (variable length)                       .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 20: Authentication Option Format

      option-code          OPTION_AUTH (11).

      option-len           11 + length of authentication information
                           field.

      protocol             The authentication protocol used in this
                           authentication option.

      algorithm            The algorithm used in the authentication
                           protocol.

      RDM                  The replay detection method used in this
                           authentication option.

      Replay detection     The replay detection information for the RDM.

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

23.12.  Server Unicast Option

   The server sends this option to a client to indicate to the client
   that it is allowed to unicast messages to the server.  The format of
   the Server Unicast option is:








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


                  Figure 21: Server Unicast Option Format

      option-code          OPTION_UNICAST (12).

      option-len           16.

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

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

   When the server sends a Unicast option to the client, some messages
   from the client will not be relayed by Relay Agents, and will not
   include Relay Agent options from the Relay Agents.  Therefore, a
   server should only send a Unicast option to a client when Relay
   Agents are not sending Relay Agent options.  A DHCP server rejects
   any messages sent inappropriately using unicast to ensure that
   messages are relayed by Relay Agents when Relay Agent options are in
   use.

   Details about when the client may send messages to the server using
   unicast are in Section 19.

23.13.  Status Code Option

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







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


                   Figure 22: Status Code Option Format

      option-code          OPTION_STATUS_CODE (13).

      option-len           2 + length of status-message.

      status-code          The numeric code for the status encoded in
                           this option.

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

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

   The status-codes values previously defined by [RFC3315] and [RFC3633]
   are:


















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   +---------------+------+--------------------------------------------+
   | Name          | Code | Description                                |
   +---------------+------+--------------------------------------------+
   | Success       |    0 | Success.                                   |
   | UnspecFail    |    1 | Failure, reason unspecified; this status   |
   |               |      | code is sent by either a client or a       |
   |               |      | server to indicate a failure not           |
   |               |      | explicitly specified in this document.     |
   | NoAddrsAvail  |    2 | Server has no addresses available to       |
   |               |      | assign to the IA(s).                       |
   | NoBinding     |    3 | Client record (binding) unavailable.       |
   | NotOnLink     |    4 | The prefix for the address is not          |
   |               |      | appropriate for the link to which the      |
   |               |      | client is attached.                        |
   | UseMulticast  |    5 | Sent by a server to a client to force the  |
   |               |      | client to send messages to the server      |
   |               |      | using the                                  |
   |               |      | All_DHCP_Relay_Agents_and_Servers address. |
   | NoPrefixAvail |    6 | Delegating router has no prefixes          |
   |               |      | available to assign to the IAPD(s).        |
   +---------------+------+--------------------------------------------+

23.14.  Rapid Commit Option

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

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


                   Figure 23: Rapid Commit Option Format

      option-code          OPTION_RAPID_COMMIT (14).

      option-len           0.

   A client MAY include this option in a Solicit message if the client
   is prepared to perform the Solicit-Reply message exchange described
   in Section 18.1.1.

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



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   DISCUSSION:

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

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

23.15.  User Class Option

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

   The format of the User Class option is:

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


                    Figure 24: User Class Option Format

      option-code          OPTION_USER_CLASS (15).

      option-len           Length of user class data field.

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

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



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   with a different printer than clients of people in the marketing
   department.  The user class information carried in this option MUST
   be configurable on the client.

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

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


                     Figure 25: User Class Data Format

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

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

23.16.  Vendor Class Option

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
















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


                   Figure 26: Vendor Class Option Format

      option-code          OPTION_VENDOR_CLASS (16).

      option-len           4 + length of vendor class data field.

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

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

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

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

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


                    Figure 27: Vendor Class Data Format

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

   Servers and clients MUST NOT include more than one instance of
   OPTION_VENDOR_CLASS with the same Enterprise Number.  Each instance
   of OPTION_VENDOR_CLASS can carry multiple sub-options.





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23.17.  Vendor-specific Information Option

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

   The format of the Vendor-specific Information option is:

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


           Figure 28: Vendor-specific Information Option Format

      option-code          OPTION_VENDOR_OPTS (17).

      option-len           4 + length of option-data field.

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

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

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

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






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


                 Figure 29: Vendor-specific Options Format

      opt-code             The code for the encapsulated option.

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

      option-data          The data area for the encapsulated option.

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

   A client that is interested in receiving a Vendor-specific
   Information Option:

   -  MUST specify the Vendor-specific Information Option in an Option
      Request Option.

   -  MAY specify an associated Vendor Class Option.

   -  MAY specify the Vendor-specific Information Option with any data.

   Severs only return the Vendor-specific Information Options if
   specified in Option Request Options from clients and:

   -  MAY use the Enterprise Numbers in the associated Vendor Class
      Options to restrict the set of Enterprise Numbers in the Vendor-
      specific Information Options returned.

   -  MAY return all configured Vendor-specific Information Options.




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   -  MAY use other information in the packet or in its configuration to
      determine which set of Enterprise Numbers in the Vendor-specific
      Information Options to return.

23.18.  Interface-Id Option

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

   The format of the Interface ID option is:

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


                   Figure 30: Interface-ID Option Format

      option-code          OPTION_INTERFACE_ID (18).

      option-len           Length of interface-id field.

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

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

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




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23.19.  Reconfigure Message Option

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

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


               Figure 31: Reconfigure Message Option Format

      option-code          OPTION_RECONF_MSG (19).

      option-len           1.

      msg-type             5 for Renew message, 6 for Rebind, 11 for
                           Information-request message.

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

23.20.  Reconfigure Accept Option

   A client uses the Reconfigure Accept option to announce to the server
   whether the client is willing to accept Reconfigure messages, and a
   server uses this option to tell the client whether or not to accept
   Reconfigure messages.  The default behavior, in the absence of this
   option, means unwillingness to accept Reconfigure messages, or
   instruction not to accept Reconfigure messages, for the client and
   server messages, respectively.  The following figure gives the format
   of the Reconfigure Accept option:

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


                Figure 32: Reconfigure Accept Option Format




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      option-code          OPTION_RECONF_ACCEPT (20).

      option-len           0.

23.21.  Identity Association for Prefix Delegation Option

   The IA_PD option is used to carry a prefix delegation identity
   association, the parameters associated with the IA_PD and the
   prefixes associated with it.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         OPTION_IA_PD          |         option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IAID (4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T1                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T2                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                          IA_PD-options                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Figure 33: Identity Association for Prefix Delegation Option Format

      option-code          OPTION_IA_PD (25).

      option-length        12 + length of IA_PD-options field.

      IAID                 The unique identifier for this IA_PD; the
                           IAID must be unique among the identifiers for
                           all of this requesting router's IA_PDs.

      T1                   The time at which the requesting router
                           should contact the delegating router from
                           which the prefixes in the IA_PD were obtained
                           to extend the lifetimes of the prefixes
                           delegated to the IA_PD; T1 is a time duration
                           relative to the current time expressed in
                           units of seconds.

      T2                   The time at which the requesting router
                           should contact any available delegating
                           router to extend the lifetimes of the



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                           prefixes assigned to the IA_PD; T2 is a time
                           duration relative to the current time
                           expressed in units of seconds.

      IA_PD-options        Options associated with this IA_PD.

   The IA_PD-options field encapsulates those options that are specific
   to this IA_PD.  For example, all of the IA_PD Prefix Options carrying
   the prefixes associated with this IA_PD are in the IA_PD-options
   field.

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

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

   Note that an IA_PD has no explicit "lifetime" or "lease length" of
   its own.  When the valid lifetimes of all of the prefixes in a IA_PD
   have expired, the IA_PD can be considered as having expired.  T1 and
   T2 are included to give delegating routers explicit control over when
   a requesting router should contact the delegating router about a
   specific IA_PD.

   In a message sent by a requesting router to a delegating router, the
   T1 and T2 fields SHOULD be set to 0.  The delegating router MUST
   ignore any values in these fields in messages received from a
   requesting router.

   In a message sent by a delegating router to a requesting router, the
   delegating router MUST use the values in the T1 and T2 fields for the
   T1 and T2 parameters, unless those values in those fields are 0.  The
   values in the T1 and T2 fields are the number of seconds until T1 and
   T2.

   The delegating router selects the T1 and T2 times to allow the
   requesting router to extend the lifetimes of any prefixes in the
   IA_PD before the lifetimes expire, even if the delegating router is
   unavailable for some short period of time.  Recommended values for T1
   and T2 are .5 and .8 times the shortest preferred lifetime of the
   prefixes in the IA_PD that the delegating router is willing to
   extend, respectively.  If the time at which the prefixes in an IA_PD
   are to be renewed is to be left to the discretion of the requesting
   router, the delegating router sets T1 and T2 to 0.  The requesting
   router MUST follow the rules defined in Section 14.2.

   If a delegating router receives an IA_PD with T1 greater than T2, and
   both T1 and T2 are greater than 0, the delegating router ignores the



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   invalid values of T1 and T2 and processes the IA_PD as though the
   requesting router had set T1 and T2 to 0.

   If a requesting router receives an IA_PD with T1 greater than T2, and
   both T1 and T2 are greater than 0, the requesting router discards the
   IA_PD option and processes the remainder of the message as though the
   requesting router had not included the IA_PD option.

23.22.  IA Prefix Option

   The IA_PD Prefix option is used to specify IPv6 address prefixes
   associated with an IA_PD.  The IA_PD Prefix option must be
   encapsulated in the IA_PD-options field of an IA_PD option.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        OPTION_IAPREFIX        |         option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      preferred-lifetime                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        valid-lifetime                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | prefix-length |                                               |
      +-+-+-+-+-+-+-+-+          IPv6 prefix                          |
      |                           (16 octets)                         |
      |                                                               |
      |                                                               |
      |                                                               |
      |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |                                               .
      +-+-+-+-+-+-+-+-+                                               .
      .                       IAprefix-options                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 34: IA Prefix Option Format

      option-code          OPTION_IAPREFIX (26).

      option-length        25 + length of IAprefix-options field.

      preferred-lifetime   The recommended preferred lifetime for the
                           IPv6 prefix in the option, expressed in units
                           of seconds.  A value of 0xFFFFFFFF represents
                           infinity.




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      valid-lifetime       The valid lifetime for the IPv6 prefix in the
                           option, expressed in units of seconds.  A
                           value of 0xFFFFFFFF represents infinity.

      prefix-length        Length for this prefix in bits.

      IPv6-prefix          An IPv6 prefix.

      IAprefix-options     Options associated with this prefix.

   In a message sent by a requesting router to a delegating router, the
   preferred and valid lifetime fields SHOULD be set to 0.  The server
   MUST ignore any received values in these lifetime fields.

   A requesting router may set the IPv6 prefix field to zero and a given
   value in the prefix-length field to indicate a preference for the
   size of the prefix to be delegated.

   In a message sent by a delegating router the preferred and valid
   lifetimes should be set to the values of AdvPreferredLifetime and
   AdvValidLifetime as specified in section 6.2.1, "Router Configuration
   Variables" of [RFC2461], unless administratively configured.

   A requesting router discards any prefixes for which the preferred
   lifetime is greater than the valid lifetime.  A delegating router
   ignores the lifetimes set by the requesting router if the preferred
   lifetime is greater than the valid lifetime and ignores the values
   for T1 and T2 set by the requesting router if those values are
   greater than the preferred lifetime.

   The values in the preferred and valid lifetimes are the number of
   seconds remaining for each lifetime.

   An IA_PD Prefix option may appear only in an IA_PD option.  More than
   one IA_PD Prefix Option can appear in a single IA_PD option.

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

23.23.  SOL_MAX_RT Option

   A DHCP server sends the SOL_MAX_RT option to a client to override the
   default value of SOL_MAX_RT.  The value of SOL_MAX_RT in the option
   replaces the default value defined in Section 6.5.  One use for the
   SOL_MAX_RT option is to set a longer value for SOL_MAX_RT, which
   reduces the Solicit traffic from a client that has not received a
   response to its Solicit messages.




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   The format of the SOL_MAX_RT option is:

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


                    Figure 35: SOL_MAX_RT Option Format

      option-code          OPTION_SOL_MAX_RT (82).

      option-len           4.

      SOL_MAX_RT value     Overriding value for SOL_MAX_RT in seconds;
                           MUST be in range: 60 <= "value" <= 86400 (1
                           day).

   A DHCP client MUST include the SOL_MAX_RT option code in any Option
   Request option (see Section 23.7) it sends.

   The DHCP server MAY include the SOL_MAX_RT option in any response it
   sends to a client that has included the SOL_MAX_RT option code in an
   Option Request option.  The SOL_MAX_RT option is sent in the main
   body of the message to client, not as an encapsulated option in,
   e.g., an IA_NA, IA_TA, or IA_PD option.

   A DHCP client MUST ignore any SOL_MAX_RT option values that are less
   than 60 or more than 86400.

   If a DHCP client receives a message containing a SOL_MAX_RT option
   that has a valid value for SOL_MAX_RT, the client MUST set its
   internal SOL_MAX_RT parameter to the value contained in the
   SOL_MAX_RT option.  This value of SOL_MAX_RT is then used by the
   retransmission mechanism defined in Section 15 and Section 18.1.2.

   Updated SOL_MAX_RT value applies only to the network interface on
   which the client received SOL_MAX_RT option.

23.24.  INF_MAX_RT Option

   A DHCP server sends the INF_MAX_RT option to a client to override the
   default value of INF_MAX_RT.  The value of INF_MAX_RT in the option
   replaces the default value defined in Section 6.5.  One use for the
   INF_MAX_RT option is to set a longer value for INF_MAX_RT, which



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   reduces the Information-request traffic from a client that has not
   received a response to its Information-request messages.

   The format of the INF_MAX_RT option is:

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


                    Figure 36: INF_MAX_RT Option Format

      option-code          OPTION_INF_MAX_RT (83).

      option-len           4.

      SOL_MAX_RT value     Overriding value for INF_MAX_RT in seconds;
                           MUST be in range: 60 <= "value" <= 86400 (1
                           day).

   A DHCP client MUST include the INF_MAX_RT option code in any Option
   Request option (see Section 23.7) it sends.

   The DHCP server MAY include the INF_MAX_RT option in any response it
   sends to a client that has included the INF_MAX_RT option code in an
   Option Request option.  The INF_MAX_RT option is sent in the main
   body of the message to client, not as an encapsulated option in,
   e.g., an IA_NA, IA_TA, or IA_PD option.

   A DHCP client MUST ignore any INF_MAX_RT option values that are less
   than 60 or more than 86400.

   If a DHCP client receives a message containing an INF_MAX_RT option
   that has a valid value for INF_MAX_RT, the client MUST set its
   internal INF_MAX_RT parameter to the value contained in the
   INF_MAX_RT option.  This value of INF_MAX_RT is then used by the
   retransmission mechanism defined in Section 15 and Section 19.1.5.

   Updated INF_MAX_RT value applies only to the network interface on
   which the client received INF_MAX_RT option.







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24.  Security Considerations

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

   Use of manually configured preshared keys for IPsec between relay
   agents and servers does not defend against replayed DHCP messages.
   Replayed messages can represent a DOS attack through exhaustion of
   processing resources, but not through mis-configuration or exhaustion
   of other resources such as assignable addresses.

   One attack specific to a DHCP client is the establishment of a
   malicious server with the intent of providing incorrect configuration
   information to the client.  The motivation for doing so may be to
   mount a "man in the middle" attack that causes the client to
   communicate with a malicious server instead of a valid server for
   some service such as DNS or NTP.  The malicious server may also mount
   a denial of service attack through misconfiguration of the client
   that causes all network communication from the client to fail.

   A malicious DHCP server might cause a client to set its SOL_MAX_RT
   and INF_MAX_RT parameters to an unreasonably high value with the
   SOL_MAX_RT and INF_MAX_RT options, which may cause an undue delay in
   a client completing its DHCP protocol transaction in the case no
   other valid response is received.  Assuming the client also receives
   a response from a valid DHCP server, large values for SOL_MAX_RT and
   INF_MAX_RT will not have any effect.

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

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




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   will also reduce the probability that such an attack will be
   successful.

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

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

   In the case where relay agents add additional options to Relay
   Forward messages, the messages exchanged between relay agents and
   servers may be used to mount a "man in the middle" or denial of
   service attack.

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

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

   Because of the opportunity for attack through the Reconfigure
   message, a DHCP client MUST discard any Reconfigure message that does
   not include authentication or that does not pass the validation
   process for the authentication protocol.

   The Reconfigure Key protocol described in Section 22.4 provides
   protection against the use of a Reconfigure message by a malicious
   DHCP server to mount a denial of service or man-in-the-middle attack
   on a client.  This protocol can be compromised by an attacker that
   can intercept the initial message in which the DHCP server sends the
   key to the client.

   Communication between a server and a relay agent, and communication
   between relay agents, can be secured through the use of IPsec, as
   described in Section 22.1.  The use of manual configuration and
   installation of static keys are acceptable in this instance because
   relay agents and the server will belong to the same administrative
   domain and the relay agents will require other specific configuration




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   (for example, configuration of the DHCP server address) as well as
   the IPsec configuration.

   A rogue delegating router can issue bogus prefixes to a requesting
   router.  This may cause denial of service due to unreachability.

   A malicious requesting router may be able to mount a denial of
   service attack by repeated requests for delegated prefixes that
   exhaust the delegating router's available prefixes.

   To guard against attacks through prefix delegation, requesting
   routers and delegating routers SHOULD use DHCP authentication as
   described in Section 22.  For point to point links, where one trusts
   that there is no man in the middle, or one trusts layer two
   authentication, DHCP authentication or IPsec may not be necessary.
   Because a requesting router and delegating routers must each have at
   least one assigned IPv6 address, the routers may be able to use IPsec
   for authentication of DHCPv6 messages.  The details of using IPsec
   for DHCPv6 are under development.

   Networks configured with delegated prefixes should be configured to
   preclude intentional or inadvertent inappropriate advertisement of
   these prefixes.

25.  IANA Considerations

   This document does not define any new DHCPv6 name spaces or
   definitions.

   IANA is requested to update the http://www.iana.org/assignments/
   dhcpv6-parameters/dhcpv6-parameters.xhtml page to add a reference to
   this document for definitions previously created by [RFC3315],
   [RFC3633], and [RFC7083].

26.  Obsoleted mechanisms

   This specification of the DHCPv6 is mostly a corrected and cleaned up
   version of the original spec [RFC3315] along with numerous additions
   from later RFCs.  However, there is a small number of mechanisms that
   didn't get much traction, were not widely deployed, underspecified or
   had other operational issues.  Those mechanisms are now considered
   decprecated.  Legacy implementations MAY support it, but
   implementations conformant to this document MUST NOT rely on them.

   The following mechanism are now obsolete:






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   Delayed Authentication.  This mechanism was underspecified and had
   significant operational burden.  As a result, after 10 years its
   adoption was extremely limited at best.

   Lifetime hints sent by a client.  Client used to be allowed to send
   lifetime values as hints.  This mechanism was not widely implemented
   and there were known misimplementations that sent remaining lifetimes
   rather than total lifetimes.  That in turn was sometimes
   misunderstood by the servers as a request for ever decreasing lease
   lifetimes, which caused issues when values started approaching zero.

27.  Acknowledgments

   The following people are authors of the original RFC 3315: Ralph
   Droms, Jim Bound, Bernie Volz, Ted Lemon, Charles Perkins, and Mike
   Carney.  The following people are authors of the original RFC 3633:
   Ole Troan and Ralph Droms.  This document is merely a refinement of
   their work and would not be possible without their original work.

   A number of additional people have contributed to identifying issues
   with RFC 3315 and RFC 3633 and proposed resolutions to these issues
   as reflected in this document (in no particular order): Ole Troan,
   Robert Marks, Leaf Yeh, Tim Winters, Michelle Cotton, Pablo Armando,
   John Brzozowski, Suresh Krishnan, Hideshi Enokihara, Alexandru
   Petrescu, Yukiyo Akisada, Tatuya Jinmei, Fred Templin.  With special
   thanks to Ralph Droms for answering many questions related to the
   original RFC 3315 work.

   The following acknowledgements are from the original RFC 3315 and RFC
   3633:

   Thanks to the DHC Working Group and the members of the IETF for their
   time and input into the specification.  In particular, thanks also
   for the consistent input, ideas, and review by (in alphabetical
   order) Bernard Aboba, Bill Arbaugh, Thirumalesh Bhat, Steve Bellovin,
   A.  K.  Vijayabhaskar, Brian Carpenter, Matt Crawford, Steve Deering,
   Francis Dupont, Dave Forster, Brian Haberman, Richard Hussong, Tatuya
   Jinmei, Kim Kinnear, Fredrik Lindholm, Tony Lindstrom, Josh
   Littlefield, Gerald Maguire, Jack McCann, Shin Miyakawa, Thomas
   Narten, Erik Nordmark, Jarno Rajahalme, Yakov Rekhter, Pekka Savola,
   Mark Stapp, Matt Thomas, Sue Thomson, Tatuya Jinmei, Bernie Volz,
   Trevor Warwick, Phil Wells and Toshi Yamasaki.

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





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

28.  References

28.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <http://www.rfc-editor.org/info/rfc768>.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <http://www.rfc-editor.org/info/rfc826>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <http://www.rfc-editor.org/info/rfc2132>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <http://www.rfc-editor.org/info/rfc2136>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <http://www.rfc-editor.org/info/rfc2464>.




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   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, DOI 10.17487/RFC2526, March 1999,
              <http://www.rfc-editor.org/info/rfc2526>.

   [RFC3118]  Droms, R. and W. Arbaugh., Ed., "Authentication for DHCP
              Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001,
              <http://www.rfc-editor.org/info/rfc3118>.

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,
              <http://www.rfc-editor.org/info/rfc3646>.

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075,
              DOI 10.17487/RFC4075, May 2005,
              <http://www.rfc-editor.org/info/rfc4075>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.






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   [RFC6221]  Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
              DOI 10.17487/RFC6221, May 2011,
              <http://www.rfc-editor.org/info/rfc6221>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <http://www.rfc-editor.org/info/rfc6355>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.

   [RFC7083]  Droms, R., "Modification to Default Values of SOL_MAX_RT
              and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC7083, November
              2013, <http://www.rfc-editor.org/info/rfc7083>.

   [RFC7227]  Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
              S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
              BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
              <http://www.rfc-editor.org/info/rfc7227>.

   [RFC7283]  Cui, Y., Sun, Q., and T. Lemon, "Handling Unknown DHCPv6
              Messages", RFC 7283, DOI 10.17487/RFC7283, July 2014,
              <http://www.rfc-editor.org/info/rfc7283>.

28.2.  Informative References

   [I-D.cui-dhc-dhcpv6-prefix-length-hint-issue]
              Cui, Y., Li, T., and C. Liu, "DHCPv6 Prefix Length Hint
              Issues", draft-cui-dhc-dhcpv6-prefix-length-hint-issue-01
              (work in progress), October 2015.

   [I-D.ietf-dhc-topo-conf]
              Lemon, T. and T. Mrugalski, "Customizing DHCP
              Configuration on the Basis of Network Topology", draft-
              ietf-dhc-topo-conf-05 (work in progress), July 2015.

   [IANA-PEN]
              IANA, "Private Enterprise Numbers registry
              http://www.iana.org/assignments/enterprise-numbers".

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              DOI 10.17487/RFC2461, December 1998,
              <http://www.rfc-editor.org/info/rfc2461>.



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   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, DOI 10.17487/RFC2462,
              December 1998, <http://www.rfc-editor.org/info/rfc2462>.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              DOI 10.17487/RFC3041, January 2001,
              <http://www.rfc-editor.org/info/rfc3041>.

   [RFC3162]  Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
              RFC 3162, DOI 10.17487/RFC3162, August 2001,
              <http://www.rfc-editor.org/info/rfc3162>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,
              <http://www.rfc-editor.org/info/rfc3633>.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, DOI 10.17487/RFC3736,
              April 2004, <http://www.rfc-editor.org/info/rfc3736>.

   [RFC3769]  Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
              Delegation", RFC 3769, DOI 10.17487/RFC3769, June 2004,
              <http://www.rfc-editor.org/info/rfc3769>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <http://www.rfc-editor.org/info/rfc4193>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <http://www.rfc-editor.org/info/rfc7084>.

   [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
              RFC 7341, DOI 10.17487/RFC7341, August 2014,
              <http://www.rfc-editor.org/info/rfc7341>.







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   [RFC7550]  Troan, O., Volz, B., and M. Siodelski, "Issues and
              Recommendations with Multiple Stateful DHCPv6 Options",
              RFC 7550, DOI 10.17487/RFC7550, May 2015,
              <http://www.rfc-editor.org/info/rfc7550>.

Appendix A.  Changes since RFC3315

   1.   Incorporated RFC3315 errata (ids: 294, 1373, 2928, 1815, 3577,
        2509, 295).

   2.   Partially incorporated RFC3315 errata id 2472 (place other IA
        options if NoAddrsAvail is sent in Advertise).

   3.   Clarified section 21.4.1 of RFC3315 by defining length of "key
        ID" field and specifying that 'DHCP realm' is Domain Name
        encoded as per section 8 of RFC3315.  Ticket #43.

   4.   Added DUID-UUID and reference to RFC6355.  Ticket #54.

   5.   Specified a minimum length for the DUID in section "9.1.  DUID
        Contents".  Ticket #39.

   6.   Removed the use of term "sub-options" from section "19.1.1.
        Creation and Transmission of Reconfigure Messages".  Ticket #40.

   7.   Added text to section 22.6 "IA Address Option" about the usage
        of unspecified address to express the client hints for Preferred
        and Valid lifetimes.  Ticket #45.

   8.   Updated text in 21.4.2 of RFC3315 ("Message Validation") as
        suggested in section 3.1 of draft-ietf-dhc-dhcpv6-clarify-auth-
        01.  Ticket #87.

   9.   Merged RFC7083, "Modification to Default Values of SOL_MAX_RT
        and INF_MAX_RT", into this document.  Ticket #51.

   10.  Incorporated RFC3315 errata (id 2471), into section 17.1.3.
        Ticket #25.

   11.  Added text that relay agents MUST NOT modify the relayed message
        to section 20.1.2.  Ticket #57.

   12.  Modified the text in section 21.4.4.5, Receiving Reply Messages,
        to remove special treatment of a Reply validation failure
        (client ignores message).  Ticket #89.

   13.  Appendix C updated: Authentication option is no longer allowed
        in Relay-forward and Relay-reply messages, ORO is no longer



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        allowed in Confirm, Release and Decline messages; Preference
        option is no longer allowed in Reply messages (only in
        Advertise).  Ticket #10.

   14.  Removed "silently" from several instances of "silently ignores"
        or "silently" discards.  It is up to software vendor if and how
        to log such events (debug log message, event log, message pop-up
        etc.).  Ticket #50.

   15.  Clarified that: there should be no more that one instance of
        Vendor Class option with a given Enterprise Number; that one
        instance of Vendor Class can contain multiple encapsulated
        options; the same applies to Vendor Specific Information option.
        Ticket #22.

   16.  Clarified relay agent definition.  Ticket #12.

   17.  Changed REL_MAX_RC and DEC_MAX_RC defaults from 5 to 4 and added
        retry to parameter description.  Ticket #84.

   18.  Clarify handling process for Vendor-specific Information Option
        and Vendor Class Option.  Ticket #20.

   19.  Replace "monotonic" with "strictly monotonic" in Section 21.3.
        Ticket #11.

   20.  Incorporate everything of RFC 6644, except for Security
        Considerations Section, which has already covered in a more
        abstracted way.  Ticket #55 & #56.

   21.  Clarify the server behavior process when a client violates
        Delayed Authentication Protocol, in Section 21.4.  Ticket #90.

   22.  Updated titles of sections 19.4.2. and 19.4.4. to include Rebind
        messages.

   23.  Applied many of the review comments from a review done by Fred
        Templin in August 2006.  Ticket #14.

   24.  Reworded the first paragraph of Section 15 to relax the "SHOULD"
        requirement to drop the messages which contain the options not
        expected in the current message.  Ticket #17.

   25.  Changed WG to DHC, added keywords

   26.  Loosened requirements for DUID-EN, so that DUID type can be used
        for virtual machines.  Ticket #16.




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   27.  Clarified that IA may contain other resources than just address.
        Ticket #93.

   28.  Clarified that most options are singletons (i.e. can appear only
        once).  Ticket #83.

   29.  Merged sections 1 (Ticket #96), 2 (Ticket #97), 3 (Ticket #98),
        4 (Ticket #99), 6 (Ticket #101), 8 (Ticket #103), 9 (Ticket
        #104), 10 (Ticket #105), 11 (Ticket #106), 13 (Ticket #108), 14
        (Ticket #109), 15 (Ticket #110), 16 (Ticket #111), 17 (Ticket
        #112) and 19 (Ticket #113) from RFC3633 (Prefix Delegation).

   30.  Clarified that encapsulated options must be requested using top
        level ORO (ticket #38).

   31.  Clarified that configuration for interface X should be requested
        over interface X (ticket #48).

   32.  CONFIRM is now an optional message (MUST send Confirm eased to
        SHOULD) (ticket #120).

   33.  Added reference to RFC7227: DHCPv6 Option Guidelines (ticket
        #121).

   34.  Added new section 5 providing an overview of DHCPv6 operational
        modes and removed two prefix delegation sections from section 1.
        See tickets #53, #100, and #102.

   35.  Addressed ticket #115 - don't use DHCPv6 for DHCPv4
        configuration.

   36.  Revised IANA Considerations based on ticket #117.

   37.  Updated IAID description in the terminology with the
        clarification that the IAID is unique among IAs of a specific
        type, rather than globally unique among all IAs (ticket #94).

   38.  Merged Section 12 from RFC3633 (ticket #107)

   39.  Clarified behavior for unknown messages (RFC7283), ticket #58.

   40.  Addressed tickets #123 and #126, and clarified that the client
        SHOULD abandon its bindings when restarts the server
        solicitation.

   41.  Clarified link-address field usage, ticket #73.





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   42.  Clarified that retransmitting at client's discretion (t1,t2=0)
        does not mean immediate transmission (ticket #71).

   43.  Merged section 4.4. of RFC7550 (stateful-issues).  This includes
        new Renew and Rebind processing by the server, i.e. a client may
        request allocation of new addresses/prefixes during Renew and
        Rebind.  This addresses tickets #62 and #63.

   44.  Merged section 4.1 and 4.2 of RFC7550 (stateful-issues).  This
        addresses tickets #59 and #60.

   45.  Added normative reference to RFC7550 and to obsoleted list.
        Also added terse text of Abstract and section 1.0 to indicate
        what this document is about.

   46.  Merged section 4.6 of RFC7550 (stateful-issues).  This addresses
        ticket #85.

   47.  Clarified that the document assumes single provisioning domain.
        This resolves ticket #66.

   48.  Removed the Delayed Auth Protocol.  This resolves ticket #86.

   49.  Added text the text suggesting to not send all-zero address/
        prefix unless the client wishes to hint the lifetimes and/or
        prefix lengths.  This resolves ticket#82.

   50.  Cleaned up Confirm text regarding rebooting client an stable
        storage.

   51.  Updated server processing section to clarify that server unicast
        check can be based on current configuration of unicast setting
        so server is not required to remember what it sent.  This
        resolves ticket #130.

   52.  Removed references to lifetime hints from the client; only
        IAPREFIX prefix-length hints are now allowed.  Servers will
        ignore any lifetime hints; clients should set lifetime to 0.
        This resolves ticket #148.  Also, removed T1/T2 hints from
        clients to servers - while not explicitly discussed, seems to
        make sense?  And, fixed some of the message processing text to
        include PD, not just addresses (though I think Marcin will
        eventually replace much of this text?  Plus a few other minor
        edits.

   53.  Clarified that the server may return different addresses in the
        Reply than requested by the client in the Request message.  Also




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        clarified that the server must not include addresses that it
        will not assign.  This resolves ticket #69.

   54.  Introduction updated, including relation to previous RFCs
        (ticket #136).

   55.  Added section with deprecated mechanisms (ticket #149).

   56.  Combined text for prefix delegation and address assignment in
        sections 18 and 19.  Also, introduced a term "lease" replacing
        "allocable resources".  This resolves ticket #146.

Appendix B.  Changes since RFC3633

   1.  Incorporated RFC3633 errata (ids: 248, 1880, 2468, 2469, 2470,
       3736)

   2.  ...

Appendix C.  Appearance of Options in Message Types

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

         Client Server IA_NA IA_PD Option Pref Elap. Relay Auth. Server
           ID     ID   IA_TA       Request     Time   Msg.       Unicast
 Solicit   *             *     *     *           *           *
 Advert.   *      *      *     *           *                 *
 Request   *      *      *     *     *           *           *
 Confirm   *             *                       *           *
 Renew     *      *      *     *     *           *           *
 Rebind    *             *     *     *           *           *
 Decline   *      *      *     *                 *           *
 Release   *      *      *     *                 *           *
 Reply     *      *      *     *                             *     *
 Reconf.   *      *                  *                       *
 Inform.   * (see note)              *           *           *
 R-forw.                                               *
 R-repl.                                               *

   NOTE:

   Only included in Information-request messages that are sent in
   response to a Reconfigure (see Section 20.4.3).







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

Appendix D.  Appearance of Options in the Options Field of DHCP Options

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

                Option  IA_NA/                        Relay  Relay
                Field   IA_TA  IAADDR IA_PD  IAPREFIX Forw.  Reply
   Client ID      *
   Server ID      *
   IA_NA/IA_TA    *
   IAADDR                 *
   IA_PD          *
   IAPREFIX                             *
   ORO            *
   Preference     *
   Elapsed Time   *
   Relay Message                                        *      *
   Authentic.     *
   Server Uni.    *
   Status Code    *       *             *
   Rapid Comm.    *
   User Class     *
   Vendor Class   *
   Vendor Info.   *                                     *      *
   Interf. ID                                           *      *
   Reconf. MSG.   *
   Reconf. Accept *

   Note: "Relay Forw" / "Relay Reply" options appear in the options
   field of the message but may only appear in these messages.





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Authors' Addresses

   Tomek Mrugalski (editor)
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   Email: tomasz.mrugalski@gmail.com


   Marcin Siodelski
   Internet Systems Consortium, Inc.
   950 Charter St.
   Redwood City, CA  94063
   USA

   Email: msiodelski@gmail.com


   Bernie Volz
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA  01719
   USA

   Email: volz@cisco.com


   Andrew Yourtchenko
   Cisco Systems, Inc.
   De Kleetlaan, 7
   Diegem  B-1831
   Belgium

   Email: ayourtch@cisco.com


   Michael C. Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z 5V7
   CA

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/





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   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

   Email: jiangsheng@huawei.com


   Ted Lemon
   Nominum, Inc.
   950 Charter St.
   Redwood City, CA  94043
   USA

   Email: Ted.Lemon@nominum.com



































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