wdiff rfc8156v1.txt rfc8156.txt

Internet Engineering Task Force (IETF) T. Mrugalski
Request for Comments: 8156 ISC
Category: Standards Track K. Kinnear
ISSN: 2070-1721 Cisco
April
June 2017

DHCPv6 Failover Protocol

Abstract

DHCPv6 as defined in "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)" (RFC 3315) does not offer server redundancy. This document
defines a protocol implementation to provide DHCPv6 failover, a
mechanism for running two servers with the capability for either
server to take over clients' leases in case of server failure or
network partition. It meets the requirements for DHCPv6 failover
detailed in "DHCPv6 Failover Requirements" (RFC 7031).

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
http://www.rfc-editor.org/info/rfc8156.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Failover Concepts and Mechanisms . . . . . . . . . . . . . . 9
4.1. Required Server Configuration . . . . . . . . . . . . . . 9
4.2. IPv6 Address and Delegable Prefix Allocation . . . . . . 9
4.2.1. Independent Allocation . . . . . . . . . . . . . . . 9
4.2.1.1. Independent Allocation Algorithm . . . . . . . . 10
4.2.2. Proportional Allocation . . . . . . . . . . . . . . . 10
4.2.2.1. Reallocating Leases . . . . . . . . . . . . . . . 12
4.3. Lazy Updates . . . . . . . . . . . . . . . . . . . . . . 13
4.4. Maximum Client Lead Time (MCLT) . . . . . . . . . . . . . 13 14
4.4.1. MCLT Example . . . . . . . . . . . . . . . . . . . . 15
5. Message and Option Definitions . . . . . . . . . . . . . . . 18
5.1. Message Framing for TCP . . . . . . . . . . . . . . . . . 18
5.2. Failover Message Format . . . . . . . . . . . . . . . . . 18
5.3. Messages . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3.1. BNDUPD . . . . . . . . . . . . . . . . . . . . . . . 19
5.3.2. BNDREPLY . . . . . . . . . . . . . . . . . . . . . . 20
5.3.3. POOLREQ . . . . . . . . . . . . . . . . . . . . . . . 20
5.3.4. POOLRESP . . . . . . . . . . . . . . . . . . . . . . 20
5.3.5. UPDREQ . . . . . . . . . . . . . . . . . . . . . . . 20
5.3.6. UPDREQALL . . . . . . . . . . . . . . . . . . . . . . 20
5.3.7. UPDDONE . . . . . . . . . . . . . . . . . . . . . . . 21
5.3.8. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 21
5.3.9. CONNECTREPLY . . . . . . . . . . . . . . . . . . . . 21
5.3.10. DISCONNECT . . . . . . . . . . . . . . . . . . . . . 21
5.3.11. STATE . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3.12. CONTACT . . . . . . . . . . . . . . . . . . . . . . . 21
5.4. Transaction-id . . . . . . . . . . . . . . . . . . . . . 22
5.5. Options . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.5.1. OPTION_F_BINDING_STATUS . . . . . . . . . . . . . . . 22
5.5.2. OPTION_F_CONNECT_FLAGS . . . . . . . . . . . . . . . 23
5.5.3. OPTION_F_DNS_REMOVAL_INFO . . . . . . . . . . . . . . 24
5.5.3.1. OPTION_F_DNS_HOST_NAME . . . . . . . . . . . . . 25
5.5.3.2. OPTION_F_DNS_ZONE_NAME . . . . . . . . . . . . . 25
5.5.3.3. OPTION_F_DNS_FLAGS . . . . . . . . . . . . . . . 26
5.5.4. OPTION_F_EXPIRATION_TIME . . . . . . . . . . . . . . 27
5.5.5. OPTION_F_MAX_UNACKED_BNDUPD . . . . . . . . . . . . . 28
5.5.6. OPTION_F_MCLT . . . . . . . . . . . . . . . . . . . . 28
5.5.7. OPTION_F_PARTNER_LIFETIME . . . . . . . . . . . . . . 29
5.5.8. OPTION_F_PARTNER_LIFETIME_SENT . . . . . . . . . . . 29
5.5.9. OPTION_F_PARTNER_DOWN_TIME . . . . . . . . . . . . . 30
5.5.10. OPTION_F_PARTNER_RAW_CLT_TIME . . . . . . . . . . . . 30
5.5.11. OPTION_F_PROTOCOL_VERSION . . . . . . . . . . . . . . 31
5.5.12. OPTION_F_KEEPALIVE_TIME . . . . . . . . . . . . . . . 32
5.5.13. OPTION_F_RECONFIGURE_DATA . . . . . . . . . . . . . . 32
5.5.14. OPTION_F_RELATIONSHIP_NAME . . . . . . . . . . . . . 33
5.5.15. OPTION_F_SERVER_FLAGS . . . . . . . . . . . . . . . . 34
5.5.16. OPTION_F_SERVER_STATE . . . . . . . . . . . . . . . . 35
5.5.17. OPTION_F_START_TIME_OF_STATE . . . . . . . . . . . . 36
5.5.18. OPTION_F_STATE_EXPIRATION_TIME . . . . . . . . . . . 37
5.6. Status Codes . . . . . . . . . . . . . . . . . . . . . . 38
6. Connection Management . . . . . . . . . . . . . . . . . . . . 39
6.1. Creating Connections . . . . . . . . . . . . . . . . . . 39
6.1.1. Sending a CONNECT Message . . . . . . . . . . . . . . 40
6.1.2. Receiving a CONNECT Message . . . . . . . . . . . . . 41
6.1.3. Receiving a CONNECTREPLY Message . . . . . . . . . . 42
6.2. Endpoint Identification . . . . . . . . . . . . . . . . . 43
6.3. Sending a STATE Message . . . . . . . . . . . . . . . . . 44
6.4. Receiving a STATE Message . . . . . . . . . . . . . . . . 44
6.5. Connection Maintenance Parameters . . . . . . . . . . . . 45
6.6. Unreachability Detection . . . . . . . . . . . . . . . . 45
7. Binding Updates and Acks . . . . . . . . . . . . . . . . . . 46
7.1. Time Skew . . . . . . . . . . . . . . . . . . . . . . . . 46
7.2. Information Model . . . . . . . . . . . . . . . . . . . . 46
7.3. Times Required for Exchanging Binding Updates . . . . . . 50
7.4. Sending Binding Updates . . . . . . . . . . . . . . . . . 51
7.5. Receiving Binding Updates . . . . . . . . . . . . . . . . 54
7.5.1. Monitoring Time Skew . . . . . . . . . . . . . . . . 54
7.5.2. Acknowledging Reception . . . . . . . . . . . . . . . 55
7.5.3. Processing Binding Updates . . . . . . . . . . . . . 55
7.5.4. Accept or Reject? . . . . . . . . . . . . . . . . . . 56
7.5.5. Accepting Updates . . . . . . . . . . . . . . . . . . 58
7.6. Sending Binding Replies . . . . . . . . . . . . . . . . . 59
7.7. Receiving Binding Acks . . . . . . . . . . . . . . . . . 61
7.8. BNDUPD/BNDREPLY Data Flow . . . . . . . . . . . . . . . . 62
8. Endpoint States . . . . . . . . . . . . . . . . . . . . . . . 63
8.1. State Machine Operation . . . . . . . . . . . . . . . . . 63
8.2. State Machine Initialization . . . . . . . . . . . . . . 66
8.3. STARTUP State . . . . . . . . . . . . . . . . . . . . . . 66
8.3.1. Operation in STARTUP State . . . . . . . . . . . . . 67
8.3.2. Transition out of STARTUP State . . . . . . . . . . . 67
8.4. PARTNER-DOWN State . . . . . . . . . . . . . . . . . . . 69
8.4.1. Operation in PARTNER-DOWN State . . . . . . . . . . . 69
8.4.2. Transition out of PARTNER-DOWN State . . . . . . . . 70
8.5. RECOVER State . . . . . . . . . . . . . . . . . . . . . . 70
8.5.1. Operation in RECOVER State . . . . . . . . . . . . . 71
8.5.2. Transition out of RECOVER State . . . . . . . . . . . 71
8.6. RECOVER-WAIT State . . . . . . . . . . . . . . . . . . . 72
8.6.1. Operation in RECOVER-WAIT State . . . . . . . . . . . 73
8.6.2. Transition out of RECOVER-WAIT State . . . . . . . . 73
8.7. RECOVER-DONE State . . . . . . . . . . . . . . . . . . . 73
8.7.1. Operation in RECOVER-DONE State . . . . . . . . . . . 73
8.7.2. Transition out of RECOVER-DONE State . . . . . . . . 74
8.8. NORMAL State . . . . . . . . . . . . . . . . . . . . . . 74
8.8.1. Operation in NORMAL State . . . . . . . . . . . . . . 74
8.8.2. Transition out of NORMAL State . . . . . . . . . . . 75
8.9. COMMUNICATIONS-INTERRUPTED State . . . . . . . . . . . . 76
8.9.1. Operation in COMMUNICATIONS-INTERRUPTED State . . . . 76
8.9.2. Transition out of COMMUNICATIONS-INTERRUPTED State . 77
8.10. POTENTIAL-CONFLICT State . . . . . . . . . . . . . . . . 78
8.10.1. Operation in POTENTIAL-CONFLICT State . . . . . . . 79
8.10.2. Transition out of POTENTIAL-CONFLICT State . . . . . 79
8.11. RESOLUTION-INTERRUPTED State . . . . . . . . . . . . . . 80
8.11.1. Operation in RESOLUTION-INTERRUPTED State . . . . . 81
8.11.2. Transition out of RESOLUTION-INTERRUPTED State . . . 81
8.12. CONFLICT-DONE State . . . . . . . . . . . . . . . . . . . 81
8.12.1. Operation in CONFLICT-DONE State . . . . . . . . . . 82
8.12.2. Transition out of CONFLICT-DONE State . . . . . . . 82
9. DNS Update Considerations . . . . . . . . . . . . . . . . . . 82
9.1. Relationship between Failover and DNS Update . . . . . . 83
9.2. Exchanging DNS Update Information . . . . . . . . . . . . 84
9.3. Adding RRs to the DNS . . . . . . . . . . . . . . . . . . 86
9.4. Deleting RRs from the DNS . . . . . . . . . . . . . . . . 86
9.5. Name Assignment with No Update of DNS . . . . . . . . . . 87
10. Security Considerations . . . . . . . . . . . . . . . . . . . 87
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 88
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 90
12.1. Normative References . . . . . . . . . . . . . . . . . . 90
12.2. Informative References . . . . . . . . . . . . . . . . . 92
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 92
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 92

1. Introduction

This document defines a DHCPv6 failover protocol, which is a
mechanism for running two DHCPv6 servers [RFC3315] with the
capability for either server to take over clients' leases in case of
server failover or network partition. For a general overview of
DHCPv6 failover problems, use cases, benefits, and shortcomings, see
[RFC7031].

The failover protocol provides a means for cooperating DHCP servers
to work together to provide a service to DHCP clients with
availability that is increased beyond the availability that could be
provided by a single DHCP server operating alone. It is designed to
protect DHCP clients against server unreachability, including server
failure and network partition. It is possible to deploy exactly two
servers that are able to continue providing a lease for an IPv6
address [RFC3315] or on an IPv6 prefix [RFC3633] without the DHCP
client experiencing lease expiration or a reassignment of a lease to
a different IPv6 address or prefix in the event of failure by one or
the other of the two servers.

The failover protocol defines an active-passive mode, sometimes also
called a "hot standby" model. This means that during normal
operation one server is active (i.e., it actively responds to
clients' requests) while the second is passive (i.e., it receives
clients' requests but responds only to those specifically directed to
it). The secondary server maintains a copy of the binding database
and is ready to take over all incoming queries in case the primary
server fails.

The failover protocol is designed to provide lease stability for
leases with valid lifetimes beyond a short period. The DHCPv6
failover protocol MUST NOT be used for new leases shorter than
30 seconds. Leases reaching the end of their lifetimes are not
affected by this restriction.

The failover protocol fulfills all DHCPv6 failover requirements
defined in [RFC7031].

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

3. Glossary

This is a supplemental glossary that should be used in combination
with the definitions in Section 2 of RFC 7031 [RFC7031].

o Absolute Time

"Absolute time" refers to the time in seconds since midnight
January 1, 2000 UTC, modulo 2^32.

o Address Lease

"Address lease" refers to a lease involving an IPv6 address.
Typically used when it is necessary to distinguish the lease for
an IPv6 address from a lease for a DHCP prefix. See the
definitions for "delegated prefix" and "prefix lease" below.

o auto-partner-down
"auto-partner-down" refers to a capability where a failover server
will move from COMMUNICATIONS-INTERRUPTED state to PARTNER-DOWN
state automatically, without operator intervention.

o Available (Lease or Prefix)

A lease or delegable prefix is available if it could be allocated
for use by a DHCP client. It is available on the main server when
it is in the FREE state and available on the secondary server when
it is in the FREE-BACKUP state. The term "available" is sometimes
used when it would be awkward to say "FREE on the primary server
and FREE-BACKUP on the secondary server".

o Binding-Status

A lease can hold a variety of states (see Section 5.5.1 for a
list); these are also referred to as the "binding-status" of the
lease.

o Delegable Prefix

"Delegable prefix" refers to a prefix from which other prefixes
may be delegated, using the mechanisms described in [RFC3633]. A
prefix that has been delegated is known as a "delegated prefix" or
a "prefix lease".

o Delegated Prefix

A delegated prefix is a prefix that has been delegated to a DHCP
client as described in [RFC3633]. Depending on the context, a
delegated prefix may also be described as a "prefix lease" when it
is necessary to distinguish it from an "address lease".

o DHCP Prefix

A DHCP prefix is part of the IPv6 address space configured to be
managed by a DHCP server.

o Failover Endpoint

The failover protocol permits a unique failover "endpoint" for
each failover relationship in which a failover server
participates. The failover relationship is defined by a
relationship name and includes

* the failover partner IP address,
* the role this server takes with respect to that partner
(primary or secondary), and

* the prefixes from which addresses can be leased, as well as
prefixes from which other prefixes can be delegated (delegable
prefixes), that are associated with that relationship.

The failover endpoint can take actions and hold unique states.
Typically, there is one failover endpoint per partner (server),
although there may be more.

o Failover Communication

"Failover communication" refers to all messages exchanged between
partners.

o Independent Allocation

"Independent allocation" refers to an allocation algorithm that
splits the available pool of address leases between the primary
and secondary servers. It is used for IPv6 address allocations.
See Section 4.2.1.

o Lease

A lease is an association of a DHCP client with an IPv6 address or
delegated prefix. This might refer to either an existing
association or a potential association.

o MCLT (Maximum Client Lead Time)

The fundamental relationship on which much of the correctness of
the failover protocol depends is that the lease expiration time
known to a DHCP client MUST NOT be greater by more than the MCLT
beyond the later of the partner lifetime acknowledged by that
server's failover partner or the current time (i.e., now). See
Section 4.4.

o Partner

The other DHCP server that participates in a failover relationship
is referred to as the "partner". When the role (primary or
secondary) is not important, the other server is referred to as a
"failover partner" or sometimes simply "partner".

o Prefix Lease
A prefix lease is a lease involving a prefix that is delegated or
could be delegated, as opposed to a lease for a single IPv6
address. A prefix lease can also be described as a "delegated
prefix".

o Primary Server

The primary server is the first of the two DHCP servers that
participate in a failover relationship. When both servers are
operating, this server handles most of the client traffic. Its
failover partner is referred to as the "secondary server".

o Proportional Allocation

"Proportional allocation" is an allocation algorithm that splits
the delegable prefixes between the primary and secondary servers
and maintains a more or less fixed proportion of the delegable
prefixes between both servers. See Section 4.2.2.

o Renew Responsive

A server that is "renew responsive" will respond to valid DHCP
client requests messages that are directed to it by having an
OPTION_SERVERID option in the message that contains the DHCP
Unique Identifier (DUID) of the renew responsive server. See
[RFC3315].

o Responsive

A server that is responsive will respond to all valid DHCP client
requests.
messages.

o Secondary Server

The secondary server is the second of the two DHCP servers that
participate in a failover relationship. Its failover partner is
referred to as the "primary server" (as defined above). When both
servers are operating, this server (the secondary) typically does
not handle client traffic and acts as a backup to the primary
server. However, it will respond to RENEW requests directed
specifically to it.

o Server

"Server" refers to a DHCP server that implements DHCPv6 failover.
"Server" and "failover endpoint" are synonymous only if the server
participates in only one failover relationship.

o State

The term "state" is used in two ways in this document. A failover
endpoint is always in some state, and there are a series of states
that a failover endpoint can move through. See Section 8 for
details of the failover endpoint states. A lease also has a
state, and this is sometimes referred to as a "binding-status".
See Section 5.5.1 for a list of the states a lease can hold.

o Unresponsive

A server that is unresponsive will not respond to DHCP client
requests.
messages.

4. Failover Concepts and Mechanisms

The following concepts and mechanisms are necessary for the operation
of the failover protocol. They are not currently employed by DHCPv6
[RFC3315]. The failover protocol provides support for all of these
concepts and mechanisms.

4.1. Required Server Configuration

Servers frequently have several kinds of leases available on a
particular network segment. The failover protocol assumes that both
the primary server and the secondary server are configured
identically with regard to the prefixes and links involved in DHCP.
For delegated delegable prefixes (involved in proportional allocation), the
primary server is responsible for allocating to the secondary server
the correct proportion of the available delegated delegable prefixes. IPv6
addresses (involved in independent allocation) are allocated to the
primary and secondary servers algorithmically and do not require an
explicit message transfer to be distributed.

4.2. IPv6 Address and Delegable Prefix Allocation

Currently, there are two allocation algorithms defined: one for
address leases and one for prefix leases.

4.2.1. Independent Allocation

In this allocation scheme, which is used for allocating individual
IPv6 addresses, available IPv6 addresses are permanently (until
server configuration changes) split between servers. Available IPv6
addresses are split between the primary and secondary servers as part
of initial connection establishment. Once IPv6 addresses are
allocated to each server, there is no need to reassign them. The
IPv6 address allocation is algorithmic in nature and does not require
a message exchange for each server to know which IPv6 addresses it
has been allocated. This algorithm is simpler than proportional
allocation, since it does not require a rebalancing mechanism. It
also assumes that the pool assigned to each server will never be
depleted.

Once each server is assigned a pool of IPv6 addresses during initial
connection establishment, it may allocate its assigned IPv6 addresses
to clients. Once a client releases a lease or its lease on an IPv6
address expires, the returned IPv6 address returns to the pool for
the server that leased it. A lease on an IPv6 address can be renewed
by a responsive server or by a renew responsive server. When an IPv6
address goes PENDING-FREE (see Section 7.2), it is owned by whichever
server it is allocated to by the independent allocation algorithm.

IPv6 addresses, which use the independent allocation approach, will
be ignored when a server processes a POOLREQ message.

During COMMUNICATIONS-INTERRUPTED events, a partner MAY continue
extending existing address leases as requested by clients. An
operational partner MUST NOT lease IPv6 addresses that were assigned
to its downed partner and later expired or that were released or
declined by a client. When it is in PARTNER-DOWN state, a server
MUST allocate new leases from its own pool. It MUST NOT use its
partner's pool to allocate new leases.

4.2.1.1. Independent Allocation Algorithm

For each address that can be allocated, the primary server MUST
allocate only IPv6 addresses when the low-order bit (i.e., bit 127)
is equal to 1, and the secondary server MUST allocate only the IPv6
addresses when the low-order bit (i.e., bit 127) is equal to 0.

4.2.2. Proportional Allocation

In this allocation scheme, each server has its own pool of prefixes
available for delegation, known as "delegable prefixes". These
delegable prefixes may be prefixes from which other prefixes can be
delegated, or they may be prefixes that are the correct size for
delegation but are not, at present, delegated to a particular client.
Remaining delegable prefixes are split between the primary and
secondary servers in a configured proportion. Note that a delegated
prefix (also known as a "prefix lease") is not "owned" by a
particular server. Only a delegable prefix that is available is
owned by a particular server -- once it has been delegated (leased)
to a client, it becomes a prefix lease and is not owned by either
failover partner. When it finally becomes available again, it will
be initially owned by the primary server, and it may or may not be
allocated to the secondary server by the primary server.

The flow of a delegable prefix is as follows: initially, the
delegable prefix is part of a set of delegable prefixes, all of which
are initially owned by the primary server. A delegable prefix may be
allocated to the secondary server, and it is then owned by the
secondary server. Either server can allocate and delegate prefixes
out of the delegable prefixes that they own. Once these prefixes are
delegated (leased) to clients, the servers cease to own them, and
they are owned by the clients to which they have been delegated
(leased). When the client releases the delegated prefix or the lease
on it expires, the prefix will again become available, will again be
a delegable prefix, and will be owned by the primary.

A server delegates prefixes only from its own pool of delegable
prefixes in all states except for PARTNER-DOWN. In PARTNER-DOWN
state, the operational partner can delegate prefixes from either pool
(both its own, and its partner's after some time constraints have
elapsed). The operational partner SHOULD allocate from its own pool
before using its partner's pool. The allocation and maintenance of
these pools of delegable prefixes are important, since the goal is to
maintain a more or less constant ratio of delegable prefixes between
the two servers.

Each server knows which delegable prefixes are in its own pool as
well as which are in its partner's pool, so that it can allocate
delegable prefixes from its partner's pool without communication with
its partner if that becomes necessary.

The initial allocation of delegable prefixes from the primary to the
secondary when the servers first integrate is triggered by the
POOLREQ message from the secondary to the primary. This is followed
(at some point) by the POOLRESP message, where the primary tells the
secondary that it received and processed the POOLREQ message. The
primary sends the allocated delegable prefixes to the secondary as
prefix leases via BNDUPD messages. The POOLRESP message may be sent
before, during, or at the completion of the BNDUPD message exchanges
that were triggered by the POOLREQ message. The POOLREQ/POOLRESP
message exchange is a trigger to the primary to perform a scan of its
database and to ensure that the secondary has enough delegable
prefixes (based on some configured ratio).

The delegable prefixes are sent to the secondary as prefix leases
using the BNDUPD message containing an OPTION_IAPREFIX with a state
of FREE-BACKUP, which indicates that the prefix lease is now
available for allocation by the secondary. Once the message is sent,
the primary MUST NOT use these prefixes for allocation to DHCP
clients (except when the server is in PARTNER-DOWN state).

The POOLREQ/POOLRESP message exchange initiated by the secondary is
valid at any time both partners remain in contact, and the primary
server SHOULD, whenever it receives the POOLREQ message, scan its
database of delegable prefixes and determine if the secondary needs
more delegable prefixes from any of the delegable prefixes that it
currently owns.

In order to support a reasonably dynamic balance of the leases
between the failover partners, the primary server needs to do
additional work to ensure that the secondary server has as many
delegable prefixes as it needs (but that it doesn't have more than it
needs).

The primary server SHOULD examine the balance of delegable prefixes
between the primary and secondary for a particular prefix whenever
the number of possibly delegable prefixes for either the primary or
secondary changes by more than a predetermined amount. Typically,
this comparison would not involve actually comparing the count of
existing instances of delegable prefixes but would instead involve
determining the number of prefixes that could be delegated given the
address ranges of the delegable prefixes allocated to each server.
The primary server SHOULD adjust the delegable prefix balance as
required to ensure the configured delegable prefix balance, except
that the primary server SHOULD employ some threshold mechanism to
such a balance adjustment in order to minimize the overhead of
maintaining this balance.

The primary server can, at any time, send an available delegable
prefix to the secondary using a BNDUPD message with the state
FREE-BACKUP. The primary server can attempt to take an available
delegable prefix away from the secondary by sending a BNDUPD message
with the state FREE. If the secondary accepts the BNDUPD message,
then the lease is now available to the primary and not available to
the secondary. Of course, the secondary MUST reject that BNDUPD
message if it has already allocated that lease to a DHCP client.

4.2.2.1. Reallocating Leases

When the server is in PARTNER-DOWN state, there is a waiting period
after which a delegated prefix can be reallocated to another client.
For delegable prefixes that are "available" when the server enters
PARTNER-DOWN state, the period is the MCLT from the entry into
PARTNER-DOWN state. For delegated prefixes that are not available
when the server enters PARTNER-DOWN state, the period is the MCLT
after the later of the following times: the acked-partner-lifetime,
the partner-lifetime (if any), the expiration-time, or the entry into
PARTNER-DOWN time plus the MCLT. time.

In any other state, a server cannot reallocate a delegated prefix
from one client to another without first notifying its partner
(through a BNDUPD message) and receiving acknowledgement (through a
BNDREPLY message) that its partner is aware that the first client is
not using the lease.

Specifically, an "available" delegable prefix on a server may be
allocated to any client. A prefix that was delegated (leased) to a
client and that expired or was released by that client would take on
a new state -- EXPIRED or RELEASED, respectively. The partner server
would then be notified that this delegated prefix was EXPIRED or
RELEASED through a BNDUPD message. When the sending server received
the BNDREPLY message for that delegated prefix showing that it was
FREE, it would move the lease from EXPIRED or RELEASED to FREE, and
the prefix would be available for allocation by the primary server to
any clients.

A server MAY reallocate a delegated prefix in the EXPIRED or RELEASED
state to the same client with no restrictions, provided it has not
sent a BNDUPD message regarding the delegated prefix to its partner.
This situation would exist if the prefix lease expired or was
released after the transition into PARTNER-DOWN state, for instance.

4.3. Lazy Updates

[RFC7031] includes the requirement that failover must not introduce
significant performance impact on server response times (see
Sections 7 and 5.2.2 of [RFC7031]). In order to realize this
requirement, a server implementing the failover protocol must be able
to respond to a DHCP client without waiting to update its failover
partner whenever the binding database changes. The "lazy update"
mechanism allows a server to allocate a new lease or extend an
existing lease, respond to the DHCP client, and then update its
failover partner as time permits.

Although the "lazy update" mechanism does not introduce additional
delays in server response times, it introduces other difficulties.
The key problem with lazy update is that when a server fails after
updating a DHCP client with a particular valid lifetime but before
updating its failover partner, the failover partner will eventually
believe that the client's lease has expired -- even though the DHCP
client still retains a valid lease on that address or prefix. It is
also possible that the failover partner will have no record at all of
the lease being assigned to the DHCP client. Both of these issues
are dealt with by using the MCLT when allocating or extending leases
(see Section 4.4).

4.4. Maximum Client Lead Time (MCLT)

In order to handle problems introduced by lazy updates (see
Section 4.3), a period of time known as the "Maximum Client Lead
Time" (MCLT) is defined and must be known to both the primary server
and the secondary server. Proper use of this time interval places an
upper bound on the difference allowed between the valid lifetime
provided to a DHCP client by a server and the valid lifetime known by
that server's failover partner.

The MCLT is typically much less than the valid lifetime that a server
has been configured to offer a client, and so some strategy must
exist to allow a server to offer the configured valid lifetime to a
client. During a lazy update, the updating server updates its
failover partner with a partner lifetime that is longer than the
valid lifetime previously given to the DHCP client and that is longer
than the valid lifetime that the server has been configured to give a
client. This allows the server to give the configured valid lifetime
to the client the next time the client renews its lease, since the
time that it will give to the client will not be longer than the MCLT
beyond the partner lifetime acknowledged by its partner or the
current time.

The fundamental relationship on which the failover protocol depends
is as follows: the lease expiration time known to a DHCP client
MUST NOT be greater by more than the MCLT beyond the later of the
partner lifetime acknowledged by that server's failover partner or
the current time.

The remainder of this section makes the above fundamental
relationship more explicit.

The failover protocol requires a DHCP server to deal with several
different lease intervals and places specific restrictions on their
relationships. The purpose of these restrictions is to allow the
partner to be able to make certain assumptions in the absence of an
ability to communicate between servers.

In the following explanation, all of the lifetimes are "valid"
lifetimes, in the context of [RFC3315].

The different times are as follows:

desired lifetime:

The desired lifetime is the lease interval that a DHCP server
would like to give to a DHCP client in the absence of any
restrictions imposed by the failover protocol. Its determination
is outside of the scope of the failover protocol. Typically, this
is the result of external configuration of a DHCP server.

actual lifetime:
The actual lifetime is the lease interval that a DHCP server gives
out to a DHCP client. It may be shorter than the desired lifetime
(as explained below).

partner lifetime:
The partner lifetime is the lease expiration interval the local
server sends to its partner in a BNDUPD message.

acknowledged partner lifetime:
The acknowledged partner lifetime is the partner lifetime the
partner server has most recently acknowledged in a BNDREPLY
message.

4.4.1. MCLT Example

The following example demonstrates the MCLT concept in practice. The
values used are arbitrarily chosen and are not a recommendation for
actual values. The MCLT in this case is 1 hour. The desired
lifetime is 3 days, and its renewal time is half the lifetime.

When a server makes an offer for a new lease on an IPv6 address to a
DHCP client, it determines the desired lifetime (in this case,
3 days). It then examines the acknowledged partner lifetime (which,
in this case, is zero) and determines the remainder of the time left
to run, which is also zero. It adds the MCLT to this value. Since
the actual lifetime cannot be allowed to exceed the remainder of the
current acknowledged partner lifetime plus the MCLT, the offer made
to the client is for the remainder of the current acknowledged
partner lifetime (i.e., zero) plus the MCLT. Thus, the actual
lifetime is 1 hour (the MCLT).

Once the server has sent the REPLY to the DHCP client, it will update
its failover partner with the lease information using a BNDUPD
message. The partner lifetime will be composed of the T1 fraction
(1/2) of the actual lifetime added to the desired lifetime. Thus,
the failover partner is updated using a BNDUPD message with a partner
lifetime of 1/2 hour + 3 days.

When the primary server receives a BNDREPLY to its update of the
secondary server's (partner's) partner lifetime, it records that as
the acknowledged partner lifetime. A server MUST NOT send a BNDREPLY
message in response to a BNDUPD message until it is sure that the
information in the BNDUPD message has been updated in its lease
database. See Section 7.5.2. Thus, the primary server in this case
can be sure that the secondary server has recorded the partner
lifetime in its stable storage when the primary server receives a
BNDREPLY message from the secondary server.

When the DHCP client attempts to renew at T1 (approximately 1/2 hour
from the start of the lease), the primary server again determines the
desired lifetime, which is still 3 days. It then compares this with
the original acknowledged partner lifetime (1/2 hour + 3 days) and
adjusts for the time passed since the secondary was last updated
(1/2 hour). Thus, the remaining time for the acknowledged partner
interval is 3 days. Adding the MCLT to this yields 3 days plus
1 hour, which is more than the desired lifetime of 3 days. So, the
client may have its lease renewed for the desired lifetime -- 3 days.

When the primary DHCP server updates the secondary DHCP server after
the DHCP client's renewal REPLY is complete, it will calculate the
partner lifetime as the T1 fraction of the actual client lifetime
(1/2 of 3 days this time = 1.5 days). To this it will add the desired lifetime
of 3 days, yielding a total partner lifetime of 4.5 days. In this
way, the primary attempts to have the secondary always "lead" the
client in its understanding of the client's lifetime so as to be able
to always offer the client the desired lifetime.

Once the initial actual client lifetime of the MCLT has passed, the
failover protocol operates effectively like DHCP does today in its
behavior concerning lifetimes. However, the guarantee that the
actual client lifetime will never exceed the partner server's
remaining acknowledged partner lifetime by more than the MCLT allows
full recovery from a variety of DHCP server failures.

Fundamental relationship:
lease time = floor( desired lifetime, acked-partner-lifetime + MCLT )

Initial conditions: MCLT = 1h, desired lifetime = 3d

DHCPv6 Primary Secondary
time Client Server Server

Figure 1: MCLT Example

5. Message and Option Definitions

5.1. Message Framing for TCP

Failover communication is conducted over a TCP connection established
between the partners. The failover protocol uses the framing format
specified in Section 5.1 of "DHCPv6 Bulk Leasequery" [RFC5460] but
uses different message types with a different message format, as
described in Section 5.2 of this document. The TCP connection
between failover servers is made to a specific port -- the
dhcp-failover port, port 647. All information is sent over the
connection as typical DHCP messages that convey DHCP options,
following the format defined in Section 22.1 of [RFC3315].

5.2. Failover Message Format

All failover messages defined below share a common format with a
fixed-size header and a variable format area for options. All values
in the message header and in any included options are in network byte
order.

The following diagram illustrates the format (which is compatible
with the format described in Section 6 of [RFC3315]) of DHCP messages
exchanged between failover partners:

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sent-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. options .
. (variable) .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

msg-type Identifies the DHCP message type; the
available message types are listed below.

transaction-id The transaction-id for this message exchange.

sent-time The time the message was transmitted (set
as close to transmission as practical),
in seconds since midnight (UTC),
January 1, 2000, modulo 2^32. Used to
determine the time skew of the failover
partners.

options Options carried in this message. These
options are all defined in the "Option Codes"
sub-registry of the "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)"
registry. A number of existing DHCPv6
options are used, and several more are
defined in this document.

5.3. Messages

The following sections list the new message types created defined for
failover communication.

5.3.1. BNDUPD

The binding update message, BNDUPD (24), is used to send the binding
lease changes to the partner. At most one OPTION_CLIENT_DATA option
may appear in a BNDUPD message. Note that not all data in a BNDUPD
message is sent in an OPTION_CLIENT_DATA option. Information about
delegable prefixes not currently allocated to a particular client is
sent in BNDUPD messages but not within OPTION_CLIENT_DATA options.
The partner is expected to respond with a BNDREPLY message.

5.3.2. BNDREPLY

The binding acknowledgement message, BNDREPLY (25), is used for
confirmation of the received BNDUPD message. It may contain a
positive or negative response (e.g., due to a detected lease
conflict).

5.3.3. POOLREQ

The pool request message, POOLREQ (26), is used by the secondary
server to request allocation of delegable prefixes from the primary
server. The primary responds with a POOLRESP message.

5.3.4. POOLRESP

The pool response message, POOLRESP (27), is used by the primary
server to indicate that it has received the secondary server's
request to ensure that delegable prefixes are balanced between the
primary and secondary servers. It does not indicate that all of the
BNDUPD messages that might be created from any rebalancing have been
sent or responded to; it only indicates reception and acceptance of
the task of ensuring that the balance is examined and corrected as
necessary.

5.3.5. UPDREQ

The update request message, UPDREQ (28), is used by one server to
request that its partner send all binding database changes that have
not yet been confirmed. The partner is expected to respond with zero
or more BNDUPD messages, followed by an UPDDONE message that signals
that all of the BNDUPD messages have been sent and a corresponding
BNDREPLY message has been received for each of them.

5.3.6. UPDREQALL

The update request all message, UPDREQALL (29), is used by one server
to request that all binding database information present in the other
server be sent to the requesting server, in order for the requesting
server to recover from a total loss of its binding database. A
server receiving this request responds with zero or more BNDUPD
messages, followed by an UPDDONE message that signals that all of the
BNDUPD messages have been sent and a corresponding BNDREPLY message
has been received for each of them.

5.3.7. UPDDONE

The update done message, UPDDONE (30), is used by the server
responding to an UPDREQ or UPDREQALL message to indicate that all
requested updates have been sent by the responding server and acked
by the requesting server.

5.3.8. CONNECT

The connect message, CONNECT (31), is used by the primary server to
establish a failover connection with the secondary server and to
transmit several important configuration attributes between the
servers. The partner is expected to confirm by responding with a
CONNECTREPLY message.

5.3.9. CONNECTREPLY

The connect acknowledgement message, CONNECTREPLY (32), is used by
the secondary server to respond to a CONNECT message from the primary
server.

5.3.10. DISCONNECT

The disconnect message, DISCONNECT (33), is used by either server
when closing a connection and shutting down. No response is required
for this message. The DISCONNECT message SHOULD contain an
OPTION_STATUS_CODE option with an appropriate status. Often, this
will be ServerShuttingDown. See Section 5.6. A server SHOULD
include a descriptive message as to what caused the disconnect
message.

5.3.11. STATE

The state message, STATE (34), is used by either server to inform its
partner about a change of failover state. In some cases, it may be
used to also inform the partner about the current state, e.g., after
connection is established in the COMMUNICATIONS-INTERRUPTED or
PARTNER-DOWN states.

5.3.12. CONTACT

The contact message, CONTACT (35), is used by either server to ensure
that its partner continues to see the connection as operational. It
MUST be transmitted periodically over every established connection if
other message traffic is not flowing, and it MAY be sent at any time.
See Section 6.5.

5.4. Transaction-id

The initiator of a message exchange MUST set the transaction-id (see
Section 5.2). This means that all of the messages above except
BNDREPLY, POOLRESP, UPDDONE, and CONNECTREPLY must set the
transaction-id. The transaction-id MUST be unique among all
currently outstanding messages sent to the failover partner. A
straightforward way to ensure this is to simply use an incrementing
value, with one caveat: The UPDREQ and UPDREQALL messages may be
separated by a considerable time prior to the receipt of an UPDDONE
message. While the usual pattern of message exchange would have the
partner doing the vast majority of message initiation, it is remotely
possible that the partner that initiated the UPDREQ or UPDREQALL
messages might also send enough messages to wrap the 24-bit
transaction-id and duplicate the transaction-id of the outstanding
UPDREQ or UPDREQALL messages. Thus, it is important to ensure that
the transaction-id of a currently outstanding UPDREQ or UPDREQALL
message is not replicated in any message initiated prior to the
receipt of the corresponding UPDDONE message.

5.5. Options

The following new options are defined.

5.5.1. OPTION_F_BINDING_STATUS

The binding-status is an implementation-independent representation of
the status (or the state) of a lease on an IPv6 address or prefix.

This is an unsigned byte.

The code for this option is 114.

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_F_BINDING_STATUS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| binding-status|
+-+-+-+-+-+-+-+-+

option-code OPTION_F_BINDING_STATUS (114)
option-len 1
binding-status The binding-status. See below:

Value binding-status
----- --------------
0 reserved
1 ACTIVE
2 EXPIRED
3 RELEASED
4 PENDING-FREE
5 FREE
6 FREE-BACKUP
7 ABANDONED
8 RESET

The binding-status values are discussed in Section 7.2.

5.5.2. OPTION_F_CONNECT_FLAGS

This option provides flags that indicate attributes of the connecting
server.

This option consists of an unsigned 16-bit integer in network byte
order.

The code for this option is 115.

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_F_CONNECT_FLAGS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_CONNECT_FLAGS (115)
option-len 2
flags flag bits. See below:

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

The bits (numbered from the most significant bit in network
byte order) are used as follows:

0-14: MBZ
Must be zero.
15 (F): FIXED_PD_LENGTH
Set to 1 to indicate that all prefixes delegated from a
given delegable prefix have the same prefix length (size).
If this is not set, the prefixes delegated from one
delegable prefix may have different sizes.

If the FIXED_PD_LENGTH bit is not set, it indicates that prefixes of
a range of sizes can be delegated from a given delegable prefix.
Note that if the FIXED_PD_LENGTH bit is set, each delegable prefix
may have its own fixed size -- this flag does not imply that all
prefixes delegated will be the same size, but rather that all
prefixes delegated from the same delegable prefix will be the same
size.

If the FIXED_PD_LENGTH bit is set, the length used for each prefix is
specified independently of the failover protocol but must be known to
both failover partners. It might be specified in the configuration
for each delegable prefix, or it might be fixed for the entire
server.

5.5.3. OPTION_F_DNS_REMOVAL_INFO

This option contains the information necessary to remove a DNS name
that was entered by the failover partner.

The code for this option is 116.

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_F_DNS_REMOVAL_INFO | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| encapsulated-options |
| (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_DNS_REMOVAL_INFO (116)
option-len variable
options Three possible encapsulated options:
OPTION_F_DNS_HOST_NAME
OPTION_F_DNS_ZONE_NAME
OPTION_F_DNS_FLAGS

5.5.3.1. OPTION_F_DNS_HOST_NAME

This option contains the hostname that was entered into the DNS by
the failover partner.

This is a DNS name encoded using the format specified in [RFC1035],
as also specified in Section 8 of [RFC3315].

The code for this option is 117.

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_F_DNS_HOST_NAME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. .
. host-name .
. (variable) .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_DNS_HOST_NAME (117)
option-len length of host-name
host-name hostname encoded per RFC 1035

5.5.3.2. OPTION_F_DNS_ZONE_NAME

This option contains the zone name that was entered into the DNS by
the failover partner.

This is a DNS name encoded using the format specified in [RFC1035],
as also specified in Section 8 of [RFC3315].

The code for this option is 118.

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_F_DNS_ZONE_NAME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. .
. zone-name .
. (variable) .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_DNS_ZONE_NAME (118)
option-len length of zone-name
zone-name zone name encoded per RFC 1035

5.5.3.3. OPTION_F_DNS_FLAGS

This option provides flags that indicate what needs to be done to
remove this DNS name.

This option consists of an unsigned 16-bit integer in network byte
order.

The code for this option is 119.

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_F_DNS_FLAGS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_DNS_FLAGS (119)
option-len 2
flags flag bits. See below:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ |U|S|R|F|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The bits (numbered from the most significant bit in network
byte order) are used as follows:

0-11: MBZ
Must be zero.
12 (U): USING_REQUESTED_FQDN
Set to 1 to indicate that the name used came from the
Fully Qualified Domain Name (FQDN) that was received
from the client.
13 (S): SYNTHESIZED_NAME
Set to 1 to indicate that the name was synthesized
based on some algorithm.
14 (R): REV_UPTODATE
Set to 1 to indicate that the reverse zone is up to date.
15 (F): FWD_UPTODATE
Set to 1 to indicate that the forward zone is up to date.

If both the U bit and the S bit are unset, then the name must have
been provided from some alternative configuration, such as client
registration in some database accessible to the DHCP server.

5.5.4. OPTION_F_EXPIRATION_TIME

This option specifies the greatest lifetime that this server has ever
acked to its partner in a BNDREPLY message for a particular lease or
prefix. This MUST be an absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 120.

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_F_EXPIRATION_TIME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| expiration-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_EXPIRATION_TIME (120)
option-len 4
expiration-time The expiration time. This MUST be an
absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.5.5. OPTION_F_MAX_UNACKED_BNDUPD

This option specifies the maximum number of BNDUPD messages that this
server is prepared to accept over the TCP connection without causing
the TCP connection to block.

This is an unsigned 32-bit integer in network byte order.

The code for this option is 121.

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_F_MAX_UNACKED_BNDUPD | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| max-unacked-bndupd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_MAX_UNACKED_BNDUPD (121)
option-len 4
max-unacked-bndupd Maximum number of unacked BNDUPD messages
allowed

5.5.6. OPTION_F_MCLT

The Maximum Client Lead Time (MCLT) is the upper bound on the
difference allowed between the valid lifetime provided to a DHCP
client by a server and the valid lifetime known by that server's
failover partner. It is an interval, measured in seconds. See
Section 4.4.

This is an unsigned 32-bit integer in network byte order.

The code for this option is 122.

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_F_MCLT | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| mclt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_MCLT (122)
option-len 4
mclt The MCLT, in seconds

5.5.7. OPTION_F_PARTNER_LIFETIME

This option specifies the time after which the partner can consider
an IPv6 address expired and is able to reuse the IPv6 address. This
MUST be an absolute time (i.e., seconds since midnight January 1,
2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 123.

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_F_PARTNER_LIFETIME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| partner-lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_PARTNER_LIFETIME (123)
option-len 4
partner-lifetime The partner lifetime. This MUST be an
absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.5.8. OPTION_F_PARTNER_LIFETIME_SENT

This option indicates the time that was received in an
OPTION_F_PARTNER_LIFETIME option (Section 5.5.7). This is an exact
duplicate (echo) of the time received in the
OPTION_F_PARTNER_LIFETIME option; it is not adjusted in any way.
This MUST be an absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 124.

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_F_PARTNER_LIFETIME_SENT | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| partner-lifetime-sent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_PARTNER_LIFETIME_SENT (124)
option-len 4
partner-lifetime-sent The partner-lifetime received in an
OPTION_F_PARTNER_LIFETIME option.
This MUST be an absolute time
(i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.5.9. OPTION_F_PARTNER_DOWN_TIME

This option specifies the time that the server most recently lost
communications with its failover partner. This MUST be an absolute
time (i.e., seconds since midnight January 1, 2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 125.

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_F_PARTNER_DOWN_TIME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| partner-down-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_PARTNER_DOWN_TIME (125)
option-len 4
partner-down-time Contains the PARTNER-DOWN time. This MUST be
an absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.5.10. OPTION_F_PARTNER_RAW_CLT_TIME

This option specifies the time when the partner most recently
interacted with the DHCP client associated with this IPv6 address or
prefix. This MUST be an absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 126.

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_F_PARTNER_RAW_CLT_TIME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| partner-raw-clt-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_PARTNER_RAW_CLT_TIME (126)
option-len 4
partner-raw-clt-time Contains the partner-raw-clt-time.
This MUST be an absolute time
(i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.5.11. OPTION_F_PROTOCOL_VERSION

The protocol version allows one failover partner to determine the
version of the protocol being used by the other partner, to allow for
changes and upgrades in the future. Two components are provided, to
allow large and small changes to be represented in one 32-bit number.
The intent is that large changes would result in an increment of the
value of major-version, while small changes would result in an
increment of the value of minor-version. As subsequent updates and
extensions of this document can define changes to these values in any
way deemed appropriate, no attempt is made to further define "large"
and "small" in this document.

This option consists of two unsigned 16-bit integers in network byte
order.

The code for this option is 127.

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_F_PROTOCOL_VERSION | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| major-version | minor-version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_PROTOCOL_VERSION (127)
option-len 4
major-version The major version of the protocol. Initially 1.
minor-version The minor version of the protocol. Initially 0.

5.5.12. OPTION_F_KEEPALIVE_TIME

This option specifies the number of seconds (an interval) within
which the server must receive a message from its partner, or it will
assume that communications from the partner are not "OK".

This is an unsigned 32-bit integer in network byte order.

The code for this option is 128.

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_F_KEEPALIVE_TIME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| keepalive-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_KEEPALIVE_TIME (128)
option-len 4
receive-time The keepalive-time. An interval of seconds.

5.5.13. OPTION_F_RECONFIGURE_DATA

This option contains the information necessary for one failover
partner to use the reconfigure-key created on the other failover
partner.

The code for this option is 129.

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_F_RECONFIGURE_DATA | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reconfigure-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. .
. reconfigure-key .
. (variable) .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_RECONFIGURE_DATA (129)
option-len 4 + length of reconfigure-key
reconfigure-time Time at which reconfigure-key was created.
This MUST be an absolute time
(i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).
reconfigure-key The reconfigure key

5.5.14. OPTION_F_RELATIONSHIP_NAME

This option specifies a name for this failover relationship. It is
used to distinguish between relationships when there are multiple
failover relationships between two failover servers.

This is a UTF-8 encoded text string suitable for display to an end
user. It MUST NOT be null-terminated.

The code for this option is 130.

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_F_RELATIONSHIP_NAME | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. .
. relationship-name .
. (variable) .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_RELATIONSHIP_NAME (130)
option-len length of relationship-name
relationship-name A UTF-8 encoded text string suitable for
display to an end user. MUST NOT be
null-terminated.

5.5.15. OPTION_F_SERVER_FLAGS

The OPTION_F_SERVER_FLAGS option specifies information associated
with the failover endpoint sending the option.

This is an unsigned byte.

The code for this option is 131.

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_F_SERVER_FLAGS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server-flags |
+-+-+-+-+-+-+-+-+

option-code OPTION_F_SERVER_FLAGS (131)
option-len 1
server-flags The server flags. See below:

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |A|S|C|
+-+-+-+-+-+-+-+-+

The bits (numbered from the most significant bit in network
byte order) are used as follows:

0-4: MBZ
Must be zero.
5 (A): ACK_STARTUP
Set to 1 to indicate that the OPTION_F_SERVER_FLAGS option
that was most recently received contained the
STARTUP bit set.
6 (S): STARTUP
MUST be set to 1 whenever the server is in STARTUP state.
7 (C): COMMUNICATED
Set to 1 to indicate that the sending server has
communicated with its partner.

5.5.16. OPTION_F_SERVER_STATE

The OPTION_F_SERVER_STATE option specifies the endpoint state of the
server sending the option.

This is an unsigned byte.

The code for this option is 132.

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_F_SERVER_STATE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server-state |
+-+-+-+-+-+-+-+-+

option-code OPTION_F_SERVER_STATE (132)
option-len 1
server-state Failover endpoint state
Value Server State
----- -------------------------------------------------------------
0 reserved
1 STARTUP Startup state (1)
2 NORMAL Normal state
3 COMMUNICATIONS-INTERRUPTED Communications interrupted
4 PARTNER-DOWN Partner down
5 POTENTIAL-CONFLICT Synchronizing
6 RECOVER Recovering bindings from partner
7 RECOVER-WAIT Waiting out MCLT after RECOVER
8 RECOVER-DONE Interlock state prior to NORMAL
9 RESOLUTION-INTERRUPTED Comm. failed during resolution
10 CONFLICT-DONE Primary resolved its conflicts

These states are discussed in detail in Section 8.

(1) The STARTUP state is never sent to the partner server; it is
indicated by the STARTUP bit in the OPTION_F_SERVER_FLAGS option
(see Section 8.3).

5.5.17. OPTION_F_START_TIME_OF_STATE

The OPTION_F_START_TIME_OF_STATE option specifies the time at which
the associated state began to hold its current value. When this
option appears in a STATE message, the state to which it refers is
the server endpoint state. When it appears in an IA_NA-options,
IA_TA-options, or IA_PD-options field, the state to which it refers
is the binding-status value in the OPTION_IA_NA, OPTION_IA_TA, or
OPTION_IA_PD option, respectively. This MUST be an absolute time
(i.e., seconds since midnight January 1, 2000 UTC, modulo 2^32).

This is an unsigned 32-bit integer in network byte order.

The code for this option is 133.

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_F_START_TIME_OF_STATE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| start-time-of-state |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_START_TIME_OF_STATE (133)
option-len 4
start-time-of-state The start time of the current failover state.
This MUST be an absolute time (i.e., seconds
since midnight January 1, 2000 UTC,
modulo 2^32).

5.5.18. OPTION_F_STATE_EXPIRATION_TIME

The OPTION_F_STATE_EXPIRATION_TIME option specifies the time at which
the current state of this lease will expire. This MUST be an
absolute time (i.e., seconds since midnight January 1, 2000 UTC,
modulo 2^32).

Note that states other than ACTIVE may have a time associated with
them. In particular, EXPIRED might have a time associated with it,
in the event that some sort of "grace period" existed where the lease
would not be reused for a period after the lease expired. The
ABANDONED state might have a time associated with it, in the event
that the servers participating in failover had a time after which an
ABANDONED lease might be placed back into a pool for allocation to a
client. In general, if there is an OPTION_STATE_EXPIRATION_TIME
associated with a particular state, that indicates that the
associated state will expire and move to a different state at that
time.

This is an unsigned 32-bit integer in network byte order.

The code for this option is 134.

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_F_STATE_EXPIRATION_TIME| option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| state-expiration-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code OPTION_F_STATE_EXPIRATION_TIME (134)
option-len 4
state-expiration-time The time at which the current state of the
lease will expire. This MUST be an
absolute time (i.e., seconds since midnight
January 1, 2000 UTC, modulo 2^32).

5.6. Status Codes

The following new status codes are defined to be used in the
OPTION_STATUS_CODE option.

AddressInUse (16)
One client on one server has leases that are in conflict with the
leases that the client has on another server. Alternatively, the
address could be associated with a different Identity Association
Identifier (IAID) on each server.

ConfigurationConflict (17)
The configuration implied by the information in a BNDUPD message
(e.g., the IPv6 address or prefix address) is in direct conflict
with the information known to the receiving server.

MissingBindingInformation (18)
There is insufficient information in a BNDUPD message to
effectively process it.

OutdatedBindingInformation (19)
The information in a server's binding database conflicts with the
information found in an incoming BNDUPD message and the server
believes that the information in its binding database more
accurately reflects reality.

ServerShuttingDown (20)
The server is undergoing an operator-directed or otherwise planned
shutdown.

DNSUpdateNotSupported (21)
A server receives a BNDUPD message with DNS update information
included and the server doesn't support DNS update.

ExcessiveTimeSkew (22)
A server detects that the time skew between its current time and
its partner's current time is greater than 5 seconds.

6. Connection Management

Communication between failover partners takes place over a long-lived
TCP connection. This connection is always initiated by the primary
server, and if the long-lived connection is lost it is the
responsibility of the primary server to attempt to reconnect to the
secondary server. The detailed process used by the primary server
when initiating a connection and by the secondary server when
responding to a connection attempt as documented in Section 6.1 is
followed each time a connection is established, regardless of any
previous connection between the failover partners.

6.1. Creating Connections

Every primary server implementing the failover protocol MUST
periodically attempt to create a TCP connection to the dhcp-failover
port (647) of all of its configured partners, where the period is
implementation dependent and SHOULD be configurable. In the event
that a connection has been rejected by a CONNECTREPLY message with an
OPTION_STATUS_CODE option contained in it or a DISCONNECT message, a
server SHOULD reduce the frequency with which it attempts to connect
to that server, but it MUST continue to attempt to connect
periodically.

Every secondary server implementing the failover protocol MUST listen
for TCP connection attempts on the dhcp-failover port (647) from a
primary server.

After a primary server successfully establishes a TCP connection to a
secondary server, it MUST continue the connection process as
described in Section 8.2 of [RFC7653]. In the language of that
section, the primary failover server operates as the "requestor" and
the secondary failover server operates as the "DHCP server". The
message that is sent over the newly established connection is a
CONNECT message, instead of an ACTIVELEASEQUERY message.

When a secondary server receives a connection attempt, the only
information that the secondary server has is the IP address of the
partner initiating a connection. If it has any relationships with
the connecting server for which it is a secondary server, it should
operate as described in Section 9.1 of [RFC7653], with the exception
that instead of waiting for an Active Leasequery message it will wait
for a CONNECT message. Once it has received the CONNECT message, it
will use the information in that message to determine which
relationship this connection is to service.

If it has no secondary relationships with the connecting server, it
MUST drop the connection.

To summarize -- a primary server MUST use a connection that it has
initiated in order to send a CONNECT message. Every server that is a
secondary server in a relationship MUST listen for CONNECT messages
from the primary server.

When the CONNECT and CONNECTREPLY exchange successfully produces a
working failover connection, the next message sent over a new
connection is a STATE message. See Section 6.3. Upon the receipt of
the STATE message, the receiver can consider communications "OK".

6.1.1. Sending a CONNECT Message

The CONNECT message is sent with information about the failover
configuration on the primary server. The message MUST contain at
least the following information in the options area:

o OPTION_F_PROTOCOL_VERSION containing the protocol version that the
primary server will use when sending failover messages.

o OPTION_F_MCLT containing the configured MCLT.

o OPTION_F_KEEPALIVE_TIME containing the number of seconds (an
interval) within which the server must receive a message from its
partner, or it will assume that communications from the partner
are not "OK".

o OPTION_F_MAX_UNACKED_BNDUPD containing the maximum number of
BNDUPD messages that this server is prepared to accept over the
failover connection without causing the connection to block. This
implements application-level flow control over the connection, so
that a flood of BNDUPD messages does not cause the connection to
block and thereby prevent other messages from being transmitted
over the connection and received by the failover partner.

o OPTION_F_RELATIONSHIP_NAME containing the name of the failover
relationship to which this connection applies. If there is no
OPTION_F_RELATIONSHIP_NAME in the CONNECT message, it indicates
that there is only a single relationship between this pair of
primary and secondary servers.

o OPTION_F_CONNECT_FLAGS containing information about certain
attributes of the connecting servers.

6.1.2. Receiving a CONNECT Message

A server receiving a CONNECT message must process the information in
the message and decide whether or not to accept the connection. The
processing is performed as follows:

o sent-time - The secondary server checks the sent-time to see if it
is within 5 seconds of its current time. See Section 7.1. If it
is not, return ExcessiveTimeSkew in the OPTION_STATUS_CODE to
reject the CONNECT message.

o OPTION_F_PROTOCOL_VERSION - The secondary server decides if the
protocol version of the primary server is supported by the
secondary server. If it is not, return NotSupported in the
OPTION_STATUS_CODE to reject the CONNECT message.

o OPTION_F_MCLT - Use this MCLT supplied by the primary server.
Remember this MCLT, and use it until a different MCLT is supplied
by some subsequent CONNECT message.

o OPTION_F_KEEPALIVE_TIME - Remember the keepalive-time as the
FO_KEEPALIVE_TIME (Section 6.5) when implementing the
Unreachability Detection algorithm described in Section 6.6.

o OPTION_F_MAX_UNACKED_BNDUPD - Ensure that the maximum amount of
unacked BNDUPD messages queued to the primary server never exceeds
the value in the OPTION_F_MAX_UNACKED_BNDUPD option.

o OPTION_F_CONNECT_FLAGS - Ensure that the secondary server can
process information from the primary server as specified in the
flags. For example, if the secondary server cannot process prefix
delegation with variable-sized prefixes delegated from the same
delegable prefix and the primary server says that it can, the
secondary should reject the connection.

A CONNECT message SHOULD always be followed by a CONNECTREPLY
message, to either (1) accept the connection or (2) reject the
connection by including an OPTION_STATUS_CODE option with a
status-code indicating the reason for the rejection. If accepting
the connection attempt, then send a CONNECTREPLY message with the
following information:

o OPTION_F_PROTOCOL_VERSION containing the protocol version being
used by the secondary server when sending failover messages.

o OPTION_F_MCLT containing the MCLT currently in use on the
secondary server. This MUST equal the MCLT that was in the
OPTION_F_MCLT option in the CONNECT message.

o OPTION_F_CONNECT_FLAGS containing information describing the
attributes of the secondary server that the primary needs to know
about.

After sending a CONNECTREPLY message to accept the primary server's
CONNECT message, the secondary server MUST send a STATE message (see
Section 6.3).

6.1.3. Receiving a CONNECTREPLY Message

A server receiving a CONNECTREPLY message must process the
information in the message and decide whether or not to continue to
employ the connection. The processing is performed as follows:

o OPTION_F_PROTOCOL_VERSION - The primary server decides if the
protocol version in use by the secondary server is supported by
the primary server. If it is not, send a DISCONNECT message and
drop the connection. If it is supported, continue processing. It
is possible that the primary and secondary servers will each be
sending different versions of the protocol to the other server.
The extent to which this is supported will be defined partly by
as-yet-unknown differences in the protocols that the versions
represent and partly by the capabilities of the two
implementations involved in the failover relationship.

o OPTION_F_MCLT - Compare the MCLT received with the configured
MCLT. If they are different, send a DISCONNECT message and drop
the connection.

o OPTION_F_KEEPALIVE_TIME - Remember the keepalive-time as the
FO_KEEPALIVE_TIME (Section 6.5) when implementing the
Unreachability Detection algorithm described in Section 6.6.

o OPTION_F_MAX_UNACKED_BNDUPD - Ensure that the maximum amount of
unacked BNDUPD messages queued to the secondary server never
exceeds the value in the OPTION_F_MAX_UNACKED_BNDUPD option.

o OPTION_F_CONNECT_FLAGS - Ensure that the primary server can
process information from the secondary server as specified in the
flags. For example, if the primary server cannot process prefix
delegation with variable-sized prefixes delegated from the same
delegable prefix and the secondary server says that it can, the
primary should drop the connection.

After receiving a CONNECTREPLY message that accepted the primary
server's CONNECT message, the primary server MUST send a STATE
message (see Section 6.3).

6.2. Endpoint Identification

A failover endpoint is always associated with a set of DHCP prefixes
that are configured on the DHCP server where the endpoint appears. A
DHCP prefix MUST NOT be associated with more than one failover
endpoint.

The failover protocol SHOULD be configured with one failover
relationship between each pair of failover servers. In this case,
there is one failover endpoint for that relationship on each failover
partner. This failover relationship MUST have a unique name.

Any failover endpoint can take actions and hold unique states.

This document frequently describes the behavior of the failover
protocol in terms of primary and secondary servers, not primary and
secondary failover endpoints. However, it is important to remember
that every "server" described in this document is in reality a
failover endpoint that resides in a particular process and that
several failover endpoints may reside in the same server process.

It is not the case that there is a unique failover endpoint for each
prefix that participates in a failover relationship. On one server,
there is (typically) one failover endpoint per partner, regardless of
how many prefixes are managed by that combination of partner and
role. On a particular server, any given prefix that participates in
failover will be associated with exactly one failover endpoint.

When a connection is received from the partner, the unique failover
endpoint to which the message is directed is determined solely by the
IPv6 address of the partner, the relationship name, and the role of
the receiving server.

6.3. Sending a STATE Message

A server MUST send a STATE message to its failover partner whenever
the state of the failover endpoint changes. Sending the occasional
duplicate STATE message will not cause any problems; note, however,
that not updating the failover partner with information about a
failover endpoint state change can, in many cases, cause the entire
failover protocol to be inoperative.

The STATE message is sent with information about the endpoint state
of the failover relationship. The STATE message MUST contain at
least the following information in the options area:

o OPTION_F_SERVER_STATE containing the state of this failover
endpoint.

o OPTION_F_SERVER_FLAGS containing the flag values associated with
this failover endpoint.

o OPTION_F_START_TIME_OF_STATE containing the time when this became
the state of this failover endpoint.

o OPTION_F_PARTNER_DOWN_TIME containing the time that this failover
endpoint went into PARTNER-DOWN state if this server is in
PARTNER-DOWN state. If this server isn't in PARTNER-DOWN state,
do not include this option.

The server sending a STATE message SHOULD ensure that this
information is written to stable storage prior to enqueuing it to its
failover partner.

6.4. Receiving a STATE Message

A server receiving a STATE message must process the information in
the message and decide how to react to the information. The
processing is performed as follows:

o OPTION_F_SERVER_STATE - If this represents a change in state for
the failover partner, react according to the instructions in
Section 8.1. If the state is not PARTNER-DOWN, clear any memory
of the partner-down-time.

o OPTION_F_SERVER_FLAGS - Remember these flags in an appropriate
data area so they can be referenced later.

o OPTION_F_START_TIME_OF_STATE - Remember this information in an
appropriate data area so it can be referenced later.

o OPTION_F_PARTNER_DOWN_TIME - If the value of the
OPTION_F_SERVER_STATE is PARTNER-DOWN, remember this information
in an appropriate data area so it can be referenced later.

A server receiving a STATE message SHOULD ensure that this
information is written to stable storage.

6.5. Connection Maintenance Parameters

The following parameters and timers are used to ensure the integrity
of the connections between two failover servers.

Parameter Default Description
---------------------------------------------------------------------
FO_KEEPALIVE_TIMER timer counts down to time
connection assumed dead
due to lack of messages

FO_KEEPALIVE_TIME 60 maximum time server will
consider connection still up
with no messages

FO_CONTACT_PER_KEEPALIVE_TIME 4 number of CONTACT messages
to send during partner's
FO_KEEPALIVE_TIME period

FO_SEND_TIMER timer counts down to time to send
next CONTACT message

FO_SEND_TIME 15 maximum time to wait between
sending CONTACT messages
if no other traffic.
Created from partner's
FO_KEEPALIVE_TIME divided by
FO_CONTACT_PER_KEEPALIVE_TIME

6.6. Unreachability Detection

Each partner MUST maintain an FO_SEND_TIMER for each failover
connection. The FO_SEND_TIMER for a particular connection is reset
to FO_SEND_TIME every time any message is transmitted on that
connection, and the timer counts down once per second. If the timer
reaches zero, a CONTACT message is transmitted on that connection and
the timer for that connection is reset to FO_SEND_TIME. The CONTACT
message may be transmitted at any time. An implementation MAY use
additional mechanisms to detect partner unreachability.

The FO_SEND_TIME is initialized from the configured FO_KEEPALIVE_TIME
divided by FO_CONTACT_PER_KEEPALIVE_TIME. When a CONNECT or
CONNECTREPLY message is received on a connection, the received
OPTION_F_KEEPALIVE_TIME option is checked, and the value in that
option is used to calculate the FO_SEND_TIME for that connection by
dividing the value received by the configured
FO_CONTACT_PER_KEEPALIVE_TIME.

Each partner MUST maintain an FO_KEEPALIVE_TIMER for each failover
connection. This timer is initialized to FO_KEEPALIVE_TIME and
counts down once per second. It is reset to FO_KEEPALIVE_TIME
whenever a message is received on that connection. If it ever
reaches zero, that connection is considered dead. In addition, the
FO_KEEPALIVE_TIME for that connection MUST be sent to the failover
partner on every CONNECT or CONNECTREPLY message in the
OPTION_F_KEEPALIVE_TIME option.

7. Binding Updates and Acks

7.1. Time Skew

Partners exchange information about known lease states. To reliably
compare a known lease state with an update received from a partner,
servers must be able to reliably compare the times stored in the
known lease state with the times received in the update. The
failover protocol adopts the simple approach of requiring that the
failover partners use some mechanism to synchronize the clocks on the
two servers to within an accuracy of roughly 5 seconds.

A mechanism to measure and track relative time differences between
servers is necessary to ensure this synchronization. To do so, each
message contains the time of the transmission in the sent-time field
of the message (see Section 5.2). The transmitting server MUST set
this as close to the actual transmission as possible. The receiving
partner MUST store its own timestamp of reception as close to the
actual reception as possible. The received timestamp information is
then compared with the local timestamp.

7.2. Information Model

In most DHCP servers, a lease on an IPv6 address or a prefix can take
on several different binding-status values, sometimes also called
"lease states". While no two DHCP server implementations will have
exactly the same possible binding-status values, [RFC3315] enforces
some commonality among the general semantics of the binding-status
values used by various DHCP server implementations.

In order to transmit binding database updates between one server and
another using the failover protocol, some common binding-status
values must be defined. It is not expected that these values
correspond to any actual implementation of DHCPv6 in a DHCP server,
but rather that the binding-status values defined in this document
should be convertible back and forth between those defined below and
those in use by many DHCP server implementations.

The lease binding-status values defined for the failover protocol are
listed below. Unless otherwise noted below, there MAY be client
information associated with each of these binding-status values.

ACTIVE - The lease is assigned to a client. Client identification
data MUST appear.

EXPIRED - This value indicates that a client's binding on a given
lease has expired. When the partner acks the BNDUPD message of an
expired lease, the server sets its internal state to PENDING-FREE.
Client identification SHOULD appear.

RELEASED - This value indicates that a client sent a RELEASE
message. When the partner acks the BNDUPD message of a released
lease, the server sets its internal state to PENDING-FREE. Client
identification SHOULD appear.

PENDING-FREE - Once a lease is expired or released, its state
becomes PENDING-FREE. Depending on which algorithm was used to
allocate a given lease, PENDING-FREE may mean either FREE or
FREE-BACKUP. Implementations do not have to implement this
PENDING-FREE state but may choose to switch to the destination
state directly. For clarity of representation, this transitional
PENDING-FREE state is treated as a separate state.

FREE - This value is used when a DHCP server needs to communicate
that a lease is unused by any client, but it was not just
released, expired, or reset by a network administrator. When the
partner acks the BNDUPD message of a FREE lease, the server marks
the lease as available for assignment by the primary server. Note
that on a secondary server running in PARTNER-DOWN state, after
waiting the MCLT, the lease MAY be allocated to a client by the
secondary server. Client identification MAY appear and indicates,
as a hint, the last client to have used this lease.

FREE-BACKUP - This value indicates that this lease can be allocated
by the secondary server to a client at any time. Note that on the
primary server running in PARTNER-DOWN state, after waiting the
MCLT, the lease MAY be allocated to a client by the primary server
if the proportional algorithm was used. Client identification MAY
appear and indicates, as a hint, the last client to have used this
lease.

ABANDONED - This value indicates that a lease is considered unusable
by the DHCP system. The primary reason for entering such a state
is the reception of a DECLINE message for the lease. Client
identification MAY appear.

RESET - This value indicates that this lease was made available by
an operator command. This is a distinct state so that the reason
that the lease became FREE can be determined. Client
identification MAY appear.

Which binding-status values are associated with a timeout is
implementation dependent. Some binding-status values, such as
ACTIVE, will have a timeout value in all implementations, while
others, such as ABANDONED, will have a timeout value in some
implementations and not in others. In some implementations, a
binding-status value may be associated with a timeout in some
circumstances and not in others. The receipt of a BNDUPD message
with a particular binding-status value and an
OPTION_F_STATE_EXPIRATION_TIME indicates that this particular
binding-status value is associated with a timeout.

The lease state machine is presented in Figure 2. Most states are
stationary, i.e., the lease stays in a given state until an external
event triggers transition to another state. The only transitive
state is PENDING-FREE. Once it is reached, the state machine
immediately transitions to either FREE or FREE-BACKUP state.

+---------+
/------------->| ACTIVE |<--------------\
| +---------+ |
| | | | |
| /--(8)--/ (3) \--(9)-\ |
| | | | |
| V V V |
| +-------+ +--------+ +---------+ |
| |EXPIRED| |RELEASED| |ABANDONED| |
| +-------+ +--------+ +---------+ |
| | | | |
| | | (10) |
| | | V |
| | | +---------+ |
| | | | RESET | |
| | | +---------+ |
| | | | |
| \--(4)--\ (4) /--(4)--/ |
| | | | |
(1) V V V (2)
| /---------\ |
| | PENDING-| |
| | FREE | |
| \---------/ |
| | | |
| /-(5)--/ \-(6)-\ |
| | | |
| V V |
| +-------+ +-----------+ |
\----| FREE |<--(7)-->|FREE-BACKUP|-----/
+-------+ +-----------+

PENDING-FREE transition

Figure 2: Lease State Machine

Transitions between states will result from the following events:

(1) The primary server allocates a lease.

(2) The secondary server allocates a lease.

(3) The client sends RELEASE, and the lease is released.

(4) The partner acknowledges the state change. This transition MAY
also occur if the server is in PARTNER-DOWN state and the MCLT
has passed since the entry into RELEASED, EXPIRED, or RESET
states.

(5) The lease belongs to a pool that is governed by proportional
allocation, or independent allocation is used and this lease
belongs to the primary server's pool.

(6) The lease belongs to a pool that is governed by independent
allocation, and the lease belongs to the secondary server.

(7) A pool rebalance event occurs (POOLREQ/POOLRESP messages are
exchanged). Delegable prefixes belonging to the primary server
can be assigned to the secondary server's pool (transition from
FREE to FREE-BACKUP) or vice versa.

(8) The lease has expired.

(9) A DECLINE message is received, or a lease is deemed unusable
for other reasons.

(10) An administrative action is taken to restore an abandoned lease
to a usable state. This transition MAY occur due to
implementation-specific handling of an ABANDONED lease. One
possible example of this is a Neighbor Discovery or ICMPv6 Echo
check to see if the address is still in use.

The lease that is no longer in use (due to expiration or release)
becomes PENDING-FREE. Depending on what allocation algorithm is
used, the lease that is no longer in use returns to the primary pool
(FREE) or the secondary pool (FREE-BACKUP). The conditions for
specific transitions are depicted in Figure 3.

Figure 3: PENDING-FREE State Transitions

7.3. Times Required for Exchanging Binding Updates

Each server must keep track of the following specific times beyond
those required by the base DHCP specification [RFC3315].

expiration-time
The greatest lifetime that this server has ever acked to its
failover partner in a BNDREPLY message.

acked-partner-lifetime
The greatest lifetime that the failover partner has ever acked to
this server in a BNDREPLY message.

partner-lifetime
The time value that will be sent (or that has been sent) to the
partner to indicate the time after which the partner can consider
the lease expired. When a BNDUPD message is received, this value
can be updated from the received OPTION_F_EXPIRATION_TIME.

client-last-transaction-time
The time when this server most recently interacted with the client
associated with this lease.

partner-raw-clt-time
The time when the partner most recently interacted with the client
associated with this lease. This time remains exactly as it was
received by this server and MUST NOT be adjusted in any way.

start-time-of-state
The time when the binding-status of this lease was changed to its
current value.

state-expiration-time
The time when the current state of this lease will expire.

7.4. Sending Binding Updates

Every BNDUPD message contains information about either (1) a single
client binding in an OPTION_CLIENT_DATA option that includes IAADDR
or IAPREFIX options associated with that client or (2) a single
prefix lease in an OPTION_IAPREFIX option for prefixes that are
currently not associated with any clients.

All information about a particular client binding MUST be contained
in a single OPTION_CLIENT_DATA option (see Section 4.1.2.2 of
[RFC5007]). The OPTION_CLIENT_DATA option contains at least the data
shown below in its client-options section:

o OPTION_CLIENTID containing the DUID of the client most recently
associated with this lease MUST appear.

o OPTION_LQ_BASE_TIME containing the absolute time that the
information was placed in this OPTION_CLIENT_DATA option (see
Section 6.3.1 of [RFC7653]) MUST appear.

o OPTION_VSS (see Section 3.4 of [RFC6607]). This option MUST NOT
appear if the information in this OPTION_CLIENT_DATA option is
associated with the global, default VPN. This option MUST appear
if the information in this OPTION_CLIENT_DATA option is associated
with a VPN other than the global, default VPN. Support of
[RFC6607] is not required, and if it is not supported, then an
OPTION_VSS MUST NOT appear. If [RFC6607] is supported, then an
OPTION_VSS MUST appear if and only if a VPN other than the global,
default VPN is used.

o OPTION_F_RECONFIGURE_DATA containing the time and reconfigure key,
if any.

o OPTION_LQ_RELAY_DATA containing information described in
Section 4.1.2.4 of [RFC5007], if any exists.

o OPTION_IA_NA or OPTION_IA_TA for an IPv6 address, or OPTION_IA_PD
for an IPv6 prefix. More than one of either of these options MAY
appear if more than one of them are associated with this client.
At least one of an OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
must appear.

* IAID - Identity Association used by the client, while obtaining
a given lease. Note that (1) one client may use many IAIDs
simultaneously and (2) IAIDs for OPTION_IA_NA, OPTION_IA_TA,
and OPTION_IA_PD are orthogonal number spaces.

* T1 time sent to client.

* T2 time sent to client.

* Inside of the IA_NA-options, IA_TA-options, or IA_PD-options
sections:

+ OPTION_IAADDR for an IPv6 address or an OPTION_IAPREFIX for
an IPv6 prefix MUST appear.

- IPv6 address or IPv6 prefix (with length).

- Preferred lifetime sent to client.

- Valid lifetime sent to client.

- Inside of the IAaddr-options or IAprefix-options:

o OPTION_F_BINDING_STATUS containing the binding-status
MUST appear.

o OPTION_F_START_TIME_OF_STATE containing the
start-time-of-state MUST appear.

o OPTION_F_STATE_EXPIRATION_TIME (absolute) containing
the state-expiration-time*.

o OPTION_CLT_TIME (relative) containing the
client-last-transaction-time. See [RFC5007] for a
description of this option.

o OPTION_F_PARTNER_LIFETIME (absolute) containing the
partner-lifetime*.

o OPTION_F_PARTNER_RAW_CLT_TIME (absolute) containing
the partner-raw-clt-time.

o OPTION_F_EXPIRATION_TIME (absolute) containing the
expiration-time*.

o DHCP_O_CLIENT_FQDN OPTION_CLIENT_FQDN containing the FQDN information
associated with this lease and client, if any.

Information about a prefix lease is contained in a single
OPTION_IAPREFIX option. Only a single OPTION_IAPREFIX option may
appear in a BNDUPD message outside of an OPTION_CLIENT_DATA option.
In detail:

o OPTION_IAPREFIX for a prefix lease.

* IPv6 prefix (with length).

* Inside of the IAprefix-options section:

+ OPTION_VSS (see Section 3.4 of [RFC6607]). This option
MUST NOT appear if the information in this OPTION_IAPREFIX
option is associated with the global, default VPN. This
option MUST appear if the information in this
OPTION_IAPREFIX option is associated with a VPN other than
the global, default VPN. Support of [RFC6607] is not
required, and if it is not supported, then an OPTION_VSS
MUST NOT appear. If [RFC6607] is supported, then an
OPTION_VSS MUST appear if and only if a VPN other than the
global, default VPN is used.

+ OPTION_LQ_BASE_TIME containing the absolute time that this
information was placed in this OPTIONS_IAPREFIX option (see
Section 6.3.1 of [RFC7653]) MUST appear.

+ OPTION_F_BINDING_STATUS containing the binding-status MUST
appear.

+ OPTION_F_START_TIME_OF_STATE containing the
start-time-of-state MUST appear.

+ OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time*.

+ OPTION_F_PARTNER_LIFETIME (absolute) containing the
partner-lifetime*.

+ OPTION_F_EXPIRATION_TIME (absolute) containing the
expiration-time*.

Items marked with a single asterisk (*) MUST appear only if the value
in the OPTION_F_BINDING_STATUS is associated with a timeout;
otherwise, it MUST NOT appear. See Section 7.2 for details.

The OPTION_CLT_TIME MUST, if it appears, be the time that the server
last interacted with the DHCP client. It MUST NOT be, for instance,
the time that the lease expired if there has been no interaction with
the DHCP client in question.

A server SHOULD be prepared to clean up DNS information once the
lease expires or is released. See Section 9 for a detailed
discussion about DNS update. Another reason the partner may be
interested in keeping additional data is to enable better support for
Leasequery [RFC5007], Bulk Leasequery [RFC5460], or Active Leasequery
[RFC7653], some of which feature queries based on Relay-ID, link
address, or Remote-ID.

7.5. Receiving Binding Updates

7.5.1. Monitoring Time Skew

The sent-time from the failover message is compared with the current
time of the receiving server as recorded when it received the
message. The difference is noted, and if it is greater than
5 seconds the receiving server SHOULD drop the connection. A message
SHOULD be logged to signal the reason for the connection being
dropped.

Any time can be before, after, or essentially the same as another
time. Any time that ends up being +/- 5 seconds of another time
SHOULD be considered to be representing the same time when performing
a comparison between two times.

7.5.2. Acknowledging Reception

Upon acceptance of a binding update, the server MUST notify its
partner that it has processed the binding update (and updated its
lease state database if necessary) by sending a BNDREPLY message. A
server MUST NOT send the BNDREPLY message before its binding database
is updated.

7.5.3. Processing Binding Updates

When a BNDUPD message is received, it MUST contain either a single
OPTION_CLIENT_DATA option or a single OPTION_IAPREFIX option.

When analyzing a BNDUPD message from a partner server, if there is
insufficient information in the BNDUPD message to process it, then it
is rejected with an OPTION_STATUS_CODE of
"MissingBindingInformation".

The server receiving a BNDUPD message from its partner must evaluate
the received information in each OPTION_CLIENT_DATA or IAPREFIX
option to see if it is consistent with the server's already-known
state and, if it is not, decide to accept or reject the information.
Section 7.5.4 provides details regarding how the server makes this
determination.

A server receiving a BNDUPD message MUST respond to the sender of
that message with a BNDREPLY message that contains the same
transaction-id as the BNDUPD message. This BNDREPLY message MUST
contain either a single OPTION_CLIENT_DATA option or a single
OPTION_IAPREFIX option, corresponding to whatever was received in the
BNDUPD message.

An OPTION_CLIENT_DATA option or an OPTION_IAPREFIX option in the
BNDREPLY message that is accepted SHOULD NOT contain an
OPTION_STATUS_CODE unless a status message needs to be sent to the
failover partner, in which case it SHOULD include an
OPTION_STATUS_CODE option with a status-code indicating success and
whatever message is needed.

To indicate rejection of the information in an OPTION_CLIENT_DATA
option or an OPTION_IAPREFIX option, an OPTION_STATUS_CODE SHOULD be
included with a status-code indicating an error in the
OPTION_CLIENT_DATA option or OPTION_IAPREFIX option in the BNDREPLY
message.

7.5.4. Accept or Reject?

The first task in processing the information in an OPTION_CLIENT_DATA
option or OPTION_IAPREFIX option is to extract the client information
(if any) and lease information out of the option and to access the
address lease or prefix lease information in the server's binding
database.

If an OPTION_VSS option is specified in the OPTION_CLIENT_DATA option
or OPTION_IAPREFIX option and the VPN specified in the OPTION_VSS
option does not appear in the configuration of the receiving server,
then reject the entire OPTION_CLIENT_DATA option or OPTION_IAPREFIX
option by including an OPTION_STATUS_CODE option with a status-code
of "ConfigurationConflict".

If the lease specified in the OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option is not a lease associated with the failover
endpoint that received the OPTION_CLIENT_DATA option, then reject it
by including an OPTION_STATUS_CODE option with a status-code of
"ConfigurationConflict".

In general, acceptance or rejection is based on the comparison of two
different time values -- one from the OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option in the BNDUPD message, and one from the
receiving server's binding database associated with the address or
prefix lease found in the BNDUPD message. The time for the BNDUPD
message where the OPTION_F_BINDING_STATUS is ACTIVE, EXPIRED, or
RELEASED is the OPTION_CLT_TIME if one appears, or the
OPTION_F_START_TIME_OF_STATE if one does not. For other
binding-status values, the time for the BNDUPD message is the
later of (1) the OPTION_CLT_TIME if one appears or (2) the
OPTION_F_START_TIME_OF_STATE. The time for the lease in the server's
binding database is the client-last-transaction-time if one appears,
or the start-time-of-state if one does not.

The basic approach is to compare these times, and if the one from the
BNDUPD message is clearly later, then accept the information in the
OPTION_CLIENT_DATA option or OPTION_IAPREFIX option. If the one from
the server's binding database is clearly later, then reject the
information in the BNDUPD message. The challenge comes when they are
essentially the same (i.e., +/- 5 seconds). In this case, they are
considered identical, despite the minor differences. Figure 4 shows
a table containing the rules for dealing with all of these
situations.

binding-status in received OPTION_CLIENT_DATA
or OPTION_IAPREFIX
binding-status in
receiving server's FREE RESET
lease state DB ACTIVE EXPIRED RELEASED FREE-BACKUP ABANDONED
---------------------------------------------------------------------
ACTIVE accept(3) time(1) accept time(1) accept
EXPIRED accept accept accept accept accept
RELEASED accept accept accept accept accept
FREE/FREE-BACKUP accept accept accept accept accept
RESET time(2) accept accept accept accept
ABANDONED accept accept accept accept accept

Figure 4: Conflict Resolution

accept: If the time value in the OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option is later than the time value in the
server's binding database, accept it, else reject it.

time(1): If the current time is later than the receiving server's
state-expiration-time, accept it, else reject it.

time(2): If the OPTION_CLT_TIME value (if it appears) in the
OPTION_CLIENT_DATA is later than the start-time-of-state in the
receiving server's binding, accept it, else reject it.

accept,time(1),time(2): If rejecting, use a status-code of
"OutdatedBindingInformation".

accept(3): If the clients in an OPTION_CLIENT_DATA option and in a
receiving server's binding differ, then if time(2) or the
receiving server is a secondary accept it, else reject it with a
status-code of "AddressInUse". If the clients match, accept the
update.

The lease update may be accepted or rejected. If a lease is rejected
with "OutdatedBindingInformation", then the flag in the lease that
indicates that the partner should be updated with the information in
this lease SHOULD be set; otherwise, it SHOULD NOT be changed. If
this flag was previously not set, then an update MAY be transmitted
immediately to the partner (though the BNDREPLY to this BNDUPD
message SHOULD be sent first). If this flag was previously set, an
update SHOULD NOT be transmitted immediately to the partner. In this
case, an update will be sent during the next periodic scan, but not
immediately, thus preventing a possible update storm should the
servers be unable to agree. Ultimately, the server with the most
recent binding information should have its update accepted by its
partner.

7.5.5. Accepting Updates

When the information in an OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option has been accepted, some of that information is
stored in the receiving server's binding database, and a
corresponding OPTION_CLIENT_DATA option or OPTION_IAPREFIX option is
entered into a BNDREPLY message. The information to enter into the
OPTION_CLIENT_DATA option or OPTION_IAPREFIX option in the BNDREPLY
message is described in Section 7.6.

The information contained in an accepted OPTION_CLIENT_DATA option is
stored in the receiving server's binding database as follows:

1. The OPTION_CLIENTID is used to find the client.

2. The other data contained in the top level of the
OPTION_CLIENT_DATA option is stored with the client as
appropriate.

3. For each of the OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
options in the OPTION_CLIENT_DATA option and for each of the
OPTION_IAADDR or OPTION_IAPREFIX options in the IA_* options:

a. OPTION_F_BINDING_STATUS is stored as the binding-status.

b. OPTION_F_PARTNER_LIFETIME is stored in the expiration-time.

c. OPTION_F_STATE_EXPIRATION_TIME is stored in the
state-expiration-time.

d. OPTION_CLT_TIME [RFC5007] is stored in the
partner-raw-clt-time.

e. OPTION_F_PARTNER_RAW_CLT_TIME replaces the
client-last-transaction-time if it is later than the current
client-last-transaction-time.

f. OPTION_F_EXPIRATION_TIME replaces the partner-lifetime if it
is later than the current partner-lifetime.

The information contained in an accepted single OPTION_IAPREFIX
option that is not contained in an OPTION_CLIENT_DATA option is
stored in the receiving server's binding database as follows:

1. The IPv6 prefix is used to find the prefix.

2. Inside of the IAprefix-options section:

a. OPTION_F_BINDING_STATUS is stored as the binding-status.

b. OPTION_F_PARTNER_LIFETIME (if any) is stored in the
expiration-time.

c. OPTION_F_STATE_EXPIRATION_TIME (if any) is stored in the
state-expiration-time.

d. OPTION_F_EXPIRATION_TIME (if any) replaces the
partner-lifetime if it is later than the current
partner-lifetime.

7.6. Sending Binding Replies

A server MUST respond to every BNDUPD message with a BNDREPLY
message. The BNDREPLY message MUST contain an OPTION_CLIENT_DATA
option if the BNDUPD message contained an OPTION_CLIENT_DATA option,
or it MUST contain an OPTION_IAPREFIX option if the BNDUPD message
contained an OPTION_IAPREFIX option. The BNDREPLY message MUST have
the same transaction-id as the BNDUPD message to which it is a
response.

Acceptance or rejection of all of or a particular part of the BNDUPD
message is signaled with an OPTION_STATUS_CODE option. An
OPTION_STATUS_CODE option containing a status-code representing an
error is significant, while an OPTION_STATUS_CODE option whose
status-code contains success is considered informational but does not
affect the processing of the BNDREPLY message when it is received by
the server that sent the BNDUPD message.

Rejection of all of or part of the information in a BNDUPD message is
signaled in a BNDREPLY message by using the OPTION_STATUS_CODE
message with an error in the status-code field. This rejection can
take place at either of two levels -- the top level of the option
hierarchy or the bottom level of the option hierarchy:

1. Entire BNDUPD: The OPTION_STATUS_CODE containing an error is
present in the outermost option of the BNDREPLY message -- either
the single OPTION_CLIENT_DATA option or the single
OPTION_IAPREFIX option. An example of this sort of error might
be that an OPTION_VSS option was present and specified a VPN that
might not exist in the receiving server.

2. Single address or prefix: The OPTION_STATUS_CODE containing an
error is present in a single IAADDR or IAPREFIX option that is
itself contained in an OPTION_IA_NA, OPTION_IA_TA, or
OPTION_IA_PD option. An example of this sort of error might be
that a particular IPv6 address was specified in an IAADDR option
that doesn't appear in the receiving server's configuration.

Rejection occurring at either of these levels indicates rejection of
all of the information contained in the option (including any other
options contained in that option) where the OPTION_STATUS_CODE option
containing an error appears. The converse is not true -- an
OPTION_STATUS_CODE option containing success does not signify that
all of the contained information has been accepted.

If the BNDREPLY message contains an OPTION_CLIENT_DATA option, then
the OPTION_CLIENT_DATA option MUST contain at least the data shown
below in its client-options section:

o OPTION_CLIENTID containing the DUID of the client most recently
associated with this IPv6 address*. address.

o OPTION_VSS from the BNDUPD message, if any.

o OPTION_IA_NA or OPTION_IA_TA for an IPv6 address or OPTION_IA_PD
for an IPv6 prefix. More than one of either of these options MAY
appear if there are more than one of them associated with this
client.

* Inside of the IA_NA-options, IA_TA-options, or IA_PD-options
sections:

+ OPTION_IAADDR for an IPv6 address or an OPTION_IAPREFIX for
an IPv6 prefix.

- IPv6 address or IPv6 prefix (with length).

- Inside of the IAaddr-options or IAprefix-options:

o OPTION_STATUS_CODE containing an error code, or
containing a success code if a message is required.
An OPTION_STATUS_CODE option SHOULD NOT appear with a
success code unless a message associated with the
success code needs to be included. The lack of an
OPTION_STATUS_CODE option is an indication of success.

o OPTION_F_BINDING_STATUS containing the binding-status
received in the BNDUPD message.

o OPTION_F_STATE_EXPIRATION_TIME (absolute) containing
the state-expiration-time received in the BNDUPD
message.

o OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a
duplicate of the OPTION_F_PARTNER_LIFETIME received in
the BNDUPD message.

If the BNDREPLY message contains a single OPTION_IAPREFIX option not
contained in an OPTION_CLIENT_DATA option, then the OPTION_IAPREFIX
option MUST contain at least the data shown below:

o IPv6 prefix (with length).

o IAprefix-options:

* OPTION_VSS from the BNDUPD message, if any.

* OPTION_STATUS_CODE containing an error code, or containing a
success code if a message is required. If the information in
the corresponding OPTION_IAPREFIX in the BNDUPD message was
accepted and no status message was required (which is the usual
case), no OPTION_STATUS_CODE option appears.

* OPTION_F_BINDING_STATUS containing the binding-status received
in the BNDREPLY message.

* OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time received in the BNDREPLY message.

* OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a
duplicate of the OPTION_F_PARTNER_LIFETIME received in the
BNDREPLY message.

7.7. Receiving Binding Acks

When a BNDREPLY message is received, the overall OPTION_CLIENT_DATA
option or the overall OPTION_IAPREFIX option may contain an
OPTION_STATUS_CODE containing an error that represents a rejection of
the entire BNDUPD message. An enclosed OPTION_IA_NA, OPTION_IA_TA,
or OPTION_IA_PD option may also contain an OPTION_STATUS_CODE
containing an error that indicates that everything in the containing
option has been rejected. Or, Alternatively, an individual IAADDR or
IAPREFIX option may contain an OPTION_STATUS_CODE option containing
an error that indicates that the IAADDR or IAPREFIX option has been
rejected. An OPTION_STATUS_CODE containing a success code has no
bearing on the acceptance status of the BNDREPLY message at any
level.

Receipt of a rejection (or a part of a BNDREPLY message that has been
rejected) requires no processing, other than remembering that it has
been encountered.

The information contained in the BNDREPLY message in an
OPTION_CLIENT_DATA that represents an acceptance is stored with the
appropriate client and lease, as follows:

1. The OPTION_CLIENTID is used to find the client.

2. For each of the OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
options in the OPTION_CLIENT_DATA option and for each of the
OPTION_IAADDR or OPTION_IAPREFIX options they contain:

a. OPTION_F_PARTNER_LIFETIME_SENT is stored in the
acked-partner-lifetime.

b. The partner-lifetime is set to 0 to indicate that no more
information needs to be sent to the partner.

Alternatively, the BNDREPLY message may contain a single
OPTION_IAPREFIX option not contained in an OPTION_CLIENT_DATA option,
representing information concerning a single prefix lease. If the
IAprefix-options section of the OPTION_IAPREFIX option contains an
OPTION_STATUS_CODE representing an error, then it is considered a
rejection of the corresponding BNDUPD message. If the
OPTION_IAPREFIX option does not contain an OPTION_STATUS_CODE option
or if the OPTION_STATUS_CODE option contains a success status, then
the three items in the following list are stored in the lease state
database, in the section associated with the prefix lease represented
by the OPTION_IAPREFIX option.

1. OPTION_F_BINDING_STATUS containing the binding-status received in
the BNDREPLY message.

2. OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time received in the BNDREPLY message.

3. OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a duplicate
of the OPTION_F_PARTNER_LIFETIME received in the BNDREPLY
message.

7.8. BNDUPD/BNDREPLY Data Flow

Figure 5 shows the relationship of the times described in Section 7.3
to the options used to transmit them. It also relates the times on
one failover partner to the other failover partner.

----------------------- BNDUPD ---------------------------------

Source on OPTION_F in Storage on
Sending Server -> BNDUPD message -> Receiving Server

[always update]

partner-lifetime PARTNER_LIFETIME expiration-time

client-last-transaction-time CLT_TIME partner-raw-clt-time
start-time-of-state START_TIME_OF_STATE start-time-of-state
state-expiration-time STATE_EXPIRATION_TIME state-expiration-time

[update only if received > current]

expiration-time EXPIRATION_TIME partner-lifetime
partner-raw-clt-time PARTNER_RAW_CLT_TIME
client-last-transaction-time

----------------------- BNDREPLY -------------------------------

Storage on OPTION_F in Storage on
Receiving Server <- BNDUPD message <- Sending Server

[always update]

acked-partner-lifetime PARTNER_LIFETIME_SENT duplicate of received
PARTNER_LIFETIME
(nothing to update) STATE_EXPIRATION_TIME state-expiration-time

----------------------------------------------------------------

Figure 5: BNDUPD and BNDREPLY Time Handling

8. Endpoint States

8.1. State Machine Operation

Each server (or, more accurately, failover endpoint) can take on a
variety of failover states. These states play a crucial role in
determining the actions that a server will perform when processing a
request from a DHCP client as well as dealing with changing external
conditions (e.g., loss of connection to a failover partner).

The failover state in which a server is running controls the
following behaviors:

o Responsiveness - the server is either responsive to DHCP client
requests, renew responsive, or unresponsive.

o Allocation Pool - which pool of addresses (or prefixes) can be
used for advertisement on receipt of a SOLICIT or allocation on
receipt of a REQUEST, RENEW, or REBIND message.

o MCLT - ensure that valid lifetimes are not beyond what the partner
has acked plus the MCLT (unless the failover state doesn't require
this restriction).

A server will transition from one failover state to another based on
the specific values held by the following state variables:

o Current failover state.

o Communications status ("OK" or not "OK").

o Partner's failover state (if known).

Whenever any of the above state variables change state, the state
machine is invoked, which may then trigger a change in the current
failover state. Thus, whenever the communications status changes,
the state machine processing is invoked. This may or may not result
in a change in the current failover state.

Whenever a server transitions to a new failover state, the new state
MUST be communicated to its failover partner in a STATE message if
the communications status is "OK". In addition, whenever a server
makes a transition into a new state, it MUST record the new state,
its current understanding of its partner's state, and the time at
which it entered the new state in stable storage.

The state transition diagram below (Figure 6) gives a condensed view
of the state machine. If there are any differences between text
describing a particular state and the information shown in Figure 6,
the text should be considered authoritative.

In Figure 6, the terms "responsive", "r-responsive", and
"unresponsive" appear in the states and refer to whether the server
in the indicated state is allowed to be responsive, renew responsive,
or unresponsive, respectively. The "+", "-", or "*" in the upper
right corner of each state is a notation about whether communication
is ongoing with the other server, with "+" meaning that
communications are "OK", "-" meaning that communications are
interrupted, and "*" meaning that communications may be either "OK"
or interrupted.

+---------------+ V +--------------+
| RECOVER * | | | STARTUP - |
|(unresponsive) | +->+(unresponsive)|
+------+--------+ +--------------+
+-Comm. OK +-----------------+
| Other State: | PARTNER-DOWN - +<---------------------+
| RESOLUTION-INTER. | (responsive) | ^
All POTENTIAL- +----+------------+ |
Others CONFLICT------------ | --------+ |
| CONFLICT-DONE Comm. OK | +--------------+ |
UPDREQ or Other State: | +--+ RESOLUTION - | |
UPDREQALL | | | | | INTERRUPTED | |
Rcv UPDDONE RECOVER All | | | (responsive) | |
| +---------------+ | Others | | +------+-----+-+ |
+->+RECOVER-WAIT * | RECOVER- | | | ^ | |
|(unresponsive) | WAIT or | | Comm. | Ext. |
+-----------+---+ DONE | | OK Comm. Cmd---->+
Comm.---+ Wait MCLT | V V V Failed |
Changed | V +---+ +---+-----+--+-+ | |
| +---+----------++ | | POTENTIAL + +-------+ |
| |RECOVER-DONE * | Wait | CONFLICT +------+ |
+->+(unresponsive) | for |(unresponsive)| Primary |
+------+--------+ Other +>+----+--------++ resolve Comm. |
Comm. OK State: | | ^ conflict Changed|
+---Other State:-+ RECOVER- | Secondary | V V | |
| | | DONE | resolve | +----+-------+--++ |
| All Others: POTENT. | | conflict | |CONFLICT-DONE * | |
| Wait for CONFLICT--|-----+ | | | (responsive) | |
| Other State: V V | +-------+--------+ |
| NORMAL or RECOVER- ++------------+---+ | Other State: NORMAL |
| | DONE | NORMAL + +<--------------+ |
| +--+----------+-->+ pri: responsive +-------External Command-->+
| ^ ^ |sec: r-responsive| | |
| | | +--------+--------+ | |
| | | | | |
| Wait for Comm. OK Comm. Failed | External
| Other Other | | Command
| State: State: Start Auto | or
| RECOVER-DONE NORMAL Partner Down Comm. OK Auto
| | COMM.-INT. Timer Other State: Partner
| Comm. OK | V All Others Down
| Other State: | +---------+--------+ | expiration
| RECOVER +--+ COMMUNICATIONS - +----+ |
| +-------------+ INTERRUPTED | |
RECOVER | (responsive) +------------------------->+
RECOVER-WAIT--------->+------------------+

Figure 6: Failover Endpoint State Machine

8.2. State Machine Initialization

The state machine is characterized by storage (in stable storage) of
at least the following information:

o Current failover state.

o Previous failover state.

o Start time of current failover state.

o Partner's failover state.

o Start time of partner's failover state.

o Time most recent message received from partner.

The state machine is initialized by reading these data items from
stable storage and restoring their values from the information saved.
If there is no information in stable storage concerning these items,
then they should be initialized as follows:

o Current failover state: Primary: PARTNER-DOWN, Secondary: RECOVER.

o Previous failover state: None.

o Start time of current failover state: Current time.

o Partner's failover state: None until reception of STATE message.

o Start time of partner's failover state: None until reception of
STATE message.

o Time most recent message received from partner: None until message
received.

8.3. STARTUP State

The STARTUP state affords an opportunity for a server to probe its
partner server before starting to service DHCP clients. When in the
STARTUP state, a server attempts to learn its partner's state and
determine (using that information if it is available) what state it
should enter.

The STARTUP state is not shown with any specific state transitions in
the state machine diagram (Figure 6) because the processing during
the STARTUP state can cause the server to transition to any of the
other states, so that specific state transition arcs would only
obscure other information.

8.3.1. Operation in STARTUP State

The server MUST NOT be responsive to DHCP clients in STARTUP state.

Whenever a STATE message is sent to the partner while in STARTUP
state, the STARTUP flag MUST be set in the OPTION_F_SERVER_FLAGS
option and the previously recorded failover state MUST be placed in
the OPTION_F_SERVER_STATE option, each of which is included in the
STATE message.

8.3.2. Transition out of STARTUP State

The algorithm below is followed every time the server initializes
itself and enters STARTUP state.

The variables PREVIOUS-STATE and CURRENT-STATE are defined for use in
the algorithm description below. PREVIOUS-STATE is simply for
storage of a state, while CURRENT-STATE not only stores the current
state but also changes the current state of the failover endpoint to
whatever state is set in CURRENT-STATE.

Step 1: If there is any record of a previous failover state in stable
storage for this server, then set the PREVIOUS-STATE to the
last recorded value in stable storage and the TIME-OF-FAILURE
to the time the server failed or a time beyond which the
server could not have been operating, and go to Step 2.

If there is no record of any previous failover state in
stable storage for this server, then set the PREVIOUS-STATE
to RECOVER, and set the TIME-OF-FAILURE to 0. This will
allow two servers that already have lease information to
synchronize themselves prior to operating.

In some cases, an existing server will be commissioned as a
failover server and brought back into operation when its
partner is not yet available. In this case, the newly
commissioned failover server will not operate until its
partner comes online -- but it has operational
responsibilities as a DHCP server nonetheless. To properly
handle this situation, a server SHOULD be configurable in
such a way as to move directly into PARTNER-DOWN state after
the startup period expires if it has been unable to contact
its partner during the startup period.

Step 2: Implementations will differ in the ways that they deal with
the state machine for failover endpoint states. In many
cases, state transitions will occur when communications go
from "OK" to failed or from failed to "OK", and some
implementations will implement a portion of their state
machine processing based on these changes.

In these cases, during startup, if the PREVIOUS-STATE is one
where communications were "OK", then set the PREVIOUS-STATE
to the state that is the result of the communication failed
state transition when in that state (if such a transition
exists -- some states don't have a communication failed state
transition, since they allow both "communications OK" and
"failed").

Step 3: Start the STARTUP state timer. The time that a server
remains in the STARTUP state (absent any communications with
its partner) is implementation dependent but SHOULD be short.
It SHOULD be long enough for a TCP connection to a heavily
loaded partner to be created across a slow network.

Step 4: If the server is a primary server, attempt to create a TCP
connection to the failover partner. If the server is a
secondary server, listen on the failover port and wait for
the primary server to connect. See Section 6.1.

Step 5: Wait for "communications OK".

When and if communications become "OK", clear the STARTUP
flag, and set the CURRENT-STATE to the PREVIOUS-STATE.

If the partner is in PARTNER-DOWN state and if the time at
which it entered PARTNER-DOWN state (as received in the
OPTION_F_START_TIME_OF_STATE option in the STATE message) is
later than the last recorded time of operation of this
server, then set CURRENT-STATE to RECOVER. If the time at
which it entered PARTNER-DOWN state is earlier than the last
recorded time of operation of this server, then set
CURRENT-STATE to POTENTIAL-CONFLICT.

Then, transition to the CURRENT-STATE and take the
"communications OK" state transition based on the
CURRENT-STATE of this server and the partner.

Step 6: If the startup time expires prior to communications becoming
"OK", the server SHOULD transition to PREVIOUS-STATE.

8.4. PARTNER-DOWN State

PARTNER-DOWN state is a state either server can enter. When in this
state, the server assumes that it is the only server operating and
serving the client base. If one server is in PARTNER-DOWN state, the
other server MUST NOT be operating.

A server can enter PARTNER-DOWN state as a result of either
(1) operator intervention (when an operator determines that the
server's partner is, indeed, down) or (2) an optional
auto-partner-down capability where PARTNER-DOWN state is entered
automatically after a server has been in COMMUNICATIONS-INTERRUPTED
state for a predetermined period of time.

8.4.1. Operation in PARTNER-DOWN State

The server MUST be responsive in PARTNER-DOWN state, regardless of
whether it is primary or secondary.

It will allow renewal of all outstanding leases.

For delegable prefixes, the server will allocate leases from its own
pool, and after a fixed period of time (the MCLT interval) has
elapsed from entry into PARTNER-DOWN state, it may allocate delegable
prefixes from the set of all available pools. The server MUST fully
deplete its own pool before starting allocations from its downed
partner's pool.

IPv6 addresses available for independent allocation by the other
server (upon entering PARTNER-DOWN state) SHOULD NOT be allocated to
a client. If one elects to do so anyway, they MUST NOT be allocated
to a new client until the MCLT beyond the entry into PARTNER-DOWN
state has elapsed.

A server in PARTNER-DOWN state MUST NOT allocate a lease to a DHCP
client different from the client to which it was allocated at the
time of entry into PARTNER-DOWN state until the MCLT beyond the
maximum of the following times: client expiration time, most recently
transmitted partner-lifetime, most recently received ack of the
partner-time from the partner, and most recently acked
partner-lifetime to the partner. If this time would be earlier than
the current time plus the MCLT, then the time the server entered
PARTNER-DOWN state plus the MCLT is used.

The server is not restricted by the MCLT when offering valid
lifetimes while in PARTNER-DOWN state.

In the unlikely case when there are two servers operating in
PARTNER-DOWN state, there is a chance that duplicate leases for the
same prefix could be assigned. This leads to a POTENTIAL-CONFLICT
(unresponsive) state when the servers reestablish contact. This
issue of duplicate leases can be prevented as long as the server
grants new leases from its own pool; therefore, the server operating
in PARTNER-DOWN state MUST use its own pool first for new leases
before assigning any leases from its downed partner's pool.

8.4.2. Transition out of PARTNER-DOWN State

When a server in PARTNER-DOWN state succeeds in establishing a
connection to its partner, its actions are conditional on the state
and flags received in the STATE message from the other server as part
of the process of establishing the connection.

If the STARTUP bit is set in the OPTION_F_SERVER_FLAGS option of a
received STATE message, a server in PARTNER-DOWN state MUST NOT take
any state transitions based on reestablishing communications. If a
server is in PARTNER-DOWN state, it ignores all STATE messages from
its partner that have the STARTUP bit set in the
OPTION_F_SERVER_FLAGS option of the STATE message.

If the STARTUP bit is not set in the OPTION_F_SERVER_FLAGS option of
a STATE message received from its partner, then a server in
PARTNER-DOWN state takes the following actions, based on the state of
the partner as received in a STATE message (either immediately after
establishing communications or at any time later when a new state is
received):

o If the partner is in NORMAL, COMMUNICATIONS-INTERRUPTED,
PARTNER-DOWN, POTENTIAL-CONFLICT, RESOLUTION-INTERRUPTED, or
CONFLICT-DONE state, then transition to POTENTIAL-CONFLICT state.

o If the partner is in RECOVER or RECOVER-WAIT state, then stay in
PARTNER-DOWN state.

o If the partner is in RECOVER-DONE state, then transition to NORMAL
state.

8.5. RECOVER State

This state indicates that the server has no information in its stable
storage or that it is reintegrating with a server in PARTNER-DOWN
state after it has been down. A server in this state MUST attempt to
refresh its stable storage from the other server.

8.5.1. Operation in RECOVER State

The server MUST NOT be responsive in RECOVER state.

A server in RECOVER state will attempt to reestablish communications
with the other server.

8.5.2. Transition out of RECOVER State

If the other server is in POTENTIAL-CONFLICT, RESOLUTION-INTERRUPTED,
or CONFLICT-DONE state when communications are reestablished, then
the server in RECOVER state will move itself to POTENTIAL-CONFLICT
state.

If the other server is in any other state, then the server in RECOVER
state will request an update of missing binding information by
sending an UPDREQ message. If the server has determined that it has
lost its stable storage because it has no record of ever having
talked to its partner even though its partner does have a record of
communicating with it, it MUST send an UPDREQALL message; otherwise,
it MUST send an UPDREQ message.

It will wait for an UPDDONE message, and upon receipt of that message
it will transition to RECOVER-WAIT state.

If communication fails during the reception of the results of the
UPDREQ or UPDREQALL message, the server will remain in RECOVER state
and will reissue the UPDREQ or UPDREQALL message when communications
are reestablished.

If an UPDDONE message isn't received within an implementation-
dependent amount of time and no BNDUPD messages are being received,
the connection SHOULD be dropped.

A B
Server Server

Figure 7: Transition out of RECOVER State

If at any time while a server is in RECOVER state communication
fails, the server will stay in RECOVER state. When communications
are restored, it will restart the process of transitioning out of
RECOVER state.

8.6. RECOVER-WAIT State

This state indicates that the server has sent an UPDREQ or UPDREQALL
message and has received the UPDDONE message indicating that it has
received all outstanding binding update information. In the
RECOVER-WAIT state, the server will wait for the MCLT in order to
ensure that any processing that this server might have done prior to
losing its stable storage will not cause future difficulties.

8.6.1. Operation in RECOVER-WAIT State

The server MUST NOT be responsive in RECOVER-WAIT state.

8.6.2. Transition out of RECOVER-WAIT State

Upon entry into RECOVER-WAIT state, the server MUST start a timer
whose expiration is set to a time equal to the time the server went
down (the TIME-OF-FAILURE from Section 8.3.2), if known, or the time
the server started (if the TIME-OF-FAILURE is unknown), plus the
MCLT. When this timer expires, the server will transition into
RECOVER-DONE state.

This allows any IPv6 addresses or prefixes that were allocated by
this server prior to the loss of its client binding information in
stable storage to contact the other server or to time out.

If the server has never before run failover, then there is no need to
wait in this state, and the server MAY transition immediately to
RECOVER-DONE state. However, to determine if this server has run
failover, it is vital that the information provided by the partner be
utilized, since the stable storage of this server may have been lost.

If communication fails while a server is in RECOVER-WAIT state, it
has no effect on the operation of this state. The server SHOULD
continue to operate its timer, and if the timer expires during the
period where communications with the other server have failed, then
the server SHOULD transition to RECOVER-DONE state. This is rare --
failover state transitions are not usually made while communications
are interrupted, but in this case there is no reason to inhibit this
transition.

8.7. RECOVER-DONE State

This state exists to allow an interlocked transition for one server
from RECOVER state and another server from PARTNER-DOWN or
COMMUNICATIONS-INTERRUPTED state into NORMAL state.

8.7.1. Operation in RECOVER-DONE State

A server in RECOVER-DONE state SHOULD be renew responsive and MAY
respond to RENEW requests but MUST only change the state of a lease
that appears in the RENEW request. It MUST NOT allocate any
additional leases when in RECOVER-DONE state and should only respond
to RENEW requests where it already has a record of the lease.

8.7.2. Transition out of RECOVER-DONE State

When a server in RECOVER-DONE state determines that its partner
server has entered NORMAL or RECOVER-DONE state, it will transition
into NORMAL state.

If the partner server enters RECOVER or RECOVER-WAIT state, this
server transitions to COMMUNICATIONS-INTERRUPTED.

If the partner server enters POTENTIAL-CONFLICT state, this server
enters POTENTIAL-CONFLICT state as well.

If communication fails while in RECOVER-DONE state, a server will
stay in RECOVER-DONE state.

8.8. NORMAL State

NORMAL state is the state used by a server when it is communicating
with the other server and any required resynchronization has been
performed. While some binding database synchronization is performed
in NORMAL state, potential conflicts are resolved prior to entry into
NORMAL state, as is binding database data loss.

When entering NORMAL state, a server will send to the other server
all currently unacknowledged binding updates as BNDUPD messages.

When the above process is complete, if the server entering NORMAL
state is a secondary server, then it will request delegable prefixes
for allocation using the POOLREQ message.

8.8.1. Operation in NORMAL State

The primary server is responsive in NORMAL state. The secondary is
renew responsive in NORMAL state.

When in NORMAL state, a primary server will operate in the following
manner:

Valid lifetime calculations
As discussed in Section 4.4, the lease interval given to a DHCP
client can never be more than the MCLT greater than the most
recently acknowledged partner lifetime received from the failover
partner or the current time, whichever is later.

As long as a server adheres to this constraint, the specifics of
the lease interval that it gives to a DHCP client or the value of
the partner lifetime sent to its failover partner are
implementation dependent.

Lazy update of partner server
After sending a REPLY that includes a lease update to a client,
the server servicing a DHCP client request attempts to update its
partner with the new binding information. See Section 4.3.

Reallocation of leases between clients
Whenever a client binding is released or expires, a BNDUPD message
must be sent to the partner, setting the binding state to RELEASED
or EXPIRED. However, until a BNDREPLY is received for this
message, the lease cannot be allocated to another client. It
cannot be allocated to the same client again if a BNDUPD message
was sent; otherwise, it can. See Section 4.2.2.1 for details.

In NORMAL state, each server receives binding updates from its
partner server in BNDUPD messages (see Section 7.5.5). It records
these in its binding database in stable storage and then sends a
corresponding BNDREPLY message to its partner server (see
Section 7.6).

8.8.2. Transition out of NORMAL State

If a server in NORMAL state receives an external command informing it
that its partner is down, it will transition immediately into
PARTNER-DOWN state. Generally, this would be an unusual situation,
where some external agency knew the partner server was down prior to
the failover server discovering it on its own.

If a server in NORMAL state fails to receive acks to messages sent to
its partner for an implementation-dependent period of time, it MAY
move into COMMUNICATIONS-INTERRUPTED state. This situation might
occur if the partner server was capable of maintaining the TCP
connection between the server and also capable of sending a CONTACT
message periodically but was (for some reason) incapable of
processing BNDUPD messages.

If it is determined that communications are not "OK" (as defined in
Section 6.6), then the server should transition into
COMMUNICATIONS-INTERRUPTED state.

If a server in NORMAL state receives any messages from its partner
where the partner has changed state from that expected by the server
in NORMAL state, then the server should transition into
COMMUNICATIONS-INTERRUPTED state and take the appropriate state
transition from there. For example, it would be expected that the
partner would transition from POTENTIAL-CONFLICT state into NORMAL
state but not that the partner would transition from NORMAL state
into POTENTIAL-CONFLICT state.

If a server in NORMAL state receives a DISCONNECT message from its
partner, then the server should transition into
COMMUNICATIONS-INTERRUPTED state.

8.9. COMMUNICATIONS-INTERRUPTED State

A server goes into COMMUNICATIONS-INTERRUPTED state whenever it is
unable to communicate with its partner. Primary and secondary
servers cycle automatically (without administrative intervention)
between NORMAL state and COMMUNICATIONS-INTERRUPTED state as the
network connection between them fails and recovers, or as the partner
server cycles between operational and non-operational. No allocation
of duplicate leases can occur while the servers cycle between these
states.

When a server enters COMMUNICATIONS-INTERRUPTED state, if it has been
configured to support an automatic transition out of
COMMUNICATIONS-INTERRUPTED state and into PARTNER-DOWN state (i.e.,
auto-partner-down has been configured), then a timer is started for
the length of the configured auto-partner-down period.

A server transitioning into the COMMUNICATIONS-INTERRUPTED state from
the NORMAL state SHOULD raise an alarm condition to alert
administrative staff to a potential problem in the DHCP subsystem.

8.9.1. Operation in COMMUNICATIONS-INTERRUPTED State

In this state, a server MUST respond to all DHCP client requests.
When allocating new leases, each server allocates from its own pool,
where the primary MUST allocate only FREE delegable prefixes and the
secondary MUST allocate only FREE-BACKUP delegable prefixes, and each
server allocates from its own independent IPv6 address ranges. When
responding to RENEW messages, each server will allow continued
renewal of a DHCP client's current lease, regardless of whether that
lease was given out by the receiving server or not, although the
renewal period MUST NOT exceed the MCLT beyond the later of (1) the
partner lifetime already acknowledged by the other server or (2) now.

However, since the server cannot communicate with its partner in this
state, the acknowledged partner lifetime will not be updated, despite
continued RENEW message processing. This is likely to eventually
cause the actual lifetimes to converge to the MCLT (unless this is
greater than the desired lease time, which would be unusual).

The server should continue to try to establish a connection with its
partner.

8.9.2. Transition out of COMMUNICATIONS-INTERRUPTED State

If the auto-partner-down timer expires while a server is in
COMMUNICATIONS-INTERRUPTED state, it will transition immediately into
PARTNER-DOWN state.

If a server in COMMUNICATIONS-INTERRUPTED state receives an external
command informing it that its partner is down, it will transition
immediately into PARTNER-DOWN state.

If communications with the other server are restored, then the server
in COMMUNICATIONS-INTERRUPTED state will transition into another
state based on the state of the partner:

o NORMAL or COMMUNICATIONS-INTERRUPTED: Transition into NORMAL
state.

o RECOVER: Stay in COMMUNICATIONS-INTERRUPTED state.

o RECOVER-DONE: Transition into NORMAL state.

o PARTNER-DOWN, POTENTIAL-CONFLICT, CONFLICT-DONE, or
RESOLUTION-INTERRUPTED: Transition into POTENTIAL-CONFLICT state.

Figure 8 illustrates the transition from NORMAL state to
COMMUNICATIONS-INTERRUPTED state and then back to NORMAL state again.

Primary Secondary
Server Server

Figure 8: Transition from NORMAL State
to COMMUNICATIONS-INTERRUPTED State and Back

8.10. POTENTIAL-CONFLICT State

This state indicates that the two servers are attempting to
reintegrate with each other but at least one of them was running in a
state that did not guarantee that automatic reintegration would be
possible. In POTENTIAL-CONFLICT state, the servers may determine
that the same lease has been offered and accepted by two different
clients.

A goal of the failover protocol is to minimize the possibility that
POTENTIAL-CONFLICT state is ever entered.

When a primary server enters POTENTIAL-CONFLICT state, it should
request that the secondary send it all updates that the primary
server has not yet acknowledged by sending an UPDREQ message to the
secondary server.

A secondary server entering POTENTIAL-CONFLICT state will wait for
the primary to send it an UPDREQ message.

8.10.1. Operation in POTENTIAL-CONFLICT State

Any server in POTENTIAL-CONFLICT state MUST NOT process any incoming
DHCP requests.

8.10.2. Transition out of POTENTIAL-CONFLICT State

If communication with the partner fails while in POTENTIAL-CONFLICT
state, then the server will transition to RESOLUTION-INTERRUPTED
state.

Whenever either server receives an UPDDONE message from its partner
while in POTENTIAL-CONFLICT state, it MUST transition to a new state.
The primary MUST transition to CONFLICT-DONE state, and the secondary
MUST transition to NORMAL state. This will cause the primary server
to leave POTENTIAL-CONFLICT state prior to the secondary, since the
primary sends an UPDREQ message and receives an UPDDONE message
before the secondary sends an UPDREQ message and receives its UPDDONE
message.

When a secondary server receives an indication that the primary
server has made a transition from POTENTIAL-CONFLICT to CONFLICT-DONE
state, it SHOULD send an UPDREQ message to the primary server.

Primary Secondary
Server Server

Figure 9: Transition out of POTENTIAL-CONFLICT State

8.11. RESOLUTION-INTERRUPTED State

This state indicates that the two servers were attempting to
reintegrate with each other in POTENTIAL-CONFLICT state but
communication failed prior to completion of reintegration.

The RESOLUTION-INTERRUPTED state exists because servers are not
responsive in POTENTIAL-CONFLICT state, and if one server drops out
of service while both servers are in POTENTIAL-CONFLICT state, the
server that remains in service will not be able to process DHCP
client requests and there will be no DHCP server available to process
client requests. The RESOLUTION-INTERRUPTED state is the state that
a server moves to if its partner disappears while it is in
POTENTIAL-CONFLICT state.

When a server enters RESOLUTION-INTERRUPTED state, it SHOULD raise an
alarm condition to alert administrative staff of a problem in the
DHCP subsystem.

8.11.1. Operation in RESOLUTION-INTERRUPTED State

In this state, a server MUST respond to all DHCP client requests.
When allocating new leases, each server SHOULD allocate from its own
pool (if that can be determined), where the primary SHOULD allocate
only FREE leases and the secondary SHOULD allocate only FREE-BACKUP
leases. When responding to renewal requests, each server will allow
continued renewal of a DHCP client's current lease, independent of
whether that lease was given out by the receiving server or not,
although the renewal period MUST NOT exceed the MCLT beyond the
later of (1) the partner lifetime already acknowledged by the other
server or (2) now.

However, since the server cannot communicate with its partner in this
state, the acknowledged partner lifetime will not be updated in any
new bindings.

8.11.2. Transition out of RESOLUTION-INTERRUPTED State

If a server in RESOLUTION-INTERRUPTED state receives an external
command informing it that its partner is down, it will transition
immediately into PARTNER-DOWN state.

If communications with the other server are restored, then the server
in RESOLUTION-INTERRUPTED state will transition into
POTENTIAL-CONFLICT state.

8.12. CONFLICT-DONE State

This state indicates that during the process where the two servers
are attempting to reintegrate with each other, the primary server has
received all of the updates from the secondary server. It makes a
transition into CONFLICT-DONE state so that it can be totally
responsive to the client load. There is no operational difference
between CONFLICT-DONE and NORMAL for the primary server, as in both
states it responds to all clients' requests. The distinction between
CONFLICT-DONE and NORMAL states is necessary in the event that a
load-balancing extension is ever defined.

8.12.1. Operation in CONFLICT-DONE State

A primary server in CONFLICT-DONE state is fully responsive to all
DHCP clients (similar to the situation in COMMUNICATIONS-INTERRUPTED
state).

If communication fails, remain in CONFLICT-DONE state. If
communication becomes "OK", remain in CONFLICT-DONE state until the
conditions for transition out of CONFLICT-DONE state are satisfied.

8.12.2. Transition out of CONFLICT-DONE State

If communication with the partner fails while in CONFLICT-DONE state,
then the server will remain in CONFLICT-DONE state.

When a primary server determines that the secondary server has made a
transition into NORMAL state, the primary server will also transition
into NORMAL state.

9. DNS Update Considerations

DHCP servers (and clients) can use "DNS update" as described in
RFC 2136 [RFC2136] to maintain DNS name mappings as they maintain
DHCP leases. Many different administrative models for DHCP-DNS
integration are possible. Descriptions of several of these models,
and guidelines that DHCP servers and clients should follow in
carrying them out, are laid out in RFC 4704 [RFC4704].

The nature of the failover protocol introduces some issues concerning
DNS updates that are not part of non-failover environments. This
section describes these issues and defines the information that
failover partners should exchange in order to ensure consistent
behavior. The presence of this section should not be interpreted as
a requirement that an implementation of the DHCPv6 failover protocol
also support DNS updates.

The purpose of this discussion is to clarify the areas where the
failover and DHCP DNS update protocols intersect for the benefit of
implementations that support both protocols, not to introduce a new
requirement into the DHCPv6 failover protocol. Thus, a DHCP server
that implements the failover protocol MAY also support DNS updates,
but if it does support DNS updates it SHOULD utilize the techniques
described here in order to correctly distribute them between the
failover partners. See RFC 4704 [RFC4704] as well as RFC 4703
[RFC4703] for information on how DHCP servers deal with potential
conflicts when updating DNS even without failover.

From the standpoint of the failover protocol, there is no reason why
a server that is utilizing the DNS update protocol to update a DNS
server should not be a partner with a server that is not utilizing
the DNS update protocol to update a DNS server. However, a server
that is not able to support DNS update or is not configured to
support DNS update SHOULD output a warning message when it receives
BNDUPD messages that indicate that its failover partner is configured
to support the DNS update protocol to update a DNS server. An
implementation MAY consider this an error and refuse to accept the
BNDUPD message by returning the status DNSUpdateNotSupported in an
OPTION_STATUS_CODE option in the BNDREPLY message, or it MAY choose
to operate anyway, having warned the administrator of the problem in
some way.

9.1. Relationship between Failover and DNS Update

The failover protocol describes the conditions under which each
failover server may renew a lease to its current DHCP clients client and
describes the conditions under which it may grant a lease to a new
DHCP client. An analogous set of conditions determines when a
failover server should initiate a DNS update, and when it should
attempt to remove records from the DNS. The failover protocol's
conditions are based on the desired external behavior: avoiding
duplicate address and prefix assignments, allowing clients to
continue using leases that they obtained from one failover partner
even if they can only communicate with the other partner, and
allowing the secondary DHCP server to grant new leases even if it is
unable to communicate with the primary server. The desired external
DNS update behavior for DHCPv6 failover servers is similar to that
described above for the failover protocol itself:

1. Allow timely DNS updates from the server that grants a lease to a
client. Recognize that there is often a DNS update "lifecycle"
that parallels the DHCP lease lifecycle. This is likely to
include the addition of records when the lease is granted and the
removal of DNS records when the lease is subsequently made
available for allocation to a different client.

2. Communicate enough information between the two failover servers
to allow one to complete the DNS update lifecycle even if the
other server originally granted the lease.

3. Avoid redundant or overlapping DNS updates where both failover
servers are attempting to perform DNS updates for the same
lease-client binding.

4. Avoid situations where one partner is attempting to add resource
records (RRs) related to a lease binding while the other partner
is attempting to remove RRs related to the same lease binding.

While DHCPv6 servers configured for DNS update typically perform
these operations on both the AAAA and the PTR RRs, this is not
required. It is entirely possible that a DHCPv6 server could be
configured to only update the DNS with PTR records, and the DHCPv6
clients could be responsible for updating the DNS with their own AAAA
records. In this case, the discussions here would apply only to the
PTR records.

9.2. Exchanging DNS Update Information

In order for either server to be able to complete a DNS update or to
remove DNS records that were added by its partner, both servers need
to know the FQDN associated with the lease-client binding. In
addition, to properly handle DNS updates, additional information is
required. All of the following information needs to be transmitted
between the failover partners:

1. The FQDN that the client requested be associated with the lease.
If the client doesn't request a particular FQDN and one is
synthesized by the failover server or if the failover server is
configured to replace a client-requested FQDN with a different
FQDN, then the server-generated value would be used.

2. The FQDN that was actually placed in the DNS for this lease. It
may differ from the client-requested FQDN due to some form of
disambiguation or other DHCP server configuration (as described
above).

3. The status of any DNS update operations in progress or completed.

4. Information sufficient to allow the failover partner to remove
the FQDN from the DNS, should that become necessary.

These data items are the minimum necessary set to reliably allow two
failover partners to successfully share the responsibility to keep
the DNS up to date with the leases allocated to clients.

This information would typically be included in BNDUPD messages sent
from one failover partner to the other. Failover servers MAY choose
not to include this information in BNDUPD messages if there has been
no change in the status of any DNS update related to the lease.

The partner server receiving BNDUPD messages containing the DNS
update information SHOULD compare the status information and the FQDN
with the current DNS update information it has associated with the
lease binding and update its notion of the DNS update status
accordingly.

Some implementations will instead choose to send a BNDUPD message
without waiting for the DNS update to complete and then will send a
second BNDUPD message once the DNS update is complete. Other
implementations will delay sending the partner a BNDUPD message until
the DNS update has been acknowledged by the DNS server, or until some
time limit has elapsed, in order to avoid sending a second BNDUPD
message.

The FQDN option contains the FQDN that will be associated with the
AAAA RR (if the server is performing a AAAA RR update for the
client). The PTR RR can be generated automatically from the IPv6
address or prefix value. The FQDN may be composed in any of several ways,
depending on server configuration and the information provided by the
client in its DHCP messages. The client may supply a hostname that
it would like the server to use in forming the FQDN, or it may supply
the entire FQDN. The server may be configured to attempt to use the
information the client supplies, it may be configured with an FQDN to
use for the client, or it may be configured to synthesize an FQDN.

Since the server interacting with the client may not have completed
the DNS update at the time it sends the first BNDUPD message about
the lease binding, there may be cases where the FQDN in later BNDUPD
messages does not match the FQDN included in earlier messages. For
example, the responsive server may be configured to handle situations
where two or more DHCP client FQDNs are identical by modifying the
most-specific label in the FQDNs of some of the clients in an attempt
to generate unique FQDNs for them (a process sometimes called
"disambiguation"). Alternatively, at sites that use some or all of
the information that clients supply to form the FQDN, it's possible
that a client's configuration may be changed so that it begins to
supply new data. The server interacting with the client may react by
removing the DNS records that it originally added for the client and
replacing them with records that refer to the client's new FQDN. In
such cases, the server SHOULD include the actual FQDN that was used
in subsequent DNS update options in any BNDUPD messages exchanged
between the failover partners. This server SHOULD include relevant
information in its BNDUPD messages. This information may be
necessary in order to allow the non-responsive partner to detect
client configuration changes that change the hostname or FQDN data
that the client includes in its DHCPv6 requests.

9.3. Adding RRs to the DNS

A failover server that is going to perform DNS updates SHOULD
initiate the DNS update when it grants a new lease to a client. The
server that did not grant the lease SHOULD NOT initiate a DNS update
when it receives the BNDUPD message after the lease has been granted.
The failover protocol ensures that only one of the partners will
grant a lease to any individual client, so it follows that this
requirement will prevent both partners from initiating updates
simultaneously. The server initiating the update SHOULD follow the
protocol in RFC 4704 [RFC4704]. The server may be configured to
perform a AAAA RR update on behalf of its clients, or not.
Ordinarily, a failover server will not initiate DNS updates when it
renews leases. In two cases, however, a failover server MAY initiate
a DNS update when it renews a lease to its existing client:

1. When the lease was granted before the server was configured to
perform DNS updates, the server MAY be configured to perform
updates when it next renews existing leases.

2. If a server is in PARTNER-DOWN state, it can conclude that its
partner is no longer attempting to perform an update for the
existing client. If the remaining server has not recorded that
an update for the binding has been successfully completed, the
server MAY initiate a DNS update. It may initiate this update
immediately upon entry into PARTNER-DOWN state, it may perform
this in the background, or it may initiate this update upon next
hearing from the DHCP client.

Note that, regardless of the use of failover, there is a use case for
updating the DNS on every lease renewal. If there is a concern that
the information in the DNS does not match the information in the DHCP
server, updating the DNS on lease renewal is one way to gradually
ensure that the DNS has information that corresponds correctly to the
information in the DHCP server.

9.4. Deleting RRs from the DNS

The failover server that makes a lease PENDING-FREE SHOULD initiate
any DNS deletes if it has recorded that DNS records were added on
behalf of the client.

A server not in PARTNER-DOWN state "makes a lease PENDING-FREE" when
it initiates a BNDUPD message with a binding-status of FREE,
FREE-BACKUP, EXPIRED, or RELEASED. Its partner confirms this status
by acking that BNDUPD message, and upon receipt of the BNDREPLY
message the server has "made the lease PENDING-FREE". Conversely, a
server in PARTNER-DOWN state "makes a lease PENDING-FREE" when it
sets the binding-status to FREE, since in PARTNER-DOWN state no
communications with the partner are required.

It is at this point that it should initiate the DNS operations to
delete RRs from the DNS. Its partner SHOULD NOT initiate DNS deletes
for DNS records related to the lease binding as part of sending the
BNDREPLY message. The partner MAY have issued BNDUPD messages with a
binding-status of FREE, EXPIRED, or RELEASED previously, but the
other server will have rejected these BNDUPD messages.

The failover protocol ensures that only one of the two partner
servers will be able to make a lease PENDING-FREE. The server making
the lease PENDING-FREE may be doing so while it is communicating in
NORMAL state with its partner, or it may be in PARTNER-DOWN state.
If a server is in PARTNER-DOWN state, it may be performing DNS
deletes for RRs that its partner added originally. This allows a
single remaining partner server to assume responsibility for all of
the DNS update activity that the two servers were undertaking.

Another implication of this approach is that no DNS RR deletes will
be performed while either server is in COMMUNICATIONS-INTERRUPTED
state, since no leases are moved into the PENDING-FREE state during
that period.

A failover server SHOULD ensure that a server failure while making a
lease PENDING-FREE and initiating a DNS delete does not somehow leave
the lease with an RR in the DNS with nothing recorded in the lease
state database to trigger a DNS delete.

9.5. Name Assignment with No Update of DNS

In some cases, a DHCP server is configured to return a name to the
DHCP client but not enter that name into the DNS. This is typically
a name that it has discovered or generated from information it has
received from the client. In this case, this name information SHOULD
be communicated to the failover partner, if only to ensure that they
will return the same name in the event the partner becomes the server
with which the DHCP client begins to interact.

10. Security Considerations

DHCPv6 failover is an extension of a standard DHCPv6 protocol, so all
security considerations from Section 23 of [RFC3315] and Section 15
of [RFC3633] related to the server apply.

The use of TCP introduces some additional concerns. Attacks that
attempt to exhaust the DHCP server's available TCP connection
resources can compromise the ability of legitimate partners to
receive service. Malicious requestors who succeed in establishing
connections but who then send invalid messages, partial messages, or
no messages at all can also exhaust a server's pool of available
connections.

DHCPv6 failover can operate in secure or insecure mode. Secure mode
(using Transport Layer Security (TLS) [RFC5246]) would be indicated
when the TCP connection between failover partners is open to external
monitoring or interception. Insecure mode should only be used when
the TCP connection between failover partners remains within a set of
protected systems. Details of such protections are beyond the scope
of this document. Failover servers MUST use the approach documented
in Section 9.1 of [RFC7653] to decide whether or not to use TLS when
connecting with the failover partner.

The threats created by using failover directly mirror those from
using DHCPv6 itself: information leakage through monitoring, and
disruption of address assignment and configuration. Monitoring the
failover TCP connection provides no additional data beyond that
available from monitoring the interactions between DHCPv6 clients and
the DHCPv6 server. Likewise, manipulating the data flow between
failover servers provides no additional opportunities to disrupt
address assignment and configuration beyond that provided by acting
as a counterfeit DHCP server. Protection from both threats is easier
than with basic DHCPv6, as only a single TCP connection needs to be
protected. Either use secure mode to protect that TCP connection or
ensure that it can only exist with a set of protected systems.

When operating in secure mode, TLS is used to secure the connection.
The recommendations in [RFC7525] SHOULD be followed when negotiating
a TLS connection.

Servers SHOULD offer configuration parameters to limit the sources of
incoming connections through validation and use of the digital
certificates presented to create a TLS connection. They SHOULD also
limit the number of accepted connections and limit the period of time
during which an idle connection will be left open.

Authentication for DHCPv6 messages [RFC3315] MUST NOT be used to
attempt to secure transmission of the messages described in this
document. If authentication is desired, secure mode using TLS SHOULD
be employed as described in Sections 8.2 and 9.1 of [RFC7653].

11. IANA Considerations

IANA has assigned values for the following new DHCPv6 message types
in the registry maintained at <http://www.iana.org/assignments/
dhcpv6-parameters>:

o BNDUPD (24)

o BNDREPLY (25)

o POOLREQ (26)

o POOLRESP (27)

o UPDREQ (28)

o UPDREQALL (29)

o UPDDONE (30)

o CONNECT (31)

o CONNECTREPLY (32)

o DISCONNECT (33)

o STATE (34)

o CONTACT (35)

IANA has assigned values for the following new DHCPv6 option codes in
the registry maintained at <http://www.iana.org/assignments/
dhcpv6-parameters>:

o OPTION_F_BINDING_STATUS (114)

o OPTION_F_CONNECT_FLAGS (115)

o OPTION_F_DNS_REMOVAL_INFO (116)

o OPTION_F_DNS_HOST_NAME (117)

o OPTION_F_DNS_ZONE_NAME (118)

o OPTION_F_DNS_FLAGS (119)

o OPTION_F_EXPIRATION_TIME (120)

o OPTION_F_MAX_UNACKED_BNDUPD (121)

o OPTION_F_MCLT (122)

o OPTION_F_PARTNER_LIFETIME (123)
o OPTION_F_PARTNER_LIFETIME_SENT (124)

o OPTION_F_PARTNER_DOWN_TIME (125)

o OPTION_F_PARTNER_RAW_CLT_TIME (126)

o OPTION_F_PROTOCOL_VERSION (127)

o OPTION_F_KEEPALIVE_TIME (128)

o OPTION_F_RECONFIGURE_DATA (129)

o OPTION_F_RELATIONSHIP_NAME (130)

o OPTION_F_SERVER_FLAGS (131)

o OPTION_F_SERVER_STATE (132)

o OPTION_F_START_TIME_OF_STATE (133)

o OPTION_F_STATE_EXPIRATION_TIME (134)

IANA has assigned values for the following new DHCPv6 status codes in
the registry maintained at <http://www.iana.org/assignments/
dhcpv6-parameters>:

o AddressInUse (16)

o ConfigurationConflict (17)

o MissingBindingInformation (18)

o OutdatedBindingInformation (19)

o ServerShuttingDown (20)

o DNSUpdateNotSupported (21)

o ExcessiveTimeSkew (22)

12. References

12.1. Normative References

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

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

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

[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.

[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<http://www.rfc-editor.org/info/rfc3633>.

[RFC4703] Stapp, M. and B. Volz, "Resolution of Fully Qualified
Domain Name (FQDN) Conflicts among Dynamic Host
Configuration Protocol (DHCP) Clients", RFC 4703,
DOI 10.17487/RFC4703, October 2006,
<http://www.rfc-editor.org/info/rfc4703>.

[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,
<http://www.rfc-editor.org/info/rfc4704>.

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

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[RFC5460] Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
DOI 10.17487/RFC5460, February 2009,
<http://www.rfc-editor.org/info/rfc5460>.

[RFC6607] Kinnear, K., Johnson, R., and M. Stapp, "Virtual Subnet
Selection Options for DHCPv4 and DHCPv6", RFC 6607,
DOI 10.17487/RFC6607, April 2012,
<http://www.rfc-editor.org/info/rfc6607>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.

[RFC7653] Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6
Active Leasequery", RFC 7653, DOI 10.17487/RFC7653,
October 2015, <http://www.rfc-editor.org/info/rfc7653>.

12.2. Informative References

[RFC7031] Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
Requirements", RFC 7031, DOI 10.17487/RFC7031, September
2013, <http://www.rfc-editor.org/info/rfc7031>.

Acknowledgements

This document extensively uses concepts, definitions, and other parts
of an effort to document failover for DHCPv4. The authors would like
to thank Shawn Routhier, Greg Rabil, Bernie Volz, and Marcin
Siodelski for their significant involvement and contributions. In
particular, Bernie Volz and Shawn Routhier provided detailed and
substantive technical reviews of the document. The RFC Editor also
caught several important technical issues. The authors would like to
thank Vithalprasad Gaitonde, Krzysztof Gierlowski, Krzysztof Nowicki,
and Michal Hoeft for their insightful comments.

Authors' Addresses

Tomasz

Tomek Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, California 94063
United States of America

Phone: +1 650 423 1345

Email: tomasz.mrugalski@gmail.com

Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Avenue
200 Beaver Brook Road
Boxborough, Massachusetts 01719
United States of America

Phone: +1 978 936 0000
Email: kkinnear@cisco.com