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rfc:rfc8415

Internet Engineering Task Force (IETF) T. Mrugalski Request for Comments: 8415 M. Siodelski Obsoletes: 3315, 3633, 3736, 4242, 7083, ISC

         7283, 7550                                            B. Volz

Category: Standards Track A. Yourtchenko ISSN: 2070-1721 Cisco

                                                         M. Richardson
                                                                   SSW
                                                              S. Jiang
                                                                Huawei
                                                              T. Lemon
                                                   Nibbhaya Consulting
                                                            T. Winters
                                                               UNH-IOL
                                                         November 2018
       Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

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 RFC 3315 (the original DHCPv6
 specification) and incorporates prefix delegation (RFC 3633),
 stateless DHCPv6 (RFC 3736), an option to specify an upper bound for
 how long a client should wait before refreshing information (RFC
 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service
 is not available (RFC 7083), and relay agent handling of unknown
 messages (RFC 7283).  In addition, this document clarifies the
 interactions between models of operation (RFC 7550).  As such, this
 document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083,
 RFC 7283, and RFC 7550.

Mrugalski, et al. Standards Track [Page 1] RFC 8415 DHCP for IPv6 November 2018

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8415.

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 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
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Mrugalski, et al. Standards Track [Page 2] RFC 8415 DHCP for IPv6 November 2018

Table of Contents

 1. Introduction ....................................................6
    1.1. Relationship to Previous DHCPv6 Standards ..................7
    1.2. Relationship to DHCPv4 .....................................8
 2. Requirements ....................................................8
 3. Background ......................................................8
 4. Terminology .....................................................9
    4.1. IPv6 Terminology ...........................................9
    4.2. DHCP Terminology ..........................................11
 5. Client/Server Exchanges ........................................16
    5.1. Client/Server Exchanges Involving Two Messages ............16
    5.2. Client/Server Exchanges Involving Four Messages ...........17
    5.3. Server/Client Exchanges ...................................18
 6. Operational Models .............................................18
    6.1. Stateless DHCP ............................................18
    6.2. DHCP for Non-temporary Address Assignment .................19
    6.3. DHCP for Prefix Delegation ................................19
    6.4. DHCP for Customer Edge Routers ............................22
    6.5. DHCP for Temporary Addresses ..............................22
    6.6. Multiple Addresses and Prefixes ...........................22
 7. DHCP Constants .................................................23
    7.1. Multicast Addresses .......................................23
    7.2. UDP Ports .................................................24
    7.3. DHCP Message Types ........................................24
    7.4. DHCP Option Codes .........................................26
    7.5. Status Codes ..............................................26
    7.6. Transmission and Retransmission Parameters ................27
    7.7. Representation of Time Values and "Infinity" as a
         Time Value ................................................28
 8. Client/Server Message Formats ..................................29
 9. Relay Agent/Server Message Formats .............................30
    9.1. Relay-forward Message .....................................31
    9.2. Relay-reply Message .......................................31
 10. Representation and Use of Domain Names ........................32
 11. DHCP Unique Identifier (DUID) .................................32
    11.1. DUID Contents ............................................33
    11.2. DUID Based on Link-Layer Address Plus Time (DUID-LLT) ....33
    11.3. DUID Assigned by Vendor Based on Enterprise
          Number (DUID-EN) .........................................35
    11.4. DUID Based on Link-Layer Address (DUID-LL) ...............36
    11.5. DUID Based on Universally Unique Identifier (DUID-UUID) ..37
 12. Identity Association ..........................................37
    12.1. Identity Associations for Address Assignment .............38
    12.2. Identity Associations for Prefix Delegation ..............38

Mrugalski, et al. Standards Track [Page 3] RFC 8415 DHCP for IPv6 November 2018

 13. Assignment to an IA ...........................................39
    13.1. Selecting Addresses for Assignment to an IA_NA ...........39
    13.2. Assignment of Temporary Addresses ........................40
    13.3. Assignment of Prefixes for IA_PD .........................41
 14. Transmission of Messages by a Client ..........................41
    14.1. Rate Limiting ............................................41
    14.2. Client Behavior when T1 and/or T2 Are 0 ..................42
 15. Reliability of Client-Initiated Message Exchanges .............43
 16. Message Validation ............................................45
    16.1. Use of Transaction IDs ...................................45
    16.2. Solicit Message ..........................................46
    16.3. Advertise Message ........................................46
    16.4. Request Message ..........................................46
    16.5. Confirm Message ..........................................47
    16.6. Renew Message ............................................47
    16.7. Rebind Message ...........................................47
    16.8. Decline Message ..........................................47
    16.9. Release Message ..........................................48
    16.10. Reply Message ...........................................48
    16.11. Reconfigure Message .....................................48
    16.12. Information-request Message .............................49
    16.13. Relay-forward Message ...................................49
    16.14. Relay-reply Message .....................................49
 17. Client Source Address and Interface Selection .................49
    17.1. Source Address and Interface Selection for
          Address Assignment .......................................49
    17.2. Source Address and Interface Selection for Prefix
          Delegation ...............................................50
 18. DHCP Configuration Exchanges ..................................50
    18.1. A Single Exchange for Multiple IA Options ................53
    18.2. Client Behavior ..........................................53
         18.2.1. Creation and Transmission of Solicit Messages .....55
         18.2.2. Creation and Transmission of Request Messages .....57
         18.2.3. Creation and Transmission of Confirm Messages .....59
         18.2.4. Creation and Transmission of Renew Messages .......60
         18.2.5. Creation and Transmission of Rebind Messages ......62
         18.2.6. Creation and Transmission of
                 Information-request Messages ......................63
         18.2.7. Creation and Transmission of Release Messages .....64
         18.2.8. Creation and Transmission of Decline Messages .....65
         18.2.9. Receipt of Advertise Messages .....................67
         18.2.10. Receipt of Reply Messages ........................68
                18.2.10.1. Reply for Solicit (with Rapid
                           Commit), Request, Renew, or Rebind ......69
                18.2.10.2. Reply for Release and Decline ...........72
                18.2.10.3. Reply for Confirm .......................72
                18.2.10.4. Reply for Information-request ...........72

Mrugalski, et al. Standards Track [Page 4] RFC 8415 DHCP for IPv6 November 2018

         18.2.11. Receipt of Reconfigure Messages ..................72
         18.2.12. Refreshing Configuration Information .............73
    18.3. Server Behavior ..........................................74
         18.3.1. Receipt of Solicit Messages .......................75
         18.3.2. Receipt of Request Messages .......................77
         18.3.3. Receipt of Confirm Messages .......................79
         18.3.4. Receipt of Renew Messages .........................79
         18.3.5. Receipt of Rebind Messages ........................81
         18.3.6. Receipt of Information-request Messages ...........83
         18.3.7. Receipt of Release Messages .......................84
         18.3.8. Receipt of Decline Messages .......................85
         18.3.9. Creation of Advertise Messages ....................85
         18.3.10. Transmission of Advertise and Reply Messages .....87
         18.3.11. Creation and Transmission of Reconfigure
                  Messages .........................................87
    18.4. Reception of Unicast Messages ............................88
 19. Relay Agent Behavior ..........................................89
    19.1. Relaying a Client Message or a Relay-forward Message .....89
         19.1.1. Relaying a Message from a Client ..................90
         19.1.2. Relaying a Message from a Relay Agent .............90
         19.1.3. Relay Agent Behavior with Prefix Delegation .......91
    19.2. Relaying a Relay-reply Message ...........................91
    19.3. Construction of Relay-reply Messages .....................91
    19.4. Interaction between Relay Agents and Servers .............92
 20. Authentication of DHCP Messages ...............................93
    20.1. Security of Messages Sent between Servers and
          Relay Agents .............................................94
    20.2. Summary of DHCP Authentication ...........................94
    20.3. Replay Detection .........................................94
    20.4. Reconfiguration Key Authentication Protocol (RKAP) .......95
         20.4.1. Use of the Authentication Option in RKAP ..........96
         20.4.2. Server Considerations for RKAP ....................96
         20.4.3. Client Considerations for RKAP ....................97
 21. DHCP Options ..................................................97
    21.1. Format of DHCP Options ...................................98
    21.2. Client Identifier Option .................................99
    21.3. Server Identifier Option .................................99
    21.4. Identity Association for Non-temporary Addresses
          Option ..................................................100
    21.5. Identity Association for Temporary Addresses Option .....102
    21.6. IA Address Option .......................................104
    21.7. Option Request Option ...................................106
    21.8. Preference Option .......................................108
    21.9. Elapsed Time Option .....................................108
    21.10. Relay Message Option ...................................109
    21.11. Authentication Option ..................................110
    21.12. Server Unicast Option ..................................111
    21.13. Status Code Option .....................................112

Mrugalski, et al. Standards Track [Page 5] RFC 8415 DHCP for IPv6 November 2018

    21.14. Rapid Commit Option ....................................114
    21.15. User Class Option ......................................115
    21.16. Vendor Class Option ....................................116
    21.17. Vendor-specific Information Option .....................117
    21.18. Interface-Id Option ....................................119
    21.19. Reconfigure Message Option .............................121
    21.20. Reconfigure Accept Option ..............................121
    21.21. Identity Association for Prefix Delegation Option ......122
    21.22. IA Prefix Option .......................................124
    21.23. Information Refresh Time Option ........................126
    21.24. SOL_MAX_RT Option ......................................127
    21.25. INF_MAX_RT Option ......................................128
 22. Security Considerations ......................................130
 23. Privacy Considerations .......................................133
 24. IANA Considerations ..........................................133
 25. Obsoleted Mechanisms .........................................138
 26. References ...................................................139
    26.1. Normative References ....................................139
    26.2. Informative References ..................................140
 Appendix A. Summary of Changes ...................................146
 Appendix B. Appearance of Options in Message Types ...............149
 Appendix C. Appearance of Options in the "options" Field of DHCP
             Options ..............................................151
 Acknowledgments ..................................................152
 Authors' Addresses ...............................................153

1. Introduction

 This document describes DHCP for IPv6 (DHCPv6), a client/server
 protocol that provides managed configuration of devices.  The basic
 operation of DHCPv6 provides configuration for clients connected to
 the same link as the server.  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; this data is 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, as 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].

Mrugalski, et al. Standards Track [Page 6] RFC 8415 DHCP for IPv6 November 2018

 DHCP can also be used just to provide other configuration options
 (i.e., no addresses or prefixes).  That implies that the server does
 not have to track any state; thus, this mode is called "stateless
 DHCPv6".  Mechanisms necessary to support stateless DHCPv6 are much
 smaller than mechanisms needed to support stateful DHCPv6.  [RFC3736]
 was written to document just those portions of DHCPv6 needed to
 support DHCPv6 stateless operation.
 The remainder of this introduction summarizes the relationship to the
 previous DHCPv6 standards (see Section 1.1) and clarifies the stance
 with regard to DHCPv4 (see 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. Relationship 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:
  1. [RFC3633] ("IPv6 Prefix Options for Dynamic Host Configuration

Protocol (DHCP) version 6")

  1. [RFC3736] ("Stateless Dynamic Host Configuration Protocol (DHCP)

Service for IPv6")

  1. [RFC4242] ("Information Refresh Time Option for Dynamic Host

Configuration Protocol for IPv6 (DHCPv6)")

  1. [RFC7083] ("Modification to Default Values of SOL_MAX_RT and

INF_MAX_RT")

  1. [RFC7283] ("Handling Unknown DHCPv6 Messages")
  1. [RFC7550] ("Issues and Recommendations with Multiple Stateful

DHCPv6 Options")

 This document provides a unified, corrected, and cleaned-up
 definition of DHCPv6 that also covers all applicable errata filed
 against older RFCs (see the list in Appendix A).  As such, it
 obsoletes the RFCs listed in the previous paragraph.  Also, there are
 a small number of mechanisms that were obsoleted; see Section 25 and
 Appendix A.

Mrugalski, et al. Standards Track [Page 7] RFC 8415 DHCP for IPv6 November 2018

1.2. Relationship to DHCPv4

 The operational models and relevant configuration information for
 DHCPv4 [RFC2131] [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 for this
 document.  [RFC4477] discusses some issues and possible strategies
 for running DHCPv4 and DHCPv6 services together.  While [RFC4477] 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
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 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, as long as its external behavior is
 consistent with that described in this document.

3. Background

 [RFC8200] ("Internet Protocol, Version 6 (IPv6) Specification")
 provides the base architecture and design of IPv6.  In addition to
 [RFC8200], related work in IPv6 that an implementer would be best
 served to study includes
  1. [RFC4291] ("IP Version 6 Addressing Architecture")
  1. [RFC4862] ("IPv6 Stateless Address Autoconfiguration")
  1. [RFC4861] ("Neighbor Discovery for IP version 6 (IPv6)")

Mrugalski, et al. Standards Track [Page 8] RFC 8415 DHCP for IPv6 November 2018

 These specifications enable DHCP to build upon the IPv6 work to
 provide robust stateful autoconfiguration.
 [RFC4291] defines the address scope that can be used in an IPv6
 implementation and also provides 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.
 [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 that replaces and enhances functions of ARP [RFC826].  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 from [RFC8200], [RFC4291], and [RFC4862] relevant to
 this specification is included below.
 address                   An IP-layer identifier for an interface or
                           a set of interfaces.
 GUA                       Global unicast address (see [RFC4291]).
 host                      Any node that is not a router.
 IP                        Internet Protocol Version 6 (IPv6).  The
                           terms "IPv4" and "IPv6" are used only in
                           contexts where it is necessary to avoid
                           ambiguity.
 interface                 A node's attachment to a link.

Mrugalski, et al. Standards Track [Page 9] RFC 8415 DHCP for IPv6 November 2018

 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); Point-to-Point Protocol (PPP) and
                           PPP over Ethernet (PPPoE) links; and
                           Internet-layer (or higher) "tunnels", such
                           as tunnels over IPv4 or IPv6 itself.
 link-layer identifier     A link-layer identifier for an interface --
                           for example, IEEE 802 addresses for
                           Ethernet or Token Ring network interfaces.
 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 IPv6
                           interface on which DHCPv6 can reasonably be
                           useful 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.
 ULA                       Unique local address (see [RFC4193]).
 unicast address           An identifier for a single interface.  A
                           packet sent to a unicast address is
                           delivered to the interface identified by
                           that address.

Mrugalski, et al. Standards Track [Page 10] RFC 8415 DHCP for IPv6 November 2018

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
                           Identity Association (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>.  See below for
                           definitions of DUID, IA, and IAID.
 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 option (see
                           Section 21.4) may contain IA Address
                           options (see Section 21.6)).

Mrugalski, et al. Standards Track [Page 11] RFC 8415 DHCP for IPv6 November 2018

 delegating router         The router that acts as a DHCP server and
                           responds to requests for delegated
                           prefixes.  This document primarily uses the
                           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               Also referred to as "client".  A node that
                           initiates requests on a link to obtain
                           configuration parameters from one or more
                           DHCP servers.  The node may act as a
                           requesting router (see below) if it
                           supports prefix delegation.
 DHCP domain               A set of links managed by DHCP and operated
                           by a single administrative entity.
 DHCP relay agent          Also referred to as "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               Also referred to as "server".  A node that
                           responds to requests from clients.  It may
                           or may not be on the same link as the
                           client(s).  Depending on its capabilities,
                           if it supports prefix delegation it may
                           also feature the functionality of a
                           delegating router.
 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.

Mrugalski, et al. Standards Track [Page 12] RFC 8415 DHCP for IPv6 November 2018

 encapsulated option       A DHCP option that is usually only
                           contained in another option.  For example,
                           the IA Address option is contained in IA_NA
                           or IA_TA options (see Section 21.5).  See
                           Section 9 of [RFC7227] for a more complete
                           definition.
 IA                        Identity Association: a collection of
                           leases assigned to a client.  Each IA has
                           an associated IAID (see below).  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 addresses (IA_TA) holds
                           temporary addresses, and an 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.
 IA option(s)              At the time of writing this document, one
                           or more IA_NA, IA_TA, and/or IA_PD options.
                           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 that belong 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").  See Section 21.4 for
                           details on the IA_NA option.
 IA_PD                     Identity Association for Prefix Delegation:
                           an IA that carries delegated prefixes.  See
                           Section 21.21 for details on the IA_PD
                           option.

Mrugalski, et al. Standards Track [Page 13] RFC 8415 DHCP for IPv6 November 2018

 IA_TA                     Identity Association for Temporary
                           Addresses: an IA that carries temporary
                           addresses (see [RFC4941]).  See
                           Section 21.5 for details on the IA_TA
                           option.
 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 for the list of
                           available message types).
 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
                           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 top-level option or at any
                           encapsulation level.  Most options are
                           singletons.

Mrugalski, et al. Standards Track [Page 14] RFC 8415 DHCP for IPv6 November 2018

 T1                        The time interval after which the client is
                           expected to contact the server that did the
                           assignment to extend (renew) the lifetimes
                           of the addresses assigned (via IA_NA
                           option(s)) and/or prefixes delegated (via
                           IA_PD option(s)) to the client.  T1 is
                           expressed as an absolute value in messages
                           (in seconds), is conveyed within IA
                           containers (currently the IA_NA and IA_PD
                           options), and is interpreted as a time
                           interval since the packet's reception.  The
                           value stored in the T1 field in IA options
                           is referred to as the T1 value.  The actual
                           time when the timer expires is referred to
                           as the T1 time.
 T2                        The time interval after which the client is
                           expected to contact any available server to
                           extend (rebind) the lifetimes of the
                           addresses assigned (via IA_NA option(s))
                           and/or prefixes delegated (via IA_PD
                           option(s)) to the client.  T2 is expressed
                           as an absolute value in messages (in
                           seconds), is conveyed within IA containers
                           (currently the IA_NA and IA_PD options),
                           and is interpreted as a time interval since
                           the packet's reception.  The value stored
                           in the T2 field in IA options is referred
                           to as the T2 value.  The actual time when
                           the timer expires is referred to as the
                           T2 time.
 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 by either a client
                           or a server.

Mrugalski, et al. Standards Track [Page 15] RFC 8415 DHCP for IPv6 November 2018

5. Client/Server Exchanges

 Clients and servers exchange DHCP messages using UDP (see [RFC768]
 and BCP 145 [RFC8085]).  The client uses a link-local address or
 addresses determined through other mechanisms for transmitting and
 receiving DHCP messages.
 A DHCP client sends most messages using a reserved, link-scoped
 multicast destination address so that the client need not be
 configured with the address or addresses of DHCP servers.
 To allow a DHCP client to send a message to a DHCP server that is not
 attached to the same link, a DHCP relay agent on the client's link
 will relay messages between the client and server.  The operation of
 the relay agent is transparent to the client.  The discussion of
 message exchanges in the remainder of this section will omit the
 description of the relaying of messages 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 IP
 addresses or delegated prefixes to it, the client can obtain other
 configuration information such as a list of available DNS servers
 [RFC3646] or NTP servers [RFC5908] 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.
 A client may also request the server to expedite address assignment
 and/or prefix delegation by using a two-message exchange instead of
 the normal four-message exchange as discussed in the next section.
 Expedited assignment can be requested by the client, and servers may
 or may not honor the request (see Sections 18.3.1 and 21.14 for more
 details and why servers may not honor this request).  Clients may
 request this expedited service in environments where it is likely
 that there is only one server available on a link and no expectation
 that a second server would become available, or when completing the
 configuration process as quickly as possible is a priority.

Mrugalski, et al. Standards Track [Page 16] RFC 8415 DHCP for IPv6 November 2018

 To request the expedited two-message exchange, 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; see Section 21.14) 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
 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.
 See Section 18 for descriptions of additional two-message exchanges
 between the client and server.

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 request an
 extension of the lifetimes assigned to addresses or delegated
 prefixes (this is a two-message exchange).

Mrugalski, et al. Standards Track [Page 17] RFC 8415 DHCP for IPv6 November 2018

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 two-message exchange as
 described earlier.  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.
 This document assumes that the DHCP servers and the client,
 communicating with the servers via a specific interface, belong to a
 single provisioning domain.
 DHCP may be extended to support additional stateful services that may
 interact with one or more of the models described below.  Such
 interaction should be considered and documented as part of any future
 protocol extension.

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 DHCP 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.

Mrugalski, et al. Standards Track [Page 18] RFC 8415 DHCP for IPv6 November 2018

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 associated preferred and valid lifetimes, which
 constitute 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.
 Clients can also request more than one address or set of addresses
 (see Sections 6.6 and 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 (1) does not have knowledge
 about the topology of the networks to which the requesting router is
 attached and (2) does not require other information aside from the
 identity of the requesting router to choose a prefix for delegation.
 This 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 such use in detail.
 The design of this prefix delegation mechanism meets the requirements
 for prefix delegation in [RFC3769].

Mrugalski, et al. Standards Track [Page 19] RFC 8415 DHCP for IPv6 November 2018

 While [RFC3633] assumes that the DHCP client is a router (hence the
 use of "requesting router") and that the DHCP server is a router
 (hence the 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 [RFC8200].  Also, in many cases (such as
 tethering or hosting virtual machines), hosts are already forwarding
 IP packets and thus are operating as routers as defined in [RFC8200].
 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 provisioned with 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 preferred lifetime and valid lifetime,
 which constitute 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.

Mrugalski, et al. Standards Track [Page 20] RFC 8415 DHCP for IPv6 November 2018

 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.
 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.

Mrugalski, et al. Standards Track [Page 21] RFC 8415 DHCP for IPv6 November 2018

 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 longer 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 [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.

6.6. Multiple Addresses and Prefixes

 DHCP 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 to which the client is attached.  In particular, the server can
 be configured to serve addresses out of multiple prefixes for a given

Mrugalski, et al. Standards Track [Page 22] RFC 8415 DHCP for IPv6 November 2018

 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 for each IA_NA option (see Section 21.4).
 A client can explicitly request multiple addresses by sending
 multiple IA_NA options (and/or IA_TA options; see Section 21.5).  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,
 DHCP allows a client to request new prefixes to be delegated by
 sending additional IA_PD options (see Section 21.21).  However, a
 typical operator usually prefers to delegate a single, larger prefix.
 In most deployments, it is recommended that the client request a
 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 out
 of scope for 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.
 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

Mrugalski, et al. Standards Track [Page 23] RFC 8415 DHCP for IPv6 November 2018

    scope to be reachable by the servers.  All servers within the site
    are members of this multicast group on the interfaces that 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.  The formats of these
 messages are provided in Sections 8 and 9.  Additional message types
 have been defined and may be defined in the future; see
 <https://www.iana.org/assignments/dhcpv6-parameters>.  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.

Mrugalski, et al. Standards Track [Page 24] RFC 8415 DHCP for IPv6 November 2018

 REBIND (6)                A client sends a Rebind message to any
                           available server to extend the lifetimes on
                           the leases assigned to the client and to
                           update other configuration parameters; this
                           message is sent after a client receives no
                           response to a Renew message.
 REPLY (7)                 A server sends a Reply message containing
                           assigned leases and configuration
                           parameters in response to a Solicit,
                           Request, Renew, or 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, Rebind/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.

Mrugalski, et al. Standards Track [Page 25] RFC 8415 DHCP for IPv6 November 2018

 RELAY-FORW (12)           A relay agent sends a Relay-forward message
                           to relay messages to servers, either
                           directly or through another relay agent.
                           The received message -- either a client
                           message or a Relay-forward message from
                           another relay agent -- is encapsulated in
                           an option in the Relay-forward message.
 RELAY-REPL (13)           A server sends a Relay-reply message to a
                           relay agent containing a message that the
                           relay agent delivers to a client.  The
                           Relay-reply message may be relayed by other
                           relay agents for delivery to the
                           destination relay agent.
                           The server encapsulates the client message
                           as an option in the Relay-reply message,
                           which the relay agent extracts and relays
                           to the client.

7.4. DHCP Option Codes

 DHCP makes extensive use of options in messages; some of these are
 defined later, in Section 21.  Additional options are defined in
 other documents or may be defined in the future (see [RFC7227] for
 guidance on new option definitions).

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 (see Section 21.13) does not appear in a
 message in which the option could appear, the status of the message
 is assumed to be Success.

Mrugalski, et al. Standards Track [Page 26] RFC 8415 DHCP for IPv6 November 2018

7.6. Transmission and Retransmission Parameters

 This section presents a table of values used to describe the message
 transmission behavior of clients and servers.  Some of the values are
 adjusted by a randomization factor and backoffs (see Section 15).
 Transmissions may also be influenced by rate limiting (see
 Section 14.1).
 +-----------------+------------------+------------------------------+
 | 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                |
 |                 |                  |                              |

Mrugalski, et al. Standards Track [Page 27] RFC 8415 DHCP for IPv6 November 2018

 | 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 | 8                | 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 |
 |                 |                  |                              |
 | MAX_WAIT_TIME   | 60 secs          | Max required time to wait    |
 |                 |                  | for a response               |
 +-----------------+------------------+------------------------------+
          Table 1: Transmission and Retransmission Parameters

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 and are expressed in units of seconds.  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 assignment of an
 address or delegation to a client and should only be used in cases
 where permanent assignments are desired.
 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.

Mrugalski, et al. Standards Track [Page 28] RFC 8415 DHCP for IPv6 November 2018

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-byte 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 number and length)                  .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 2: Client/Server Message Format
    msg-type             Identifies the DHCP message type; the
                         available message types are listed in
                         Section 7.3.  A 1-octet field.
    transaction-id       The transaction ID for this message exchange.
                         A 3-octet 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).

Mrugalski, et al. Standards Track [Page 29] RFC 8415 DHCP for IPv6 November 2018

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-byte or 4-byte boundaries).
 There are two relay agent messages (Relay-forward and Relay-reply),
 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

Mrugalski, et al. Standards Track [Page 30] RFC 8415 DHCP for IPv6 November 2018

 The following sections describe the use of the relay agent message
 header.

9.1. Relay-forward Message

 The following table defines the use of message fields in a
 Relay-forward message.
    msg-type             RELAY-FORW (12).  A 1-octet field.
    hop-count            Number of relay agents that have already
                         relayed this message.  A 1-octet 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 scoped
                         unicast address (i.e., GUA or ULA), but see
                         the discussion in Section 19.1.1.  A 16-octet
                         field.
    peer-address         The address of the client or relay agent from
                         which the message to be relayed was received.
                         A 16-octet field.
    options              MUST include a Relay Message option (see
                         Section 21.10); MAY include other options,
                         such as the Interface-Id option (see
                         Section 21.18), 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 of how the link-address field
 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 1-octet field.
    hop-count            Copied from the Relay-forward message.
                         A 1-octet field.
    link-address         Copied from the Relay-forward message.
                         A 16-octet field.

Mrugalski, et al. Standards Track [Page 31] RFC 8415 DHCP for IPv6 November 2018

    peer-address         Copied from the Relay-forward message.
                         A 16-octet field.
    options              MUST include a Relay Message option (see
                         Section 21.10); MAY include other options,
                         such as the Interface-Id option (see
                         Section 21.18).  A 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 Sections 21.2 and 21.3 for details regarding 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.  Also, such an
 attempt will be more and more unreliable as more clients adopt
 privacy measures such as those defined in [RFC7844].  If this
 capability is required, it is recommended to rely on the Client
 Link-Layer Address option instead [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 or changes to virtual interfaces (e.g.,

Mrugalski, et al. Standards Track [Page 32] RFC 8415 DHCP for IPv6 November 2018

 logical PPP (over Ethernet) interfaces that may come and go in
 Customer Premises 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
 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 2-octet type code represented in network byte
 order, followed by a variable number of octets that make up the
 actual identifier.  The length of the DUID (not including the type
 code) is at least 1 octet and at most 128 octets.  The following
 types are currently defined:
    +------+------------------------------------------------------+
    | Type | Description                                          |
    +------+------------------------------------------------------+
    | 1    | Link-layer address plus time                         |
    | 2    | Vendor-assigned unique ID based on Enterprise Number |
    | 3    | Link-layer address                                   |
    | 4    | Universally Unique Identifier (UUID) [RFC6355]       |
    +------+------------------------------------------------------+
                          Table 2: DUID Types
 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 2-octet type field containing the
 value 1, a 2-octet hardware type code, and 4 octets containing a time
 value, followed by the 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 IANA; see [IANA-HARDWARE-TYPES].  Both the time and the
 hardware type are stored in network byte order.  For Ethernet
 hardware types, the link-layer address is stored in canonical form,
 as described in [RFC2464].

Mrugalski, et al. Standards Track [Page 33] RFC 8415 DHCP for IPv6 November 2018

 The following diagram illustrates the format of a DUID-LLT:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         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; 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.

Mrugalski, et al. Standards Track [Page 34] RFC 8415 DHCP for IPv6 November 2018

 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.

11.3. DUID Assigned by Vendor Based on Enterprise Number (DUID-EN)

 The vendor assigns this form of DUID to the device.  This DUID
 consists of the 4-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 the manufacturing process
 in the case of physical devices or, in the case of virtual machines,
 when the image is created or booted for the first time.  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.

Mrugalski, et al. Standards Track [Page 35] RFC 8415 DHCP for IPv6 November 2018

 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
 This example includes the 2-octet type of 2 and the Enterprise Number
 (9), followed by 8 octets of identifier data (0x0CC084D303000912).

11.4. DUID Based on Link-Layer Address (DUID-LL)

 This type of DUID consists of 2 octets containing a DUID type of 3
 and a 2-octet network hardware type code, followed by the link-layer
 address of any one network interface that is permanently connected to
 the client or server device.  For example, a 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 IANA; see [IANA-HARDWARE-TYPES].  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.

Mrugalski, et al. Standards Track [Page 36] RFC 8415 DHCP for IPv6 November 2018

 A 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.  A 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.

11.5. DUID Based on Universally Unique Identifier (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 (e.g., an IA_NA with
 an IAID of 0 and an IA_PD with an IAID of 0 are each considered
 unique).  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 by
 either storing the IAID in non-volatile storage or 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.

Mrugalski, et al. Standards Track [Page 37] RFC 8415 DHCP for IPv6 November 2018

 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.  The client then creates
 a new IAID, to be used in future messages to obtain leases for the
 new IA.

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 such IA must be associated with exactly one
 interface.
 The configuration information in an IA_NA option consists of one or
 more IPv6 addresses along with the T1 and T2 values for the IA.  See
 Section 21.4 for details regarding the representation of an IA_NA in
 a DHCP message.
 The configuration information in an IA_TA option consists of one or
 more IPv6 addresses.  See Section 21.5 for details regarding 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 (see Section 21.6).
 The lifetimes apply to the use of addresses; see 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 configured to request delegated
 prefixes 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 option consists of one or
 more prefixes along with the T1 and T2 values for the IA_PD.  See
 Section 21.21 for details regarding the representation of an IA_PD in
 a DHCP message.

Mrugalski, et al. Standards Track [Page 38] RFC 8415 DHCP for IPv6 November 2018

 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 (see
 Section 21.22).  The lifetimes apply to the use of delegated
 prefixes; see 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
 administrator and the specific information the server determines
 about the client from some combination of the following sources:
  1. 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).
  1. The DUID supplied by the client.

Mrugalski, et al. Standards Track [Page 39] RFC 8415 DHCP for IPv6 November 2018

  1. Other information in options supplied by the client, e.g., IA

Address options (see Section 21.6) that include the client's

    requests for specific addresses.
  1. Other information in options supplied by the relay agent.
 By default, DHCP server implementations SHOULD NOT generate
 predictable addresses (see Section 4.7 of [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].
 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 IA_TA option (see
 Section 21.5).  Each IA_TA option typically contains at least 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 of
 [RFC4941]).
 A DHCP server implementation MAY generate temporary addresses,
 referring to the algorithm defined in Section 3.2.1 of [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].

Mrugalski, et al. Standards Track [Page 40] RFC 8415 DHCP for IPv6 November 2018

 On the clients, by default, temporary addresses are preferred in
 source address selection, according to Rule 7 in Section 5 of
 [RFC6724].  However, this policy can be overridden.
 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, and selection based on an external
 authority such as a RADIUS server using the Framed-IPv6-Prefix option
 as described in [RFC3162].

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 check to see whether 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 when this
 option is not supplied.
 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 or retransmits.  One example is that a client
 obtains an address or delegated prefix but does not like the
 response, so it reverts back to the 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

Mrugalski, et al. Standards Track [Page 41] RFC 8415 DHCP for IPv6 November 2018

 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 (see Appendix A of [RFC3290]), 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.
 A transmission rate limit SHOULD be configurable.  A possible default
 could be 20 packets in 20 seconds.
 For a device that has multiple interfaces, the limit MUST be enforced
 on a per-interface basis.
 Rate limiting of forwarded DHCP messages and server-side messages is
 out of scope for this specification.

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

 In certain cases, T1 and/or T2 values may be set to 0.  Currently,
 there are three such cases:
 1.  a client received an IA_NA option (see Section 21.4) with a zero
     value
 2.  a client received an IA_PD option (see Section 21.21) with a zero
     value
 3.  a client received an IA_TA option (see Section 21.5) (which does
     not contain T1 and T2 fields and these leases are not generally
     renewed)
 This is an indication that the renew and rebind times are left to the
 discretion of the client.  However, they are not completely
 discretionary.
 When T1 and/or T2 values are set to 0, the client MUST choose a time
 to avoid packet storms.  In particular, it MUST NOT transmit
 immediately.  If the client received multiple IA options, it SHOULD
 pick renew and/or rebind transmission times so all IA options are
 handled in one exchange, if possible.  The client MUST choose renew
 and rebind times to not violate rate-limiting restrictions as defined
 in Section 14.1.

Mrugalski, et al. Standards Track [Page 42] RFC 8415 DHCP for IPv6 November 2018

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 (1) the
 client successfully receives the appropriate response or responses
 from a server or servers or (2) 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 IA Address
 options (see Section 21.6) or IA Prefix options (see Section 21.22)
 if a valid lifetime for any of the client's leases 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
 Specific values for each of these parameters relevant to the various
 messages are given in the subsections of Section 18.2, using values
 defined in Table 1 in Section 7.6.  The algorithm for RAND is common
 across all message transmissions.

Mrugalski, et al. Standards Track [Page 43] RFC 8415 DHCP for IPv6 November 2018

 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 includes a randomization factor
 (RAND), which is a random number chosen with a uniform distribution
 between -0.1 and +0.1.  The randomization factor is included to
 minimize synchronization of messages transmitted by DHCP clients.
 The algorithm for choosing a random number does not need to be
 cryptographically sound.  The algorithm SHOULD produce a different
 sequence of random numbers from each invocation of the DHCP client.
 RT for the first message transmission is based on IRT:
    RT = IRT + RAND*IRT
 RT for each subsequent message transmission is based on the previous
 value of RT:
    RT = 2*RTprev + RAND*RTprev
 MRT specifies an upper bound on the value of RT (disregarding the
 randomization added by the use of RAND).  If MRT has a value of 0,
 there is no upper limit on the value of RT.  Otherwise:
    if (RT > MRT)
       RT = MRT + RAND*MRT
 MRC specifies an upper bound on the number of times a client may
 retransmit a message.  Unless MRC is zero, the message exchange fails
 once the client has transmitted the message MRC times.
 MRD specifies an upper bound on the length of time a client may
 retransmit a message.  Unless MRD is zero, the message exchange fails
 once MRD seconds have elapsed since the client first transmitted the
 message.
 If both MRC and MRD are non-zero, the message exchange fails whenever
 either of the conditions specified in the previous two paragraphs
 is met.
 If both MRC and MRD are zero, the client continues to transmit the
 message until it receives a response.

Mrugalski, et al. Standards Track [Page 44] RFC 8415 DHCP for IPv6 November 2018

 A client is not expected to listen for a response during the entire
 RT period and may turn off listening capabilities after waiting at
 least the shorter of RT and MAX_WAIT_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 and explains what to do when a client or server
 receives a message that contains known options that are invalid for
 that message.  For example, an IA option is not allowed to appear in
 an Information-request message.
 Clients and servers MAY choose to either (1) extract information from
 such a message if the information is of use to the recipient or
 (2) ignore such a message completely and just discard it.
 If a server receives a message that it considers invalid, it MAY send
 a Reply message (or Advertise message, as appropriate) with a Server
 Identifier option (see Section 21.3), a Client Identifier option (see
 Section 21.2) (if one was included in the message), and a Status Code
 option (see Section 21.13) with status UnspecFail.
 Clients, relay agents, and servers MUST NOT discard messages that
 contain unknown options (or instances of vendor options with unknown
 enterprise-number values).  These should be ignored as if they were
 not present.  This is critical to provide for future extensions of
 DHCP.
 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.

Mrugalski, et al. Standards Track [Page 45] RFC 8415 DHCP for IPv6 November 2018

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:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. the message does not include a Client Identifier option (see

Section 21.2).

  1. the contents of the Client Identifier option do not match the

client's DUID.

  1. the "transaction-id" field value does not match the value the

client used in its Solicit message.

 Servers and relay agents MUST discard any received Advertise
 messages.

16.4. Request Message

 Clients MUST discard any received Request messages.
 Servers MUST discard any received Request message that meets any of
 the following conditions:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. the contents of the Server Identifier option do not match the

server's DUID.

  1. the message does not include a Client Identifier option (see

Section 21.2).

Mrugalski, et al. Standards Track [Page 46] RFC 8415 DHCP for IPv6 November 2018

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 (see Section 21.2) or that do
 include a Server Identifier option (see Section 21.3).

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:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. the contents of the Server Identifier option do not match the

server's identifier.

  1. the message does not include a Client Identifier option (see

Section 21.2).

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 (see Section 21.2) or that do include a
 Server Identifier option (see Section 21.3).

16.8. Decline Message

 Clients MUST discard any received Decline messages.
 Servers MUST discard any received Decline message that meets any of
 the following conditions:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. the contents of the Server Identifier option do not match the

server's identifier.

  1. the message does not include a Client Identifier option (see

Section 21.2).

Mrugalski, et al. Standards Track [Page 47] RFC 8415 DHCP for IPv6 November 2018

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:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. the contents of the Server Identifier option do not match the

server's identifier.

  1. the message does not include a Client Identifier option (see

Section 21.2).

16.10. Reply Message

 Clients MUST discard any received Reply message that meets any of the
 following conditions:
  1. the message does not include a Server Identifier option (see

Section 21.3).

  1. 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 (see Section 21.2)
 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.  If the client did not include a
 Client Identifier option in the original message, the Reply message
 MUST NOT include a Client Identifier option.
 Servers and relay agents MUST discard any received Reply messages.

16.11. Reconfigure Message

 Servers and relay agents MUST discard any received Reconfigure
 messages.
 Clients MUST discard any Reconfigure message that meets any of the
 following conditions:
  1. the message was not unicast to the client.
  1. the message does not include a Server Identifier option (see

Section 21.3).

Mrugalski, et al. Standards Track [Page 48] RFC 8415 DHCP for IPv6 November 2018

  1. the message does not include a Client Identifier option (see

Section 21.2) that contains the client's DUID.

  1. the message does not include a Reconfigure Message option (see

Section 21.19).

  1. the Reconfigure Message option msg-type is not a valid value.
  1. 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:
  1. the message includes a Server Identifier option (see

Section 21.3), and the DUID in the option does not match the

    server's DUID.
  1. 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

 The client's behavior regarding interface selection is different,
 depending on the purpose of the configuration.

17.1. Source Address and 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 for
 which configuration is being requested is a logical interface without
 direct link attachment or the client is certain that two interfaces
 are attached to the same link.

Mrugalski, et al. Standards Track [Page 49] RFC 8415 DHCP for IPv6 November 2018

 When a client sends a DHCP message directly to a server using unicast
 (after receiving the Server Unicast option (see Section 21.12) 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 that is suitable for use by
 the server in responding to the client.

17.2. Source Address and 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 (typically, connected to an ISP network); see
 [RFC7084].  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 (see Section 21.12) from
 that server), the source address SHOULD be an address that is from
 the upstream interface and that 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),
 3.  when a Router Advertisement indicates that DHCPv6 is available
     for address configuration (see Section 4.2 of [RFC4861]),
 4.  as required to extend the lifetime of address(es) and/or
     delegated prefix(es), using Renew and Rebind messages, or
 5.  upon the receipt of a Reconfigure message, when requested to do
     so by a server.

Mrugalski, et al. Standards Track [Page 50] RFC 8415 DHCP for IPv6 November 2018

 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 that
 it had obtained from the previous server (see Section 18.2.7 for more
 details on what "stop using" means in this context) and that it is
 now trying to obtain from a new server.
 A DHCP client that does not need to have a DHCP server assign IP
 addresses or delegated prefixes to it can obtain configuration
 information such as a list of available DNS servers [RFC3646] or NTP
 servers [RFC5908] 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
 that respond to the client's message; the client then 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 that 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.
 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.

Mrugalski, et al. Standards Track [Page 51] RFC 8415 DHCP for IPv6 November 2018

 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 starts with the four-message Solicit/Advertise/
 Request/Reply exchange to obtain the lease(s), followed by a
 two-message Renew/Reply exchange to extend the lifetime on the
 lease(s), and then ends with a two-message Release/Reply exchange to
 end the client's use of 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        |
   exchange     |               |               |
                |    Initialization complete    |
                |               |               |
                .               .               .
                .               .               .
                |   T1 (renewal) timer expires  |
                |               |               |

Mrugalski, et al. Standards Track [Page 52] RFC 8415 DHCP for IPv6 November 2018

   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

 This document assumes that a client SHOULD use a single transaction
 for all of the IA options required on an interface; this simplifies
 the client implementation and reduces the potential number of
 transactions required (for the background on this design choice,
 refer to Section 4 of [RFC7550]).  To facilitate a client's use of a
 single transaction for all IA options, servers MUST return the same
 T1/T2 values for all IA options in a Reply (see Sections 18.3.2,
 18.3.4, and 18.3.5) so that the client will generate a single
 transaction when renewing or rebinding its leases.  However, because
 some servers may not yet conform to this requirement, a client MUST
 be prepared to select appropriate T1/T2 times as described in
 Section 18.2.4.

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 lifecycle of addresses and delegated prefixes.

Mrugalski, et al. Standards Track [Page 53] RFC 8415 DHCP for IPv6 November 2018

 When a client detects that 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.  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 that the client will 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 Information-request message as
 indicated by the Reconfigure Message option (see Section 21.19).  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 (see 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
 client.  The server should enable the use of unicast only when relay
 agent options will not be used.

Mrugalski, et al. Standards Track [Page 54] RFC 8415 DHCP for IPv6 November 2018

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 (see Section 21.2)
 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 (see Section 21.4) to request the
 assignment of non-temporary addresses, IA_TA options (see
 Section 21.5) to request the assignment of temporary addresses, and
 IA_PD options (see Section 21.21) to request prefix delegation.
 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 (see
 Section 21.6) 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 (see
 Section 21.22) 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 (ORO) (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.

Mrugalski, et al. Standards Track [Page 55] RFC 8415 DHCP for IPv6 November 2018

 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
 random delay helps desynchronize clients that start a DHCP session at
 the same time, such as after recovery from a power failure or after a
 router outage after seeing that DHCP is available in Router
 Advertisement messages (see Section 4.2 of [RFC4861]).
 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 two-message exchange
 includes a Rapid Commit option (see Section 21.14) 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 does one of the following:
  1. processes the Reply message as described in Section 18.2.10 and

discards any Reply messages received in response to the Request

    message
  1. 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
 described in Section 15 is modified as follows for use in the
 transmission of Solicit messages.  The message exchange is not
 terminated by the receipt of an Advertise before the first RT has
 elapsed.  Rather, the client collects valid Advertise messages until

Mrugalski, et al. Standards Track [Page 56] RFC 8415 DHCP for IPv6 November 2018

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

Mrugalski, et al. Standards Track [Page 57] RFC 8415 DHCP for IPv6 November 2018

 The client MUST include the identifier of the destination server in a
 Server Identifier option (see Section 21.3).
 The client MUST include a Client Identifier option (see Section 21.2)
 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 the following:
  1. Select another server from a list of servers known to the client
    1. - for example, servers that responded with an Advertise message.
  1. Initiate the server discovery process described in Section 18.
  1. Terminate the configuration process and report failure.

Mrugalski, et al. Standards Track [Page 58] RFC 8415 DHCP for IPv6 November 2018

18.2.3. Creation and Transmission of Confirm Messages

 The client uses a Confirm message when it 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 (see Section 21.2)
 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 (see Section 21.4) and the
 preferred-lifetime and valid-lifetime fields in the IA Address
 options (see Section 21.6) 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.

Mrugalski, et al. Standards Track [Page 59] RFC 8415 DHCP for IPv6 November 2018

18.2.4. Creation and Transmission of Renew Messages

 To extend the preferred and valid 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; the Renew message includes IA options for the IAs
 whose lease lifetimes are to be extended.  The client includes IA
 Address options (see Section 21.6) within IA_NA (see Section 21.4)
 and IA_TA (see Section 21.5) options for the addresses assigned to
 the IAs.  The client includes IA Prefix options (see Section 21.22)
 within IA_PD options (see Section 21.21) 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 values assigned to an IA.  However, as the client SHOULD
 renew/rebind all IAs from the server at the same time, the client
 MUST select T1 and T2 times from all IA options that will guarantee
 that the client initiates transmissions of 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, at its discretion, send a Renew or Rebind message,
 respectively.  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 (see Section 21.3)
 in the Renew message, identifying the server with which the client
 most recently communicated.
 The client MUST include a Client Identifier option (see Section 21.2)
 to identify itself to the server.  The client adds any appropriate
 options, including one or more IA options.

Mrugalski, et al. Standards Track [Page 60] RFC 8415 DHCP for IPv6 November 2018

 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 an 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 an IA Prefix option with the "IPv6-prefix" field set to 0
 unless it is supplying a hint for the prefix length.
 The client includes 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
 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 the earliest time T2 is
 reached.  While the client is responding to a Reconfigure, the client
 ignores and discards any additional Reconfigure messages it may
 receive.
 The message exchange is terminated when the earliest time T2 is
 reached, at which point the client begins the Rebind message exchange
 (see Section 18.2.5).

Mrugalski, et al. Standards Track [Page 61] RFC 8415 DHCP for IPv6 November 2018

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:
  1. The client sets the "msg-type" field to REBIND.
  1. The client does not include the Server Identifier option (see

Section 21.3) 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.

Mrugalski, et al. Standards Track [Page 62] RFC 8415 DHCP for IPv6 November 2018

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.
 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 (see
 Section 21.2) to identify itself to the server (however, 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 (see Section 21.3) 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

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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.
 The client places the identifier of the server that allocated the
 lease(s) in a Server Identifier option (see Section 21.3).
 The client MUST include a Client Identifier option (see Section 21.2)
 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 0
 in a Router Advertisement message as described in part (e) of
 Section 5.5.3 of [RFC4862]; also see requirement 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.

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 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
 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; thus, the
 client MUST NOT include IA_PD options (see Section 21.21) 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 (see Section 21.3).
 The client MUST include a Client Identifier option (see Section 21.2)
 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.

Mrugalski, et al. Standards Track [Page 65] RFC 8415 DHCP for IPv6 November 2018

 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 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
    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 (due to the conflict) as equivalent
 to not having received the binding, insofar as how it behaves when
 sending Renew and Rebind messages.

Mrugalski, et al. Standards Track [Page 66] RFC 8415 DHCP for IPv6 November 2018

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.
  1. Those Advertise messages with the highest server preference value

SHOULD be preferred over all other Advertise messages. 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.
  1. 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.
 Once a client has selected Advertise message(s), the client will
 typically store information about each server, such as the 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 for 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 any SOL_MAX_RT option (see Section 21.24) and
 INF_MAX_RT option (see Section 21.25) present in an Advertise
 message, even if the message contains a Status Code option (see
 Section 21.13) indicating a failure, and the Advertise message will
 be discarded by 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 (see Section 21.6) encapsulated in
 IA_NA options (see Section 21.4) or IA_TA options (see Section 21.5))
 and no delegated prefixes (IA Prefix options (see Section 21.22)
 encapsulated in IA_PD options (see Section 21.21)), with the
 exception that the client:
  1. MUST process an included SOL_MAX_RT option and
  1. MUST process an included INF_MAX_RT option.

Mrugalski, et al. Standards Track [Page 67] RFC 8415 DHCP for IPv6 November 2018

 A client can record in an activity log or display to the user any
 associated status message(s).
 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 (see Section 21.14), Request, Confirm,
 Renew, Rebind, or Information-request message, the client extracts
 the top-level Status Code option (see Section 21.13) if present.
 The client MUST process any SOL_MAX_RT option (see Section 21.24) and
 INF_MAX_RT option (see Section 21.25) present 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.
 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 and
 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

Mrugalski, et al. Standards Track [Page 68] RFC 8415 DHCP for IPv6 November 2018

 example, a client that previously received a Client FQDN option (see
 [RFC4704]) 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 anymore 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 options, 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; see Section 21.14) or a
 Request, the client can either reissue 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:
  1. Calculate T1 and T2 times (based on T1 and T2 values sent in the

packet and the packet reception time), if appropriate for the

    IA type.
  1. Add any new leases in the IA option to the IA as recorded by the

client.

  1. Update lifetimes for any leases in the IA option that the client

already has recorded in the IA.

  1. 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.

  1. Leave unchanged any information about leases the client has

recorded in the IA but that were not included in the IA from the

    server.

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 If the client can operate with the addresses and/or prefixes obtained
 from the server:
  1. 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.
  1. The client MUST perform duplicate address detection as per

Section 5.4 of [RFC4862], 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.
  1. For each assigned address that does not have any associated

reachability information (see the definition of "on-link" in

    Section 2.1 of [RFC4861]), 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 an IA_NA or IA_TA MUST NOT be
    used to form an implicit prefix with a length other than 128.
  1. For each delegated prefix, the client assigns a subnet to each of

the links to which the associated interfaces are attached.

    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
    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 longer than the
    remaining preferred and valid lifetimes, respectively, for the
    delegated prefix at any time.  In particular, if the delegated

Mrugalski, et al. Standards Track [Page 70] RFC 8415 DHCP for IPv6 November 2018

    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.
 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:
  1. Sends a Request message to the server that responded if any of the

IAs in the Reply message contain the NoBinding status code. 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.
  1. Sends a Renew/Rebind if any of the IAs are not in the Reply

message, but as this likely indicates that the server that

    responded does not support that IA type, sending immediately is
    unlikely to produce a different result.  Therefore, the client
    MUST rate-limit its transmissions (see Section 14.1) and MAY just
    wait for the normal retransmission time (as if the Reply message
    had not been received).  The client continues to use other
    bindings for which the server did return information.
  1. 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.

Mrugalski, et al. Standards Track [Page 71] RFC 8415 DHCP for IPv6 November 2018

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 (see Section 21.13) 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 status of
 Success (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 Reply messages 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 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
 a Renew message, a Rebind message, or an Information-request message
 as indicated by the Reconfigure Message option (see 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 (1) trigger the exchange (if the first
 Reconfigure was not received by the client) or (2) 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) that it arrives
 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
 the following:
  1. The client reboots (and has stable storage and persistent DHCP

state).

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

leases.

  1. The client returns from sleep mode.
  1. The client changes access points (e.g., if using Wi-Fi

technology).

 When the client detects that it may have moved to a new link 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.

Mrugalski, et al. Standards Track [Page 73] RFC 8415 DHCP for IPv6 November 2018

 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
 (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
 an 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 prefixes are added or existing prefixes 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 configurations 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 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 (see Section 21.14)
 when the server is configured to respond with committed lease
 assignments.
 These Advertise and Reply messages MUST always contain the Server
 Identifier option (see Section 21.3) containing the server's DUID and
 the Client Identifier option (see Section 21.2) 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

Mrugalski, et al. Standards Track [Page 74] RFC 8415 DHCP for IPv6 November 2018

 as discussed in Section 5 of [RFC8200].  If the client included an
 Option Request option (see Section 21.7) 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.
 Any message sent from a client may arrive at the server encapsulated
 in one or more Relay-forward messages.  The server MUST use the
 received message to construct the proper Relay-reply message to allow
 the response to the received message to be relayed through the same
 relay agents (in reverse order) as the original client message; see
 Section 19.3 for more details.  The server may also need to record
 this information with each client in case it is needed to send a
 Reconfigure message at a later time, unless the server has been
 configured with addresses that can be used to send Reconfigure
 messages directly to the client (see Section 18.3.11).  Note that
 servers that support leasequery [RFC5007] also need to record this
 information.
 By sending Reconfigure messages, the server MAY initiate a
 configuration exchange 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
 (see 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 details regarding the handling of Solicit
 messages received via unicast.  Unicast transmission of Solicit
 messages is not allowed, regardless of whether the Server Unicast
 option (see Section 21.12) 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

Mrugalski, et al. Standards Track [Page 75] RFC 8415 DHCP for IPv6 November 2018

 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 (1) the server is permitted to respond to the client, (2) the
 client has not included a Rapid Commit option (see Section 21.14) in
 the Solicit message, or (3) the server has not been configured to
 respond with committed assignments 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
 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 that 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.

Mrugalski, et al. Standards Track [Page 76] RFC 8415 DHCP for IPv6 November 2018

18.3.2. Receipt of Request Messages

 See Section 18.4 for details regarding the handling of Request
 messages 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 in the Reply message a Server Identifier
 option (see Section 21.3) containing the server's DUID and the Client
 Identifier option (see Section 21.2) from the Request message.
 The server examines all IAs in the message from the client.
 For each IA_NA option (see Section 21.4) and IA_TA option (see
 Section 21.5) 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 are 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 (see Section 21.13) 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.
 For any IA_PD option (see Section 21.21) 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_NAs and
 IA_TAs), prefixes (for IA_PDs), 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 that the server
 does not assign to the client.

Mrugalski, et al. Standards Track [Page 77] RFC 8415 DHCP for IPv6 November 2018

 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 (see
 Section 21.20) if the server policy enables the reconfigure mechanism
 and the client supports it.  Currently, sending this option in a
 Reply is technically redundant, as the use of the reconfiguration
 mechanism requires authentication; at present, the only defined
 mechanism is RKAP (see Section 20.4), and the presence of the
 reconfigure key signals support for the acceptance of Reconfigure
 messages.  However, there may be better security mechanisms defined
 in the future that would cause RKAP to not be used anymore.
 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" field value
 matches a previous transmission.  The server MUST NOT cache its
 responses.
 DISCUSSION:
    Cached replies are bad because lifetimes need to be updated
    (either decrease the timers by the amount of time elapsed since
    the original transmission or keep the lifetime values and update
    the lease information in the server's database).  Also, if the
    message uses any security protection (such as the Replay Detection
    Method (RDM), as described in Section 20.3), its value must be
    updated.  Additionally, any digests must be updated.  Given all of
    the above, caching replies is far more complex than simply sending
    the same buffer as before, and it is easy to miss some of those
    steps.

Mrugalski, et al. Standards Track [Page 78] RFC 8415 DHCP for IPv6 November 2018

18.3.3. Receipt of Confirm Messages

 See Section 18.4 for details regarding the handling of Confirm
 messages received via unicast.  Unicast transmission of Confirm
 messages is not allowed, regardless of whether the Server Unicast
 option (see Section 21.12) 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 (see Section 21.6).
 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 in the Reply message a Server Identifier
 option (see Section 21.3) containing the server's DUID and the Client
 Identifier option (see Section 21.2) from the Confirm message.  The
 server includes a Status Code option (see Section 21.13) indicating
 the status of the Confirm message.

18.3.4. Receipt of Renew Messages

 See Section 18.4 for details regarding the handling of Renew messages
 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 the
 IA back 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 option (see Section 21.6) for that address or IA
 Prefix option (see Section 21.22) for that delegated prefix.  If the
 server chooses to include the IA Address or IA Prefix option for such

Mrugalski, et al. Standards Track [Page 79] RFC 8415 DHCP for IPv6 November 2018

 an address or delegated prefix, the server SHOULD set T1 and T2
 values to the valid lifetime for the IA option unless the server also
 includes other addresses or delegated prefixes that the server is
 able to extend for the IA.  Setting T1 and T2 to values equal to the
 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:
  1. 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 (see Section 21.21) with a zero
    value in the "IPv6-prefix" field and a 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).
  1. 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 (see Section 21.13) with the NoAddrsAvail or
    NoPrefixAvail status code and a status message for a user.
  1. 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.

Mrugalski, et al. Standards Track [Page 80] RFC 8415 DHCP for IPv6 November 2018

 The server constructs a Reply message by setting the "msg-type" field
 to REPLY and copying the transaction ID from the Renew message into
 the "transaction-id" field.
 The server MUST include in the Reply message a Server Identifier
 option (see Section 21.3) containing the server's DUID and the Client
 Identifier option (see Section 21.2) from the Renew 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 values set in each applicable IA option for a Reply MUST be
 the same across all IAs.  The server MUST determine the T1/T2 values
 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 details regarding the handling of Rebind
 messages received via unicast.  Unicast transmission of Rebind
 messages is not allowed, regardless of whether the Server Unicast
 option (see Section 21.12) 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 the IA back to the client with new lifetimes and,
 if applicable, T1/T2 values.  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 (see Section 21.6) 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
 (see Section 21.22) for this prefix.

Mrugalski, et al. Standards Track [Page 81] RFC 8415 DHCP for IPv6 November 2018

 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 those
 addresses or delegated prefixes to the client with lifetimes of 0.
 If the server cannot find a client entry for the IA, the server
 checks if the IA contains addresses (for IA_NAs and IA_TAs) or
 delegated prefixes (for IA_PDs).  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 that is not appropriate
 for the link to which the client's interface is attached, the server
 MAY include the IA Address option with lifetimes of 0.  For any
 delegated prefix that is not appropriate for the link to which the
 client's interface is attached, the server MAY include the IA Prefix
 option with 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:
  1. If the server is configured to create new bindings as a result of

processing Rebind messages (also see the note below about the

    Rapid Commit option (Section 21.14)), the server SHOULD create a
    binding and return the IA with allocated leases with lifetimes
    and, if applicable, T1/T2 values and other information requested
    by the client.  The server MUST NOT return any addresses or
    delegated prefixes in the IA that the server does not assign to
    the client.
  1. 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 (see Section 21.13) with the NoAddrsAvail or
    NoPrefixAvail status code and a status message for a user.

Mrugalski, et al. Standards Track [Page 82] RFC 8415 DHCP for IPv6 November 2018

  1. 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" text in Section 21.14.
 Therefore, the server SHOULD only create new bindings during
 processing of a Rebind message if the server is configured to respond
 with a Reply message to a Solicit message containing the Rapid Commit
 option.
 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 in the Reply message a Server Identifier
 option (see Section 21.3) containing the server's DUID and the Client
 Identifier option (see Section 21.2) from the Rebind 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 or T2 values set in each applicable IA option for a Reply MUST
 be the same values across all IAs.  The server MUST determine the T1
 or T2 values 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 details regarding the handling of
 Information-request messages 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.

Mrugalski, et al. Standards Track [Page 83] RFC 8415 DHCP for IPv6 November 2018

 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 (see Section 21.3)
 containing the server's DUID in the Reply message.  If the client
 included a Client Identifier option (see Section 21.2) 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
 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 details regarding the handling of Release
 messages 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 IAs,
 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 (see Section 21.13)
 with the value Success, a Server Identifier option (see Section 21.3)
 with the server's DUID, and a Client Identifier option (see
 Section 21.2) 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.

Mrugalski, et al. Standards Track [Page 84] RFC 8415 DHCP for IPv6 November 2018

 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 details regarding the handling of Decline
 messages 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
 the IAs (though it may choose to log an error if it finds such
 addresses).
 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 (see Section 21.13)
 with the value Success, a Server Identifier option (see Section 21.3)
 with the server's DUID, and a Client Identifier option (see
 Section 21.2) 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 (see
 Section 21.3) and copies the Client Identifier option (see
 Section 21.2) from the Solicit message into the Advertise message.

Mrugalski, et al. Standards Track [Page 85] RFC 8415 DHCP for IPv6 November 2018

 The server MAY add a Preference option (see Section 21.8) 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 0
 unless otherwise configured by the server administrator.
 The server includes a Reconfigure Accept option (see Section 21.20)
 if the server wants to indicate that it supports the 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 (see
 Section 21.7) in the Solicit message that the server has been
 configured to return to the 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 Address options (see
 Section 21.6) 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 options (see
 Section 21.22), the server MAY use the prefix contained in the
 "IPv6-prefix" field and/or the prefix length contained in the
 "prefix-length" field as 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_NA or IA_TA in
 subsequent Request messages from the client, the server MUST include
 the IA option in the Advertise message with no addresses in that IA
 and a Status Code option (see Section 21.13) encapsulated in the IA
 option containing status code NoAddrsAvail.

Mrugalski, et al. Standards Track [Page 86] RFC 8415 DHCP for IPv6 November 2018

 If the server will not assign any prefixes to an IA_PD in subsequent
 Request messages from the client, the server MUST include the IA_PD
 option (see Section 21.21) in the Advertise message with no prefixes
 in the IA_PD option and a Status Code option encapsulated in the
 IA_PD containing status code NoPrefixAvail.
 Transmission of Advertise messages 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-forward messages included an Interface-Id option (see
 Section 21.18), 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 and sets the
 "transaction-id" field to 0.  The server includes a Server Identifier
 option (see Section 21.3) containing its DUID and a Client Identifier
 option (see Section 21.2) containing the client's DUID in the
 Reconfigure message.
 Because of the risk of denial-of-service (DoS) 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 (see Section 20.4).
 The server MUST include a Reconfigure Message option (see
 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
 message, except as specifically allowed in the definition of
 individual options.

Mrugalski, et al. Standards Track [Page 87] RFC 8415 DHCP for IPv6 November 2018

 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
 (see Section 18.3) 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,
 Rebind/Reply, or Information-request/Reply message exchange with the
 server.  The server interprets the receipt of a Renew, Rebind, or
 Information-request message (whichever was specified in the original
 Reconfigure message) from the client as satisfying the Reconfigure
 message request.
 When transmitting the Reconfigure message, the server sets the
 retransmission time (RT) to REC_TIMEOUT.  If the server does not
 receive a Renew, Rebind, or Information-request message from the
 client before the RT elapses, the server retransmits the Reconfigure
 message, doubles the RT 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 the subsections of Section 18.3 that
 discuss the receipt of specific messages, the server is not supposed
 to accept unicast traffic when it is not explicitly configured to do
 so.  For example, unicast transmission is not allowed for Solicit,
 Confirm, and Rebind messages (see Sections 18.3.1, 18.3.3, and
 18.3.5, respectively), even if the Server Unicast option (see
 Section 21.12) is configured.  For Request, Renew,
 Information-request, Release, and Decline messages, it is allowed
 only if the Server Unicast option is configured.

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 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 do so), the server discards that message and responds
 with an Advertise (when responding to a Solicit message) or Reply
 message (when responding to any other messages) containing a Status
 Code option (see Section 21.13) with the value UseMulticast, a Server
 Identifier option (see Section 21.3) containing the server's DUID,
 the Client Identifier option (see Section 21.2) 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 that includes 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 8.
 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 the message 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
 Relay-forward message, a recognized message type for which it is not
 the intended target, or an unrecognized message type [RFC7283], 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 (see
 Section 21.10) 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.

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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 globally scoped unicast address (i.e., GUA
 or ULA) from 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 on which the original message
 was received.  This is not recommended, as it may require that
 additional information 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 value 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 earlier in this
 subsection, regardless of whether the relay agent includes an
 Interface-Id option in the Relay-forward message.

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 value 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 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 globally scoped unicast address (i.e., GUA or ULA), the
 relay agent sets the link-address field to 0; otherwise, the relay
 agent sets the link-address field to a globally scoped unicast
 address (i.e., GUA or ULA) assigned to the interface on which the
 message was received or includes an Interface-Id option (see
 Section 21.18) to identify the interface on which the message was
 received.

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19.1.3. Relay Agent Behavior with Prefix Delegation

 A relay agent forwards messages containing prefix delegation options
 in the same way as it would relay addresses (i.e., per
 Sections 19.1.1 and 19.1.2).
 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 (see Section 21.10).
 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 (see
 Section 21.18), 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 0, 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.

19.3. Construction of Relay-reply Messages

 A server uses a Relay-reply message to (1) return a response to a
 client if the original message from the client was relayed to the
 server in a Relay-forward message or (2) 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 (see Section 21.10) 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

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 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-REPL
    hop-count:      1
    link-address:   0
    peer-address:   A
    Relay Message option containing the following:
       msg-type:     RELAY-REPL
       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.

19.4. Interaction between Relay Agents and Servers

 Each time a packet is relayed by a relay agent towards a server, a
 new encapsulation level is added around the packet.  Each relay is
 allowed to insert additional options on the encapsulation level it
 added but MUST NOT change anything in the packet being encapsulated.
 If there are multiple relays between a client and a server, multiple
 encapsulations are used.  Although it makes packet processing
 slightly more complex, it provides the major advantage of having a
 clear indication as to which relay inserted which option.  The
 response packet is expected to travel through the same relays, but in
 reverse order.  Each time a response packet is relayed back towards a
 client, one encapsulation level is removed.

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 In certain cases, relays can add one or more options.  These options
 can be added for several reasons:
  1. First, relays can provide additional information about the client.

That source of information is usually more trusted by a server

    administrator, as it comes from the network infrastructure rather
    than the client and cannot be easily spoofed.  These options can
    be used by the server to determine its allocation policy.
  1. Second, a relay may need some information to send a response back

to the client. Relay agents are expected to be stateless (not

    retain any state after a packet has been processed).  A relay
    agent may include the Interface-Id option (see Section 21.18),
    which will be echoed back in the response.  It can include other
    options and ask the server to echo one or more of the options back
    in the response.  These options can then be used by the relay
    agent to send the response back to the client, or for other needs.
    The client will never see these options.  See [RFC4994] for
    details.
  1. Third, sometimes a relay is the best device to provide values for

certain options. A relay can insert an option into the packet

    being forwarded to the server and ask the server to pass that
    option back to the client.  The client will receive that option.
    It should be noted that the server is the ultimate authority here,
    and -- depending on its configuration -- it may or may not send
    the option back to the client.  See [RFC6422] for details.
 For various reasons, servers may need to retain the relay information
 after the packet processing is completed.  One is a bulk leasequery
 mechanism that may ask for all addresses and/or prefixes that were
 assigned via a specific relay.  A second is for the reconfigure
 mechanism.  The server may choose to not send the Reconfigure message
 directly to the client but rather to send it via relays.  This
 particular behavior is considered an implementation detail and is out
 of scope for this document.

20. Authentication of DHCP Messages

 This document introduces two security mechanisms for the
 authentication of DHCP messages: (1) authentication (and encryption)
 of messages sent between servers and relay agents using IPsec and
 (2) 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).

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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, RKAP, 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
 RDM field specifies the type of replay detection used in the replay
 detection field.

20.3. Replay Detection

 The RDM field of the Authentication option (see Section 21.11)
 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 64-bit
 unsigned integer (modulo 2^64).  Using this technique can reduce the
 danger of replay attacks.  This method MUST be supported by all
 Authentication option protocols.  One choice might be to use the
 64-bit NTP timestamp format [RFC5905]).
 A client that receives a message with the RDM field set to 0x00 MUST
 compare its replay detection field with the previous value sent by
 that same server (based on the Server Identifier option; see
 Section 21.3) and only accept the message if the received value is
 greater and record this as the new value.  If this is the first time

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 a client processes an Authentication option sent by a server, the
 client MUST record the replay detection value and skip the replay
 detection check.
 Servers that support the reconfigure mechanism MUST ensure that the
 replay detection value is retained between restarts.  Failing to do
 so may cause clients to refuse Reconfigure messages sent by the
 server, effectively rendering the reconfigure mechanism useless.

20.4. Reconfiguration Key Authentication Protocol (RKAP)

 RKAP 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 a Hashed Message
 Authentication Code (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 (see
 Section 21.11).  The format of the authentication information is
 defined in the following section.
 RKAP is used (initiated by the server) only if the client and server
 have negotiated to use Reconfigure messages.

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20.4.1. Use of the Authentication Option in RKAP

 The following fields are set in an Authentication option (see
 Section 21.11) for RKAP:
    protocol   3
    algorithm  1
    RDM        0
 The format of the authentication information for RKAP 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     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 the Reply
                             message).
                        2    HMAC-MD5 digest of the message (used in
                             the Reconfigure message).
                     A 1-octet field.
    Value            Data as defined by the Type field.  A 16-octet
                     field.

20.4.2. Server Considerations for RKAP

 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 (see Section 21.11) in
 the Reply message.

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 The reconfigure key is 128 bits long and MUST be a cryptographically
 strong random or pseudorandom 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 0 for the HMAC-MD5 computation.  The server includes
 the HMAC-MD5 in the authentication information field in an
 Authentication option included in the Reconfigure message sent to the
 client.

20.4.3. Client Considerations for RKAP

 The client will receive a reconfigure key from the server in an
 Authentication option (see Section 21.11) 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 Reconfigure message, with zeroes substituted for
 the HMAC-MD5 field, 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 the
 definition of new options.  See Section 24 for additional information
 about the DHCPv6 "Option Codes" 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.

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 Options that are allowed to appear only once are called "singleton
 options".  The only non-singleton options defined in this document
 are the 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.

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 2-octet field.
    option-len           An unsigned integer giving the length of the
                         option-data field in this option in octets.
                         A 2-octet 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 Sections 21.4, 21.5, and 21.6.

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21.2. Client Identifier Option

 The Client Identifier option is used to carry a DUID (see Section 11)
 that identifies the client.  The format of the Client Identifier
 option is:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        OPTION_CLIENTID        |          option-len           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                              DUID                             .
    .                        (variable length)                      .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 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)
 that identifies the 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

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    option-code          OPTION_SERVERID (2).
    option-len           Length of DUID in octets.
    DUID                 The DUID for the server.

21.4. Identity Association for Non-temporary Addresses Option

 The Identity Association for Non-temporary Addresses (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 4-octet field containing
                         an unsigned integer.

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    T1                   The time interval after 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 expressed in
                         units of seconds.  A 4-octet field containing
                         an unsigned integer.
    T2                   The time interval after 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 4-octet field containing an
                         unsigned integer.
    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 (see
 Section 21.6) 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.
 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 (see Section 21.13) 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.

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 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 times, unless values
 in those fields are 0.  The values in the T1 and T2 fields are the
 number of seconds until T1 and T2 and are calculated since reception
 of the message.
 As per Section 7.7, the value 0xffffffff is taken to mean "infinity"
 and should be used carefully.
 The server selects the T1 and T2 values to allow the client to extend
 the lifetimes of any addresses in the IA_NA before the lifetimes
 expire, even if the server is unavailable for some short period of
 time.  Recommended values for T1 and T2 are 0.5 and 0.8 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 the T1 and T2 values 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.

21.5. Identity Association for Temporary Addresses Option

 The Identity Association for 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

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    option-code          OPTION_IA_TA (4).
    option-len           4 + length of IA_TA-options field.
    IAID                 The unique identifier for this IA_TA; the
                         IAID must be unique among the identifiers for
                         all of this client's IA_TAs.  The number
                         space for IA_TA IAIDs is separate from the
                         number space for other IA option types (i.e.,
                         IA_NA and IA_PD).  A 4-octet field containing
                         an unsigned integer.
    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 (see
 Section 21.6) 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 (see Section 21.13) 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 an 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 a deprecated state eventually.  Existing
 connections could continue, but no new ones would be created using
 that address.

Mrugalski, et al. Standards Track [Page 103] RFC 8415 DHCP for IPv6 November 2018

 The client obtains new temporary addresses by sending an IA_TA option
 with a new IAID to a server.  Requesting new temporary addresses from
 the server is the equivalent of generating new temporary addresses as
 described in [RFC4941].  The server will generate new temporary
 addresses and return them to the client.  The client should request
 new temporary addresses before the lifetimes on the previously
 assigned addresses expire.
 A server MUST return the same set of temporary addresses 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 IA_NA-options field of an IA_NA option (see Section 21.4) or the
 IA_TA-options field of an IA_TA option (see Section 21.5).  The
 IAaddr-options field encapsulates those options that are specific to
 this address.
 The format of the IA Address option is:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          OPTION_IAADDR        |          option-len           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                         IPv6-address                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      preferred-lifetime                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        valid-lifetime                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                        IAaddr-options                         .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 17: IA Address Option Format

Mrugalski, et al. Standards Track [Page 104] RFC 8415 DHCP for IPv6 November 2018

    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.  A 16-octet field.
    preferred-lifetime   The preferred lifetime for the address in the
                         option, expressed in units of seconds.  A
                         4-octet field containing an unsigned integer.
    valid-lifetime       The valid lifetime for the address in the
                         option, expressed in units of seconds.  A
                         4-octet field containing an unsigned integer.
    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-lifetime 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-lifetime and valid-lifetime fields for the
 preferred and valid lifetimes.  The values in these fields 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, if the valid lifetime of an address is
 0xffffffff, it 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.

Mrugalski, et al. Standards Track [Page 105] RFC 8415 DHCP for IPv6 November 2018

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:
     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.  Each option code is a
                              2-octet field containing an unsigned
                              integer.
 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 4 for details.
 The Option Request option MUST NOT include the following options:
  1. Client Identifier (see Section 21.2)
  1. Server Identifier (see Section 21.3)
  1. IA_NA (see Section 21.4)
  1. IA_TA (see Section 21.5)
  1. IA_PD (see Section 21.21)
  1. IA Address (see Section 21.6)
  1. IA Prefix (see Section 21.22)

Mrugalski, et al. Standards Track [Page 106] RFC 8415 DHCP for IPv6 November 2018

  1. Option Request (this section)
  1. Elapsed Time (see Section 21.9)
  1. Preference (see Section 21.8)
  1. Relay Message (see Section 21.10)
  1. Authentication (see Section 21.11)
  1. Server Unicast (see Section 21.12)
  1. Status Code (see Section 21.13)
  1. Rapid Commit (see Section 21.14)
  1. User Class (see Section 21.15)
  1. Vendor Class (see Section 21.16)
  1. Interface-Id (see Section 21.18)
  1. Reconfigure Message (see Section 21.19)
  1. Reconfigure Accept (see Section 21.20)
 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 that 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 4.

Mrugalski, et al. Standards Track [Page 107] RFC 8415 DHCP for IPv6 November 2018

21.8. Preference Option

 The Preference option is sent by a server to a client to control the
 selection of a server by the client.
 The format of the Preference option is:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       OPTION_PREFERENCE       |          option-len           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  pref-value   |
    +-+-+-+-+-+-+-+-+
                  Figure 19: Preference Option Format
    option-code          OPTION_PREFERENCE (7).
    option-len           1.
    pref-value           The preference value for the server in this
                         message.  A 1-octet unsigned integer.
 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 information regarding the use of the Preference option by the
 client and the interpretation of the 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

Mrugalski, et al. Standards Track [Page 108] RFC 8415 DHCP for IPv6 November 2018

    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 2-octet field containing
                         an unsigned integer.
 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 that controls 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 a 16-bit
 (2-octet) unsigned 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

Mrugalski, et al. Standards Track [Page 109] RFC 8415 DHCP for IPv6 November 2018

    option-code          OPTION_RELAY_MSG (9).
    option-len           Length of DHCP-relay-message field.
    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.

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 (see Section 25).  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
                                 1-octet unsigned integer.

Mrugalski, et al. Standards Track [Page 110] RFC 8415 DHCP for IPv6 November 2018

    algorithm                    The algorithm used in the
                                 authentication protocol.  A 1-octet
                                 unsigned integer.
    RDM                          The replay detection method used in
                                 this Authentication option.  A
                                 1-octet unsigned integer.
    replay detection             The replay detection information for
                                 the RDM.  A 64-bit (8-octet) field.
    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 the
                                 option-len field).
 IANA maintains a registry for the protocol, algorithm, and RDM values
 at <https://www.iana.org/assignments/auth-namespaces>.

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.

Mrugalski, et al. Standards Track [Page 111] RFC 8415 DHCP for IPv6 November 2018

 The server specifies in the server-address field the address to which
 the client is to send unicast messages.  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 Server 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 Server Unicast option to a client when
 relay agents are not sending relay agent options.  A DHCP server
 rejects any messages sent inappropriately using unicast to ensure
 that messages are relayed by relay agents when relay agent options
 are in use.
 Details about when the client may send messages to the server using
 unicast are provided 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 field.
    status-code          The numeric code for the status encoded in
                         this option.  A 2-octet field containing an
                         unsigned integer.

Mrugalski, et al. Standards Track [Page 112] RFC 8415 DHCP for IPv6 November 2018

    status-message       A UTF-8 encoded [RFC3629] text string
                         suitable for display to an end user.
                         MUST NOT be null-terminated.  A
                         variable-length field (2 octets less than the
                         value in the option-len field).
 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:
 +---------------+------+--------------------------------------------+
 | 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 | The 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 | The server has no prefixes available to    |
 |               |      | assign to the IA_PD(s).                    |
 +---------------+------+--------------------------------------------+
                   Table 3: Status Code Definitions
 See the "Status Codes" registry at <https://www.iana.org/assignments/
 dhcpv6-parameters> for the current list of status codes.

Mrugalski, et al. Standards Track [Page 113] RFC 8415 DHCP for IPv6 November 2018

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      |         option-len            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 25: Rapid Commit Option Format
    option-code          OPTION_RAPID_COMMIT (14).
    option-len           0.
 A client MAY include this option in a Solicit message if the client
 is prepared to perform the Solicit/Reply message exchange described
 in Section 18.2.1.
 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 but 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, all but one server will commit leases that are not
    actually used by the client; this could result in incorrect
    address information in DNS if the DHCP servers update DNS
    [RFC4704], and responses to leasequery requests [RFC5007] may
    include information on leases not in use by the client.
    The problem of unused leases can be minimized by designing the
    DHCP service so that only one server responds to the Solicit or by
    using relatively short lifetimes for newly assigned leases.

Mrugalski, et al. Standards Track [Page 114] RFC 8415 DHCP for IPv6 November 2018

21.15. User Class Option

 The User Class option is used by a client to identify the type or
 category of users 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).
    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 information.  Each instance of
 user-class-data is formatted as follows:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
    |        user-class-len         |          opaque-data          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
              Figure 27: Format of user-class-data Field

Mrugalski, et al. Standards Track [Page 115] RFC 8415 DHCP for IPv6 November 2018

 The user-class-len field is 2 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 may
 include 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:
     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
                         maintained by IANA [IANA-PEN].  A 4-octet
                         field containing an unsigned integer.
    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 the option-len field).

Mrugalski, et al. Standards Track [Page 116] RFC 8415 DHCP for IPv6 November 2018

 The vendor-class-data field 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 vendor-class-data is formatted as follows:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
    |       vendor-class-len        |          opaque-data          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
             Figure 29: Format of vendor-class-data Field
 The vendor-class-len field is 2 octets long and specifies the length
 of the opaque vendor-class-data in network byte order.
 Servers and clients MUST NOT include more than one instance of
 OPTION_VENDOR_CLASS with the same Enterprise Number.  Each instance
 of OPTION_VENDOR_CLASS can carry multiple 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 vendor-option-data field.

Mrugalski, et al. Standards Track [Page 117] RFC 8415 DHCP for IPv6 November 2018

    enterprise-number    The vendor's registered Enterprise Number as
                         maintained by IANA [IANA-PEN].  A 4-octet
                         field containing an unsigned integer.
    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 the option-len field).
 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's IPv6 stack to be
 functional.
 The vendor-option-data field MUST be encoded as a sequence of
 code/length/value fields of format identical to the DHCP options (see
 Section 21.1).  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 2-octet
                         field.
    sub-option-len       An unsigned integer giving the length of the
                         sub-option-data field in this sub-option in
                         octets.  A 2-octet field.
    sub-option-data      The data area for the sub-option.  The
                         length, in octets, is specified by
                         sub-option-len.

Mrugalski, et al. Standards Track [Page 118] RFC 8415 DHCP for IPv6 November 2018

 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 the Vendor-specific Information option with
 the same Enterprise Number.  Each instance of the Vendor-specific
 Information option MAY contain multiple sub-options.
 A client that is interested in receiving a Vendor-specific
 Information option:
  1. MUST specify the Vendor-specific Information option in an Option

Request option.

  1. MAY specify an associated Vendor Class option (see Section 21.16).
  1. 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:
  1. 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.
  1. MAY return all configured Vendor-specific Information options.
  1. 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.

Mrugalski, et al. Standards Track [Page 119] RFC 8415 DHCP for IPv6 November 2018

 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.
 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 field for parameter assignment
 policies.  The interface-id value SHOULD be considered an opaque
 value, with policies based on exact match only; that is, the
 interface-id field 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 value changes, a server will not be able to use it
 reliably in parameter assignment policies.

Mrugalski, et al. Standards Track [Page 120] RFC 8415 DHCP for IPv6 November 2018

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 the Reconfigure 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_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 message,
                         11 for Information-request message.  A
                         1-octet unsigned integer.
 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
 Reconfigure messages.  In the absence of this option, the default
 behavior is that the client is unwilling to accept Reconfigure
 messages.  The format of the Reconfigure Accept 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_ACCEPT      |         option-len            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 34: Reconfigure Accept Option Format
    option-code          OPTION_RECONF_ACCEPT (20).
    option-len           0.

Mrugalski, et al. Standards Track [Page 121] RFC 8415 DHCP for IPv6 November 2018

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 the IA_PD 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-len          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         IAID (4 octets)                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              T1                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              T2                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                          IA_PD-options                        .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Figure 35: Identity Association for Prefix Delegation Option Format
    option-code          OPTION_IA_PD (25).
    option-len           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 4-octet field containing
                         an unsigned integer.
    T1                   The time interval after 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 message reception time expressed in units
                         of seconds.  A 4-octet field containing an
                         unsigned integer.

Mrugalski, et al. Standards Track [Page 122] RFC 8415 DHCP for IPv6 November 2018

    T2                   The time interval after 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
                         message reception time expressed in units of
                         seconds.  A 4-octet field containing an
                         unsigned integer.
    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 (see
 Section 21.22) 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 (see Section 21.13) 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 an IA_PD
 have expired, the IA_PD can be considered as having expired.  T1 and
 T2 fields are included to give the server explicit control over when
 a client should contact the server about a specific IA_PD.
 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 timers, unless
 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 0.5 and 0.8 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.

Mrugalski, et al. Standards Track [Page 123] RFC 8415 DHCP for IPv6 November 2018

 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 server 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 (see Section 21.21).
     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-len          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      preferred-lifetime                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        valid-lifetime                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | prefix-length |                                               |
    +-+-+-+-+-+-+-+-+          IPv6-prefix                          |
    |                           (16 octets)                         |
    |                                                               |
    |                                                               |
    |                                                               |
    |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               |                                               .
    +-+-+-+-+-+-+-+-+                                               .
    .                       IAprefix-options                        .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 36: IA Prefix Option Format
    option-code          OPTION_IAPREFIX (26).
    option-len           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"
                         (see Section 7.7).  A 4-octet field
                         containing an unsigned integer.

Mrugalski, et al. Standards Track [Page 124] RFC 8415 DHCP for IPv6 November 2018

    valid-lifetime       The valid lifetime for the prefix in the
                         option, expressed in units of seconds.  A
                         value of 0xffffffff represents "infinity".  A
                         4-octet field containing an unsigned integer.
    prefix-length        Length for this prefix in bits.  A 1-octet
                         unsigned integer.
    IPv6-prefix          An IPv6 prefix.  A 16-octet field.
    IAprefix-options     Options associated with this prefix.  A
                         variable-length field (25 octets less than
                         the value in the option-len field).
 In a message sent by a client to a server, the preferred-lifetime and
 valid-lifetime fields SHOULD be set to 0.  The server MUST ignore any
 received values in these lifetime fields.
 The client SHOULD NOT send an IA Prefix option with 0 in the
 "prefix-length" field (and an unspecified value (::) in the
 "IPv6-prefix" field).  A client MAY send a non-zero value in the
 "prefix-length" field and the unspecified value (::) in the
 "IPv6-prefix" field to indicate a preference for the size of the
 prefix to be delegated.  See [RFC8168] for further details on prefix-
 length hints.
 The client MUST discard any prefixes for which the preferred lifetime
 is greater than the valid lifetime.
 The values in the preferred-lifetime and valid-lifetime fields are
 the number of seconds remaining in 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 value of 0xffffffff for the preferred
 lifetime or the valid lifetime 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 (see Section 21.13) in the
 IAprefix-options field.

Mrugalski, et al. Standards Track [Page 125] RFC 8415 DHCP for IPv6 November 2018

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 a 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
    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
                               4-octet field containing an unsigned
                               integer.
 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 Option Request option in
 any other messages.
 A server sending a Reply to an Information-request message SHOULD
 include this option if it is requested in the Option Request option
 of the Information-request.  The option value MUST NOT be smaller
 than IRT_MINIMUM.  This option MUST only appear in the top-level
 options 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 the value
 IRT_DEFAULT was provided.

Mrugalski, et al. Standards Track [Page 126] RFC 8415 DHCP for IPv6 November 2018

 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,
 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 higher value for SOL_MAX_RT; this
 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_SOL_MAX_RT        |         option-len            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       SOL_MAX_RT value                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 38: SOL_MAX_RT Option Format

Mrugalski, et al. Standards Track [Page 127] RFC 8415 DHCP for IPv6 November 2018

    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 this range: 60 <= "value" <= 86400
                         (1 day).  A 4-octet field containing an
                         unsigned integer.
 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.
 A DHCP client MUST ignore any SOL_MAX_RT option values that are less
 than 60 or more than 86400.
 If a DHCP client receives a message containing a SOL_MAX_RT option
 that has a valid value for SOL_MAX_RT, the client MUST set its
 internal SOL_MAX_RT parameter to the value contained in the
 SOL_MAX_RT option.  This value of SOL_MAX_RT is then used by the
 retransmission mechanism defined in Sections 15 and 18.2.1.
 The purpose of this mechanism is to give network administrators a way
 to avoid excessive DHCP traffic if all DHCP servers become
 unavailable.  Therefore, this value is expected to be retained for as
 long as practically possible.
 An updated SOL_MAX_RT value applies only to the network interface on
 which the client received the 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 higher value for INF_MAX_RT; this
 reduces the Information-request traffic from a client that has not
 received a response to its Information-request messages.

Mrugalski, et al. Standards Track [Page 128] RFC 8415 DHCP for IPv6 November 2018

 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_INF_MAX_RT        |         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 this range: 60 <= "value" <= 86400
                         (1 day).  A 4-octet field containing an
                         unsigned integer.
 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.
 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 Sections 15 and 18.2.6.
 An updated INF_MAX_RT value applies only to the network interface on
 which the client received the INF_MAX_RT option.

Mrugalski, et al. Standards Track [Page 129] RFC 8415 DHCP for IPv6 November 2018

22. Security Considerations

 This section discusses security considerations that are not related
 to privacy.  See Section 23 for a discussion dedicated to privacy.
 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.
 DHCP lacks end-to-end encryption between clients and servers; thus,
 hijacking, tampering, and eavesdropping attacks are all possible as a
 result.  Some network environments (discussed below) can be secured
 through various means to minimize these attacks.
 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 DoS attack through misconfiguration of the client; this
 attack would cause 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 (see Section 21.24) and INF_MAX_RT (see Section 21.25)
 options; this may cause an undue delay in a client completing its
 DHCP protocol transaction in the case where no other valid response
 is received.  Assuming that 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.
 A malicious server can also send a Server Unicast option (see
 Section 21.12) to a client in an Advertise message, thus potentially
 causing the client to bypass relays and communicate only with the
 malicious server for subsequent Request and Renew messages.
 Another threat to DHCP clients originates from mistakenly or
 accidentally configured DHCP servers that answer DHCP client requests
 with unintentionally incorrect configuration parameters.
 A DHCP client may also be subject to attack through the receipt of a
 Reconfigure message from a malicious server that causes the client to
 obtain incorrect configuration information from that server.  Note
 that although a client sends its response (Renew, Rebind, or
 Information-request message) through a relay agent and, therefore,

Mrugalski, et al. Standards Track [Page 130] RFC 8415 DHCP for IPv6 November 2018

 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.
 RKAP, described in Section 20.4, provides protection against the use
 of a Reconfigure message by a malicious DHCP server to mount a DoS 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 attacks by rogue servers can be mitigated by making use
 of the mechanisms 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-
 exhaustion" DoS attack.  These attacks typically involve the
 exhaustion of available assigned addresses or delegatable prefixes,
 or the exhaustion of CPU or network bandwidth, and are present any
 time 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 DoS 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].

Mrugalski, et al. Standards Track [Page 131] RFC 8415 DHCP for IPv6 November 2018

 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 misconfiguration
 or exhaustion of other resources such as assignable addresses and
 delegatable prefixes.
 Various network environments also offer levels of security if
 deployed as described below.
  1. In enterprise and factory networks, use of authentication per

[IEEE-802.1x] 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.
  1. 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.  Also, the server's (or relay's)
    unicast replies are only delivered to the target client's port --
    not all ports.
  1. In public networks (such as a Wi-Fi 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 from server to client) if
    the privacy considerations provided in Section 23 are followed.
    Even for devices that do not follow the privacy considerations,
    there is little that can be learned that would not be available
    from subsequent communications anyway (such as the device's Media
    Access Control (MAC) address).  Also, because all clients will
    typically receive similar configuration details, a bad actor that
    initiates a DHCP request itself can learn much of such
    information.  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 easily
    prevented by disallowing direct client-to-client communication on
    these networks or using [RFC7610] and [RFC7513].

Mrugalski, et al. Standards Track [Page 132] RFC 8415 DHCP for IPv6 November 2018

23. Privacy Considerations

 For an extended discussion about privacy considerations for the
 client, see [RFC7824]:
  1. In particular, its Section 3 discusses various identifiers that

could be misused to track the client.

  1. Its Section 4 discusses existing mechanisms that may have an

impact on a client's privacy.

  1. Finally, its Section 5 discusses potential attack vectors.
 For recommendations regarding how to address or mitigate those
 issues, see [RFC7844].
 This specification does not define any allocation strategies for
 servers.  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 the list in [RFC7824] is not
 exhaustive; there are certainly other possible strategies.  Readers
 are also encouraged to read [RFC7707] -- in particular, its
 Section 4.1.2, which 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
 <https://www.iana.org/assignments/dhcpv6-parameters>.
 IANA has updated <https://www.iana.org/assignments/dhcpv6-parameters>
 to add a reference to this document for definitions previously
 created by [RFC3315], [RFC3633], [RFC4242], and [RFC7083].
 IANA has added two columns to the DHCPv6 "Option Codes" registry at
 <https://www.iana.org/assignments/dhcpv6-parameters> to indicate
 which options are allowed to appear in a client's Option Request
 option (see Section 21.7) and which options are singleton options

Mrugalski, et al. Standards Track [Page 133] RFC 8415 DHCP for IPv6 November 2018

 (only allowed to appear once as a top-level or encapsulated option;
 see Section 16 of [RFC7227]).  Table 4 provides the data for the
 options assigned by IANA at the time of writing this document.
 +---------+--------------------------+------------------+-----------+
 |  Option | Option Name ("OPTION"    | Client ORO (1)   | Singleton |
 |         | 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                  | No               | 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_TIME | Required for     | Yes       |
 |         |                          | 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       |

Mrugalski, et al. Standards Track [Page 134] RFC 8415 DHCP for IPv6 November 2018

 |      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       |
 |      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 | IPv6_Address-MoS         | Yes              | Yes       |
 |      55 | 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       |

Mrugalski, et al. Standards Track [Page 135] RFC 8415 DHCP for IPv6 November 2018

 |      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       |
 |      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              | No        |
 |     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               | No               | Yes       |
 |     113 | V6_PREFIX64              | Yes              | No        |
 |     114 | F_BINDING_STATUS         | No               | Yes       |
 |     115 | F_CONNECT_FLAGS          | No               | Yes       |
 |     116 | F_DNS_REMOVAL_INFO       | No               | Yes       |
 |     117 | F_DNS_HOST_NAME          | No               | Yes       |
 |     118 | F_DNS_ZONE_NAME          | No               | Yes       |
 |     119 | F_DNS_FLAGS              | No               | Yes       |
 |     120 | F_EXPIRATION_TIME        | No               | Yes       |
 |     121 | F_MAX_UNACKED_BNDUPD     | No               | Yes       |
 |     122 | F_MCLT                   | No               | Yes       |
 |     123 | F_PARTNER_LIFETIME       | No               | Yes       |
 |     124 | F_PARTNER_LIFETIME_SENT  | No               | Yes       |
 |     125 | F_PARTNER_DOWN_TIME      | No               | Yes       |
 |     126 | F_PARTNER_RAW_CLT_TIME   | No               | Yes       |
 |     127 | F_PROTOCOL_VERSION       | No               | Yes       |
 |     128 | F_KEEPALIVE_TIME         | No               | Yes       |
 |     129 | F_RECONFIGURE_DATA       | No               | Yes       |
 |     130 | F_RELATIONSHIP_NAME      | No               | Yes       |
 |     131 | F_SERVER_FLAGS           | No               | Yes       |
 |     132 | F_SERVER_STATE           | No               | Yes       |
 |     133 | F_START_TIME_OF_STATE    | No               | Yes       |

Mrugalski, et al. Standards Track [Page 136] RFC 8415 DHCP for IPv6 November 2018

 |     134 | F_STATE_EXPIRATION_TIME  | No               | Yes       |
 |     135 | RELAY_PORT               | No               | Yes       |
 |     143 | IPv6_Address-ANDSF       | Yes              | Yes       |
 +---------+--------------------------+------------------+-----------+
                       Table 4: Updated Options
 Notes for Table 4:
 (1)  In the "Client ORO" column, a "Yes" for an option means that the
      client includes this option code in the Option Request option
      (see Section 21.7) if it desires that configuration information,
      and a "No" means that the option MUST NOT be included (and
      servers SHOULD silently ignore that option code if it appears in
      a client's Option Request option).
 (2)  For each Enterprise Number, there MUST only be a single
      instance.
 (3)  See [RFC7598] for details.
 IANA has corrected the range of possible status codes in the "Status
 Codes" table at <https://www.iana.org/assignments/dhcpv6-parameters>
 by replacing 23-255 (as Unassigned) with 23-65535 (the codes are
 16-bit unsigned integers).
 IANA has updated 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 <https://www.iana.org/assignments/
 ipv6-multicast-addresses> to reference this document instead of
 [RFC3315].
 IANA has added an "Obsolete" annotation in the "DHCPv6 Delayed
 Authentication" entry in the "Authentication Suboption (value 8) -
 Protocol identifier values" registry at
 <https://www.iana.org/assignments/bootp-dhcp-parameters> and has
 added an "Obsolete" annotation in the "Delayed Authentication" entry
 in the "Protocol Name Space Values" registry at
 <https://www.iana.org/assignments/auth-namespaces>.  IANA has also
 updated these pages to reference this document instead of [RFC3315].
 IANA has added a reference to this document for the RDM value of 0 to
 the "RDM Name Space Values" registry at
 <https://www.iana.org/assignments/auth-namespaces>.

Mrugalski, et al. Standards Track [Page 137] RFC 8415 DHCP for IPv6 November 2018

 IANA has updated the "Service Name and Transport Protocol Port Number
 Registry" at <https://www.iana.org/assignments/
 service-names-port-numbers> as follows:
          546/udp      This document
          547/udp      This document
          547/tcp      [RFC5460]
          647/tcp      [RFC8156]

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
    presented a significant operational burden.  As a result, after
    10 years its adoption was extremely limited at best.
 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 issues similar to those for
    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 T1/T2
    values supplied by a client.

Mrugalski, et al. Standards Track [Page 138] RFC 8415 DHCP for IPv6 November 2018

26. References

26.1. Normative References

 [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            DOI 10.17487/RFC0768, August 1980,
            <https://www.rfc-editor.org/info/rfc768>.
 [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>.
 [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>.
 [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>.

Mrugalski, et al. Standards Track [Page 139] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
            Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
            March 2017, <https://www.rfc-editor.org/info/rfc8085>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", STD 86, RFC 8200,
            DOI 10.17487/RFC8200, July 2017,
            <https://www.rfc-editor.org/info/rfc8200>.
 [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>.

26.2. Informative References

 [IANA-HARDWARE-TYPES]
            IANA, "Hardware Types",
            <https://www.iana.org/assignments/arp-parameters>.
 [IANA-PEN] IANA, "Private Enterprise Numbers",
            <https://www.iana.org/assignments/enterprise-numbers>.
 [IANA-RESERVED-IID]
            IANA, "Reserved IPv6 Interface Identifiers",
            <https://www.iana.org/assignments/ipv6-interface-ids>.
 [IEEE-802.1x]
            IEEE, "IEEE Standard for Local and metropolitan area
            networks--Port-Based Network Access Control",
            IEEE 802.1X-2010, DOI 10.1109/IEEESTD.2010.5409813,
            <https://ieeexplore.ieee.org/servlet/
            opac?punumber=5409757>.
 [RFC826]   Plummer, D., "An 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>.

Mrugalski, et al. Standards Track [Page 140] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [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>.
 [RFC3290]  Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
            Informal Management Model for Diffserv Routers", RFC 3290,
            DOI 10.17487/RFC3290, May 2002,
            <https://www.rfc-editor.org/info/rfc3290>.
 [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>.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
            ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
            November 2003, <https://www.rfc-editor.org/info/rfc3629>.
 [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>.

Mrugalski, et al. Standards Track [Page 141] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [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>.
 [RFC4994]  Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
            "DHCPv6 Relay Agent Echo Request Option", RFC 4994,
            DOI 10.17487/RFC4994, September 2007,
            <https://www.rfc-editor.org/info/rfc4994>.
 [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>.
 [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
            DOI 10.17487/RFC5460, February 2009,
            <https://www.rfc-editor.org/info/rfc5460>.

Mrugalski, et al. Standards Track [Page 142] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [RFC5908]  Gayraud, R. and B. Lourdelet, "Network Time Protocol (NTP)
            Server Option for DHCPv6", RFC 5908, DOI 10.17487/RFC5908,
            June 2010, <https://www.rfc-editor.org/info/rfc5908>.
 [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
            RFC 6422, DOI 10.17487/RFC6422, December 2011,
            <https://www.rfc-editor.org/info/rfc6422>.
 [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>.
 [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>.
 [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>.
 [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>.

Mrugalski, et al. Standards Track [Page 143] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [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>.
 [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>.

Mrugalski, et al. Standards Track [Page 144] RFC 8415 DHCP for IPv6 November 2018

 [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>.
 [RFC8156]  Mrugalski, T. and K. Kinnear, "DHCPv6 Failover Protocol",
            RFC 8156, DOI 10.17487/RFC8156, June 2017,
            <https://www.rfc-editor.org/info/rfc8156>.
 [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>.
 [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>.

Mrugalski, et al. Standards Track [Page 145] RFC 8415 DHCP for IPv6 November 2018

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).
 2.   New sections were added to discuss the relationship to previous
      DHCPv6 documents and also to DHCPv4.
 3.   Sections 2 ("Requirements") and 3 ("Background") had very minor
      edits.
 4.   Section 4 ("Terminology") had minor edits.
 5.   Section 4.2 ("DHCP Terminology") was expanded to incorporate
      definitions from RFC 3633, add T1/T2 definitions, add
      definitions useful in describing combined address assignment and
      prefix delegation operations, and improve some existing
      definitions.
 6.   Section 5 ("Client/Server Exchanges") was added from material
      previously in Section 1 of RFC 3315 ("Introduction and
      Overview") and was expanded.
 7.   Section 6 ("Operational Models") is new.  It provides
      information on the kinds of DHCP clients and how they operate.
 8.   Section 7 ("DHCP Constants") was primarily updated to add
      constants from RFC 4242 and RFC 7083.  Note that the default
      HOP_COUNT_LIMIT value was reduced from 32 to 8.
 9.   Sections 8 ("Client/Server Message Formats"), 9 ("Relay Agent/
      Server Message Formats"), and 10 ("Representation and Use of
      Domain Names") had only very minor changes.
 10.  Section 11 ("DHCP Unique Identifier (DUID)") now discourages,
      rather than disallows, a server to parse the DUID; now includes
      some information on the DUID-UUID (RFC 6355); and had other
      minor edits.
 11.  Section 12 ("Identity Association") was expanded to better
      explain the concept and to also include prefix delegation.

Mrugalski, et al. Standards Track [Page 146] RFC 8415 DHCP for IPv6 November 2018

 12.  Section 13 ("Assignment to an IA") incorporates material from
      two sections (11 and 12) of RFC 3315 and also includes a section
      on prefix delegation.
 13.  Section 14 ("Transmission of Messages by a Client") was expanded
      to include rate limiting by clients and how clients should
      handle T1 or T2 values of 0.
 14.  Section 15 ("Reliability of Client-Initiated Message Exchanges")
      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.
 15.  Section 16 ("Message Validation") had minor edits.
 16.  Section 17 ("Client Source Address and Interface Selection") was
      expanded to include prefix delegation.
 17.  Section 18 ("DHCP Configuration Exchanges") consolidates what
      used to be in the following sections in RFC 3315: "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 it incorporates prefix delegation from RFC 3633
      and other changes from RFC 4242, RFC 7083, and RFC 7550.  A few
      changes of note:
      A.  The Option Request option is no longer optional for some
          messages (Solicit and Information-request), as RFC 7083
          requires clients to request SOL_MAX_RT or INF_MAX_RT
          options.
      B.  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.
      C.  The lifetime and T1/T2 hints should not be sent by a client
          (it should send values of 0 in these fields), and any
          non-zero values should be ignored by the server.
      D.  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.

Mrugalski, et al. Standards Track [Page 147] RFC 8415 DHCP for IPv6 November 2018

      Also, Section 18.2.12 ("Refreshing Configuration Information")
      was added to indicate use cases for when a client should try to
      refresh network information.
 18.  Section 19 ("Relay Agent Behavior") incorporates RFC 7283 and
      had minor edits.  A new section, "Interaction between Relay
      Agents and Servers" (Section 19.4), was added.
 19.  Section 20 ("Authentication of DHCP Messages") includes
      significant changes: IPsec materials were mostly removed and
      replaced with a reference to RFC 8213, and the delayed
      authentication protocol has been obsoleted (see Section 25).
      Note that RKAP is still considered current.
 20.  Section 21 ("DHCP Options") was expanded to incorporate
      OPTION_IA_PD and OPTION_IAPREFIX from RFC 3633, the Information
      Refresh Time option (OPTION_INFORMATION_REFRESH_TIME) from
      RFC 4242, and the SOL_MAX_RT and INF_MAX_RT options from
      RFC 7083.  Some additional edits were made to clarify option
      handling, such as which options should not be in an Option
      Request option.
 21.  The security considerations (Section 22) were updated to expand
      the discussion of security threats and include material from the
      incorporated documents, primarily RFC 3633.
 22.  New privacy considerations were added (Section 23) to account
      for privacy issues.
 23.  Section 24 ("IANA Considerations") 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.
 24.  Section 25 ("Obsoleted Mechanisms") is a new section that
      documents the mechanisms obsoleted by this specification.
 25.  Appendices B ("Appearance of Options in Message Types") and C
      ("Appearance of Options in the "options" Field of DHCP Options")
      were updated to reflect the incorporated options from RFC 3633,
      RFC 4242, and RFC 7083.
 26.  Where appropriate, informative references have been added to
      provide further background and guidance throughout the document
      (as can be noted by the vast increase in references).

Mrugalski, et al. Standards Track [Page 148] RFC 8415 DHCP for IPv6 November 2018

 27.  Changes were made to incorporate the following errata for
      RFC 3315: Erratum IDs 294, 295, 1373, 1815, 2471, 2472, 2509,
      2928, 3577, 5450; RFC 3633: Erratum IDs 248, 2468, 2469, 2470,
      3736; and RFC 3736: Erratum ID 3796.  Note that Erratum ID 1880
      for RFC 3633 no longer applies, as servers (delegating routers)
      ignore received T1/T2 hints (see (C) in item 17 above).
 28.  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.
 29.  It should be noted that this document does not refer to all
      DHCPv6 functionality and specifications.  Readers of this
      specification should visit <https://www.iana.org/assignments/
      dhcpv6-parameters> 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 indicate with a "*" the options that are allowed
 in each DHCP message type.
 These tables are informational.  If they conflict with text earlier
 in this document, that text should be considered authoritative.
       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: The Server Identifier option (see Section 21.3) is only
 included in Information-request messages that are sent in response to
 a Reconfigure (see Section 18.2.6).

Mrugalski, et al. Standards Track [Page 149] RFC 8415 DHCP for IPv6 November 2018

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

Mrugalski, et al. Standards Track [Page 150] RFC 8415 DHCP for IPv6 November 2018

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 can be encapsulated
 in other options defined in this document.  Other RFCs may define
 additional situations where options defined in this document are
 encapsulated in other options.
 This table is informational.  If it conflicts with text earlier in
 this document, that text should be considered authoritative.
                 Top-    IA_NA/                        RELAY-  RELAY-
                 Level   IA_TA  IAADDR IA_PD  IAPREFIX FORW    REPL
 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" and "RELAY-REPL" columns appear in the
 "options" field of the Relay-forward and Relay-reply messages (see
 Section 9).

Mrugalski, et al. Standards Track [Page 151] RFC 8415 DHCP for IPv6 November 2018

Acknowledgments

 This document is merely a refinement of earlier work by the authors
 of the following documents and would not be possible without their
 original work:
  1. RFC 3315 (Ralph Droms, Jim Bound, Bernie Volz, Ted Lemon, Charles

Perkins, and Mike Carney)

  1. RFC 3633 (Ole Troan and Ralph Droms)
  1. RFC 3736 (Ralph Droms)
  1. RFC 4242 (Stig Venaas, Tim Chown, and Bernie Volz)
  1. RFC 7083 (Ralph Droms)
  1. RFC 7283 (Yong Cui, Qi Sun, and Ted Lemon)
  1. RFC 7550 (Ole Troan, Bernie Volz, and Marcin Siodelski)
 A number of additional people have contributed to identifying issues
 with RFC 3315 and RFC 3633 and proposed resolutions to these issues
 as reflected in this document (listed here 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, Lorenzo Colitti, Tianxiang Li, Ian Farrer, Yogendra Pal, Kim
 Kinnear, Shawn Routhier, Michayla Newcombe, Alissa Cooper, Allison
 Mankin, Adam Roach, Kyle Rose, Elwyn Davies, Eric Rescorla, Ben
 Campbell, Warren Kumari, and Kathleen Moriarty.
 Also, special thanks to Ralph Droms for answering many questions
 related to the original RFC 3315 and RFC 3633 work and for
 shepherding this document through the IETF process.

Mrugalski, et al. Standards Track [Page 152] RFC 8415 DHCP for IPv6 November 2018

Authors' Addresses

 Tomek Mrugalski
 Internet Systems Consortium, Inc.
 950 Charter Street
 Redwood City, CA  94063
 United States of America
 Email: tomasz.mrugalski@gmail.com
 Marcin Siodelski
 Internet Systems Consortium, Inc.
 950 Charter Street
 Redwood City, CA  94063
 United States of America
 Email: msiodelski@gmail.com
 Bernie Volz
 Cisco Systems, Inc.
 1414 Massachusetts Ave.
 Boxborough, MA  01719
 United States of America
 Email: volz@cisco.com
 Andrew Yourtchenko
 Cisco Systems, Inc.
 De kleetlaan 6a
 Diegem  BRABANT 1831
 Belgium
 Email: ayourtch@cisco.com
 Michael C. Richardson
 Sandelman Software Works
 470 Dawson Avenue
 Ottawa, ON  K1Z 5V7
 Canada
 Email: mcr+ietf@sandelman.ca
 URI:   http://www.sandelman.ca/

Mrugalski, et al. Standards Track [Page 153] RFC 8415 DHCP for IPv6 November 2018

 Sheng Jiang
 Huawei Technologies Co., Ltd
 Q14, Huawei Campus, No. 156 Beiqing Road
 Hai-Dian District, Beijing  100095
 China
 Email: jiangsheng@huawei.com
 Ted Lemon
 Nibbhaya Consulting
 P.O. Box 958
 Brattleboro, VT  05301-0958
 United States of America
 Email: mellon@fugue.com
 Timothy Winters
 University of New Hampshire, Interoperability Lab (UNH-IOL)
 Durham, NH
 United States of America
 Email: twinters@iol.unh.edu

Mrugalski, et al. Standards Track [Page 154]

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