wdiff rfc8415v1.txt rfc8415.txt

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
August
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.

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.

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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Relationship to Previous DHCPv6 Standards . . . . . . . . 6
1.2. Relationship to DHCPv4 . . . . . . . . . . . . . . . . . 7
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . . 9
4.2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . 10
5. Client/Server Exchanges . . . . . . . . . . . . . . . . . . . 16 15
5.1. Client/Server Exchanges Involving Two Messages . . . . . 16 15
5.2. Client/Server Exchanges Involving Four Messages . . . . . 17 16
5.3. Server/Client Exchanges . . . . . . . . . . . . . . . . . 17
6. Operational Models . . . . . . . . . . . . . . . . . . . . . 18 17
6.1. Stateless DHCP . . . . . . . . . . . . . . . . . . . . . 18 17
6.2. DHCP for Non-temporary Address Assignment . . . . . . . . 18
6.3. DHCP for Prefix Delegation . . . . . . . . . . . . . . . 19 18
6.4. DHCP for Customer Edge Routers . . . . . . . . . . . . . 22 21
6.5. DHCP for Temporary Addresses . . . . . . . . . . . . . . 22 21
6.6. Multiple Addresses and Prefixes . . . . . . . . . . . . . 22 21
7. DHCP Constants . . . . . . . . . . . . . . . . . . . . . . . 23 22
7.1. Multicast Addresses . . . . . . . . . . . . . . . . . . . 23 22
7.2. UDP Ports . . . . . . . . . . . . . . . . . . . . . . . . 24 23
7.3. DHCP Message Types . . . . . . . . . . . . . . . . . . . 24 23
7.4. DHCP Option Codes . . . . . . . . . . . . . . . . . . . . 26 25
7.5. Status Codes . . . . . . . . . . . . . . . . . . . . . . 26 25
7.6. Transmission and Retransmission Parameters . . . . . . . 26 25
7.7. Representation of Time Values and "Infinity" as a Time
Value . . . . . . . . . . . . . . . . . . . . . . . . . . 28 27
8. Client/Server Message Formats . . . . . . . . . . . . . . . . 28 27
9. Relay Agent/Server Message Formats . . . . . . . . . . . . . 29 28
9.1. Relay-forward Message . . . . . . . . . . . . . . . . . . 30 29
9.2. Relay-reply Message . . . . . . . . . . . . . . . . . . . 31 30
10. Representation and Use of Domain Names . . . . . . . . . . . 31 30
11. DHCP Unique Identifier (DUID) . . . . . . . . . . . . . . . . 31 30
11.1. DUID Contents . . . . . . . . . . . . . . . . . . . . . 32 31
11.2. DUID Based on Link-Layer Address Plus Time (DUID-LLT) . 33 32
11.3. DUID Assigned by Vendor Based on Enterprise Number
(DUID-EN) . . . . . . . . . . . . . . . . . . . . . . . 34 33
11.4. DUID Based on Link-Layer Address (DUID-LL) . . . . . . . 35 34
11.5. DUID Based on Universally Unique Identifier (DUID-UUID) 36 35
12. Identity Association . . . . . . . . . . . . . . . . . . . . 37 36
12.1. Identity Associations for Address Assignment . . . . . . 37 36
12.2. Identity Associations for Prefix Delegation . . . . . . 38 37
13. Assignment to an IA . . . . . . . . . . . . . . . . . . . . . 38 37
13.1. Selecting Addresses for Assignment to an IA_NA . . . . . 38 37
13.2. Assignment of Temporary Addresses . . . . . . . . . . . 39 38
13.3. Assignment of Prefixes for IA_PD . . . . . . . . . . . . 40 39
14. Transmission of Messages by a Client . . . . . . . . . . . . 40 39
14.1. Rate Limiting . . . . . . . . . . . . . . . . . . . . . 41 40
14.2. Client Behavior when T1 and/or T2 Are 0 . . . . . . . . 41 40
15. Reliability of Client-Initiated Message Exchanges . . . . . . 42 41
16. Message Validation . . . . . . . . . . . . . . . . . . . . . 44 43
16.1. Use of Transaction IDs . . . . . . . . . . . . . . . . . 45 44
16.2. Solicit Message . . . . . . . . . . . . . . . . . . . . 45 44
16.3. Advertise Message . . . . . . . . . . . . . . . . . . . 45 44
16.4. Request Message . . . . . . . . . . . . . . . . . . . . 45 44
16.5. Confirm Message . . . . . . . . . . . . . . . . . . . . 46 45
16.6. Renew Message . . . . . . . . . . . . . . . . . . . . . 46 45
16.7. Rebind Message . . . . . . . . . . . . . . . . . . . . . 46 45
16.8. Decline Message . . . . . . . . . . . . . . . . . . . . 46 45
16.9. Release Message . . . . . . . . . . . . . . . . . . . . 47 46
16.10. Reply Message . . . . . . . . . . . . . . . . . . . . . 47 46
16.11. Reconfigure Message . . . . . . . . . . . . . . . . . . 47 46
16.12. Information-request Message . . . . . . . . . . . . . . 48 47
16.13. Relay-forward Message . . . . . . . . . . . . . . . . . 48 47
16.14. Relay-reply Message . . . . . . . . . . . . . . . . . . 48 47
17. Client Source Address and Interface Selection . . . . . . . . 48 47
17.1. Source Address and Interface Selection for Address
Assignment . . . . . . . . . . . . . . . . . . . . . . . 49 48
17.2. Source Address and Interface Selection for Prefix
Delegation . . . . . . . . . . . . . . . . . . . . . . . 49 48
18. DHCP Configuration Exchanges . . . . . . . . . . . . . . . . 49 48
18.1. A Single Exchange for Multiple IA Options . . . . . . . 52 51
18.2. Client Behavior . . . . . . . . . . . . . . . . . . . . 53 52
18.2.1. Creation and Transmission of Solicit Messages . . . 54 53
18.2.2. Creation and Transmission of Request Messages . . . 56 55
18.2.3. Creation and Transmission of Confirm Messages . . . 58 57
18.2.4. Creation and Transmission of Renew Messages . . . . 59 58
18.2.5. Creation and Transmission of Rebind Messages . . . . 61 60
18.2.6. Creation and Transmission of Information-request
Messages . . . . . . . . . . . . . . . . . . . . . . 61 60
18.2.7. Creation and Transmission of Release Messages . . . 62 61
18.2.8. Creation and Transmission of Decline Messages . . . 64 63
18.2.9. Receipt of Advertise Messages . . . . . . . . . . . 65 64
18.2.10. Receipt of Reply Messages . . . . . . . . . . . . . 66 65
18.2.10.1. Reply for Solicit (with Rapid Commit), Request,
Renew, or Rebind . . . . . . . . . . . . . . . 67 66
18.2.10.2. Reply for Release and Decline . . . . . . . . . 70 69
18.2.10.3. Reply for Confirm . . . . . . . . . . . . . . . 70 69
18.2.10.4. Reply for Information-request . . . . . . . . . 70 69
18.2.11. Receipt of Reconfigure Messages . . . . . . . . . . 70 69
18.2.12. Refreshing Configuration Information . . . . . . . . 71 70
18.3. Server Behavior . . . . . . . . . . . . . . . . . . . . 72 71
18.3.1. Receipt of Solicit Messages . . . . . . . . . . . . 74 73
18.3.2. Receipt of Request Messages . . . . . . . . . . . . 75 74
18.3.3. Receipt of Confirm Messages . . . . . . . . . . . . 77 76
18.3.4. Receipt of Renew Messages . . . . . . . . . . . . . 77 76
18.3.5. Receipt of Rebind Messages . . . . . . . . . . . . . 79 78
18.3.6. Receipt of Information-request Messages . . . . . . 81 80
18.3.7. Receipt of Release Messages . . . . . . . . . . . . 82 81
18.3.8. Receipt of Decline Messages . . . . . . . . . . . . 83 82
18.3.9. Creation of Advertise Messages . . . . . . . . . . . 83 82
18.3.10. Transmission of Advertise and Reply Messages . . . . 85 84
18.3.11. Creation and Transmission of Reconfigure Messages . 85 84
18.4. Reception of Unicast Messages . . . . . . . . . . . . . 86 85
19. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 87 86
19.1. Relaying a Client Message or a Relay-forward Message . . 87 86
19.1.1. Relaying a Message from a Client . . . . . . . . . . 87 86
19.1.2. Relaying a Message from a Relay Agent . . . . . . . 88 87
19.1.3. Relay Agent Behavior with Prefix Delegation . . . . 88 87
19.2. Relaying a Relay-reply Message . . . . . . . . . . . . . 89 88
19.3. Construction of Relay-reply Messages . . . . . . . . . . 89 88
19.4. Interaction between Relay Agents and Servers . . . . . . 90 89
20. Authentication of DHCP Messages . . . . . . . . . . . . . . . 91 90
20.1. Security of Messages Sent between Servers and Relay
Agents . . . . . . . . . . . . . . . . . . . . . . . . . 91 90
20.2. Summary of DHCP Authentication . . . . . . . . . . . . . 92 91
20.3. Replay Detection . . . . . . . . . . . . . . . . . . . . 92 91
20.4. RKAP . . . . . . . . . . . . . . . . . . . . . . . Reconfiguration Key Authentication Protocol (RKAP) . . . 93 92
20.4.1. Use of the Authentication Option in RKAP . . . . . . 93 92
20.4.2. Server Considerations for RKAP . . . . . . . . . . . 94 93
20.4.3. Client Considerations for RKAP . . . . . . . . . . . 95 94
21. DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . 95 94
21.1. Format of DHCP Options . . . . . . . . . . . . . . . . . 95 94
21.2. Client Identifier Option . . . . . . . . . . . . . . . . 96 95
21.3. Server Identifier Option . . . . . . . . . . . . . . . . 97 96
21.4. Identity Association for Non-temporary Addresses Option 97 96
21.5. Identity Association for Temporary Addresses Option . . 100 99
21.6. IA Address Option . . . . . . . . . . . . . . . . . . . 102 101
21.7. Option Request Option . . . . . . . . . . . . . . . . . 103 102
21.8. Preference Option . . . . . . . . . . . . . . . . . . . 105 104
21.9. Elapsed Time Option . . . . . . . . . . . . . . . . . . 106 105
21.10. Relay Message Option . . . . . . . . . . . . . . . . . . 107 105
21.11. Authentication Option . . . . . . . . . . . . . . . . . 107 106
21.12. Server Unicast Option . . . . . . . . . . . . . . . . . 109 108
21.13. Status Code Option . . . . . . . . . . . . . . . . . . . 110 109
21.14. Rapid Commit Option . . . . . . . . . . . . . . . . . . 111 110
21.15. User Class Option . . . . . . . . . . . . . . . . . . . 112 111
21.16. Vendor Class Option . . . . . . . . . . . . . . . . . . 113 112
21.17. Vendor-specific Information Option . . . . . . . . . . . 115 114
21.18. Interface-Id Option . . . . . . . . . . . . . . . . . . 117 116
21.19. Reconfigure Message Option . . . . . . . . . . . . . . . 118 117
21.20. Reconfigure Accept Option . . . . . . . . . . . . . . . 118 117
21.21. Identity Association for Prefix Delegation Option . . . 119 118
21.22. IA Prefix Option . . . . . . . . . . . . . . . . . . . . 121 120
21.23. Information Refresh Time Option . . . . . . . . . . . . 122 121
21.24. SOL_MAX_RT Option . . . . . . . . . . . . . . . . . . . 124 123
21.25. INF_MAX_RT Option . . . . . . . . . . . . . . . . . . . 125 124
22. Security Considerations . . . . . . . . . . . . . . . . . . . 126 125
23. Privacy Considerations . . . . . . . . . . . . . . . . . . . 129 128
24. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 130 129
25. Obsoleted Mechanisms . . . . . . . . . . . . . . . . . . . . 134 133
26. References . . . . . . . . . . . . . . . . . . . . . . . . . 135 134
26.1. Normative References . . . . . . . . . . . . . . . . . . 135 134
26.2. Informative References . . . . . . . . . . . . . . . . . 136 135
Appendix A. Summary of Changes . . . . . . . . . . . . . . . . . 141 140
Appendix B. Appearance of Options in Message Types . . . . . . . 145 144
Appendix C. Appearance of Options in the "options" Field of DHCP
Options . . . . . . . . . . . . . . . . . . . . . . 146 145
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 147 146
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 148 147

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].

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] DHCPv6. [RFC3736]
was written to document just those portions of DHCPv6 needed to
support DHCPv6 stateless operation). 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:

- [RFC3633] ("IPv6 Prefix Options for Dynamic Host Configuration
Protocol (DHCP) version 6")

- [RFC3736] ("Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6")

- [RFC4242] ("Information Refresh Time Option for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)")

- [RFC7083] ("Modification to Default Values of SOL_MAX_RT and
INF_MAX_RT")

- [RFC7283] ("Handling Unknown DHCPv6 Messages")

- [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.

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

- [RFC4291] ("IP Version 6 Addressing Architecture")

- [RFC4862] ("IPv6 Stateless Address Autoconfiguration")

- [RFC4861] ("Neighbor Discovery for IP version 6 (IPv6)")

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 [RFC0826]. To
understand IPv6 and stateless address autoconfiguration, it is
strongly recommended that implementers understand IPv6 Neighbor
Discovery.

4. Terminology

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

4.1. IPv6 Terminology

IPv6 terminology 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.

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.

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)).

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 (or 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 (or 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 (or 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.

encapsulated option A DHCPv6 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.

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.

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.

5. Client/Server Exchanges

Clients and servers exchange DHCP messages using UDP (see [RFC0768]
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 Simple Network Time Protocol (SNTP) NTP servers [RFC4075] [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.

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).

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.

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.
For example, this
This mechanism would be used is appropriate for use by a service provider an ISP to
assign delegate a prefix
to a Customer Edge Router device acting as a router
between subscriber, where the subscriber's internal network delegated prefix would possibly be
subnetted and assigned to the service provider's
core 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].

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.

This prefix delegation mechanism is appropriate for use by an ISP to
delegate a prefix to a subscriber, where the delegated prefix would
possibly be subnetted and assigned to the links within the
subscriber's network. [RFC7084] and [RFC7368] describe such use in
detail.

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.

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 SNTP 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
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
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.

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
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.

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. 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.

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).

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
of the prefix 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.

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).

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

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 unique
address (including a unique local scoped
unicast address
[RFC4193]), (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.

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]. It If this
capability is required, it is recommended to rely on the mechanism defined in 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.,
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].

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.

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.

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.

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_TA 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.

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.

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:

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

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

* If the server receives the message from a forwarding relay
agent, then the client is on the same link as the one to which
the interface, identified by the link-address field in the
message from the relay agent, is attached. According to
[RFC6221], the server MUST ignore any link-address field whose
value is zero. The link-address in this case may come from any
of the Relay-forward messages encapsulated in the received
Relay-forward, and in general the most encapsulated (closest
Relay-forward to the client) has the most useful value.

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

- The DUID supplied by the client.

- Other information in options supplied by the client, e.g., IA
Address options (see Section 21.6) that include the client's
requests for specific addresses.

- Other information in options supplied by the relay agent.

By default, DHCP server implementations SHOULD NOT generate
predictable addresses (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].

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 care 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
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, 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.

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.

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.

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.

16.2. Solicit Message

Clients MUST discard any received Solicit messages.

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

16.3. Advertise Message

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

- the message does not include a Server Identifier option (see
Section 21.3).

- the message does not include a Client Identifier option (see
Section 21.2).

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

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

Servers and relay agents MUST discard any received Advertise
messages.

16.4. Request Message

Clients MUST discard any received Request messages.

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

- the message does not include a Server Identifier option (see
Section 21.3).

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

- the message does not include a Client Identifier option (see
Section 21.2).

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:

- the message does not include a Server Identifier option (see
Section 21.3).

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

- 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:

- the message does not include a Server Identifier option (see
Section 21.3).

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

- the message does not include a Client Identifier option (see
Section 21.2).

16.9. Release Message

Clients MUST discard any received Release messages.

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

- the message does not include a Server Identifier option (see
Section 21.3).

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

- 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:

- the message does not include a Server Identifier option (see
Section 21.3).

- 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, either (1) the Reply message MUST include a Client
Identifier option, and the contents of the Client Identifier option
MUST match the DUID of the client or (2) if 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:

- the message was not unicast to the client.

- the message does not include a Server Identifier option (see
Section 21.3).

- the message does not include a Client Identifier option (see
Section 21.2) that contains the client's DUID.

- the message does not include a Reconfigure Message option (see
Section 21.19).

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

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

16.12. Information-request Message

Clients MUST discard any received Information-request messages.

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

- the message includes a Server Identifier option (see
Section 21.3), and the DUID in the option does not match the
server's DUID.

- the message includes an IA option.

16.13. Relay-forward Message

Clients MUST discard any received Relay-forward messages.

16.14. Relay-reply Message

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

17. Client Source Address and Interface Selection

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.

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.

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 SNTP NTP
servers [RFC4075] [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.

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)

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.
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.

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.

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:

- processes the Reply message as described in Section 18.2.10 and
discards any Reply messages received in response to the Request
message

- 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
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.

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:

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

- Initiate the server discovery process described in Section 18.

- Terminate the configuration process and report failure.

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.

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.

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).

18.2.5. Creation and Transmission of Rebind Messages

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

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

The client constructs the Rebind message as described in
Section 18.2.4, with the following differences:

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

- The client does not include the Server Identifier option (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.

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

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.

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.

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

18.2.9. Receipt of Advertise Messages

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

- Those Advertise messages with the highest server preference value
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.

- 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:

- MUST process an included SOL_MAX_RT option and

- MUST process an included INF_MAX_RT option.

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
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 SNTP NTP server address from the DHCP
server and does not receive it anymore MUST stop using the configured
SNTP
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:

- 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.

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

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

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

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

If the client can operate with the addresses and/or prefixes obtained
from the server:

- The client uses the addresses, delegated prefixes, and other
information from any IAs that do not contain a Status Code option
with the NoAddrsAvail or NoPrefixAvail status code. The client
MAY include the IAs for which it received the NoAddrsAvail or
NoPrefixAvail status code, with no addresses or prefixes, in
subsequent Renew and Rebind messages sent to the server, to retry
obtaining the addresses or prefixes for these IAs.

- The client MUST perform duplicate address detection as per
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.

- 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.

- 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
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:

- 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.

- 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.

- 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.

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.

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:

- The client reboots (and has stable storage and persistent DHCP
state).

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

- The client returns from sleep mode.

- The client changes access points (e.g., if using a wireless 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.

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
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
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.

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.

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.

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
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:

- 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).

- 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.

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

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

The server MUST include 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.

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:

- 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.

- 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.

- 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.

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.

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.

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.

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.

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.

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.

19.1.1. Relaying a Message from a Client

If the relay agent received the message to be relayed from a client,
the relay agent places a global address (including a unique local globally scoped unicast address [RFC4193]) with (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 global address (including a unique local globally scoped unicast address
[RFC4193]), (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 global globally scoped unicast
address (including a unique local 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.

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
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.

In certain cases, relays can add one or more options. These options
can be added for several reasons:

- 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.

- 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.

- 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).

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
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. RKAP 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.

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.

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.

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.

21.2. Client Identifier Option

The Client Identifier option is used to carry a DUID (see Section 11)
to identify
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)
to identify
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

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.

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.

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 those 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

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.

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

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. Also, a client MUST NOT
assume that any length of prefix that matches
this address is on-link (see [RFC7421]). 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.

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:

- Client Identifier (see Section 21.2)

- Server Identifier (see Section 21.3)

- IA_NA (see Section 21.4)

- IA_TA (see Section 21.5)

- IA_PD (see Section 21.21)

- IA Address (see Section 21.6)

- IA Prefix (see Section 21.22)

- Option Request (this section)

- Elapsed Time (see Section 21.9)

- Preference (see Section 21.8)

- Relay Message (see Section 21.10)

- Authentication (see Section 21.11)

- Server Unicast (see Section 21.12)

- Status Code (see Section 21.13)

- Rapid Commit (see Section 21.14)

- User Class (see Section 21.15)
- Vendor Class (see Section 21.16)

- Interface-Id (see Section 21.18)

- Reconfigure Message (see Section 21.19)

- 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.

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

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

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.

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.

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.

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:

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.

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.

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

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
includes 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).

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.

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.

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:

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

- MAY specify an associated Vendor Class option (see Section 21.16).

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

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

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

- MAY return all configured Vendor-specific Information options.

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

21.18. Interface-Id Option

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

The format of the Interface-Id option is:

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

Figure 32: Interface-Id Option Format

option-code OPTION_INTERFACE_ID (18).

option-len Length of interface-id field.

interface-id An opaque value of arbitrary length generated
by the relay agent to identify one of the
relay agent's interfaces. The length, in
octets, is specified by option-len.

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.

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.

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.

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 those
values in those fields are 0. The values in the T1 and T2 fields are
the number of seconds until T1 and T2.

The server selects the T1 and T2 times to allow the client to extend
the lifetimes of any prefixes in the IA_PD before the lifetimes
expire, even if the server is unavailable for some short period of
time. Recommended values for T1 and T2 are 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.

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.

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.

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.

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

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.

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.

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,
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].

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.

- 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.

- 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.

- 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. 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].

23. Privacy Considerations

This section focuses on privacy considerations for the server.

For an extended discussion about privacy considerations for the
client, see [RFC7824]:

- In particular, its Section 3 discusses various identifiers that
could be misused to track the client.

- Its Section 4 discusses existing mechanisms that may have an
impact on a client's privacy.

- 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. 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
(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_" ("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 |
| 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 |
| 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 | 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 (TEMPORARY) | 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 |
| 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>.

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.

26. References

26.1. Normative References

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

[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>.

[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>.

[RFC0826] 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>.

[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>.

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

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

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

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

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

[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>.

[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>.

[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
RFC 7341, DOI 10.17487/RFC7341, August 2014,
<https://www.rfc-editor.org/info/rfc7341>.

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

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

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

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

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

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

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

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

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

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

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

[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>.

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.

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.

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).

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).

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.

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).

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:

- RFC 3315 (Ralph Droms, Jim Bound, Bernie Volz, Ted Lemon, Charles
Perkins, and Mike Carney)

- RFC 3633 (Ole Troan and Ralph Droms)

- RFC 3736 (Ralph Droms)

- RFC 4242 (Stig Venaas, Tim Chown, and Bernie Volz)

- RFC 7083 (Ralph Droms)
- RFC 7283 (Yong Cui, Qi Sun, and Ted Lemon)

- 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, Tatuya Jinmei, Lorenzo Colitti, Tianxiang Li, Ian Farrer, Yogendra Pal, Kim
Kinnear, Shawn Routhier, Tim Chown, 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.

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/

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