





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

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   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.




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RFC 8156                DHCPv6 Failover Protocol               June 2017


Table of Contents

   1. Introduction ....................................................5
   2. Requirements Language ...........................................5
   3. Glossary ........................................................6
   4. Failover Concepts and Mechanisms ...............................10
      4.1. Required Server Configuration .............................10
      4.2. IPv6 Address and Delegable Prefix Allocation ..............10
           4.2.1. Independent Allocation .............................10
                  4.2.1.1. Independent Allocation Algorithm ..........11
           4.2.2. Proportional Allocation ............................11
                  4.2.2.1. Reallocating Leases .......................13
      4.3. Lazy Updates ..............................................14
      4.4. Maximum Client Lead Time (MCLT) ...........................14
           4.4.1. MCLT Example .......................................16
   5. Message and Option Definitions .................................19
      5.1. Message Framing for TCP ...................................19
      5.2. Failover Message Format ...................................19
      5.3. Messages ..................................................20
           5.3.1. BNDUPD .............................................20
           5.3.2. BNDREPLY ...........................................20
           5.3.3. POOLREQ ............................................20
           5.3.4. POOLRESP ...........................................21
           5.3.5. UPDREQ .............................................21
           5.3.6. UPDREQALL ..........................................21
           5.3.7. UPDDONE ............................................21
           5.3.8. CONNECT ............................................21
           5.3.9. CONNECTREPLY .......................................22
           5.3.10. DISCONNECT ........................................22
           5.3.11. STATE .............................................22
           5.3.12. CONTACT ...........................................22
      5.4. Transaction-id ............................................22
      5.5. Options ...................................................23
           5.5.1. OPTION_F_BINDING_STATUS ............................23
           5.5.2. OPTION_F_CONNECT_FLAGS .............................24
           5.5.3. OPTION_F_DNS_REMOVAL_INFO ..........................25
                  5.5.3.1. OPTION_F_DNS_HOST_NAME ....................26
                  5.5.3.2. OPTION_F_DNS_ZONE_NAME ....................26
                  5.5.3.3. OPTION_F_DNS_FLAGS ........................27
           5.5.4. OPTION_F_EXPIRATION_TIME ...........................28
           5.5.5. OPTION_F_MAX_UNACKED_BNDUPD ........................29
           5.5.6. OPTION_F_MCLT ......................................29
           5.5.7. OPTION_F_PARTNER_LIFETIME ..........................30
           5.5.8. OPTION_F_PARTNER_LIFETIME_SENT .....................30
           5.5.9. OPTION_F_PARTNER_DOWN_TIME .........................31
           5.5.10. OPTION_F_PARTNER_RAW_CLT_TIME .....................32
           5.5.11. OPTION_F_PROTOCOL_VERSION .........................32




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           5.5.12. OPTION_F_KEEPALIVE_TIME ...........................33
           5.5.13. OPTION_F_RECONFIGURE_DATA .........................34
           5.5.14. OPTION_F_RELATIONSHIP_NAME ........................35
           5.5.15. OPTION_F_SERVER_FLAGS .............................36
           5.5.16. OPTION_F_SERVER_STATE .............................37
           5.5.17. OPTION_F_START_TIME_OF_STATE ......................38
           5.5.18. OPTION_F_STATE_EXPIRATION_TIME ....................38
      5.6. Status Codes ..............................................39
   6. Connection Management ..........................................40
      6.1. Creating Connections ......................................40
           6.1.1. Sending a CONNECT Message ..........................41
           6.1.2. Receiving a CONNECT Message ........................42
           6.1.3. Receiving a CONNECTREPLY Message ...................43
      6.2. Endpoint Identification ...................................44
      6.3. Sending a STATE Message ...................................45
      6.4. Receiving a STATE Message .................................46
      6.5. Connection Maintenance Parameters .........................46
      6.6. Unreachability Detection ..................................47
   7. Binding Updates and Acks .......................................47
      7.1. Time Skew .................................................47
      7.2. Information Model .........................................48
      7.3. Times Required for Exchanging Binding Updates .............52
      7.4. Sending Binding Updates ...................................53
      7.5. Receiving Binding Updates .................................56
           7.5.1. Monitoring Time Skew ...............................56
           7.5.2. Acknowledging Reception ............................56
           7.5.3. Processing Binding Updates .........................57
           7.5.4. Accept or Reject? ..................................57
           7.5.5. Accepting Updates ..................................59
      7.6. Sending Binding Replies ...................................61
      7.7. Receiving Binding Acks ....................................63
      7.8. BNDUPD/BNDREPLY Data Flow .................................65
   8. Endpoint States ................................................66
      8.1. State Machine Operation ...................................66
      8.2. State Machine Initialization ..............................69
      8.3. STARTUP State .............................................70
           8.3.1. Operation in STARTUP State .........................70
           8.3.2. Transition out of STARTUP State ....................70
      8.4. PARTNER-DOWN State ........................................72
           8.4.1. Operation in PARTNER-DOWN State ....................72
           8.4.2. Transition out of PARTNER-DOWN State ...............73
      8.5. RECOVER State .............................................74
           8.5.1. Operation in RECOVER State .........................74
           8.5.2. Transition out of RECOVER State ....................74
      8.6. RECOVER-WAIT State ........................................76
           8.6.1. Operation in RECOVER-WAIT State ....................76
           8.6.2. Transition out of RECOVER-WAIT State ...............76




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      8.7. RECOVER-DONE State ........................................77
           8.7.1. Operation in RECOVER-DONE State ....................77
           8.7.2. Transition out of RECOVER-DONE State ...............77
      8.8. NORMAL State ..............................................77
           8.8.1. Operation in NORMAL State ..........................78
           8.8.2. Transition out of NORMAL State .....................78
      8.9. COMMUNICATIONS-INTERRUPTED State ..........................79
           8.9.1. Operation in COMMUNICATIONS-INTERRUPTED State ......80
           8.9.2. Transition out of COMMUNICATIONS-INTERRUPTED
                  State ..............................................80
      8.10. POTENTIAL-CONFLICT State .................................82
           8.10.1. Operation in POTENTIAL-CONFLICT State .............82
           8.10.2. Transition out of POTENTIAL-CONFLICT State ........82
      8.11. RESOLUTION-INTERRUPTED State .............................83
           8.11.1. Operation in RESOLUTION-INTERRUPTED State .........84
           8.11.2. Transition out of RESOLUTION-INTERRUPTED State ....84
      8.12. CONFLICT-DONE State ......................................84
           8.12.1. Operation in CONFLICT-DONE State ..................85
           8.12.2. Transition out of CONFLICT-DONE State .............85
   9. DNS Update Considerations ......................................85
      9.1. Relationship between Failover and DNS Update ..............86
      9.2. Exchanging DNS Update Information .........................87
      9.3. Adding RRs to the DNS .....................................89
      9.4. Deleting RRs from the DNS .................................90
      9.5. Name Assignment with No Update of DNS .....................91
   10. Security Considerations .......................................91
   11. IANA Considerations ...........................................92
   12. References ....................................................94
      12.1. Normative References .....................................94
      12.2. Informative References ...................................96
   Acknowledgements ..................................................96
   Authors' Addresses ................................................96



















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RFC 8156                DHCPv6 Failover Protocol               June 2017


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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.





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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".







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   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.








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   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.









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   o  Renew Responsive

      A server that is "renew responsive" will respond to valid DHCP
      client 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
      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
      messages.









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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 delegable prefixes (involved in proportional allocation), the
   primary server is responsible for allocating to the secondary server
   the correct proportion of the available 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.



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   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.





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   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).




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   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.

   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



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   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



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   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.







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   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





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   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 = 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.
































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 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

               | >-SOLICIT------>    |                    |
               |  acknowledged partner lifetime = 0       |
               |  lease time = floor( 3d, 0 + 1h ) = 1h   |
               |   <-----ADVERTISE-< |                    |
               |    lease-time = 1h  |                    |
               | >-REQUEST------>    |                    |
        t      |   <---------REPLY-< |                    |
               |    lease-time = 1h  |                    |
               |                     |  >-BNDUPD------>   |
               |                     |  partner-lifetime = 1/2h + 3d
               |                     |    <----BNDREPLY-< |
               |                     |  partner-lifetime = 1/2h + 3d
               |acknowledged partner lifetime = 1/2h + 3d |
  1/2h passes ...                   ...                  ...
     t+1/2h    | >-RENEW-------->    |                    |
               |   acknowledged partner lifetime = 3d     |
               |   lease time = floor( 3d, 3d + 1h ) = 3d |
               |   <---------REPLY-< |                    |
               |   lease-time = 3d   |                    |
               |                     | >-BNDUPD------->   |
               |                     |  partner-lifetime = 1.5d + 3d
               |                     |    <----BNDREPLY-< |
               |                     |  partner-lifetime = 1.5d + 3d
               |acknowledged partner lifetime = 1.5d + 3d |
  1.5d passes ...                   ...                  ...
               |                     |                    |
 t+1.5d + 1/2h | >-RENEW-------->    |                    |
               |  acknowledged partner lifetime = 3d      |
               |   lease time = floor( 3d, 3d + 1h ) = 3d |
               |   <---------REPLY-< |                    |
               |   lease-time = 3d   |                    |
               |                     | >-BNDUPD------->   |
               |                     |  partner-lifetime = 1.5d + 3d
               |                     |    <----BNDREPLY-< |
               |                     |  partner-lifetime = 1.5d + 3d
               |acknowledged partner lifetime = 1.5d + 3d |

                          Figure 1: MCLT Example





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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.









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    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 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.









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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.






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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



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   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.









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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.





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   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























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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)                          .
    .                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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    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.



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   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).























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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





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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                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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    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).




















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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.













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   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.












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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






















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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.























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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.











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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).











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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 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.



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   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.



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   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



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   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.









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   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.








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   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_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_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.




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      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.



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   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.









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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






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   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.





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   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.









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   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.





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   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










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   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.
















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   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.

                 +----------------+---------+-----------+
                 | \   Lease owner|         |           |
                 |  \----------\  | Primary | Secondary |
                 |Algorithm     \ |         |           |
                 +----------------+---------+-----------+
                 | Proportional   | FREE    |FREE-BACKUP|
                 | Independent    | FREE    |    FREE   |
                 +----------------+---------+-----------+

                 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.







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   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.









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   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.




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

               o  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*.





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   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.











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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,



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   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



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   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.








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   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.






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       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.



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   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.

   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.










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   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.  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.







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   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.














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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











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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.






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   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.





































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       +---------------+  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



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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.
















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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.





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           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.















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   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.



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   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.










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   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.



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   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

                   |                                        |
                RECOVER                               PARTNER-DOWN
                   |                                        |
                   | >--UPDREQ-------------------->         |
                   |                                        |
                   |        <---------------------BNDUPD--< |
                   | >--BNDREPLY------------------>         |
                  ...                                      ...
                   |                                        |
                   |        <---------------------BNDUPD--< |
                   | >--BNDREPLY------------------>         |
                   |                                        |
                   |        <--------------------UPDDONE--< |
                   |                                        |
              RECOVER-WAIT                                  |
                   |                                        |
                   | >--STATE-(RECOVER-WAIT)------>         |
                   |                                        |
                   |                                        |
          Wait MCLT from last known                         |
             time of failover operation                     |
                   |                                        |
              RECOVER-DONE                                  |
                   |                                        |
                   | >--STATE-(RECOVER-DONE)------>         |
                   |                                     NORMAL
                   |        <-------------(NORMAL)-STATE--< |
                NORMAL                                      |
                   | >---- State-(NORMAL)--------------->   |
                   |                                        |
                   |                                        |

                 Figure 7: Transition out of RECOVER State






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   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.






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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.








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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.






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   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.






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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.






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

             Primary                                Secondary
              Server                                  Server

              NORMAL                                  NORMAL
                | >--CONTACT------------------->         |
                |        <--------------------CONTACT--< |
                |         [TCP connection broken]        |
           COMMUNICATIONS-         :              COMMUNICATIONS-
             INTERRUPTED           :                INTERRUPTED
                |      [attempt new TCP connection]      |
                |         [connection succeeds]          |
                |                                        |
                | >--CONNECT------------------->         |
                |        <---------------CONNECTREPLY--< |
                | >--STATE--------------------->         |
                |                                     NORMAL
                |        <-------------------STATE-----< |
              NORMAL                                     |
                |                                        |
                | >--BNDUPD-------------------->         |
                |        <-------------------BNDREPLY--< |
                |                                        |
                |        <---------------------BNDUPD--< |
                | >------BNDREPLY-------------->         |
               ...                                      ...
                |                                        |
                |        <--------------------POOLREQ--< |
                | >--POOLRESP------------------>         |
                |                                        |
                | >--BNDUPD-(#1)--------------->         |
                |        <-------------------BNDREPLY--< |
                |                                        |
                | >--BNDUPD-(#2)--------------->         |
                |        <-------------------BNDREPLY--< |
                |                                        |

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










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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.








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             Primary                                Secondary
             Server                                  Server

               |                                        |
         POTENTIAL-CONFLICT                    POTENTIAL-CONFLICT
               |                                        |
               | >--UPDREQ-------------------->         |
               |                                        |
               |        <---------------------BNDUPD--< |
               | >--BNDREPLY------------------>         |
              ...                                      ...
               |                                        |
               |        <---------------------BNDUPD--< |
               | >--BNDREPLY------------------>         |
               |                                        |
               |        <--------------------UPDDONE--< |
         CONFLICT-DONE                                  |
               | >--STATE--(CONFLICT-DONE)---->         |
               |        <---------------------UPDREQ--< |
               |                                        |
               | >--BNDUPD-------------------->         |
               |        <-------------------BNDREPLY--< |
              ...                                      ...
               | >--BNDUPD-------------------->         |
               |        <-------------------BNDREPLY--< |
               |                                        |
               | >--UPDDONE------------------->         |
               |                                     NORMAL
               |        <------------STATE--(NORMAL)--< |
            NORMAL                                      |
               | >--STATE--(NORMAL)----------->         |
               |                                        |
               |        <--------------------POOLREQ--< |
               | >------POOLRESP-------------->         |
               |                                        |

           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



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   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.




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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.





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   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 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.






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   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.






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   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 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.





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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.
















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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.











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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.





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   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)







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   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)






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   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>.






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   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <http://www.rfc-editor.org/info/rfc8174>.








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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

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

   Email: tomasz.mrugalski@gmail.com


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

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














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