Dynamic Host Configuration Protocol for IPv6 (DHCPv6) bis
draft-ietf-dhc-rfc3315bis-10

Versions: (draft-dhcwg-dhc-rfc3315bis)  00 01 02         Standards Track
          03 04 05 06 07 08 09 10                                       
Dynamic Host Configuration (DHC)                            T. Mrugalski
Internet-Draft                                              M. Siodelski
Obsoletes: 3315,3633,3736,4242,7083,7550                             ISC
           (if approved)                                         B. Volz
Intended status: Standards Track                          A. Yourtchenko
Expires: March 9, 2018                                             Cisco
                                                           M. Richardson
                                                                     SSW
                                                                S. Jiang
                                                                  Huawei
                                                                T. Lemon
                                                                 Nominum
                                                              T. Winters
                                                                 UNH-IOL
                                                       September 5, 2017


       Dynamic Host Configuration Protocol for IPv6 (DHCPv6) bis
                      draft-ietf-dhc-rfc3315bis-10

Abstract

   This document describes the Dynamic Host Configuration Protocol for
   IPv6 (DHCPv6): an extensible mechanism for configuring nodes with
   network configuration parameters, IP addresses, and prefixes.
   Parameters can be provided statelessly, or in combination with
   stateful assignment of one or more IPv6 addresses and/or IPv6
   prefixes.  DHCPv6 can operate either in place of or in addition to
   stateless address autoconfiguration (SLAAC).

   This document updates the text from RFC3315, the original DHCPv6
   specification, and incorporates prefix delegation (RFC3633),
   stateless DHCPv6 (RFC3736), an option to specify an upper bound for
   how long a client should wait before refreshing information
   (RFC4242), a mechanism for throttling DHCPv6 clients when DHCPv6
   service is not available (RFC7083), and clarifies the interactions
   between modes of operation (RFC7550).  As such, this document
   obsoletes RFC3315, RFC3633, RFC3736, RFC4242, RFC7083, and RFC7550.

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 https://datatracker.ietf.org/drafts/current/.



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   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
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   This Internet-Draft will expire on March 9, 2018.

Copyright Notice

   Copyright (c) 2017 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
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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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   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  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.  Relation to Previous DHCPv6 standards . . . . . . . . . .   6
     1.2.  Relation to DHCP in IPv4  . . . . . . . . . . . . . . . .   7
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .   8
     4.2.  DHCP Terminology  . . . . . . . . . . . . . . . . . . . .  10
   5.  Client-Server Exchanges . . . . . . . . . . . . . . . . . . .  13
     5.1.  Client-server Exchanges Involving Two Messages  . . . . .  14
     5.2.  Client-server Exchanges Involving Four Messages . . . . .  15
     5.3.  Server-client Exchanges . . . . . . . . . . . . . . . . .  15



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   6.  Operational Models  . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Stateless DHCP  . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  DHCP for Non-Temporary Address Assignment . . . . . . . .  16
     6.3.  DHCP for Prefix Delegation  . . . . . . . . . . . . . . .  17
     6.4.  DHCP for Customer Edge Routers  . . . . . . . . . . . . .  19
     6.5.  DHCP for Temporary Addresses  . . . . . . . . . . . . . .  19
     6.6.  Multiple Addresses and Prefixes . . . . . . . . . . . . .  20
   7.  DHCP Constants  . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Multicast Addresses . . . . . . . . . . . . . . . . . . .  20
     7.2.  UDP Ports . . . . . . . . . . . . . . . . . . . . . . . .  21
     7.3.  DHCP Message Types  . . . . . . . . . . . . . . . . . . .  21
     7.4.  DHCP Option Codes . . . . . . . . . . . . . . . . . . . .  23
     7.5.  Status Codes  . . . . . . . . . . . . . . . . . . . . . .  23
     7.6.  Transmission and Retransmission Parameters  . . . . . . .  23
     7.7.  Representation of Time Values and "Infinity" as a Time
           Value . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Client/Server Message Formats . . . . . . . . . . . . . . . .  25
   9.  Relay Agent/Server Message Formats  . . . . . . . . . . . . .  26
     9.1.  Relay-forward Message . . . . . . . . . . . . . . . . . .  27
     9.2.  Relay-reply Message . . . . . . . . . . . . . . . . . . .  27
   10. Representation and Use of Domain Names  . . . . . . . . . . .  28
   11. DHCP Unique Identifier (DUID) . . . . . . . . . . . . . . . .  28
     11.1.  DUID Contents  . . . . . . . . . . . . . . . . . . . . .  29
     11.2.  DUID Based on Link-layer Address Plus Time, DUID-LLT . .  29
     11.3.  DUID Assigned by Vendor Based on Enterprise Number,
            DUID-EN  . . . . . . . . . . . . . . . . . . . . . . . .  31
     11.4.  DUID Based on Link-layer Address, DUID-LL  . . . . . . .  32
     11.5.  DUID Based on Universally Unique IDentifier (UUID),
            DUID-UUID  . . . . . . . . . . . . . . . . . . . . . . .  33
   12. Identity Association  . . . . . . . . . . . . . . . . . . . .  33
     12.1.  Identity Associations for Address Assignment . . . . . .  34
     12.2.  Identity Associations for Prefix Delegation  . . . . . .  34
   13. Assignment to an IA . . . . . . . . . . . . . . . . . . . . .  34
     13.1.  Selecting Addresses for Assignment to an IA_NA . . . . .  34
     13.2.  Assignment of Temporary Addresses  . . . . . . . . . . .  36
     13.3.  Assignment of Prefixes for IA_PD . . . . . . . . . . . .  36
   14. Transmission of Messages by a Client  . . . . . . . . . . . .  37
     14.1.  Rate Limiting  . . . . . . . . . . . . . . . . . . . . .  37
     14.2.  Client Behavior when T1 and/or T2 are 0  . . . . . . . .  38
   15. Reliability of Client Initiated Message Exchanges . . . . . .  38
   16. Message Validation  . . . . . . . . . . . . . . . . . . . . .  40
     16.1.  Use of Transaction IDs . . . . . . . . . . . . . . . . .  41
     16.2.  Solicit Message  . . . . . . . . . . . . . . . . . . . .  41
     16.3.  Advertise Message  . . . . . . . . . . . . . . . . . . .  41
     16.4.  Request Message  . . . . . . . . . . . . . . . . . . . .  42
     16.5.  Confirm Message  . . . . . . . . . . . . . . . . . . . .  42
     16.6.  Renew Message  . . . . . . . . . . . . . . . . . . . . .  42
     16.7.  Rebind Message . . . . . . . . . . . . . . . . . . . . .  42



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     16.8.  Decline Messages . . . . . . . . . . . . . . . . . . . .  43
     16.9.  Release Message  . . . . . . . . . . . . . . . . . . . .  43
     16.10. Reply Message  . . . . . . . . . . . . . . . . . . . . .  43
     16.11. Reconfigure Message  . . . . . . . . . . . . . . . . . .  44
     16.12. Information-request Message  . . . . . . . . . . . . . .  44
     16.13. Relay-forward Message  . . . . . . . . . . . . . . . . .  44
     16.14. Relay-reply Message  . . . . . . . . . . . . . . . . . .  44
   17. Client Source Address and Interface Selection . . . . . . . .  44
     17.1.  Address, Interface Selection for Address Assignment  . .  45
     17.2.  Address, Interface Selection for Prefix Delegation . . .  45
   18. DHCP Configuration Exchanges  . . . . . . . . . . . . . . . .  45
     18.1.  A Single Exchange for Multiple IA Options  . . . . . . .  48
     18.2.  Client Behavior  . . . . . . . . . . . . . . . . . . . .  49
       18.2.1.  Creation and Transmission of Solicit Messages  . . .  50
       18.2.2.  Creation and Transmission of Request Messages  . . .  52
       18.2.3.  Creation and Transmission of Confirm Messages  . . .  54
       18.2.4.  Creation and Transmission of Renew Messages  . . . .  55
       18.2.5.  Creation and Transmission of Rebind Messages . . . .  57
       18.2.6.  Creation and Transmission of Information-request
                Messages . . . . . . . . . . . . . . . . . . . . . .  57
       18.2.7.  Creation and Transmission of Release Messages  . . .  58
       18.2.8.  Creation and Transmission of Decline Messages  . . .  60
       18.2.9.  Receipt of Advertise Messages  . . . . . . . . . . .  61
       18.2.10. Receipt of Reply Messages  . . . . . . . . . . . . .  62
         18.2.10.1.  Reply for Solicit (with Rapid Commit), Request,
                     Renew or Rebind . . . . . . . . . . . . . . . .  63
         18.2.10.2.  Reply for Release and Decline . . . . . . . . .  66
         18.2.10.3.  Reply for Confirm . . . . . . . . . . . . . . .  66
         18.2.10.4.  Reply for Information-request . . . . . . . . .  66
       18.2.11. Receipt of Reconfigure Messages  . . . . . . . . . .  66
       18.2.12. Refreshing Configuration Information . . . . . . . .  67
     18.3.  Server Behavior  . . . . . . . . . . . . . . . . . . . .  68
       18.3.1.  Receipt of Solicit Messages  . . . . . . . . . . . .  69
       18.3.2.  Receipt of Request Messages  . . . . . . . . . . . .  70
       18.3.3.  Receipt of Confirm Messages  . . . . . . . . . . . .  72
       18.3.4.  Receipt of Renew Messages  . . . . . . . . . . . . .  72
       18.3.5.  Receipt of Rebind Messages . . . . . . . . . . . . .  74
       18.3.6.  Receipt of Information-request Messages  . . . . . .  76
       18.3.7.  Receipt of Release Messages  . . . . . . . . . . . .  77
       18.3.8.  Receipt of Decline Messages  . . . . . . . . . . . .  77
       18.3.9.  Creation of Advertise Messages . . . . . . . . . . .  78
       18.3.10. Transmission of Advertise and Reply Messages . . . .  79
       18.3.11. Creation and Transmission of Reconfigure Messages  .  80
     18.4.  Reception of Unicast Messages  . . . . . . . . . . . . .  81
   19. Relay Agent Behavior  . . . . . . . . . . . . . . . . . . . .  81
     19.1.  Relaying a Client Message or a Relay-forward Message . .  82
       19.1.1.  Relaying a Message from a Client . . . . . . . . . .  82
       19.1.2.  Relaying a Message from a Relay Agent  . . . . . . .  83



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       19.1.3.  Relay Agent Behavior with Prefix Delegation  . . . .  83
     19.2.  Relaying a Relay-reply Message . . . . . . . . . . . . .  83
     19.3.  Construction of Relay-reply Messages . . . . . . . . . .  84
   20. Authentication of DHCP Messages . . . . . . . . . . . . . . .  85
     20.1.  Security of Messages Sent Between Servers and Relay
            Agents . . . . . . . . . . . . . . . . . . . . . . . . .  85
     20.2.  Summary of DHCP Authentication . . . . . . . . . . . . .  85
     20.3.  Replay Detection . . . . . . . . . . . . . . . . . . . .  85
     20.4.  Reconfigure Key Authentication Protocol  . . . . . . . .  86
       20.4.1.  Use of the Authentication Option in the Reconfigure
                Key Authentication Protocol  . . . . . . . . . . . .  86
       20.4.2.  Server Considerations for Reconfigure Key
                Authentication Protocol  . . . . . . . . . . . . . .  87
       20.4.3.  Client Considerations for Reconfigure Key
                Authentication Protocol  . . . . . . . . . . . . . .  88
   21. DHCP Options  . . . . . . . . . . . . . . . . . . . . . . . .  88
     21.1.  Format of DHCP Options . . . . . . . . . . . . . . . . .  89
     21.2.  Client Identifier Option . . . . . . . . . . . . . . . .  89
     21.3.  Server Identifier Option . . . . . . . . . . . . . . . .  90
     21.4.  Identity Association for Non-temporary Addresses Option   91
     21.5.  Identity Association for Temporary Addresses Option  . .  93
     21.6.  IA Address Option  . . . . . . . . . . . . . . . . . . .  95
     21.7.  Option Request Option  . . . . . . . . . . . . . . . . .  96
     21.8.  Preference Option  . . . . . . . . . . . . . . . . . . .  97
     21.9.  Elapsed Time Option  . . . . . . . . . . . . . . . . . .  98
     21.10. Relay Message Option . . . . . . . . . . . . . . . . . .  99
     21.11. Authentication Option  . . . . . . . . . . . . . . . . . 100
     21.12. Server Unicast Option  . . . . . . . . . . . . . . . . . 101
     21.13. Status Code Option . . . . . . . . . . . . . . . . . . . 102
     21.14. Rapid Commit Option  . . . . . . . . . . . . . . . . . . 103
     21.15. User Class Option  . . . . . . . . . . . . . . . . . . . 104
     21.16. Vendor Class Option  . . . . . . . . . . . . . . . . . . 105
     21.17. Vendor-specific Information Option . . . . . . . . . . . 107
     21.18. Interface-Id Option  . . . . . . . . . . . . . . . . . . 109
     21.19. Reconfigure Message Option . . . . . . . . . . . . . . . 110
     21.20. Reconfigure Accept Option  . . . . . . . . . . . . . . . 110
     21.21. Identity Association for Prefix Delegation Option  . . . 111
     21.22. IA Prefix Option . . . . . . . . . . . . . . . . . . . . 113
     21.23. Information Refresh Time Option  . . . . . . . . . . . . 115
     21.24. SOL_MAX_RT Option  . . . . . . . . . . . . . . . . . . . 117
     21.25. INF_MAX_RT Option  . . . . . . . . . . . . . . . . . . . 118
   22. Security Considerations . . . . . . . . . . . . . . . . . . . 119
   23. Privacy Considerations  . . . . . . . . . . . . . . . . . . . 122
   24. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 122
   25. Obsoleted Mechanisms  . . . . . . . . . . . . . . . . . . . . 126
   26. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 127
   27. References  . . . . . . . . . . . . . . . . . . . . . . . . . 127
     27.1.  Normative References . . . . . . . . . . . . . . . . . . 127



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     27.2.  Informative References . . . . . . . . . . . . . . . . . 129
   Appendix A.  Summary of Changes . . . . . . . . . . . . . . . . . 133
   Appendix B.  Appearance of Options in Message Types . . . . . . . 136
   Appendix C.  Appearance of Options in the Options Field of DHCP
                Options  . . . . . . . . . . . . . . . . . . . . . . 138
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 139

1.  Introduction

   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 also provides a mechanism for automated delegation of IPv6
   prefixes using DHCPv6, originally specified in [RFC3633].  Through
   this mechanism, a delegating router can delegate prefixes to
   requesting routers.  Use of this mechanism is specified as part of
   [RFC7084] and by [TR-187].

   DHCPv6 can also provide only other configuration options (i.e., no
   addresses or prefixes).  That implies that the server does not have
   to track any state, and thus this 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 the relationship to the
   previous DHCPv6 standards in Section 1.1 and clarifies the stance
   with regards to DHCPv4 in Section 1.2.  Section 5 describes the
   message exchange mechanisms to illustrate DHCP operation rather than
   provide an exhaustive list of all possible interactions and Section 6
   provides an overview of common operational models.  Section 18
   explains 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 documents were published over the years: IPv6
   Prefix Options for Dynamic Host Configuration Protocol (DHCP) version
   6 [RFC3633], Stateless Dynamic Host Configuration Protocol (DHCP)
   Service for IPv6s [RFC3736], Information Refresh Time Option for
   Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC4242],



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   Modification to Default Values of SOL_MAX_RT and INF_MAX_RT
   [RFC7083], and Issues and Recommendations with Multiple Stateful
   DHCPv6 Options [RFC7550].  This document provides a unified,
   corrected, and cleaned up definition of DHCPv6 that also covers all
   errata filled against older RFCs.  As such, it obsoletes a number of
   aforementioned RFCs.  And, there are a small number of mechanisms
   that were obsoleted, listed in Section 25.  Also see Appendix A.

1.2.  Relation to DHCP in IPv4

   The operational models and relevant configuration information for
   DHCPv4 ([RFC2131] and [RFC2132]) 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.  For IPv6-only networks, [RFC7341] describes a transport
   mechanism to carry DHCPv4 messages using the DHCPv6 protocol for the
   dynamic provisioning of IPv4 address and configuration information.

   Merging DHCPv4 and DHCPv6 configuration is out of scope of this
   document.  [RFC4477] discusses some issues and possible strategies
   for running DHCPv4 and DHCPv6 services together.  While this document
   is a bit dated, it provides a good overview of the issues at hand.

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] when they appear in ALL CAPS.  When these words are not in
   ALL CAPS (such as "should" or "Should"), they have their usual
   English meanings, and are not to be interpreted as [RFC2119] key
   words.

   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.







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3.  Background

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

   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.  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 implementers understand IPv6 Neighbor
   Discovery.

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.




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   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); PPP and PPPoE links; and Internet
                             (or higher) layer "tunnels", such as
                             tunnels over IPv4 or IPv6 itself.

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

   link-local address        An IPv6 address having 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.





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

4.2.  DHCP Terminology

   Terminology specific to DHCP can be found below.

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

   binding                   A binding (or, client binding) is a group
                             of server data records containing the
                             information the server has about the
                             addresses or delegated prefixes in an IA or
                             configuration information explicitly
                             assigned to the client.  Configuration
                             information that has been returned to a
                             client through a policy, such as 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 lease
                             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.

   container option          An option that encapsulates other options
                             (for example, the IA_NA may contain IAADDR
                             options).

   delegating router         The router that acts as a DHCP server, and
                             responds to requests for delegated
                             prefixes.  This document primarily uses the



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                             term "DHCP server" or "server" when
                             discussing the "delegating router"
                             functionality of prefix delegation (see
                             Section 1).

   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 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 11 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 lease; for example, an
                             identity association for temporary



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                             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 three 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
                             addresses that are not temporary addresses
                             (see "IA_TA").

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

   IA_PD                     Identity Association for Prefix Delegation:
                             An IA that carries delegated prefixes.

   lease                     A contract by which the server grants the
                             use of an address or delegated prefix to
                             the client for a specified period of time.

   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 (see Section 7.3).

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

   requesting router         The router that acts as a DHCP client and
                             is requesting prefix(es) to be assigned.
                             This document primarily uses the term "DHCP



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                             client" or "client" when discussing the
                             "requesting router" functionality of prefix
                             delegation (see Section 1).

   retransmission            Another attempt to send the same DHCP
                             message by a client or server, as a result
                             of not receiving a valid response to the
                             previously sent messages.  The
                             retransmitted message is typically modified
                             prior to sending, as required by the DHCP
                             specifications.  In particular, the client
                             updates the value of the Elapsed Time
                             option in the retransmitted message.

   RKAP                      The Reconfiguration Key Authentication
                             Protocol, see Section 20.4.

   singleton option          An option that is allowed to appear only
                             once as a the top-level option or at any
                             encapsulation level.  Most options are
                             singletons.

   T1                        The time at which the client contacts the
                             server from which the addresses in the
                             IA_NA or prefixes in the IA_PD were
                             obtained to extend the lifetimes of the
                             addresses assigned to the IA_NA or prefixes
                             delegated to the IA_PD.

   T2                        The time at which the client contacts any
                             available server to extend the lifetimes of
                             the addresses assigned to the IA_NA or
                             prefixes delegated to the IA_PD.

   top-level option          An option conveyed in a DHCP message
                             directly, i.e., not encapsulated in any
                             other option, as described in Section 9 of
                             [RFC7227].

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

5.  Client-Server Exchanges

   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.



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   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.

5.1.  Client-server Exchanges Involving Two Messages

   When a DHCP client does not need to have a DHCP server assign it IP
   addresses or delegated prefixes, the client can obtain other
   configuration information such as a list of available DNS servers
   [RFC3646] or NTP servers [RFC4075] through a single message and reply
   exchange with a DHCP server.  To obtain other 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 other configuration
   information for the client.

   When a server has addresses and/or delegated prefixes 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 multicast address requesting the
   assignment of addresses and/or delegated prefixes and other
   configuration information.  This message includes an indication (the
   Rapid Commit option) 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 and/or delegated prefixes to
   the client immediately responds with a Reply message.  The
   configuration information and the addresses and/or delegated prefixes
   in the Reply message are then immediately available for use by the
   client.

   Each address or delegated prefix 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 or
   delegated prefix, the client sends a Renew message to the server.
   The server sends a Reply message to the client with the new



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   lifetimes, allowing the client to continue to use the address or
   delegated prefix without interruption.  If the server is unable to
   extend the lifetime of an address or delegated prefix, it indicates
   this by returning the address or delegated prefix with lifetimes of
   0.  At the same time, the server may assign other addresses or
   delegated prefixes.

   There are additional two message exchanges between the client and
   server described later in this document.

5.2.  Client-server Exchanges Involving Four Messages

   To request the assignment of one or more addresses and/or delegated
   prefixes, a client first locates a DHCP server and then requests the
   assignment of addresses and/or delegated prefixes and other
   configuration information from the server.  The client sends a
   Solicit message to the All_DHCP_Relay_Agents_and_Servers multicast
   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/or delegated
   prefixes and other configuration information.  The server responds
   with a Reply message that contains the confirmed addresses, delegated
   prefixes, and configuration.

   As described in the previous section, the client can also request an
   extension of the lifetimes assigned to addresses or delegated
   prefixes (this is a two message exchange).

5.3.  Server-client Exchanges

   A server that has previously communicated with a client and
   negotiated for the client to listen for Reconfigure messages, may
   send the client a Reconfigure message to initiate the client to
   update its configuration by sending an Information-request, Renew, or
   Rebind message.  The client then performs the earlier described two
   message exchange.  This can be used to expedite configuration changes
   to a client, such as the need to renumber a network (see [RFC6879]).

6.  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.




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

6.1.  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-forward and Relay-reply messages to
   accommodate operation when clients and servers are not on the same
   link.

6.2.  DHCP for Non-Temporary Address Assignment

   This model of operation was the original motivation for DHCP.  It is
   appropriate for situations where stateless address autoconfiguration
   alone is insufficient or impractical, e.g., because of network
   policy, additional requirements such as dynamic updates to the DNS,
   or client-specific requirements.

   The model of operation for non-temporary address assignment is as
   follows.  The server is provided with 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
   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
   DNS name server addresses and domain search lists, when requesting
   addresses.




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   Clients can also request more than one address or set of addresses
   (see Section 6.6 and Section 12).

6.3.  DHCP for Prefix Delegation

   The prefix delegation mechanism, originally described in [RFC3633],
   is another stateful mode of operation and was originally 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].

   While [RFC3633] assumed that the DHCP client is a router (hence use
   of "requesting router") and that the DHCP server was a router (hence
   use of "delegating router"), DHCP prefix delegation itself does not
   require that the client forward IP packets not addressed to itself,
   and thus does not require that the client (or server) be a router as
   defined in [RFC2460].  Also, in many cases (such as tethering or
   hosting virtual machines), hosts are already forwarding IP packets
   and thus operating as routers as defined in [RFC2460].  Therefore,
   this document mostly replaces "requesting router" with client and
   "delegating router" with server.

   The model of operation for prefix delegation is as follows.  A server
   is provided prefixes to be delegated to clients.  A client requests
   prefix(es) from the server, as described in Section 18.  The server
   chooses prefix(es) for delegation, and responds with prefix(es) to
   the client.  The client is then responsible for the delegated
   prefix(es).  For example, the client 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
   client is allowed to use the prefix.  A client 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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   This prefix delegation mechanism is 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.  [RFC7084] and [RFC7368] describe in detail
   such use.

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

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

                    Figure 1: Prefix Delegation Network

   In this example, the server (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 client (requesting router)
   requests configuration information through DHCP.  The DHCP messages
   from the client are received by the server in the aggregation device.
   When the server receives the request, it selects an available prefix
   or prefixes for delegation to the client.  The server then returns
   the prefix or prefixes to the client.



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   The client 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 client 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 client.
   The client may, in turn, provide DHCP service to nodes attached to
   the internal network.  For example, the client may obtain the
   addresses of DNS and NTP servers from the ISP server, and then pass
   that configuration information on to the subscriber hosts through a
   DHCP server in the client (requesting router).

   If the client uses a delegated prefix to configure addresses on
   interfaces on itself or other nodes behind it, the preferred and
   valid lifetimes of those addresses MUST be no larger than the
   remaining preferred and valid lifetimes, respectively, for the
   delegated prefix at any time.  In particular, if the delegated prefix
   or a prefix derived from it is advertised for stateless address
   autoconfiguration [RFC4862], the advertised preferred and valid
   lifetimes MUST NOT exceed the corresponding remaining lifetimes of
   the delegated prefix.

6.4.  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 6.2) and prefix delegation (see
   Section 6.3).  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.

6.5.  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 6.2), 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.






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6.6.  Multiple Addresses and Prefixes

   The protocol allows a client to receive multiple addresses.  During
   typical operation, a client sends one instance of an IA_NA option and
   the server assigns at most one address from each prefix assigned to
   the link the client is attached to.  In particular, the server can be
   configured to serve addresses out of multiple prefixes for a given
   link.  This is useful in cases such as when a network renumbering
   event is in progress.  In a typical deployment the server will grant
   one address per each IA_NA option.

   A client can explicitly request multiple addresses by sending
   multiple IA_NA (and/or IA_TA) options.  A client can send multiple
   IA_NA (and/or IA_TA) options in its initial transmissions.
   Alternatively, it can send an extra Request message with additional
   new IA_NA (and/or IA_TA) options (or include them in a Renew
   message).

   The same principle also applies to Prefix Delegation.  In principle
   the protocol allows a client to request new prefixes to be delegated
   by sending additional IA_PD options.  However, a typical operator
   usually prefers to delegate a single, larger prefix.  In most
   deployments it recommended for the client to request larger prefix in
   its initial transmissions rather than request additional prefixes
   later on.

   The exact behavior of the server (whether to grant additional
   addresses and prefixes or not) is up to the server policy and is
   outside of scope of this document.

   For more information on how the server distinguishes between IA
   option instances, see Section 12.

7.  DHCP Constants

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

7.1.  Multicast Addresses

   DHCP makes use of the following multicast addresses:

   All_DHCP_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.




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   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 on the interfaces which are
                   within the site.

7.2.  UDP Ports

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

7.3.  DHCP Message Types

   DHCP defines the following message types.  More detail on these
   message types can be found in Section 8 and Section 9.  Additional
   message types have been defined and may be defined in the future -
   see https://www.iana.org/assignments/dhcpv6-parameters/
   dhcpv6-parameters.xhtml.  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 addresses and/or
                   delegated prefixes, 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 leases and
                   configuration parameters to extend the lifetimes on
                   the leases 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 leases assigned



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                   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
                   leases 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 leases to the client to indicate that the
                   client will no longer use one or more of the assigned
                   leases.

   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 leases 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



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                   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.

7.4.  DHCP Option Codes

   DHCP makes extensive use of options in messages and some of these are
   defined later in Section 21.  Additional options are defined in other
   documents or may be defined in the future.

7.5.  Status Codes

   DHCP 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 21.13.

   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.

7.6.  Transmission and Retransmission Parameters

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






















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   +-----------------+------------------+------------------------------+
   | 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              |
   | IRT_DEFAULT     | 86400 secs (24   | Default information refresh  |
   |                 | hours)           | time                         |
   | IRT_MINIMUM     | 600 secs         | Min information refresh time |
   +-----------------+------------------+------------------------------+

7.7.  Representation of Time Values and "Infinity" as a Time Value

   All time values for lifetimes, T1 and T2 are unsigned 32-bit
   integers.  The value 0xffffffff is taken to mean "infinity" when used
   as a lifetime (as in [RFC4861]) or a value for T1 or T2.

   Setting the valid lifetime of an address or a delegated prefix to
   0xffffffff ("infinity") amounts to a permanent address or delegation
   of the prefix to a client and should only be used in cases were
   permanent assignments are desired.





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   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.

8.  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.

   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 7.3.  A one octet long field.

      transaction-id       The transaction ID for this message exchange.
                           A three octets long field.

      options              Options carried in this message; options are
                           described in Section 21.  A variable length
                           field (4 octets less than the size of the
                           message).




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9.  Relay Agent/Server Message Formats

   Relay agents exchange messages with other relay agents and 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:


       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.








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9.1.  Relay-forward Message

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

      msg-type             RELAY-FORW (12).  A one octet long field.

      hop-count            Number of relay agents that have already
                           relayed this message.  A one octet long
                           field.

      link-address         An address that may be used by the server to
                           identify the link on which the client is
                           located.  This is typically a globally unique
                           address (including unique local address,
                           [RFC4193]), but see discussion in
                           Section 19.1.1.  A 16 octets long field

      peer-address         The address of the client or relay agent from
                           which the message to be relayed was received.
                           A 16 octets long field.

      options              MUST include a "Relay Message option" (see
                           Section 21.10); MAY include other options
                           added by the relay agent.  A variable length
                           field (34 octets less than the size of the
                           message).

   See Section 13.1 for an explanation how link-address is used.

9.2.  Relay-reply Message

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

      msg-type             RELAY-REPL (13).  A one octet long field.

      hop-count            Copied from the Relay-forward message.  A one
                           octet long field.

      link-address         Copied from the Relay-forward message.  A 16
                           octets long field.

      peer-address         Copied from the Relay-forward message.  A 16
                           octets long field.

      options              MUST include a "Relay Message option"; see
                           Section 21.10; MAY include other options.  A



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                           variable length field (34 octets less than
                           the size of the message).

10.  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].

11.  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 21.2 and Section 21.3 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 SHOULD 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.  It should be noted that an attempt to
   parse a DUID to obtain a client's link-layer address is unreliable as
   there is no guarantee that the client is still using the same link-
   layer address as when it generated its DUID.  And, such an attempt
   will be more and more unreliable as more clients adopt privacy
   measures, such as those defined in [RFC7844].  It is recommended to
   rely on the mechanism defined in [RFC6939].

   The DUID is carried in an option because it may be variable in length
   and because it is not required in all DHCP messages.  The DUID is
   designed to be unique across all DHCP clients and servers, and stable
   for any specific client or server - that is, the DUID used by a
   client or server SHOULD NOT change over time if at all possible; for
   example, a device's DUID should not change as a result of a change in
   the device's network hardware.  The stability of the DUID includes
   changes to virtual interfaces, such as logical PPP (over Ethernet)
   interfaces that may come and go in Customer Premise Equipment
   routers.  The client may change its DUID as specified in [RFC7844].

   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




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   any persistent storage.  Retaining a generated DUID in such a device
   is not possible, so the DUID scheme must accommodate such devices.

11.1.  DUID Contents

   A DUID consists of a two octets 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 three 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.

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

   This type of DUID consists of a two octets type field containing the
   value 1, a two octets 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         DUID-Type (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.

   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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11.3.  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 four octet 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         DUID-Type (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 octets type of 2, the Enterprise Number
   (9), followed by eight octets of identifier data
   (0x0CC084D303000912).

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

   This type of DUID consists of two octets containing the DUID type 3,
   a two octets 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 node 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         DUID-Type (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 SHOULD 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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11.5.  DUID Based on Universally Unique IDentifier (UUID), DUID-UUID

   This type of DUID consists of 16 octets containing a 128-bit UUID.
   [RFC6355] details when to use this type, and how to pick an
   appropriate source of the UUID.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         DUID-Type (4)         |        UUID (128 bits)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                                               |
      |                                                               |
      |                                -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                        Figure 8: DUID-UUID format

12.  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 (i.e., IA_NA with IAID
   0 is unique from IA_TA with IAID 0).  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.

   If the client wishes to obtain a distinctly new address or prefix and
   deprecate the existing one, the client sends a Release message to the
   server for the IAs using the original IAID.  Then the client creates
   a new IAID, to be used in future messages to obtain leases for the
   new IA.






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12.1.  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_NA consists of one or more
   IPv6 addresses along with the T1 and T2 times for the IA.  See
   Section 21.4 for the representation of an IA_NA in a DHCP message.

   The configuration information in an IA_TA consists of one or more
   IPv6 addresses.  See Section 21.5 for the representation of an IA_TA
   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 Address option.  The lifetimes apply
   to the use of addresses, as described in section 5.5.4 of [RFC4862].

12.2.  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 client, with a set of interfaces or with
   exactly one interface.  A client 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 server.

   The configuration information in an IA_PD consists of one or more
   prefixes along with the T1 and T2 times for the IA_PD.  See
   Section 21.21 for the representation of an IA_PD in a DHCP message.

   Each delegated prefix 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 Prefix option.  The
   lifetimes apply to the use of delegated prefixes, as described in
   section 5.5.4 of [RFC4862].

13.  Assignment to an IA

13.1.  Selecting Addresses for Assignment to an IA_NA

   A server selects addresses to be assigned to an IA_NA according to
   the address assignment policies determined by the server




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   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 in this case may come from any
         of the Relay-forward messages encapsulated in the received
         Relay-forward, and in general the most encapsulated (closest
         Relay-forward to the client) has the most useful value.

      *  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.

   By default, DHCP server implementations SHOULD NOT generate
   predictable addresses [RFC7721].  Server implementers are encouraged
   to review [RFC4941], [RFC7824], and [RFC7707] as to possible
   considerations for how to generate addresses.

   A server MUST NOT assign an address that is otherwise reserved for
   some other purpose.  For example, a server MUST NOT assign addresses
   that use a reserved IPv6 Interface Identifier ([RFC5453], [RFC7136],
   [IANA-RESERVED-IID]).



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   See [RFC7969] for a more detailed discussion on how servers determine
   a client's location on the network.

13.2.  Assignment of Temporary Addresses

   A client may request the assignment of temporary addresses (see
   [RFC4941] for the definition of temporary addresses).  DHCP 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 21.5).  Each IA_TA
   option contains at lease 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 21.6) encapsulated 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]).

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

   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 a temporary address is to
   make it difficult to link the address to different actions over time.
   So, it is NOT RECOMMENDED for a client to renew temporary addresses,
   though DHCP provides for such a possibility (see Section 21.5).

13.3.  Assignment of Prefixes for IA_PD

   The mechanism through which the server 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].




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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 multicast address.

   DHCP servers SHOULD NOT care if the layer-2 address used was
   multicast or not, as long as the layer-3 address was correct.

   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 21.3) 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 21.12.

14.1.  Rate Limiting

   In order to avoid prolonged message bursts that may be caused by
   possible logic loops, a DHCP client MUST limit the rate of DHCP
   messages it transmits.  One example is that a client obtains an
   address or delegated prefix, but does not like the response; so it
   reverts back to Solicit procedure, discovers the same (sole) server,
   requests an address or delegated prefix and gets the same address or
   delegated prefix as before (as the server has this previously
   requested lease assigned to this client).  This loop 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 interval.  This method of bounding
   burstiness also guarantees that the long-term transmission rate will
   not be exceeded.

      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.





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

14.2.  Client Behavior when T1 and/or T2 are 0

   In certain cases, T1 and/or T2 time may be set to zero.  Currently
   there are three such cases:

   1.    a client received an IA_NA option with a zero value

   2.    a client received an IA_PD option with a zero value

   3.    a client received an IA_TA option (which does not contain T1
         and T2 fields and are not generally renewed)

   This is an indication that the renew and rebind times are left at the
   client's discretion.  However, they are not completely discretionary.

   When T1 and/or T2 times are set to zero, the client MUST choose a
   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 times so all IA
   containers are handled in one exchange, if possible.  The client MUST
   choose renew and rebind 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.  If a
   DHCP client fails to receive an expected response from a server, the
   client must retransmit its message according to the retransmission
   strategy described in this section.

   Note that the procedure described in this section is slightly
   modified when used with the Solicit message.  The modified procedure
   is described in Section 18.2.1.

   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 MUST update an "elapsed-time" value within an Elapsed Time
   option (see Section 21.9) in the retransmitted message.  In some
   cases, the client may also need to modify values in the IA Address or
   IA Prefix options if a valid lifetime for any of the client's leases



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   expires before retransmission.  Thus, whenever this document refers
   to a "retransmission" of a client's message, it refers to both
   modifying the original message and sending this new message instance
   to the server.

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

      RT      Retransmission timeout

      IRT     Initial retransmission time

      MRC     Maximum retransmission count

      MRT     Maximum retransmission time

      MRD     Maximum retransmission duration

      RAND    Randomization factor

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

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

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

   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:





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

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

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

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

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

   A client is not expected to listen for a response during the entire
   RT period and may turn off listening capabilities after a certain
   time due to power consumption saving or other reasons.  Of course, a
   client MUST listen for a Reconfigure if it has negotiated for its use
   with the server.

16.  Message Validation

   This section describes which options are valid in which kinds of
   message types.  Should a client or server receive messages which
   contain known options which are invalid for the message, this section
   explains how to process it.  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 discard it.

   If a server receives a message that it considers invalid, 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 were not
   present.  This is critical to provide for later extension of the DHCP
   protocol.




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   A server MUST discard any Solicit, Confirm, Rebind or Information-
   request messages it receives with a layer-3 unicast destination
   address.

   A client or server MUST discard any received DHCP 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.

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.








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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.

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.








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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.

   -  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.





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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 include a Reconfigure Message option.

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

   -  the message does not include authentication (such as RKAP, see
      Section 20.4) or fails authentication validation.

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 regarding interface selection is different
   depending on the purpose of the configuration.





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17.1.  Address, Interface Selection for Address Assignment

   When a client sends a DHCP message to the
   All_DHCP_Relay_Agents_and_Servers multicast address, it SHOULD send
   the message through the interface for which configuration information
   (including the addresses) is being requested.  However, the client
   MAY send the message through another interface if the interface which
   configuration is being requested for 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.

17.2.  Address, Interface Selection for Prefix Delegation

   Delegated prefixes are not associated with a particular interface in
   the same way as addresses are for address assignment, as mentioned in
   Section 17.1 above.

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

18.  DHCP Configuration Exchanges

   A client initiates a message exchange with a server or servers to
   acquire or update configuration information of interest.  A client
   has many reasons to initiate the configuration exchange.  Some of the
   more common ones are:

   1.    as part of the operating system configuration/bootstrap
         process,

   2.    when requested to do so by the application layer (through an
         operating system specific API),




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   3.    when Router Advertisement indicates DHCPv6 is available for
         address configuration (see [RFC4861] Section 4.2),

   4.    as required to extend the lifetime of address(es) and/or
         delegated prefix(es), using Renew and Rebind messages,

   5.    or when requested to do so by a server - upon the receipt of a
         Reconfigure message.

   The client is responsible for creating IAs and requesting that a
   server assign 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 discovery process
   to obtain the same 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 (see Section 18.2.7 for more
   details on what stop using means), and which it is now trying to
   obtain from a new server.

   A DHCP client that does not need to have a DHCP server assign it IP
   addresses or delegated prefixes, can obtain configuration information
   such as a list of available DNS servers [RFC3646] or NTP servers
   [RFC4075] through a single message and reply exchange with a DHCP
   server.  To obtain configuration information the client first sends
   an Information-request message (see Section 18.2.6) to the
   All_DHCP_Relay_Agents_and_Servers multicast address.  Servers respond
   with a Reply message containing the configuration information for the
   client (see Section 18.3.6).

   To request the assignment of one or more addresses or delegated
   prefixes, a client first locates a DHCP server and then requests the
   assignment of addresses/prefixes and other configuration information
   from the server.  The client does this by sending the Solicit message
   (see Section 18.2.1) to the All_DHCP_Relay_Agents_and_Servers
   multicast address and collecting Advertise messages from the servers
   which respond to the client's message and selects a server from which
   it wants to obtain configuration information.  This process is
   referred to as server discovery.  When the client has selected the
   server it sends a Request message to this server as described in
   Section 18.2.2.

   A client willing to perform the Solicit/Reply message exchange
   described in Section 18.2.1 includes a Rapid Commit option (see
   Section 21.14) in its Solicit message.



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   Servers that can assign addresses or delegated prefixes to the IAs
   respond to the client with an Advertise message or Reply message if
   the client included a Rapid Commit option and the server is
   configured to accept it.

   If the server responds with an Advertise message, the client
   initiates a configuration exchange as described in Section 18.2.2.

   A server may initiate a message exchange with a client by sending a
   Reconfigure message to cause the client to send a Renew, Rebind or
   Information-request message to refresh its configuration information
   as soon as the Reconfigure message is received by the client.

   Figure 9 shows a timeline diagram of the messages exchanged between a
   client and two servers for the typical lifecycle of one or more
   leases.  This is a combination of the 4-message exchange (to select a
   server and assign the lease(s) to the client) followed by two
   2-message exchanges (to extend the lifetime on the lease(s) and
   eventually release the lease(s)).

                Server                          Server
            (not selected)      Client        (selected)

                  v               v               v
                  |               |               |
                  |     Begins initialization     |
                  |               |               |
     start of     | _____________/|\_____________ |
     4-message    |/ Solicit      | Solicit      \|
     exchange     |               |               |
              Determines          |          Determines
             configuration        |         configuration
                  |               |               |
                  |\              |  ____________/|
                  | \________     | /Advertise    |
                  | Advertise\    |/              |
                  |           \   |               |
                  |      Collects Advertises      |
                  |             \ |               |
                  |     Selects configuration     |
                  |               |               |
                  | _____________/|\_____________ |
                  |/ Request      |  Request     \|
                  |               |               |
                  |               |     Commits configuration
                  |               |               |
     end of       |               | _____________/|
     4-message    |               |/ Reply        |



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     exchange     |               |               |
                  |    Initialization complete    |
                  |               |               |
                  .               .               .
                  .               .               .
                  |   T1 (Renewal) Timer Expires  |
                  |               |               |
     2-message    | _____________/|\_____________ |
     exchange     |/ Renew        |  Renew       \|
                  |               |               |
                  |               | Commits extended lease(s)
                  |               |               |
                  |               | _____________/|
                  |               |/ Reply        |
                  .               .               .
                  .               .               .
                  |               |               |
                  |      Graceful shutdown        |
                  |               |               |
     2-message    | _____________/|\_____________ |
     exchange     |/ Release      |  Release     \|
                  |               |               |
                  |               |         Discards lease(s)
                  |               |               |
                  |               | _____________/|
                  |               |/ Reply        |
                  |               |               |
                  v               v               v

   Figure 9: Timeline diagram of the messages exchanged between a client
      and two servers for the typical lifecycle of one or more leases

18.1.  A Single Exchange for Multiple IA Options

   The client and server use the IA_PD option to exchange information
   about prefix(es) in much the same way as IA_NA and IA_TA options are
   used for assigned 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.

   Clients SHOULD use a single transaction for all of its IA options.
   Servers SHOULD assign the same T1/T2 values to all IA options
   configured for a client, so the client will generate a single
   transaction when renewing its leases.  For a rationale of this
   approach, see Section 4.2 of [RFC7550].






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18.2.  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.

   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.

   When a client requests multiple IA option types or multiple instances
   of the same IA types in a Solicit, Request, Renew, or Rebind, it is
   possible that the available server(s) may only be configured to offer
   a subset of them.  When possible, the client SHOULD use the best
   configuration available and continue to request the additional IAs in
   subsequent messages [RFC7550].  This allows the client to maintain a
   single session and state machine.  In practice, especially in the
   case of handling IA_NA and IA_PD requests [RFC7084], this situation
   should be rare or a result of a temporary operational error.  Thus,
   it is more likely for the client to get all configuration if it
   continues, in each subsequent configuration exchange, to request all
   the configuration information it is programmed to try to obtain,
   including any stateful configuration options for which no results
   were returned in previous message exchanges.

   Upon receipt of a Reconfigure message from the server, a client
   responds with a Renew, Rebind or an Information-request message as
   indicated by the Reconfigure Message option.  The client SHOULD be
   suspicious of the Reconfigure message (they may be faked), and it
   MUST NOT abandon any resources it might have already obtained.  The
   client SHOULD treat the Reconfigure message as if the T1 timer had
   expired.  The client will expect the server to send IAs and/or other
   configuration information to the client in a Reply message.

   If the client has a source address of sufficient scope that can be
   used by the server as a return address, and the client has received a
   Server Unicast option Section 21.12 from the server, the client
   SHOULD unicast any Request, Renew, Release and Decline messages to
   the server.

   Use of unicast may avoid delays due to the relaying of messages by
   relay agents, as well as avoid overhead on servers due to the
   delivery of client messages to multiple servers.  However, 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



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   client.  The server should enable the use of unicast only when relay
   agent options will not be used.

18.2.1.  Creation and Transmission 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 MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   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 MAY include addresses in IA Address options encapsulated
   within 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 IA Prefix options encapsulated
   within IA_PD options as hints for the delegated prefix and/or prefix
   length for which the client has a preference.  See Section 18.2.4 for
   more on prefix length hints.

   The client MUST include an Option Request option (see Section 21.7)
   to request the SOL_MAX_RT option (see Section 21.24) 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 21.20)
   if the client is willing to accept Reconfigure messages from the
   server.

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




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   The first Solicit message from the client on the interface SHOULD be
   delayed by a random amount of time between 0 and SOL_MAX_DELAY.  This
   mechanism is essential for devices that are not battery powered, as
   they may suffer from power failure.  After recovering from a power
   outage, many devices may start their transmission at the same time.
   This is much less of a concern for battery powered devices.  This
   random delay also helps descynronize clients which start DHCP session
   by seeing it available in Router Advertisement messages (see
   [RFC4861] Section 4.2).

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

      IRT     SOL_TIMEOUT

      MRT     SOL_MAX_RT

      MRC     0

      MRD     0

   A client that wishes to use the Rapid Commit 2-message exchange
   includes a Rapid Commit option in its Solicit message.  The client
   may receive a number of different replies from different servers.
   The client will make note of any valid Advertise messages that it
   receives.  The client will discard any Reply messages that do not
   contain the Rapid Commit option.

   Upon receipt of a valid Reply with the Rapid Commit option, the
   client processes the message as described in Section 18.2.10

   At the end of the first RT period, if no suitable Reply messages are
   received, but the client has valid Advertise messages, then the
   client processes the Advertise as described in Section 18.2.9.

   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 18.2.10, 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.

   If the client is waiting for an Advertise message, the mechanism in
   Section 15 is modified as follows for use in the transmission of



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   Solicit messages.  The message exchange is not terminated by the
   receipt of an Advertise before the first RT has elapsed.  Rather, the
   client collects valid 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 valid Advertise messages for the first RT
   seconds, unless it receives a valid Advertise message with a
   preference value of 255.  The preference value is carried in the
   Preference option (see Section 21.8).  Any valid Advertise that does
   not include a Preference option is considered to have a preference
   value of 0.  If the client receives a valid Advertise message that
   includes a Preference option with a preference value of 255, the
   client immediately begins a client-initiated message exchange (as
   described in Section 18.2.2) by sending a Request message to the
   server from which the Advertise message was received.  If the client
   receives a valid 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 a valid Advertise message, the client SHOULD continue
   with a client-initiated message exchange by sending a Request
   message.

   If the client does not receive any valid 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 valid 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.

18.2.2.  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.



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   The client MUST include 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.

   The client MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   The client MUST include an Option Request option (see Section 21.7)
   to request the SOL_MAX_RT option (see Section 21.24) 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 21.20)
   if 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.








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18.2.3.  Creation and Transmission of Confirm Messages

   The client uses a Confirm message when is has only addresses (no
   delegated prefixes) assigned by a DHCP server to determine if it is
   still connected to the same link when the client detects a change in
   network information as described in Section 18.2.12.

   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 MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   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 leases, using the last known lifetimes for those
   leases, and SHOULD continue to use any other previously obtained
   configuration parameters.







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18.2.4.  Creation and Transmission of Renew Messages

   To extend the valid and preferred lifetimes for the leases assigned
   to the IAs and obtain new addresses or delegated prefixes for 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 controls the time at which the client should contact 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 (earliest
   T1/T2).

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

   A client MUST also initiate a Renew/Reply message exchange before
   time T1 if the client's link-local address used in previous
   interactions with the server is no longer valid and it is willing to
   receive Reconfigure messages.

   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 MUST include a Server Identifier option in the Renew
   message, identifying the server with which the client most recently
   communicated.

   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.






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   The client MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   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
   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 includes Option Request option (see Section 21.7) to
   request the SOL_MAX_RT option (see Section 21.24) 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 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 earliest T2

   The message exchange is terminated when earliest time T2 is reached.
   If the client is responding to a Reconfigure, the client ignores and
   discards the Reconfigure message.  In this case, the client continues
   to operate as if Reconfigure message was not received, i.e., it uses
   T1/T2 times associated with the client's leases to determine when it
   should send Renew or Rebind to the server. server.  The client begins
   a Rebind message exchange (see Section 18.2.5) when the earliest time
   T2 is reached.





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18.2.5.  Creation and Transmission of Rebind Messages

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

   A Rebind is also used to verify delegated prefix bindings but with
   different retransmission parameters as described in Section 18.2.3.

   The client constructs the Rebind message as described in
   Section 18.2.4 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.

   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 leases 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.  If the terminated Rebind exchange
   was initiated as a result of receiving a Reconfigure message, the
   client ignores and discards the Reconfigure message.

18.2.6.  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.





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   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 (see section 4.3.1 of [RFC7844] for reasons why
   a client may not want to include this option).  If the client does
   not include a Client Identifier option, the server will not be able
   to return any client-specific options to the client, or the server
   may choose not to respond to the message at all.

   The client MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   The client MUST include an Option Request option (see Section 21.7)
   to request the INF_MAX_RT option (see Section 21.25), the Information
   Refresh Time option (see Section 21.23), 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.

   When responding to a Reconfigure, the client includes a Server
   Identifier option with the identifier from the Reconfigure message to
   which the client is responding.

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

18.2.7.  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.




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   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 MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   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 stop using all of the leases being released before
   the client begins the Release message exchange process.  For an
   address, this means the address MUST have been removed from the
   interface.  For a delegated prefix, this means the prefix MUST have
   been advertised with a Preferred Lifetime and a Valid Lifetime of
   zero in a Router Advertisement message as described in (e) of
   Section 5.5.3 of [RFC4862] - also see L-13 in Section 4.3 of
   [RFC7084].

   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

      MRD     0

   If leases are released but the Reply from a DHCP server is lost, the
   client will retransmit the Release message, and the server may



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   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.

18.2.8.  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 MUST include an Elapsed Time option (see Section 21.9) to
   indicate how long the client has been trying to complete the current
   DHCP message exchange.

   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

      MRC     DEC_MAX_RC



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      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.

18.2.9.  Receipt of Advertise Messages

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

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

   -  Within a group of Advertise messages with the same server
      preference value, a client MAY select those servers whose
      Advertise messages advertise information of interest to the
      client.

   -  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.

   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



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   the client.  A client SHOULD only update its SOL_MAX_RT and
   INF_MAX_RT values if all received Advertise messages that contained
   the corresponding option specified the same value, otherwise it
   should use the default value (see Section 7.6).

   The client MUST ignore any Advertise message that contains no
   addresses (IA Address options encapsulated in IA_NA or IA_TA options)
   and no delegated prefixes (IA Prefix 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 an Advertise message MUST NOT restart the Solicit
   retransmission timer.

18.2.10.  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 top-level Status
   Code option if present.

   The client MUST process SOL_MAX_RT and INF_MAX_RT options in a Reply
   message, even if the message contains a Status Code option indicating
   a failure.

   If the client receives a Reply message with a status code of
   UnspecFail, the server is indicating that it was unable to process
   the client's 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).

   If the client receives a Reply message with a status code of
   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.

   Otherwise (no status code or another status code), the client
   processes the Reply as described below based on the original message
   for which the Reply was received.



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   The client MAY choose to report any status code or message from the
   Status Code option in the Reply message.

   When a client received a configuration option in an earlier Reply,
   then sends a Renew, Rebind or Information-request and the requested
   option is not present in the Reply, the client SHOULD stop using the
   previously received configuration information.  In other words, the
   client should behave as if it never received this configuration
   option and return to the relevant default state.  If there is no
   viable way to stop using the received configuration information, the
   values received/configured from the option MAY persist if there are
   no other sources for that data and they have no external impact.  For
   example, a client that previously received a Client FQDN option and
   used it to set up its hostname is allowed to continue using it if
   there is no reasonable way for a node to unset its hostname and it
   has no external impact.  As a counter example, a client that
   previously received an NTP server address from the DHCP server and
   does not receive it any more, MUST stop using the configured NTP
   server address.  The client SHOULD be open to other sources of the
   same configuration information.  This behavior does not apply to any
   IA containers, as their processing is described in detail in the next
   section.

   When a client receives a requested option that has an updated value
   from what was previously received, the client SHOULD make use of that
   updated value as soon as possible for its configuration information.

18.2.10.1.  Reply for Solicit (with Rapid Commit), Request, Renew or
            Rebind

   If 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).

   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, if appropriate for the IA type.

   -  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.




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   -  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.

   If the client can operate with the addresses and/or 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
      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.

   -  The client MUST perform duplicate address detection as per
      [RFC4862] Section 5.4, which does list some exceptions, 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 any 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 18.2.8.

   -  For each assigned address, which does not have any associated
      reachability information, in order to avoid the problems described
      in [RFC4943], the client MUST NOT assume that any addresses are
      reachable on-link as a result of receiving an IA_NA or IA_TA.
      Addresses obtained from IA_NA or IA_TA MUST NOT be used to form an
      implicit prefix with a length other than 128.

   -  For each delegated prefix, the client assigns a subnet to each of
      the links to which the associated interfaces are attached, with
      the following exception: the client MUST NOT advertise any
      delegated prefixes or subnets from the delegated prefix(es) to the
      link through which it received the DHCP message from the server
      (see [RFC6603] for exceptions).

      When a client subnets a delegated prefix, it must assign
      additional bits to the prefix to generate unique, longer prefixes.
      For example, if the client in Figure 1 were delegated
      2001:db8:0::/48, it might generate 2001:db8:0:1::/64 and
      2001:db8:0:2::/64 for assignment to the two links in the
      subscriber network.  If the client were delegated 2001:db8:0::/48



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      and 2001:db8:5::/48, it might assign 2001:db8:0:1::/64 and
      2001:db8:5:1::/64 to one of the links, and 2001:db8:0:2::/64 and
      2001:db8:5:2::/64 for assignment to the other link.

      If the client uses a delegated prefix to configure addresses on
      interfaces on itself or other nodes behind it, the preferred and
      valid lifetimes of those addresses MUST be no larger than the
      remaining preferred and valid lifetimes, respectively, for the
      delegated prefix at any time.  In particular, if the delegated
      prefix or a prefix derived from it is advertised for stateless
      address autoconfiguration [RFC4862], the advertised valid and
      preferred lifetimes MUST NOT exceed the corresponding remaining
      lifetimes of the delegated prefix.

   Management of the specific configuration information is detailed in
   the definition of each option in Section 21.

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

   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 to the server that responded.
      The client places IA options in this message for all IAs.  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.

   Whenever a client restarts the DHCP server discovery process or
   selects an alternate server, as described in Section 18.2.9, the
   client SHOULD stop using all the addresses and delegated prefixes for
   which it has bindings and try to obtain all required leases from the
   new server.  This facilitates the client using a single state machine
   for all bindings.







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18.2.10.2.  Reply for Release and Decline

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

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

18.2.10.3.  Reply for Confirm

   If the client receives any Reply messages that indicate a success
   status (explicit or implicit), the client can use the addresses in
   the IA and ignore any messages that indicate a NotOnLink status.
   When the client only receives one or more Replies with the NotOnLink
   status in response to a Confirm message, the client performs DHCP
   server discovery as described in Section 18.

18.2.10.4.  Reply for Information-request

   Refer to Section 21.23 for details on how the Information Refresh
   Time option (whether or not present in the Reply) should be handled
   by the client.

18.2.11.  Receipt of Reconfigure Messages

   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.

   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 21.19).  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.

   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, Rebind, or Information-request message if
   necessary); the client MUST ignore any additional Reconfigure
   messages until the exchange is complete.




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   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 MAY
   discontinue retransmission of Reconfigure messages to the client once
   the server receives the Renew, Rebind 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.

18.2.12.  Refreshing Configuration Information

   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 stable storage and persisted DHCP
      state).

   o  The client is reconnected to a link on which it has obtained
      leases.

   o  The client returns from sleep mode.

   o  The client changes access points (such as if using a wireless
      technology).

   When the client detects one of the above conditions and it has
   obtained addresses and no delegated prefixes from a server, 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, the client MUST initiate a Rebind/Reply message exchange as
   described in Section 18.2.5, with the exception that the
   retransmission parameters should be set as for the Confirm message



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   (see Section 18.2.3).  The client includes IA_NAs, IA_TAs, and
   IA_PDs, along with the associated leases, in its Rebind message.

   If the client has only obtained network information using
   Information-request/Reply message exchanges, the client MUST initiate
   a Information-request/Reply message exchange as described in
   Section 18.2.6.

   If not associated with one of the above mentioned conditions, a
   client SHOULD initiate a Renew/Reply exchange (as if the T1 time
   expired) as described in Section 18.2.4 or an Information-request/
   Reply exchange as described in Section 18.2.6 if the client detects a
   significant change regarding the prefixes available on the link (when
   new are added or existing are deprecated) as this may indicate a
   configuration change.  However, a client MUST rate limit such
   attempts to avoid flooding a server with requests when there are link
   issues (for example, only doing one of these at most every 30
   seconds).

18.3.  Server Behavior

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

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

   In most cases, the server will send a Reply in response to a Request,
   Confirm, Renew, Rebind, Decline, Release, and Information-request
   messages sent by a client.  The server will also send a Reply in
   response to a Solicit with a Rapid Commit option, when the server is
   configured to respond with committed lease assignments.

   These Advertise and Reply messages 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 response 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 response
   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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   The server MAY initiate a configuration exchange, by sending
   Reconfigure messages, to cause DHCP clients to obtain new addresses,
   prefixes 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 or when other
   configuration options are updated, perhaps because servers are moved,
   added, or removed.

   When a client receives a Reconfigure message from the server, the
   client initiates sending a Renew, Rebind or Information-request
   message as indicated by msg-type in the Reconfigure Message option
   (as defined in Section 21.19).  The server sends IAs and/or other
   configuration information to the client in a Reply message.  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 client's message.

18.3.1.  Receipt of Solicit Messages

   See Section 18.4 for handling Solicit message received via unicast.
   Unicast transmission of Solicit is not allowed, regardless if Server
   Unicast option is configured or not.

   The server determines the information about the client and its
   location as described in Section 13 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 21.20) in the Solicit message, the server
   discards the Solicit message.

   If the server is permitted to respond to the client, the client has
   not included a Rapid Commit option in the Solicit message or the
   server has not been configured to respond with committed assignment
   of leases and other resources, the server sends an Advertise message
   to the client as described in Section 18.3.9.

   If the client has included a Rapid Commit option in the Solicit
   message and the server has been configured to respond with committed
   assignments of leases and other resources, the server responds to the
   Solicit with a Reply message.  The server produces the Reply message
   as though it had received a Request message, as described in
   Section 18.3.2.  The server transmits the Reply message as described
   in Section 18.3.10.  The server MUST commit the assignment of any
   addresses and delegated prefixes or other configuration information
   before sending a Reply message to a client.  In this case the server



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   includes a Rapid Commit option in the Reply message to indicate that
   the Reply is 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
      leases.

      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.

18.3.2.  Receipt of Request Messages

   See Section 18.4 for handling Request message received via unicast.

   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 in the Request message the server checks if
   the prefixes of included addresses are appropriate for the link to
   which the client is connected.  If any of the prefixes of the
   included 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 option in the Reply
   message with no addresses in the IA and a Status Code option
   containing status code NoAddrsAvail in the IA.




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   For any IA_PD in the Request message, to which the server cannot
   assign any delegated prefixes, the server MUST return the IA_PD
   option in the Reply message with no prefixes in the IA_PD and with a
   Status Code option containing status code NoPrefixAvail in the IA_PD.

   The server MAY assign different addresses and/or delegated prefixes
   to an IA than those included within the IA of the client's Request
   message.

   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
   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 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.

   The server SHOULD include a Reconfigure Accept option if the server
   policy enables reconfigure mechanism and the client supports it.
   Currently sending this option in Reply is technically redundant, as
   the use of the reconfiguration mechanism requires authentication and
   currently the only defined one is the Reconfigure Key Authentication
   Protocol (see Section 20.4) and the presence of the reconfigure key
   signals support for Reconfigure acceptance.  However, there may be
   better security mechanisms defined in the future that would cause
   RKAP to not be used anymore.  One of such mechanisms being worked on
   is mentioned in Section 22.

   The server includes other options containing configuration
   information to be returned to the client as described in
   Section 18.3.

   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 server sends a Reply message with
   existing bindings, possibly with updated lifetimes.  The server may
   update the bindings according to its local policies, but the server
   SHOULD generate the response again and not simply retransmit
   previously sent information, even if the transaction-id matches a
   previous transmission.  The server MUST NOT cache its responses.






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18.3.3.  Receipt of Confirm Messages

   See Section 18.4 for handling Confirm message received via unicast.
   Unicast transmission of Confirm is not allowed, regardless if Server
   Unicast option is configured or not.

   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.

18.3.4.  Receipt of Renew Messages

   See Section 18.4 for handling Renew message received via unicast.

   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.  If the server chooses to include the IA Address or
   IA Prefix option for such an address or delegated prefix, the server
   SHOULD set T1 and T2 to the valid lifetime for the IA option unless
   the server also includes other addresses or delegated prefixes which
   the server is able to extend for the IA.  Setting T1 and T2 to values



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   equal to valid lifetime informs the client that the leases associated
   with said IA will not be extended, so there is no point in trying.
   Also, it avoids generating unnecessary traffic as the remaining
   lifetime approaches 0.

   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 zero value in the "IPv6
      prefix" field and non-zero value in the "prefix-length" field, the
      server MAY use the "prefix-length" value as a hint for the length
      of the prefixes to be assigned (see [RFC8168] 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.

   -  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.




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   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 18.3.

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

   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.

18.3.5.  Receipt of Rebind Messages

   See Section 18.4 for handling Rebind message received via unicast.
   Unicast transmission of Rebind is not allowed, regardless if Server
   Unicast option is configured or not.

   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.  If the server chooses to not include any IAs
   containing IA Address or IA Prefix options with lifetimes of 0 and
   the server does not include any other IAs with leases and/or status
   codes, the server does not send a Reply message.  In this situation
   the server 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 - see the Discussion in Section 21.14.  Therefore, the
   server SHOULD only create new bindings during processing of a Rebind




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   message if the server is configured to respond with a Reply message
   to a Solicit message containing the Rapid Commit option.

   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 18.3.

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

   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.

18.3.6.  Receipt of Information-request Messages

   See Section 18.4 for handling Information-request message received
   via unicast.

   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 18.3.  The server




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   MAY include additional options that were not requested by the client
   in the Information-request message.

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

18.3.7.  Receipt of Release Messages

   See Section 18.4 for handling Release message received via unicast.

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

   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.

18.3.8.  Receipt of Decline Messages

   See Section 18.4 for handling Decline message received via unicast.

   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



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   the IA (though it may choose to log an error if it finds such an
   address).

   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 by setting the "msg-type" field to REPLY, and copying
   the transaction ID from the Decline message into the transaction-id
   field.  The client 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.

18.3.9.  Creation 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 option 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 indicate it supports Reconfigure mechanism.  This information may
   be used by the client during the server selection process.

   The server includes the 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.  The server MUST include
   options in the Advertise message containing configuration parameters
   for all of the options identified in the Option Request option in the
   Solicit message that the server has been configured to return to the



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   client.  If the Option Request option includes a container option the
   server MUST include all the options that are eligible to be
   encapsulated in the container.  The Option Request option MAY be used
   to signal support for a feature even when that option is encapsulated
   as in the case of the Prefix Exclude option [RFC6603].  In this case,
   special processing is required by the server.  The server MAY return
   additional options to the client if it has been configured to do so.

   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_TA)
   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.

   Transmission of the Advertise message is described in the next
   section.

18.3.10.  Transmission of Advertise and Reply Messages

   If the original message was received directly by the server, the
   server unicasts the Advertise or 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 Advertise or 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 21.10).  If the Relay-



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   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.  See Section 19.3 for more details on
   the construction of Relay-reply messages.

18.3.11.  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.

   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 21.19) 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 19.3) 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
   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



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   original Reconfigure message) from the client as satisfying the
   Reconfigure message request.

   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 7.6.

18.4.  Reception of Unicast Messages

   Unless otherwise stated in sections dedicated to specific messages
   reception (see dedicated sections in Section 18.3), the server is not
   supposed to accept unicast traffic when it is not explicitly
   configured to do so.  For some messages (Solicit, Rebind, and
   Confirm) unicast transmission is not allowed, even if Server Unicast
   option is configured.  For Request, Renew, Informaton-request,
   Release, and Decline messages, it is allowed only if Server Unicast
   option is configured.

   When the server receives a message via unicast from a client to which
   the server has not sent a Server Unicast option (or is not currently
   configured to send a Server Unicast option to the client), the server
   discards that message and responds with an Advertise (when responding
   to Solicit) or Reply (when responding to any other messages) 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 (if any), and no other options.

19.  Relay Agent Behavior

   The relay agent SHOULD be configured to use a list of destination
   addresses, which include unicast addresses.  The list of destination
   addresses MAY include 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.






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   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 19.1.1.

19.1.  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
   of the IP datagram in which the message was received into 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.

19.1.1.  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 address (including unique local
   address, [RFC4193]) with a prefix assigned to the link on which the
   client should be assigned leases into the link-address field.  If
   such an address is not available, the relay agent may set the link-
   address field to a link-local address from the interface the original
   message was received on.  That is not recommended as it may require
   additional information to be provided in the server configuration.
   See Section 3.2 of [RFC7969] for a detailed discussion.

   This address will be used by the server to determine the link from
   which the client should be assigned leases 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 21.18) in the Relay-forward message.  The server will
   include the Interface-id option in its Relay-reply message.  The
   relay agent sets the link-address field as described in the earlier
   paragraphs regardless of whether the relay agent includes an
   Interface-id option in the Relay-forward message.




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19.1.2.  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.

   If the source address from the IP datagram header of the received
   message is a global address (including unique local address,
   [RFC4193]), the relay agent sets the link-address field to 0;
   otherwise the relay agent sets the link-address field to a global
   address (including unique local 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.

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

19.2.  Relaying a Relay-reply Message

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

   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.



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19.3.  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.

   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 10: 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.




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20.  Authentication of DHCP Messages

   Within this document, two security mechanisms are introduced for the
   authentication of DHCP messages: authentication (and encryption) of
   messages sent between servers and relay agents using IPsec, and
   protection against misconfiguration of a client caused by a
   Reconfigure message sent by a malicious DHCP server.

   The delayed authentication protocol, defined in [RFC3315], has been
   obsoleted by this document (see Section 25).

20.1.  Security of Messages Sent Between Servers and Relay Agents

   Relay agents and servers that exchange messages can use IPsec as
   detailed in [RFC8213].

20.2.  Summary of DHCP Authentication

   Authentication of DHCP messages is accomplished through the use of
   the Authentication option (see Section 21.11).  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.  One such protocol, the Reconfigure key
   authentication protocol, is defined in Section 20.4.  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.

20.3.  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.



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   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.

20.4.  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 have negotiated to use Reconfigure messages.

20.4.1.  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 11: RKAP Authentication Information

      Type             Type of data in the 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).

                       A one octet long field.

      Value            Data as defined by the Type field.  A 16 octets
                       long field.

20.4.2.  Server Considerations for Reconfigure Key Authentication
         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



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   Authentication option included in the Reconfigure message sent to the
   client.

20.4.3.  Client Considerations for Reconfigure Key Authentication
         Protocol

   The client will receive a Reconfigure Key from the server in an
   Authentication option 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.

21.  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 21.1.  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.  See Section 24 for additional information about
   a registry maintained by IANA.

   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 21.4), IA_TA (see Section 21.5), Vendor Class (see
   Section 21.16), Vendor-specific Information (see Section 21.17), and
   IA_PD (see Section 21.21) options.  Also, IA Address (see
   Section 21.6) and IA Prefix (see Section 21.22) may appear in their
   respective IA options more than once.








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21.1.  Format of DHCP Options

   The format of DHCP options is:


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


                         Figure 12: Option Format

      option-code          An unsigned integer identifying the specific
                           option type carried in this option.  A two
                           octets long field.

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

      option-data          The data for the option; the format of this
                           data depends on the definition of the option.
                           A variable length field (the length, in
                           octets, is specified by option-len).

   DHCP 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 21.4 and Section 21.6.

21.2.  Client Identifier Option

   The Client Identifier option is used to carry a DUID (see Section 11)
   identifying a client between a client and a server.  The format of
   the Client Identifier option is:











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


                Figure 13: Client Identifier Option Format

      option-code          OPTION_CLIENTID (1).

      option-len           Length of DUID in octets.

      DUID                 The DUID for the client.

21.3.  Server Identifier Option

   The Server Identifier option is used to carry a DUID (see Section 11)
   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 14: Server Identifier Option Format

      option-code          OPTION_SERVERID (2).

      option-len           Length of DUID in octets.

      DUID                 The DUID for the server.





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21.4.  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 21.5).

   The format of the IA_NA 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_NA         |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        IAID (4 octets)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T1                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T2                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      .                         IA_NA-options                         .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Figure 15: 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 other IA option types (i.e.,
                           IA_TA and IA_PD).  A four octets long field.

      T1                   The time at which the client should contact
                           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



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                           expressed in units of seconds.  A four octets
                           long field.

      T2                   The time at which the client should contact
                           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.  A four octets
                           long field.

      IA_NA-options        Options associated with this IA_NA.  A
                           variable length field (12 octets less than
                           the value in the option-len field).

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

   Each IA_NA carries one "set" of non-temporary addresses; it is up to
   the server policy to determine how many addresses are assigned, but
   typically at most one address is assigned from each prefix assigned
   to the link to which the client is attached to.

   An IA_NA option may only appear in the options area of a DHCP
   message.  A DHCP message may contain multiple IA_NA options (though
   each must have a unique IAID).

   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



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

   As per Section 7.7, the value 0xffffffff is taken to ("infinity") and
   should be used carefully.

21.5.  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 16: 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



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                           all of this client's IA_TAs.  The number
                           space for IA_TA IAIDs is separate from the
                           number space for other IA option types (i.e.,
                           IA_NA and IA_PD).  A four octets long field.

      IA_TA-options        Options associated with this IA_TA.  A
                           variable length field (4 octets less than the
                           value in the option-len field).

   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.  It is up to the
   server policy to determine how many addresses are assigned.

   An IA_TA option may only appear in the options area of a DHCP
   message.  A DHCP message may contain multiple IA_TA options (though
   each must have a unique IAID).

   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 valid lifetime 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 valid lifetime of a temporary address to allow an
   application to continue to use an established TCP connection.
   Extending only the valid, but not the preferred lifetime means the
   address will end up in deprecated state eventually.  Existing
   connections could continue, but no new ones would be created using
   that address.

   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.





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   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.

21.6.  IA Address Option

   The IA Address option is used to specify an address 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 IAaddr-options
   fields encapsulates those options that are specific to this address.

   The format of the IA Address option is:


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


                    Figure 17: IA Address Option Format

      option-code          OPTION_IAADDR (5).

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

      IPv6-address         An IPv6 address.  A client MUST NOT form an
                           implicit prefix with a length other than 128
                           for this address.  And, a client MUST NOT
                           assume any length of prefix that matches this
                           address is on-link (see [RFC7421]).  A 16
                           octets long field.



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      preferred-lifetime   The preferred lifetime for the address in the
                           option, expressed in units of seconds.  A
                           four octets long field.

      valid-lifetime       The valid lifetime for the address in the
                           option, expressed in units of seconds.  A
                           four octets long field.

      IAaddr-options       Options associated with this address.  A
                           variable length field (24 octets less than
                           the value in the option-len field).

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

   The client MUST discard any addresses for which the preferred
   lifetime is greater than the valid lifetime.

   As per Section 7.7, the valid lifetime of an address 0xffffffff is
   taken to mean "infinity" and should be used carefully.

   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 21.13.

21.7.  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:









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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_ORO          |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    requested-option-code-1    |    requested-option-code-2    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 18: 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.  The length, in octets, is
                           specified by option-len.

   A client MUST include an Option Request option in a Solicit, Request,
   Renew, Rebind, or Information-request message to inform the server
   about options the client wants the server to send to the client.  For
   certain message types, some option codes MUST be included in the
   Option Request option, see Table 1 for details.

   The Option Request option MUST NOT include the following options:
   Server Identifier, Client Identifier, IA_NA, IA_PD, IA_TA, IA
   Address, IA Prefix, Option Request, Elapsed Time, Preference, Relay
   Message, Authentication, Server Unicast, Rapid Commit, User Class,
   Vendor Class, Interface-Id, Reconfigure Message, and Reconfigure
   Accept.  Other top-level options MUST appear in the Option Request
   option or they will not be sent by the server.  Only top-level
   options MAY appear in the Option Request option.  Options
   encapsulated in a container option SHOULD NOT appear in an Option
   Request option; see [RFC7598] for an example of container options.
   However, options MAY be defined which specify exceptions to this
   restriction on including encapsulated options in an Option Request
   option.  For example, the Option Request option MAY be used to signal
   support for a feature even when that option is encapsulated, as in
   the case of the Prefix Exclude option [RFC6603].  See Table 1.

21.8.  Preference Option

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




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   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 19: Preference Option Format

      option-code          OPTION_PREFERENCE (7).

      option-len           1.

      pref-value           The preference value for the server in this
                           message.  A one octet long field.

   A server MAY include a Preference option in an Advertise message to
   control the selection of a server by the client.  See Section 18.2.9
   for the use of the Preference option by the client and the
   interpretation of Preference option data value.

21.9.  Elapsed Time Option


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


                   Figure 20: 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 two octets long field.



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   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.

21.10.  Relay Message Option

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

   The format of the Relay Message option is:


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


                  Figure 21: 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.  The length, in octets, is specified
                           by option-len.



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21.11.  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 20.  The delayed
   authentication protocol, defined in [RFC3315], has been obsoleted by
   this document, due to lack of usage.  The format of the
   Authentication option is:


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


                  Figure 22: 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.  A one octet long
                           field.

      algorithm            The algorithm used in the authentication
                           protocol.  A one octet long field.

      RDM                  The replay detection method used in this
                           authentication option.  A one octet long
                           field.

      Replay detection     The replay detection information for the RDM.
                           A 64-bit (8 octets) long field





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      authentication information  The authentication information, as
                           specified by the protocol and algorithm used
                           in this authentication option.  A variable
                           length field (11 octets less than the value
                           in option-len).

21.12.  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:


       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 23: Server Unicast Option Format

      option-code          OPTION_UNICAST (12).

      option-len           16.

      server-address       The 128-bit address to which the client
                           should send messages delivered using unicast.

   The server specifies the 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 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.



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

21.13.  Status Code Option

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


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


                   Figure 24: 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.  A two octets long field.

      status-message       A UTF-8 encoded text string suitable for
                           display to an end user, which MUST NOT be
                           null-terminated.  A variable length field (2
                           octets less than the value in option-len).

   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-code 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          |
   |               |      | multicast address.                         |
   | NoPrefixAvail |    6 | Server has no prefixes available to assign |
   |               |      | to the IA_PD(s).                           |
   +---------------+------+--------------------------------------------+

   See Section 24 for additional information about the registry
   maintained by IANA with the complete list of status codes.

21.14.  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 25: Rapid Commit Option Format

      option-code          OPTION_RAPID_COMMIT (14).

      option-len           0.






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   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 18.2.1.

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

   DISCUSSION:

      Each server that responds with a Reply to a Solicit that includes
      a Rapid Commit option will commit the leases 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 leases that are not actually used
      by the client, which could result in bad information in the DNS
      server if the DHCP server updates DNS [RFC4704] or in response to
      leasequery requests [RFC5007].

      The problem of unused leases can be minimized by designing the
      DHCP service so that only one server responds to the Solicit, by
      using relatively short lifetimes for newly assigned leases, or by
      having DHCP clients release unused leases using the Release
      message.

21.15.  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 26: User Class Option Format

      option-code          OPTION_USER_CLASS (15).



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      option-len           Length of user class data field.

      user-class-data      The user classes carried by the client.  The
                           length, in octets, is specified by option-
                           len.

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

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


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


                     Figure 27: 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.

21.16.  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 28: 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].  A four
                           octets long field.

      vendor-class-data    The hardware configuration of the node on
                           which the client is running.  A variable
                           length field (4 octets less than the value in
                           option-len).

   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 29: 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.





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   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 vendor-class-data
   instances.

21.17.  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                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                       vendor-option-data                      .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


           Figure 30: 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].  A four
                           octets long field.

      vendor-option-data   Vendor options, interpreted by vendor-
                           specific code on the clients and servers.  A
                           variable length field (4 octets less than the
                           value in option-len).

   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 node device's IPv6 stack to be
   functional.



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


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          sub-opt-code         |         sub-option-len        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                        sub-option-data                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 31: Vendor-specific Options Format

      sub-opt-code         The code for the sub-option.  A two octets
                           long field.

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

      sub-option-data      The data area for the sub-option.  The
                           length, in octets, is specified by sub-
                           option-len.

   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 sub-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.





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   -  MAY specify the Vendor-specific Information option with
      appropriate data.

   Servers 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.

   -  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.

21.18.  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 32: 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 length, in
                           octets, is specified by option-len.



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

   Servers MAY use the Interface-ID for parameter assignment policies.
   The Interface-ID SHOULD be considered an opaque value, with policies
   based on exact 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.

21.19.  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 33: 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.  A one octet
                           long field.

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

21.20.  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



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   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 34: Reconfigure Accept Option Format

      option-code          OPTION_RECONF_ACCEPT (20).

      option-len           0.

21.21.  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.  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_IA_PD          |         option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IAID (4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T1                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              T2                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                          IA_PD-options                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Figure 35: Identity Association for Prefix Delegation Option Format

      option-code          OPTION_IA_PD (25).




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      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 client's IA_PDs.  The number
                           space for IA_PD IAIDs is separate from the
                           number space for other IA option types (i.e.,
                           IA_NA and IA_TA).  A four octets long field.

      T1                   The time at which the client should contact
                           the server 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.  A four octets
                           long field.

      T2                   The time at which the client should contact
                           any available server to extend the lifetimes
                           of the prefixes assigned to the IA_PD; T2 is
                           a time duration relative to the current time
                           expressed in units of seconds.  A four octets
                           long field.

      IA_PD-options        Options associated with this IA_PD.  A
                           variable length field (12 octets less than
                           the value in the option-len field).

   The IA_PD-options field encapsulates those options that are specific
   to this IA_PD.  For example, all of the IA 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 (though
   each must have a unique IAID).

   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 the server explicit control over when a
   client should contact the server about a specific IA_PD.






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   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 prefixes in the IA_PD 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 prefixes in the IA_PD that the
   server 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 client, the server sets T1 and T2 to 0.  The client
   MUST follow the rules defined in Section 14.2.

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

21.22.  IA Prefix Option

   The IA Prefix option is used to specify a prefix associated with an
   IA_PD.  The IA Prefix option must be encapsulated in the IA_PD-
   options field of an IA_PD 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_IAPREFIX        |         option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      preferred-lifetime                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        valid-lifetime                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | prefix-length |                                               |
      +-+-+-+-+-+-+-+-+          IPv6-prefix                          |
      |                           (16 octets)                         |
      |                                                               |
      |                                                               |
      |                                                               |
      |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |                                               .
      +-+-+-+-+-+-+-+-+                                               .
      .                       IAprefix-options                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 36: IA Prefix Option Format

      option-code          OPTION_IAPREFIX (26).

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

      preferred-lifetime   The preferred lifetime for the prefix in the
                           option, expressed in units of seconds.  A
                           value of 0xFFFFFFFF represents infinity.  A
                           four octets long field.

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

      prefix-length        Length for this prefix in bits.  A one octet
                           long field.

      IPv6-prefix          An IPv6 prefix.  A 16 octets long field.

      IAprefix-options     Options associated with this prefix.  A
                           variable length field (25 octets less than
                           the value in the option-len field).




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   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 in these lifetime fields.

   A client 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.

   The client MUST discard any prefixes for which the preferred lifetime
   is greater than the valid lifetime.

   The values in the preferred and valid lifetimes are the number of
   seconds remaining for each lifetime.  See Section 18.2.10.1 for more
   details on how these values are used for delegated prefixes.

   As per Section 7.7, the preferred and valid lifetime values of
   0xffffffff is taken to mean "infinity" and should be used carefully.

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

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

21.23.  Information Refresh Time Option

   This option is requested by clients and returned by servers to
   specify an upper bound for how long a client should wait before
   refreshing information retrieved from a DHCP server.  It is only used
   in Reply messages in response to Information-request messages.  In
   other messages there will usually be other information that indicates
   when the client should contact the server, e.g., T1/T2 times and
   lifetimes.  This option is useful when the configuration parameters
   change or during renumbering event as clients running in the
   stateless mode will be able to update their configuration.

   The format of the Information Refresh Time 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_INFORMATION_REFRESH_TIME|         option-len            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   information-refresh-time                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 37: Information Refresh Time Option Format




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      option-code          OPTION_INFORMATION_REFRESH_TIME (32).

      option-len           4.

      information-refresh-time  Time duration relative to the current
                           time, expressed in units of seconds.  A four
                           octets long field.

   A DHCP client MUST request this option in the Option Request option
   (see Section 21.7) when sending Information-request messages.  A
   client MUST NOT request this option in the ORO in any other messages.

   A server sending a Reply to an Information-request message SHOULD
   include this option if it is requested in the ORO of the Information-
   Request.  The option value MUST NOT be smaller than IRT_MINIMUM.
   This option MUST only appear in the top-level option area of Reply
   messages.

   If the Reply to an Information-request message does not contain this
   option, the client MUST behave as if the option with value
   IRT_DEFAULT was provided.

   A client MUST use the refresh time IRT_MINIMUM if it receives the
   option with a value less than IRT_MINIMUM.

   As per Section 7.7, the value 0xffffffff is taken to mean "infinity"
   and implies that the client should not refresh its configuration data
   without some other trigger (such as detecting movement to a new
   link).

   If a client contacts the server to obtain new data or refresh some
   existing data before the refresh time expires, then it SHOULD also
   refresh all data covered by this option.

   When the client detects that the refresh time has expired, it SHOULD
   try to update its configuration data by sending an Information-
   Request as specified in Section 18.2.6, except that the client MUST
   delay sending the first Information-request by a random amount of
   time between 0 and INF_MAX_DELAY.

   A client MAY have a maximum value for the refresh time, where that
   value is used whenever the client receives this option with a value
   higher than the maximum.  This also means that the maximum value is
   used when the received value is "infinity".  A maximum value might
   make the client less vulnerable to attacks based on forged DHCP
   messages.  Without a maximum value, a client may be made to use wrong
   information for a possibly infinite period of time.  There may




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   however be reasons for having a very long refresh time, so it may be
   useful for this maximum value to be configurable.

21.24.  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 7.6.  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.

   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 38: 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 four octets long field.

   A DHCP client MUST include the SOL_MAX_RT option code in any Option
   Request option (see Section 21.7) it sends in a Solicit message.

   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 as a top-level
   option in the message to the 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



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   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 18.2.1.

   The purpose of this mechanism is to give network administrator a way
   to avoid large DHCP traffic if all DHCP servers become unavailable.
   Therefore this value is expected to be retained for as long as
   practically possible.

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

21.25.  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 7.6.  One use for the
   INF_MAX_RT option is to set a longer value for INF_MAX_RT, which
   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 39: INF_MAX_RT Option Format

      option-code          OPTION_INF_MAX_RT (83).

      option-len           4.

      INF_MAX_RT value     Overriding value for INF_MAX_RT in seconds;
                           MUST be in range: 60 <= "value" <= 86400 (1
                           day).  A four octets long field.

   A DHCP client MUST include the INF_MAX_RT option code in any Option
   Request option (see Section 21.7) it sends in an Information-request
   message.





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   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 a top-level option
   in the message to the client.

   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 18.2.6.

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

22.  Security Considerations

   This section discusses security considerations that are not related
   to privacy.  For dedicated privacy discussion, see Section 23.

   The threat to DHCP is inherently an insider threat (assuming a
   properly configured network where DHCP 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.

   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.



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

   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 20.4 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 "in plain text" to the client.

   Many of these rogue server attacks can be mitigated by making use of
   the mechanism described in [RFC7610] and [RFC7513].

   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 assigned address or delegatable prefixes,
   or the exhaustion of CPU or network bandwidth, and are present
   anytime there is a shared resource.  Some forms of these exhaustion
   attacks can be partially mitigated by appropriate server policy,
   e.g., limiting the maximum number of leases any one client can get.

   The messages exchanged between relay agents and servers may be used
   to mount a "man in the middle" or denial of service attack.
   Communication between a server and a relay agent, and communication
   between relay agents, can be authenticated and encrypted through the
   use of IPsec, as described in [RFC8213].



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   However, the use of manually configured pre-shared 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 address and
   delegatable prefixes.

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

   Various network environments also offer levels of security if
   deployed as described below.

   In enterprise and factory networks, use of [IEEE-802.1x]
   authentication can prevent unknown or untrusted clients from
   connecting to the network.  However, this does not necessarily assure
   that the connected client will be a good DHCP or network actor.

   For wired networks where clients typically are connected to a switch
   port, snooping DHCP multicast (or unicast traffic) becomes difficult
   as the switches limit the traffic delivered to a port.  The client's
   DHCP multicast packets (with destination address fe02::1:2) are only
   forwarded to the DHCP server's (or relay's) switch port - not all
   ports.  And the server's (or relay's) unicast replies are only
   delivered to the target client's port - not all ports.

   In public networks (such as a WiFi network in a coffee shop or
   airport), it is possible for others within radio range to snoop DHCP
   and other traffic.  But in these environments, there is very little
   if anything that can be learned from the DHCP traffic itself (either
   from client to server, or server to client) if the privacy
   considerations (see Section 23) are followed.  For devices that do
   not follow the privacy considerations, there is also little that can
   be learned that would not be available from subsequent communications
   anyway (such as the device's mac-address).  Or, that cannot be
   inferred by the bad actor initiating a DHCP request itself (since all
   clients will typically receive similar configuration details).  As
   mentioned above, one threat is that the RKAP key for a client can be
   learned (if the initial Solicit / Advertise / Request / Reply
   exchange is monitored) and trigger a premature reconfiguration - but
   this is relatively easy to prevent by disallowing direct client-to-
   client communication on these networks or using [RFC7610] and
   [RFC7513].







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23.  Privacy Considerations

   This section focuses on the server considerations.  For extended
   discussion about privacy considerations for the client, see
   [RFC7824].  In particular, Section 3 of that document discusses
   various identifiers that could be misused to track the client.
   Section 4 discusses existing mechanisms that may have an impact on
   client's privacy.  Finally, Section 5 discusses potential attack
   vectors.  For recommendations how to address or mitigate those
   issues, see [RFC7844].

   This specification does not define any allocation strategies.
   Implementers are expected to develop their own algorithm for the
   server to choose a resource out of the available pool.  Several
   possible allocation strategies are mentioned in Section 4.3 of
   [RFC7824].  Please keep in mind that this list is not exhaustive and
   there are certainly other possible strategies.  Readers are also
   encouraged to read [RFC7707], in particular Section 4.1.2 that
   discusses the problems with certain allocation strategies.

24.  IANA Considerations

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

   The publication of this document does not change the assignment rules
   for new values for message types, option codes, DUID types or status
   codes.

   The list of assigned values used in DHCPv6 is available at
   http://www.iana.org/assignments/dhcpv6-parameters/
   dhcpv6-parameters.xml

   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], [RFC4242] and [RFC7083].

   IANA is requested to add two columns to the DHCPv6 Option table at
   http://www.iana.org/assignments/dhcpv6-parameters/
   dhcpv6-parameters.xhtml to indicate which options are allowed to
   appear in a client's ORO option (see Section 21.7) and which options
   are singleton options (only allowed to appear once as a top-level or
   encapsulated option - see Section 16 of [RFC7227]).  Table 1 provides
   the data for the options assigned by IANA at the time of writing.

   +-------+-------------------------+---------------------+-----------+
   | Optio | Option Name (OPTION     | Client ORO (1)      | Singleton |



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   |     n | prefix removed)         |                     | Option    |
   +-------+-------------------------+---------------------+-----------+
   |     1 | CLIENTID                | No                  | Yes       |
   |     2 | SERVERID                | No                  | Yes       |
   |     3 | IA_NA                   | No                  | No        |
   |     4 | IA_TA                   | No                  | No        |
   |     5 | IAADDR                  | No                  | No        |
   |     6 | ORO                     | No                  | Yes       |
   |     7 | PREFERENCE              | No                  | Yes       |
   |     8 | ELAPSED_TIME            | No                  | Yes       |
   |     9 | RELAY_MSG               | No                  | Yes       |
   |    11 | AUTH                    | No                  | Yes       |
   |    12 | UNICAST                 | Yes                 | Yes       |
   |    13 | STATUS_CODE             | No                  | Yes       |
   |    14 | RAPID_COMMIT            | No                  | Yes       |
   |    15 | USER_CLASS              | No                  | Yes       |
   |    16 | VENDOR_CLASS            | No                  | No (2)    |
   |    17 | VENDOR_OPTS             | Optional            | No (2)    |
   |    18 | INTERFACE_ID            | No                  | Yes       |
   |    19 | RECONF_MSG              | No                  | Yes       |
   |    20 | RECONF_ACCEPT           | No                  | Yes       |
   |    21 | SIP_SERVER_D            | Yes                 | Yes       |
   |    22 | SIP_SERVER_A            | Yes                 | Yes       |
   |    23 | DNS_SERVERS             | Yes                 | Yes       |
   |    24 | DOMAIN_LIST             | Yes                 | Yes       |
   |    25 | IA_PD                   | No                  | No        |
   |    26 | IAPREFIX                | No                  | No        |
   |    27 | NIS_SERVERS             | Yes                 | Yes       |
   |    28 | NISP_SERVERS            | Yes                 | Yes       |
   |    29 | NIS_DOMAIN_NAME         | Yes                 | Yes       |
   |    30 | NISP_DOMAIN_NAME        | Yes                 | Yes       |
   |    31 | SNTP_SERVERS            | Yes                 | Yes       |
   |    32 | INFORMATION_REFRESH_TIM | Required for        | Yes       |
   |       | E                       | Information-request |           |
   |    33 | BCMCS_SERVER_D          | Yes                 | Yes       |
   |    34 | BCMCS_SERVER_A          | Yes                 | Yes       |
   |    36 | GEOCONF_CIVIC           | Yes                 | Yes       |
   |    37 | REMOTE_ID               | No                  | Yes       |
   |    38 | SUBSCRIBER_ID           | No                  | Yes       |
   |    39 | CLIENT_FQDN             | Yes                 | Yes       |
   |    40 | PANA_AGENT              | Yes                 | Yes       |
   |    41 | NEW_POSIX_TIMEZONE      | Yes                 | Yes       |
   |    42 | NEW_TZDB_TIMEZONE       | Yes                 | Yes       |
   |    43 | ERO                     | No                  | Yes       |
   |    44 | LQ_QUERY                | No                  | Yes       |
   |    45 | CLIENT_DATA             | No                  | Yes       |
   |    46 | CLT_TIME                | No                  | Yes       |
   |    47 | LQ_RELAY_DATA           | No                  | Yes       |



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   |    48 | LQ_CLIENT_LINK          | No                  | Yes       |
   |    49 | MIP6_HNIDF              | Yes                 | Yes       |
   |    50 | MIP6_VDINF              | Yes                 | Yes       |
   |    51 | V6_LOST                 | Yes                 | Yes       |
   |    52 | CAPWAP_AC_V6            | Yes                 | Yes       |
   |    53 | RELAY_ID                | No                  | Yes       |
   |    54 | OPTION-IPv6_Address-MoS | Yes                 | Yes       |
   |    55 | OPTION-IPv6_FQDN-MoS    | Yes                 | Yes       |
   |    56 | NTP_SERVER              | Yes                 | Yes       |
   |    57 | V6_ACCESS_DOMAIN        | Yes                 | Yes       |
   |    58 | SIP_UA_CS_LIST          | Yes                 | Yes       |
   |    59 | OPT_BOOTFILE_URL        | Yes                 | Yes       |
   |    60 | OPT_BOOTFILE_PARAM      | Yes                 | Yes       |
   |    61 | CLIENT_ARCH_TYPE        | No                  | Yes       |
   |    62 | NII                     | Yes                 | Yes       |
   |    63 | GEOLOCATION             | Yes                 | Yes       |
   |    64 | AFTR_NAME               | Yes                 | Yes       |
   |    65 | ERP_LOCAL_DOMAIN_NAME   | Yes                 | Yes       |
   |    66 | RSOO                    | No                  | Yes       |
   |    67 | PD_EXCLUDE              | Yes                 | Yes       |
   |    68 | VSS                     | No                  | Yes       |
   |    69 | MIP6_IDINF              | Yes                 | Yes       |
   |    70 | MIP6_UDINF              | Yes                 | Yes       |
   |    71 | MIP6_HNP                | Yes                 | Yes       |
   |    72 | MIP6_HAA                | Yes                 | Yes       |
   |    73 | MIP6_HAF                | Yes                 | Yes       |
   |    74 | RDNSS_SELECTION         | Yes                 | No        |
   |    75 | KRB_PRINCIPAL_NAME      | Yes                 | Yes       |
   |    76 | KRB_REALM_NAME          | Yes                 | Yes       |
   |    77 | KRB_DEFAULT_REALM_NAME  | Yes                 | Yes       |
   |    78 | KRB_KDC                 | Yes                 | Yes       |
   |    79 | CLIENT_LINKLAYER_ADDR   | No                  | Yes       |
   |    80 | LINK_ADDRESS            | No                  | Yes       |
   |    81 | RADIUS                  | No                  | Yes       |
   |    82 | SOL_MAX_RT              | Required for        | Yes       |
   |       |                         | Solicit             |           |
   |    83 | INF_MAX_RT              | Required for        | Yes       |
   |       |                         | Information-request |           |
   |    84 | ADDRSEL                 | Yes                 | Yes       |
   |    85 | ADDRSEL_TABLE           | Yes                 | Yes       |
   |    86 | V6_PCP_SERVER           | Yes                 | No        |
   |    87 | DHCPV4_MSG              | No                  | Yes       |
   |    88 | DHCP4_O_DHCP6_SERVER    | Yes                 | Yes       |
   |    89 | S46_RULE                | No                  | No (3)    |
   |    90 | S46_BR                  | No                  | No        |
   |    91 | S46_DMR                 | No                  | Yes       |
   |    92 | S46_V4V6BIND            | No                  | Yes       |
   |    93 | S46_PORTPARAMS          | No                  | Yes       |



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   |    94 | S46_CONT_MAPE           | Yes                 | No        |
   |    95 | S46_CONT_MAPT           | Yes                 | Yes       |
   |    96 | S46_CONT_LW             | Yes                 | Yes       |
   |    97 | 4RD                     | Yes                 | Yes       |
   |    98 | 4RD_MAP_RULE            | Yes                 | Yes       |
   |    99 | 4RD_NON_MAP_RULE        | Yes                 | Yes       |
   |   100 | LQ_BASE_TIME            | No                  | Yes       |
   |   101 | LQ_START_TIME           | No                  | Yes       |
   |   102 | LQ_END_TIME             | No                  | Yes       |
   |   103 | DHCP Captive-Portal     | Yes                 | Yes       |
   |   104 | MPL_PARAMETERS          | Yes                 | Yes       |
   |   105 | ANI_ATT                 | No                  | Yes       |
   |   106 | ANI_NETWORK_NAME        | No                  | Yes       |
   |   107 | ANI_AP_NAME             | No                  | Yes       |
   |   108 | ANI_AP_BSSID            | No                  | Yes       |
   |   109 | ANI_OPERATOR_ID         | No                  | Yes       |
   |   110 | ANI_OPERATOR_REALM      | No                  | Yes       |
   |   111 | S46_PRIORITY            | Yes                 | Yes       |
   |   112 | MUD_URL_V6 (TEMPORARY)  | No                  | Yes       |
   |   113 | V6_PREFIX64             | Yes                 | No        |
   |   114 | F_BINDING_STATUS        | No                  | (4)       |
   |   115 | F_CONNECT_FLAGS         | No                  | (4)       |
   |   116 | F_DNS_REMOVAL_INFO      | No                  | (4)       |
   |   117 | F_DNS_HOST_NAME         | No                  | (4)       |
   |   118 | F_DNS_ZONE_NAME         | No                  | (4)       |
   |   119 | F_DNS_FLAGS             | No                  | (4)       |
   |   120 | F_EXPIRATION_TIME       | No                  | (4)       |
   |   121 | F_MAX_UNACKED_BNDUPD    | No                  | (4)       |
   |   122 | F_MCLT                  | No                  | (4)       |
   |   123 | F_PARTNER_LIFETIME      | No                  | (4)       |
   |   124 | F_PARTNER_LIFETIME_SENT | No                  | (4)       |
   |   125 | F_PARTNER_DOWN_TIME     | No                  | (4)       |
   |   126 | F_PARTNER_RAW_CLT_TIME  | No                  | (4)       |
   |   127 | F_PROTOCOL_VERSION      | No                  | (4)       |
   |   128 | F_KEEPALIVE_TIME        | No                  | (4)       |
   |   129 | F_RECONFIGURE_DATA      | No                  | (4)       |
   |   130 | F_RELATIONSHIP_NAME     | No                  | (4)       |
   |   131 | F_SERVER_FLAGS          | No                  | (4)       |
   |   132 | F_SERVER_STATE          | No                  | (4)       |
   |   133 | F_START_TIME_OF_STATE   | No                  | (4)       |
   |   134 | F_STATE_EXPIRATION_TIME | No                  | (4)       |
   |   143 | IPv6_ADDRESS-ANDSF      | Yes                 | Yes       |
   +-------+-------------------------+---------------------+-----------+

                      Table 1: Updated Options Table

   Notes for Table 1:




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   (1)   For the "Client ORO" column: a "Yes" for an option means that
         the client includes this option code if it desires that
         configuration information; a "No" means that the option MUST
         NOT be included (and servers SHOULD silently ignore that option
         code if it appears in ORO).

   (2)   For each enterprise-number, there MUST only be a single
         instance.

   (3)   See [RFC7598] for details.

   (4)   See RFC8156 for details.

   IANA is requested to update the All_DHCP_Relay_Agents_and_Servers
   (ff02::1:2) and All_DHCP_Servers (ff05::1:3) table entries in the
   IPv6 multicast address space registry at
   http://www.iana.org/assignments/ipv6-multicast-addresses/ipv6-
   multicast-addresses.xhtml to reference this document instead of
   [RFC3315].

   IANA is requested to update the http://www.iana.org/assignments/
   bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#authentication-
   protocol-id page to add an "Obsolete" annotation into the "DHCPv6
   Delayed Authentication" entity in the "Authentication Suboption
   (value 8) - Protocol identifier values" registry, and
   https://www.iana.org/assignments/auth-namespaces/auth-
   namespaces.xhtml page to add an "Obsolete" annotation into the
   "Delayed Authentication" entity in the "Protocol Name Space Values"
   registry.  IANA is also requested to update these pages to reference
   this document instead of [RFC3315].

25.  Obsoleted Mechanisms

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

   The following mechanisms are now obsolete:

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





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   Lifetime hints sent by a client.  Clients used to be allowed to send
   lifetime values as hints.  This mechanism was not widely implemented
   and there were known misimplementations that sent the remaining
   lifetimes rather than total desired lifetimes.  That in turn was
   sometimes misunderstood by servers as a request for ever decreasing
   lease lifetimes, which caused issues when values started approaching
   zero.  Clients now SHOULD set lifetimes to 0 in IA Address and IA
   Prefix options, and servers MUST ignore any requested lifetime value.

   T1/T2 hints sent by a client.  These had similar issues to the
   lifetime hints.  Clients now SHOULD set the T1/T2 values to 0 in
   IA_NA and IA_PD options, and servers MUST ignore any client supplied
   T1/T2 values.

26.  Acknowledgments

   This document is merely a refinement of earlier work by the authors
   of RFC3315 (Ralph Droms, Jim Bound, Bernie Volz, Ted Lemon, Charles
   Perkins, and Mike Carney), RFC3633 (Ole Troan and Ralph Droms),
   RFC3736 (Ralph Droms), RFC4242 (Stig Venaas, Tim Chown, and Bernie
   Volz), RFC7083 (Ralph Droms), and RFC7550 (Ole Troan, Bernie Volz,
   and Marcin Siodelski) and would not be possible without their
   original work.

   A number of additional people have contributed to identifying issues
   with RFC3315 and RFC3633 and proposed resolutions to these issues as
   reflected in this document (in no particular order): Ole Troan,
   Robert Marks, Leaf Yeh, Michelle Cotton, Pablo Armando, John
   Brzozowski, Suresh Krishnan, Hideshi Enokihara, Alexandru Petrescu,
   Yukiyo Akisada, Tatuya Jinmei, Fred Templin and Christian Huitema.

   We also thank the following, not otherwise acknowledged and in no
   particular order, for their review comments: Jeremy Reed, Francis
   Dupont, Tatuya Jinmei, Lorenzo Colitti, Tianxiang Li, Ian Farrer,
   Yogendra Pal, Kim Kinnear, Shawn Routhier, Tim Chown, and Michayla
   Newcombe.

   And, special thanks to Ralph Droms for answering many questions
   related to the original RFC3315 and RFC3633 work and for being the
   document shepherd.

27.  References

27.1.  Normative References

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



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   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://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,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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

   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc4862>.

   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6355>.

   [RFC7083]  Droms, R., "Modification to Default Values of SOL_MAX_RT
              and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC7083, November
              2013, <https://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,
              <https://www.rfc-editor.org/info/rfc7227>.






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   [RFC7283]  Cui, Y., Sun, Q., and T. Lemon, "Handling Unknown DHCPv6
              Messages", RFC 7283, DOI 10.17487/RFC7283, July 2014,
              <https://www.rfc-editor.org/info/rfc7283>.

27.2.  Informative References

   [IANA-PEN]
              IANA, "Private Enterprise Numbers registry
              http://www.iana.org/assignments/enterprise-numbers".

   [IANA-RESERVED-IID]
              IANA, "Reserved IPv6 Interface Identifiers
              http://www.iana.org/assignments/ipv6-interface-ids/
              ipv6-interface-ids.xml".

   [IEEE-802.1x]
              IEEE, "802.1X-2010 - IEEE Standard for Local and
              metropolitan area networks--Port-Based Network Access
              Control", February 2010,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=5409757>.

   [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,
              <https://www.rfc-editor.org/info/rfc826>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://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,
              <https://www.rfc-editor.org/info/rfc2132>.

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

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              DOI 10.17487/RFC3041, January 2001,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3162>.



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   [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, <https://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,
              <https://www.rfc-editor.org/info/rfc3633>.

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

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, DOI 10.17487/RFC3736,
              April 2004, <https://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,
              <https://www.rfc-editor.org/info/rfc3769>.

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

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, DOI 10.17487/RFC4242, November
              2005, <https://www.rfc-editor.org/info/rfc4242>.

   [RFC4477]  Chown, T., Venaas, S., and C. Strauf, "Dynamic Host
              Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack
              Issues", RFC 4477, DOI 10.17487/RFC4477, May 2006,
              <https://www.rfc-editor.org/info/rfc4477>.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,
              <https://www.rfc-editor.org/info/rfc4704>.





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

   [RFC4943]  Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
              Discovery On-Link Assumption Considered Harmful",
              RFC 4943, DOI 10.17487/RFC4943, September 2007,
              <https://www.rfc-editor.org/info/rfc4943>.

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, DOI 10.17487/RFC5007,
              September 2007, <https://www.rfc-editor.org/info/rfc5007>.

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers",
              RFC 5453, DOI 10.17487/RFC5453, February 2009,
              <https://www.rfc-editor.org/info/rfc5453>.

   [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,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC6603]  Korhonen, J., Ed., Savolainen, T., Krishnan, S., and O.
              Troan, "Prefix Exclude Option for DHCPv6-based Prefix
              Delegation", RFC 6603, DOI 10.17487/RFC6603, May 2012,
              <https://www.rfc-editor.org/info/rfc6603>.

   [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,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC6879]  Jiang, S., Liu, B., and B. Carpenter, "IPv6 Enterprise
              Network Renumbering Scenarios, Considerations, and
              Methods", RFC 6879, DOI 10.17487/RFC6879, February 2013,
              <https://www.rfc-editor.org/info/rfc6879>.

   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939,
              May 2013, <https://www.rfc-editor.org/info/rfc6939>.

   [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,
              <https://www.rfc-editor.org/info/rfc7084>.





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   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <https://www.rfc-editor.org/info/rfc7136>.

   [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,
              <https://www.rfc-editor.org/info/rfc7341>.

   [RFC7368]  Chown, T., Ed., Arkko, J., Brandt, A., Troan, O., and J.
              Weil, "IPv6 Home Networking Architecture Principles",
              RFC 7368, DOI 10.17487/RFC7368, October 2014,
              <https://www.rfc-editor.org/info/rfc7368>.

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,
              <https://www.rfc-editor.org/info/rfc7421>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <https://www.rfc-editor.org/info/rfc7513>.

   [RFC7550]  Troan, O., Volz, B., and M. Siodelski, "Issues and
              Recommendations with Multiple Stateful DHCPv6 Options",
              RFC 7550, DOI 10.17487/RFC7550, May 2015,
              <https://www.rfc-editor.org/info/rfc7550>.

   [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              Configuration of Softwire Address and Port-Mapped
              Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
              <https://www.rfc-editor.org/info/rfc7598>.

   [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
              Protecting against Rogue DHCPv6 Servers", BCP 199,
              RFC 7610, DOI 10.17487/RFC7610, August 2015,
              <https://www.rfc-editor.org/info/rfc7610>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <https://www.rfc-editor.org/info/rfc7707>.







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   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC7824]  Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              Considerations for DHCPv6", RFC 7824,
              DOI 10.17487/RFC7824, May 2016,
              <https://www.rfc-editor.org/info/rfc7824>.

   [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              Profiles for DHCP Clients", RFC 7844,
              DOI 10.17487/RFC7844, May 2016,
              <https://www.rfc-editor.org/info/rfc7844>.

   [RFC7969]  Lemon, T. and T. Mrugalski, "Customizing DHCP
              Configuration on the Basis of Network Topology", RFC 7969,
              DOI 10.17487/RFC7969, October 2016,
              <https://www.rfc-editor.org/info/rfc7969>.

   [RFC8168]  Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint
              Issues", RFC 8168, DOI 10.17487/RFC8168, May 2017,
              <https://www.rfc-editor.org/info/rfc8168>.

   [RFC8213]  Volz, B. and Y. Pal, "Security of Messages Exchanged
              between Servers and Relay Agents", RFC 8213,
              DOI 10.17487/RFC8213, August 2017,
              <https://www.rfc-editor.org/info/rfc8213>.

   [TR-187]   Broadband Forum, "TR-187 - IPv6 for PPP Broadband Access",
              February 2013, <https://www.broadband-
              forum.org/technical/download/TR-187_Issue-2.pdf>.

Appendix A.  Summary of Changes

   This appendix provides a summary of the significant changes made to
   this updated DHCPv6 specification.

   1.   The Introduction Section 1 was reorganized and updated.  In
        particular, the client/server message exchanges were moved into
        a new (and expanded) section on their own (see Section 5).  And,
        new sections were added to discuss the relation to previous
        DHCPv6 documents and also to DHCPv4.

   2.   The Requirements Section 2 and Background Section 3 had very
        minor edits.

   3.   The Terminology Section 4 had minor edits.



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   4.   The DHCP Terminology Section 4.2 was expanded to incorporate
        definitions from RFC3633, add T1/T2 definitions, add a few new
        definitions useful in a document that combined address and
        prefix delegation assignments, and improve some existing
        definitions.

   5.   The Client-Server Exchanges Section 5 was added from material
        previously in the Introduction Section 1 of RFC3315 and was
        expanded.

   6.   The Operational Models Section 6 is new and provides information
        on the kinds of DHCP clients and how they operate.

   7.   The DHCP Constants Section 7 was primarily updated to add
        constants from RFC4242 and RFC7083.

   8.   The Client/Server Message Formats Section 8, Relay Agent/Server
        Message Formats Section 9, and Representation and Use of Domain
        Names Section 10 had only very minor changes.

   9.   The DHCP Unique Identifier (DUID) Section 11 now discourages,
        rather than disallows, a server to parse the DUID, now includes
        some information on the DUID-UUID (RFC6355), and has other minor
        edits.

   10.  The Identity Association Section 12 was expanded to better
        explain the concept and also included prefix delegation.

   11.  The Assignment to an IA Section 13 incorporates material from
        two sections (11 and 12) of RFC3315 and also includes a section
        on prefix delegation.

   12.  The Transmission of Messages by a Client Section 14 was expanded
        to include rate limiting by clients and how clients should
        handle T1 or T2 values of 0.

   13.  The Reliability of Client Initiated Message Exchanges Section 15
        was expanded to clarify that the Elapsed Time option must be
        updated in retransmitted messages and that a client is not
        required to listen for DHCP traffic for the entire
        retransmission period.

   14.  The Message Validation Section 16 had minor edits.

   15.  The Client Source Address and Interface Selection Section 17 was
        expanded to include prefix delegation.





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   16.  The DHCP Configuration Exchanges Section 18 consolidates what
        used to be in the RFC3315 DHCP Server Solicitation Section 17,
        DHCP Client-Initiated Configuration Exchange Section 18, and
        DHCP Server-Initiated Configuration Exchange Section 19.  This
        material was reorganized and enhanced, and incorporates prefix
        delegation from RFC3633 and other changes from RFC4242, RFC7083,
        and RFC7550.  A few changes of note:

        1.  The Option Request option is no longer optional for some
            messages (Solicit and Information-request) as RFC7083
            requires clients to request SOL_MAX_RT or INF_MAX_RT
            options.

        2.  The Reconfigure message should no longer contain IA_NA/
            IA_PD, ORO, or other options to indicate to the client what
            was reconfigured.  The client should request everything it
            needs in the response to the Reconfigure.

        3.  The lifetime and T1/T2 hints should not be sent by a client
            (it should send 0 values in these fields) and any non-zero
            values should be ignored by the server.

        4.  Clarified that a server may return different addresses in
            the Reply than requested by a client in the Request message.
            Also clarified that a server must not include addresses that
            it will not assign.

        Also, a Refreshing Configuration Information Section 18.2.12 was
        added indicating use cases for when a client should try to
        refresh network information.

   17.  The Relay Agent Behavior Section 19 had minor edits.

   18.  The Authentication of DHCP Messages Section 20 had significant
        changes: IPsec materials were mostly removed and replaced with a
        reference to [RFC8213], and the Delay Authentication Protocol
        was removed (see Section 25).  Note that the Reconfigure Key
        Authentication Protocol is retained.

   19.  The DHCP Options Section 21 was expanded to incorporate the
        prefix delegation options from RFC3633, the Information Refresh
        Time option from RFC4242, and the SOL_MAX_RT and INF_MAX_RT
        options from RFC7083.  In addition, some additional edits were
        made to clarify option handling, such as which options should
        not be in an Option Request option.






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   20.  The Security Considerations Section 22 were updated to expand
        the discussion of security threats and incorporate material from
        the incorporated documents, primarily RFC3633.

   21.  The new Privacy Considerations Section 23 was added to consider
        privacy issues.

   22.  The IANA Considerations Section 24 was rewritten to reflect the
        changes requested for this document as other documents have
        already made the message, option, DUID, and status code
        assignments and this document does not add any new assignments.

   23.  The new Obsoleted Mechanisms Section 25 documents what this
        specification obsoletes.

   24.  The Appearance of Options in Message Types Appendix B and
        Appearance of Options in the Options Field of DHCP Appendix C
        were updated to reflect the incorporated options from RFC3633,
        RFC4242, and RFC7083.

   25.  Where appropriate, informational references have been added to
        provide further background and guidance throughout the document
        (as can be noted by the vast increase in references).

   26.  General changes to other IPv6 specifications, such as removing
        the use of site-local unicast addresses and adding unique local
        addresses, were made to the document.  Note that in a few
        places, older obsoleted RFCs (such as RFC2462 related to M and O
        bit handling) are still referenced as the material cited was not
        added in the replacement RFC.

   27.  It should be noted that this document does not refer to all
        DHCPv6 functionality and specifications.  Readers of this
        specification should visit http://www.iana.org/assignments/
        dhcpv6-parameters/dhcpv6-parameters.xml and
        https://datatracker.ietf.org/wg/dhc/ to learn of the RFCs that
        define DHCPv6 messages, options, status-codes, and more.

Appendix B.  Appearance of Options in Message Types

   The following tables indicates with a "*" the options are allowed in
   each DHCP message type.

   These tables are informational and should they conflict with text
   earlier in this document, that text should be considered
   authoritative.





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         Client Server IA_NA/                  Elap. Relay       Server
           ID     ID   IA_TA IA_PD  ORO   Pref Time   Msg. Auth. Unicast
 Solicit   *             *     *     *           *
 Advert.   *      *      *     *           *
 Request   *      *      *     *     *           *
 Confirm   *             *                       *
 Renew     *      *      *     *     *           *
 Rebind    *             *     *     *           *
 Decline   *      *      *     *                 *
 Release   *      *      *     *                 *
 Reply     *      *      *     *                             *     *
 Reconf.   *      *                                          *
 Inform.   * (see note)              *           *
 R-forw.                                               *
 R-repl.                                               *

   NOTE: Server ID option is only included in Information-request
   messages that are sent in response to a Reconfigure (see
   Section 18.2.6).

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















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           SOL_MAX_RT  INF_MAX_RT
   Solicit
   Advert.    *
   Request
   Confirm
   Renew
   Rebind
   Decline
   Release
   Reply      *           *
   Reconf.
   Inform.
   R-forw.
   R-repl.

Appendix C.  Appearance of Options in the Options Field of DHCP Options

   The following table indicates with a "*" where options defined in
   this document can appear as top-level options or encapsulated in
   other options defined in this document.  Other RFC's may define
   additional situations where options defined in this document are
   encapsulated in other options.

   This table is informational and should it conflict with text earlier
   in this document, that text should be considered authoritative.


























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                   Top-    IA_NA/                        RELAY-  RELAY-
                   Level   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     *
   Info Refresh Time  *
   SOL_MAX_RT         *
   INF_MAX_RT         *

   Notes: Options asterisked in the "Top-Level" column appear in the
   options field of client messages (see Section 8).  Options asterisked
   in the "RELAY-FORW" / "RELAY-REPLY" column appear in the options
   field of the Relay-forward and Relay-reply messages (see Section 9).

Authors' Addresses

   Tomek Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   Email: tomasz.mrugalski@gmail.com











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   Marcin Siodelski
   Internet Systems Consortium, Inc.
   950 Charter Street
   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/


   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







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   Ted Lemon
   Nominum, Inc.
   800 Bridge St.
   Redwood City, CA  94043
   USA

   Email: Ted.Lemon@nominum.com


   Timothy Winters
   University of New Hampshire, Interoperability Lab (UNH-IOL)
   Durham, NH
   USA

   Email: twinters@iol.unh.edu




































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