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Versions: (draft-ietf-dhc-dhcpv6-failover-design) 00 01 02 03 04 05 06 RFC 8156

Dynamic Host Configuration (DHC)                            T. Mrugalski
Internet-Draft                                                       ISC
Intended status: Standards Track                              K. Kinnear
Expires: April 18, 2016                                            Cisco
                                                        October 16, 2015


                        DHCPv6 Failover Protocol
               draft-ietf-dhc-dhcpv6-failover-protocol-00

Abstract

   DHCPv6 defined in "Dynamic Host Configuration Protocol for IPv6
   (DHCPv6)" does not offer server redundancy.  This document defines a
   specific protocol implementation to provide for DHCPv6 failover, a
   mechanism for running two servers on the same network with 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".

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 18, 2016.

Copyright Notice

   Copyright (c) 2015 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



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Failover Concepts and Mechanisms  . . . . . . . . . . . . . .   7
     4.1.  Required Server Configuration . . . . . . . . . . . . . .   7
     4.2.  IPv6 Address and Delegable Prefix Allocation  . . . . . .   8
       4.2.1.  Independent Allocation  . . . . . . . . . . . . . . .   8
       4.2.2.  Proportional Allocation . . . . . . . . . . . . . . .   9
     4.3.  Lazy Updates  . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Maximum Client Lead Time (MCLT) . . . . . . . . . . . . .  12
       4.4.1.  MCLT example  . . . . . . . . . . . . . . . . . . . .  13
   5.  Message and Option Definitions  . . . . . . . . . . . . . . .  14
     5.1.  Message Framing for TCP . . . . . . . . . . . . . . . . .  15
     5.2.  Failover Message Format . . . . . . . . . . . . . . . . .  15
     5.3.  Messages  . . . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.1.  BNDUPD  . . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.2.  BNDACK  . . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.3.  POOLREQ . . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.4.  POOLRESP  . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.5.  UPDREQ  . . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.6.  UPDREQALL . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.7.  UPDDONE . . . . . . . . . . . . . . . . . . . . . . .  17
       5.3.8.  CONNECT . . . . . . . . . . . . . . . . . . . . . . .  17
       5.3.9.  CONNECTACK  . . . . . . . . . . . . . . . . . . . . .  17
       5.3.10. DISCONNECT  . . . . . . . . . . . . . . . . . . . . .  17
       5.3.11. STATE . . . . . . . . . . . . . . . . . . . . . . . .  17
       5.3.12. CONTACT . . . . . . . . . . . . . . . . . . . . . . .  17
     5.4.  Options . . . . . . . . . . . . . . . . . . . . . . . . .  18
       5.4.1.  OPTION_F_BINDING_STATUS . . . . . . . . . . . . . . .  18
       5.4.2.  OPTION_F_DNS_REMOVAL_INFO . . . . . . . . . . . . . .  19
       5.4.3.  OPTION_F_DNS_HOST_NAME  . . . . . . . . . . . . . . .  19
       5.4.4.  OPTION_F_DNS_ZONE_NAME  . . . . . . . . . . . . . . .  20
       5.4.5.  OPTION_F_DNS_FLAGS  . . . . . . . . . . . . . . . . .  21
       5.4.6.  OPTION_F_EXPIRATION_TIME  . . . . . . . . . . . . . .  21
       5.4.7.  OPTION_F_MAX_UNACKED_BNDUPD . . . . . . . . . . . . .  22
       5.4.8.  OPTION_F_MCLT . . . . . . . . . . . . . . . . . . . .  23
       5.4.9.  OPTION_F_PARTNER_LIFETIME . . . . . . . . . . . . . .  23
       5.4.10. OPTION_F_PARTNER_LIFETIME_SENT  . . . . . . . . . . .  24
       5.4.11. OPTION_F_PARTNER_DOWN_TIME  . . . . . . . . . . . . .  24
       5.4.12. OPTION_F_PARTNER_RAW_CLT_TIME . . . . . . . . . . . .  25
       5.4.13. OPTION_F_PROTOCOL_VERSION . . . . . . . . . . . . . .  25
       5.4.14. OPTION_F_RECEIVE_TIME . . . . . . . . . . . . . . . .  26



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       5.4.15. OPTION_F_RECONFIGURE_DATA . . . . . . . . . . . . . .  26
       5.4.16. OPTION_F_RELATIONSHIP_NAME  . . . . . . . . . . . . .  27
       5.4.17. OPTION_F_SERVER_FLAGS . . . . . . . . . . . . . . . .  28
       5.4.18. OPTION_F_SERVER_STATE . . . . . . . . . . . . . . . .  29
       5.4.19. OPTION_F_START_TIME_OF_STATE  . . . . . . . . . . . .  30
       5.4.20. OPTION_F_STATE_EXPIRATION_TIME  . . . . . . . . . . .  31
     5.5.  Status Codes  . . . . . . . . . . . . . . . . . . . . . .  31
   6.  Connection Management . . . . . . . . . . . . . . . . . . . .  32
     6.1.  Creating Connections  . . . . . . . . . . . . . . . . . .  32
       6.1.1.  Sending a CONNECT message . . . . . . . . . . . . . .  33
       6.1.2.  Receiving a CONNECT message . . . . . . . . . . . . .  33
       6.1.3.  Receiving a CONNECTACK message  . . . . . . . . . . .  34
     6.2.  Endpoint Identification . . . . . . . . . . . . . . . . .  35
     6.3.  Sending a STATE message . . . . . . . . . . . . . . . . .  35
     6.4.  Receiving a STATE message . . . . . . . . . . . . . . . .  36
     6.5.  Connection Maintenance Parameters . . . . . . . . . . . .  37
     6.6.  Unreachability detection  . . . . . . . . . . . . . . . .  37
   7.  Binding Updates and Acks  . . . . . . . . . . . . . . . . . .  38
     7.1.  Time Skew . . . . . . . . . . . . . . . . . . . . . . . .  38
     7.2.  Information model . . . . . . . . . . . . . . . . . . . .  38
     7.3.  Times Required for Exchanging Binding Updates . . . . . .  42
     7.4.  Sending Binding Updates . . . . . . . . . . . . . . . . .  43
     7.5.  Receiving Binding Updates . . . . . . . . . . . . . . . .  45
       7.5.1.  Correcting Time Skew  . . . . . . . . . . . . . . . .  45
       7.5.2.  Processing Binding Updates  . . . . . . . . . . . . .  46
       7.5.3.  Accept or Reject? . . . . . . . . . . . . . . . . . .  47
       7.5.4.  Accepting Updates . . . . . . . . . . . . . . . . . .  48
     7.6.  Sending Binding Acks  . . . . . . . . . . . . . . . . . .  49
     7.7.  Receiving Binding Acks  . . . . . . . . . . . . . . . . .  50
     7.8.  Acknowledging Reception . . . . . . . . . . . . . . . . .  51
     7.9.  BNDUPD/BNDACK Data Flow . . . . . . . . . . . . . . . . .  51
   8.  Endpoint States . . . . . . . . . . . . . . . . . . . . . . .  52
     8.1.  State Machine Operation . . . . . . . . . . . . . . . . .  52
     8.2.  State Machine Initialization  . . . . . . . . . . . . . .  55
     8.3.  STARTUP State . . . . . . . . . . . . . . . . . . . . . .  55
       8.3.1.  Operation in STARTUP State  . . . . . . . . . . . . .  56
       8.3.2.  Transition Out of STARTUP State . . . . . . . . . . .  56
     8.4.  PARTNER-DOWN State  . . . . . . . . . . . . . . . . . . .  58
       8.4.1.  Operation in PARTNER-DOWN State . . . . . . . . . . .  58
       8.4.2.  Transition Out of PARTNER-DOWN State  . . . . . . . .  59
     8.5.  RECOVER State . . . . . . . . . . . . . . . . . . . . . .  59
       8.5.1.  Operation in RECOVER State  . . . . . . . . . . . . .  60
       8.5.2.  Transition Out of RECOVER State . . . . . . . . . . .  60
     8.6.  RECOVER-WAIT State  . . . . . . . . . . . . . . . . . . .  61
       8.6.1.  Operation in RECOVER-WAIT State . . . . . . . . . . .  62
       8.6.2.  Transition Out of RECOVER-WAIT State  . . . . . . . .  62
     8.7.  RECOVER-DONE State  . . . . . . . . . . . . . . . . . . .  62
       8.7.1.  Operation in RECOVER-DONE State . . . . . . . . . . .  62



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       8.7.2.  Transition Out of RECOVER-DONE State  . . . . . . . .  63
     8.8.  NORMAL State  . . . . . . . . . . . . . . . . . . . . . .  63
       8.8.1.  Operation in NORMAL State . . . . . . . . . . . . . .  63
       8.8.2.  Transition Out of NORMAL State  . . . . . . . . . . .  64
     8.9.  COMMUNICATIONS-INTERRUPTED State  . . . . . . . . . . . .  65
       8.9.1.  Operation in COMMUNICATIONS-INTERRUPTED State . . . .  65
       8.9.2.  Transition Out of COMMUNICATIONS-INTERRUPTED State  .  66
     8.10. POTENTIAL-CONFLICT State  . . . . . . . . . . . . . . . .  67
       8.10.1.  Operation in POTENTIAL-CONFLICT State  . . . . . . .  68
       8.10.2.  Transition Out of POTENTIAL-CONFLICT State . . . . .  68
     8.11. RESOLUTION-INTERRUPTED State  . . . . . . . . . . . . . .  69
       8.11.1.  Operation in RESOLUTION-INTERRUPTED State  . . . . .  70
       8.11.2.  Transition Out of RESOLUTION-INTERRUPTED State . . .  70
     8.12. CONFLICT-DONE State . . . . . . . . . . . . . . . . . . .  70
       8.12.1.  Operation in CONFLICT-DONE State . . . . . . . . . .  71
       8.12.2.  Transition Out of CONFLICT-DONE State  . . . . . . .  71
   9.  Dynamic DNS Considerations  . . . . . . . . . . . . . . . . .  71
     9.1.  Relationship between failover and dynamic DNS update  . .  72
     9.2.  Exchanging DDNS Information . . . . . . . . . . . . . . .  73
     9.3.  Adding RRs to the DNS . . . . . . . . . . . . . . . . . .  74
     9.4.  Deleting RRs from the DNS . . . . . . . . . . . . . . . .  75
     9.5.  Name Assignment with No Update of DNS . . . . . . . . . .  76
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  76
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  77
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  78
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  79
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  79
     13.2.  Informative References . . . . . . . . . . . . . . . . .  80
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  80

1.  Requirements Language

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

2.  Glossary

   This is a supplemental glossary that should be combined with
   definitions in Section 3 of RFC 7031 [RFC7031].

   o  Absolute Time

      The time in seconds since midnight January 1, 2000 UTC, modulo
      2^32).

   o  auto-partner-down




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      A capability where a failover server will move from
      COMMUNICATIONS-INTERRUPTED state to PARTNER-DOWN state
      automatically, without operator intervention.

   o  DDNS

      Dynamic DNS.  Typically used as an acronym referring to dynamic
      update of the DNS.

   o  Delegable Prefix

      A prefix from which other prefixes may be delegated, as described
      in [RFC3633].

   o  Failover endpoint

      The failover protocol allows for there to be 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 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

      All messages exchanged between partners.

   o  Independent Allocation

      An allocation algorithm that splits the available pool of
      resources between the primary and secondary servers that is
      particularly well suited for vast pools (i.e. when available
      resources are not expected to deplete).  It is used for IPv6
      address allocations.  See Section 4.2.1.

   o  Lease

      An association of a DHCPv6 client with an IPv6 address or
      delegated prefix.

   o  MCLT

      Maximum Client Lead Time.  The fundamental relationship on which
      much of the correctness of this protocol depends is that the lease
      expiration time known to a DHCPv6 client MUST NOT be greater by



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      more than the MCLT beyond the partner lifetime time acknowledged
      by that servers's failover partner.  See Section 4.4.

   o  Partner

      Name of the other DHCPv6 server that participates in failover
      relationship.  When the role (primary or secondary) is not
      important, the other server is referred to as a "failover partner"
      or somtimes simply "partner".

   o  Primary Server

      First out of two DHCPv6 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 secondary server.

   o  Proportional Allocation

      An allocation algorithm that splits the available resources
      between the primary and secondary servers and maintains a more or
      less fixed proportion of the available resources between both
      servers.  It is particularly well suited for more limited
      resources.  It is used for allocations of delegated prefixes.  See
      Section 4.2.2.

   o  Resource

      Any type of resource that is managed by DHCPv6.  Currently there
      are three types of such resources defined: a non-temporary IPv6
      address, a temporary IPv6 address, and an IPv6 delegated prefix.
      Only the non-temporary IPv6 addresses and IPv6 delegated prefixes
      are involved in DHCPv6 failover.

   o  Responsive

      A server that is responsive will respond to DHCPv6 client
      requests.

   o  Secondary Server

      Second of two DHCPv6 servers that participate in a failover
      relationship.  Its failover partner is referred to as the primary
      server.  When both servers are operating this server (the
      secondary) typically does not handle client traffic and acts as a
      backup to the primary server.

   o  Server



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      A DHCPv6 server that implements DHCPv6 failover.  'Server' and
      'failover endpoint' are synonymous only if the server participates
      in only one failover relationship.

   o  Unresponsive

      A server that is unresponsive will not respond to DHCPv6 client
      requests.

3.  Introduction

   The failover protocol provides a means for cooperating DHCPv6 servers
   to work together to provide a DHCPv6 service with availability that
   is increased beyond that which could be provided by a single DHCPv6
   server operating alone.  It is designed to protect DHCPv6 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 on an IPv6 address [RFC3315] or on
   an IPv6 prefix [RFC3633] without the DHCPv6 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.

   This 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. actively responds to clients' requests) while
   the second is passive (i.e. it receives clients' requests, but does
   not respond to them and only maintains a copy on the binding database
   and is ready to take over incoming queries in case of primary server
   failure).

   The failover protocol is designed to provide lease stability for
   leases with lease times beyond a short period.  Due in part to the
   additional overhead required as well as requirements to handle time
   skew between failover partners (See Section 7.1) failover is not
   suitable for leases shorter than 30 seconds.  The DHCPv6 Failover
   protocol MUST NOT be used for leases shorter than 30 seconds.

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

4.  Failover Concepts and Mechanisms

4.1.  Required Server Configuration

   Servers frequently have several kinds of resources available on a
   particular network segment.  The failover protocol assumes that both
   primary and secondary servers are configured identically with regard



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   to the prefixes and links involved in DHCPv6.  For delegated prefixes
   (involved in proportional allocation) the primary server is
   responsible for allocating to the secondary server the correct
   proportion of the available delegated 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 for resources
   (IPv6 addresses or delegable prefixes).

4.2.1.  Independent Allocation

   In this allocation scheme, used for allocating individual DHCPv6 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 IPv6 address allocated.  This
   algorithm is simpler than proportional allocation since it does not
   require a rebalancing mechanism.  It assumes that the pool assigned
   to each server will never deplete.

   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 resource 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 any responsive server.  When an IPv6 address goes FREE* it
   is owned by whichever server it is allocated to by the independent
   allocation algorithm.

   IPv6 addresses (which use the independent allocation approach) are
   ignored when a server processes a POOLREQ message.

   During COMMUNICATION-INTERRUPTED events, a partner MAY continue
   extending existing leases when requested by clients.  A healthy
   partner MUST NOT lease IPv6 addresses that were assigned to its
   downed partner and later released by a client unless it is in
   PARTNER-DOWN state.  When it is in PARTNER-DOWN state, a server
   SHOULD use its own pool first and then it can start making new
   assignments from its downed partner's pool.





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4.2.1.1.  Independent Allocation Algorithm

   For every prefix from which IPv6 addresses can be allocated, the
   primary server MUST allocate only IPv6 addresses when the low-order
   bit (i.e., bit 15) is equal to 1, and the secondary server MUST
   allocate only the IPv6 addresses when the low-order bit (i.e., bit
   15) is equal to 0.

4.2.2.  Proportional Allocation

   In this allocation scheme, each server has its own pool of prefixes
   available for delegation.  Remaining available delegeable prefixes
   are split between the primary and secondary servers in a configured
   proportion.  Note that a delegable prefix is not "owned" by a
   particular server throughout its entire lifetime.  Only a delegable
   prefix which is available is "owned" by a particular server -- once
   it has been leased to a client, it is not owned by either failover
   partner.  When it finally becomes available again, it will be owned
   initially 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 a delegable
   prefix is owned by the primary server.  It may be allocated to the
   secondary server if it is available, and then it is owned by the
   secondary server.  Either server can allocate available delegable
   prefixes which they own to clients, in which case they cease to own
   them.  When the client releases the delegated prefix or the lease on
   it expires, it will again become available and will be owned by the
   primary.

   Pools governed by proportional allocation are used for allocation
   when the server is in all states, except PARTNER-DOWN.  In PARTNER-
   DOWN state the operational partner can allocate from either pool
   (both its own, and its partner's after some time constraints have
   elapsed).  The allocation and maintenance of these address pools is
   important, since the goal is to maintain a more or less constant
   ratio of available addresses between the two servers.

   The initial allocation 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 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




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   database and to ensure that the secondary has enough delegable
   prefixes (based on some configured ratio).

   The delegable prefixes are sent to the secondary using the BNDUPD
   message with a state of FREE_BACKUP, which indicates the delegable
   prefix is now available for allocation by the secondary.  Once the
   message is sent, the primary MUST NOT use these prefixes for
   allocation to DHCPv6 clients.

   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 prefixes and determine if the secondary needs more
   delegable prefixes from any of the delegable prefixes which it
   currently owns.

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

   The primary server SHOULD examine the balance of delegable prefixes
   between the primary and secondary for a particular prefix whenever
   the number of available prefixes for either the primary or secondary
   changes by more than a configured limit.  The primary server SHOULD
   adjust the delegable prefix balance as required to ensure the
   configured delegable prefix balance, excepting that the primary
   server SHOULD employ some threshold mechanism to such a balance
   adjustment in order to minimize the overhead of maintaining this
   balance.

   An example of a threshold approach is: do not attempt to re-balance
   the prefixes on the primary and secondary until the out of balance
   value exceeds a configured value.

   The primary server can, at any time, send an available delegable
   prefix to the secondary using a BNDUPD with the state FREE_BACKUP.
   The primary server can attempt to take an available delegable prefix
   away from the secondary by sending a BNDUPD with the state FREE.  If
   the secondary accepts the BNDUPD, then the resource is now available
   to the primary and not available to the secondary.  Of course, the
   secondary MUST reject that BNDUPD if it has already used that
   resource for a DHCPv6 client.







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4.2.2.1.  Re-allocating Leases

   When in PARTNER-DOWN state there is a waiting period after which a
   delegated prefix can be re-allocated to another client.  For
   delegable prefixes which are available when the server enters
   PARTNER-DOWN state, the period is the MCLT from the entry into
   PARTNER-DOWN state.  For delegated prefixes which 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), and the expiration-time.  If this time
   would be earlier than the current time plus the MCLT, then the time
   the server entered PARTNER-DOWN state plus the maximum-client-lead-
   time is used.

   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
   BNDACK message) that its partner is aware that the first client is
   not using the resource.

   This may be modeled in the following way.

   An "available" delegable prefix on a server may be allocated to any
   client.  A prefix which was delegated (leased) to a client and which
   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.  When the sending server received the BNDACK for that
   delegated prefix showing it was FREE, it would move the resource from
   EXPIRED or RELEASED to FREE, and it 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 to its partner.  This situation would exist if
   the lease expired or was released after the transition into PARTNER-
   DOWN state, for instance.

4.3.  Lazy Updates

   The DHCPv6 Failover Requirements document 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 respond to a DHCPv6 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




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   extend an existing lease, respond to the DHCPv6 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 DHCPv6 client with a particular lease time and before
   updating its failover partner, the failover partner will eventually
   believe that the client's lease has expired -- even though the DHCPv6
   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 of the resource to the DHCPv6 client.  Both of these issues
   are dealt with by use of 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 and
   secondary servers.  Proper use of this time interval places an upper
   bound on the difference allowed between the lease time provided to a
   DHCPv6 client by a server and the lease time known by that server's
   failover partner.

   The MCLT is typically much less than the lease time that a server has
   been configured to offer a client, and so some strategy must exist to
   allow a server to offer the configured lease time to a client.
   During a lazy update the updating server updates its failover partner
   with a partner lifetime which is longer than the lease time
   previously given to the DHCPv6 client and which is longer than the
   lease time that the server has been configured to give a client.
   This allows the server to give the configured lease time 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.

   The fundamental relationship on which this protocol depends is: the
   lease expiration time known to a DHCPv6 client MUST NOT be greater by
   more than the MCLT beyond the partner lifetime acknowledged by that
   server's failover partner.

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

   This protocol requires a DHCPv6 server to deal with several different
   lease intervals and places specific restrictions on their
   relationships.  The purpose of these restrictions is to allow the



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   other server in the pair 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:

   desired lifetime:
      The desired lifetime is the lease interval that a DHCPv6 server
      would like to give to a DHCPv6 client in the absence of any
      restrictions imposed by the failover protocol.  Its determination
      is outside of the scope of this protocol.  Typically this is the
      result of external configuration of a DHCPv6 server.

   actual lifetime:
      The actual lifetime is the lease interval that a DHCPv6 server
      gives out to a DHCPv6 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 tells to its partner in a BNDUPD message.

   acknowledged partner lifetime:
      The acknowledged partner lifetime is the partner lifetime the
      partner server has most recently acknowledged in a BNDACK 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
   DHCPv6 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 DHCPv6 client, it will
   update its failover partner with the lease information using a BNDUPD



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   message.  However, the desired partner lifetime will be composed of
   one half of the current actual lifetime added to the desired
   lifetime.  Thus, the failover partner is updated with a BNDUPD with a
   partner lifetime of 1/2 hour + 3 days.

   When the primary server receives a BNDACK 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 BNDACK
   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 7.8.  Thus, the primary server in this case can be sure that
   the secondary server has recorded the parnter lease interval in its
   stable storage when the primary server receives a BNDACK message from
   the secondary server.

   When the DHCPv6 client attempts to renew at T1 (approximately one
   half an 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 time remaining of the
   acknowledged partner interval is 3 days.  Adding the MCLT to this
   yields 3 days plus 1 hour, which is more than the desired lifetime of
   3 days.  So the client may have its lease renewed for the desired
   lifetime -- 3 days.

   When the primary DHCPv6 server updates the secondary DHCPv6 server
   after the DHCPv6 client's renewal REPLY is complete, it will
   calculate the desired partner lifetime as the T1 fraction of the
   actual client lifetime (1/2 of 3 days this time = 1.5 days).  To this
   it will add the desired client lifetime of 3 days, yielding a total
   desired 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 client lifetime.

   Once the initial actual client lifetime of the MCLT is past, the
   protocol operates effectively like the DHCPv6 protocol does today in
   its behavior concerning lifetimes.  However, the guarantee that the
   actual client lifetime will never exceed the remaining acknowledged
   partner server partner lifetime by more than the MCLT allows full
   recovery from a variety of DHCPv6 server failures.

5.  Message and Option Definitions







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5.1.  Message Framing for TCP

   Failover communication is conducted over a TCP connection established
   between the partners.  The 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, described in
   Section 5.2.  All information is sent over the connection as typical
   DHCPv6 messages that convey DHCPv6 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 of DHCPv6 messages
   exchanged between failover partners (which is compatible with the
   format described in Section 6 of [RFC3315]):

       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 DHCPv6 message type; the
                           available message types are listed in
                           below.

      transaction-id       The transaction ID for this message exchange.

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

      options              Options carried in this message.



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5.3.  Messages

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

5.3.1.  BNDUPD

   The binding update message BNDUPD (TBD1) is used to send the binding
   lease changes to the partner.  One message may contain one or more
   lease updates.  The partner is expected to respond with a BNDACK
   message.

5.3.2.  BNDACK

   The binding acknowledgement message BNDACK (TBD2) is used for
   confirmation of the received BNDUPD message.  It may contain a
   positive or negative response (e.g. due to detected lease conflict).

5.3.3.  POOLREQ

   The Pool Request message POOLREQ (TBD3) is used by the secondary
   server to request allocation of resources (addresses or prefixes)
   from the primary server.  The primary responds with POOLRESP.

5.3.4.  POOLRESP

   The Pool Response POOLRESP (TBD4) message is used by the primary
   server to indicate that it has responded to the secondary's request
   for resource allocation.

5.3.5.  UPDREQ

   The update request message UPDREQ (TBD5) 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
   BNDACK message has been received for each of them.

5.3.6.  UPDREQALL

   The update request all UPDREQALL (TBD6) 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 to recover from a
   total loss of its binding database by the requesting server.  A
   server receiving this request responds with zero or more BNDUPD
   messages, followed by an UPDDONE that signals that all of the BNDUPD




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   messages have been sent and a corresponding BNDACK message has been
   received for each of them.

5.3.7.  UPDDONE

   The update done message UPDDONE (TBD7) is used by the server
   responding to an UPDREQ or UPDREQALL 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 (TBD8) is used by the primary server to
   establish a failover connection with the secondary server, and to
   transmit several important configuration data items between the
   servers.  The partner is expected to confirm by responding with
   CONNECTACK message.

5.3.9.  CONNECTACK

   The connect acknowledgement message CONNECTACK (TBD9) is used by the
   secondary server to respond to a CONNECT message from the primary
   server.

5.3.10.  DISCONNECT

   The disconnect message DISCONNECT (TBD10) 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 5.5.  A server SHOULD
   include a descriptive message as to the reasons causing the
   disconnect message.

5.3.11.  STATE

   The state message STATE (TBD11) 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 COMMUNICATIONS-INTERRUPTED or
   PARTNER-DOWN states.

5.3.12.  CONTACT

   The contact message CONTACT (TBD12) is used by either server to
   ensure that its partner continues to see the connection as
   operational.  It MUST be transmitted periodically over every




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   established connection if other message traffic is not flowing, and
   it MAY be sent at any time.  See Section 6.5.

5.4.  Options

   The following new options are defined.

5.4.1.  OPTION_F_BINDING_STATUS

   The binding-status represents an implementation independent
   representation of the status (or the state) of a resource (IPv6
   address or prefix).

   This is an unsigned byte.

   The code for this option is TBD13.


       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 (TBD13).
      option-len        1.
      binding-status    The binding status.  See below.

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

   The binding-status values are discussed in Section 7.2








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5.4.2.  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 TBD14.


       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          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          sub-options                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_DNS_REMOVAL_INFO (TBD14).
      option-len        4.
      sub-options       Three possible sub-options:
                        OPTION_F_DNS_HOST_NAME
                        OPTION_F_DNS_ZONE_NAME
                        OPTION_F_DNS_FLAGS

5.4.3.  OPTION_F_DNS_HOST_NAME

   Contains the host name that was entered into DNS by the failover
   partner.

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

   This is a suboption of OPTION_F_DNS_REMOVAL_INFO.  The suboption code
   for this suboption is 1.


















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     OPTION_F_DNS_HOST_NAME    |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               .
      .                                                               .
      .                           host-name                           .
      .                           (variable)                          .
      .                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_DNS_HOST_NAME (1).
      option-len        0 + length of host-name.
      host-name         RFC 1035 encoded host-name.

5.4.4.  OPTION_F_DNS_ZONE_NAME

   Contains the zone name that was entered into DNS by the failover
   partner.

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

   This is a suboption of OPTION_F_DNS_REMOVAL_INFO.  The suboption code
   for this suboption is 2.


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     OPTION_F_DNS_ZONE_NAME    |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               .
      .                                                               .
      .                           zone-name                           .
      .                           (variable)                          .
      .                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_DNS_ZONE_NAME (2).
      option-len        0 + length of zone-name.
      zone-name         RFC 1035 encoded zone name.








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5.4.5.  OPTION_F_DNS_FLAGS

   Flags which indicate what needs to be done to remove this DNS name.

   This consists an unsigned 16 bit value in network byte order.

   This is a suboption of OPTION_F_DNS_REMOVAL_INFO.  The suboption code
   for this suboption is 3.


       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 (3).
      option-len        2.
      flags             flag bits, see below:

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

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

      4 (U): USING_REQUESTED_FQDN
             Set to 1 to indicate that name used came from the
             FQDN that was received from the client.
      5 (S): SYNTHESIZED_NAME
             Set to 1 to indicate that the name was synthesized
             based on some algorithm.
      6 (R): REV_UPTODATE
             Set to 1 to indicate that the reverse zone is up to date.
      7 (F): FWD_UPTODATE
             Set to 1 to indicate that the forward zone is up to date.
      0-3  : MBZ
             Must be zero

5.4.6.  OPTION_F_EXPIRATION_TIME

   The greatest lifetime that this server has ever acked to its partner
   in a BNDACK.  This MUST be an absolute time (i.e. seconds since
   midnight January 1, 2000 UTC, modulo 2^32).



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

   The code for this option is TBD15.


       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 (TBD15).
      option-len        4.
      expiration-time The expiration time. This MUST be an
                      absolute time (i.e. seconds since midnight
                      January 1, 2000 UTC, modulo 2^32).


5.4.7.  OPTION_F_MAX_UNACKED_BNDUPD

   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 TBD16.


       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 (TBD16).
      option-len         4.
      max-unacked-bndupd Maximum number of unacked BNDUPD message
                         allowed.








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5.4.8.  OPTION_F_MCLT

   The maximum-client-lead-time (MCLT) is the is the upper bound on the
   difference allowed between the lease time provided to a DHCPv6 client
   by a server and the lease time 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 TBD17.


       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 (TBD17).
      option-len        4.
      mclt              The maximum-client-lease-time, in seconds.

5.4.9.  OPTION_F_PARTNER_LIFETIME

   The time after which the partner can consider an IPv6 address expired
   and is able to re-use 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 TBD18.


       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_PARTER_LIFETIME (TBD18).
      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).



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5.4.10.  OPTION_F_PARTNER_LIFETIME_SENT

   The time that was received in an OPTION_F_PARTNER_LIFETIME
   Section 5.4.9 option.  This is an exact duplicate (echo) of the time
   received in the OPTION_F_PARTNER_LIFETIME option, uncorrected and
   unadjusted 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 TBD19.


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |OPTION_F_PARTNER_LIFETIME_SENT |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      partner-lifetime-sent                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_PARTNER_LIFETIME_SENT (TBD19).
      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).

5.4.11.  OPTION_F_PARTNER_DOWN_TIME

   The time that the partner most recently lost commmunications 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 TBD20.













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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_F_PARTNER_DOWN_TIME  |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       partner-down-time                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_PARTNER_DOWN_TIME (TBD20).
      option-len        4.
      partner-down-time Contains the partner-down-time. This MUST be an
                        absolute time (i.e. seconds since midnight
                        January 1, 2000 UTC, modulo 2^32).

5.4.12.  OPTION_F_PARTNER_RAW_CLT_TIME

   The time when the partner most recently interacted with the DHCPv6
   client associated with this IPv6 address.  This MUST be an absolute
   time (i.e. seconds since midnight January 1, 2000 UTC, modulo 2^32).
   This time is uncorrected for clock skew, and remains in the time
   context of the partner server.

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

   The code for this option is TBD21.


       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 (TBD21).
      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).

5.4.13.  OPTION_F_PROTOCOL_VERSION

   The protocol version allows the 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.

   This is an unsigned integer in network byte order.



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   The code for this option is TBD22.


       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          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         protocol-version                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_PROTOCOL_VERSION (TBD22).
      option-len        4.
      protocol-version  The version of the protocol.

5.4.14.  OPTION_F_RECEIVE_TIME

   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 is not ok.

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

   The code for this option is TBD23.


       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_RECEIVE_TIME     |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          receive-time                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_RECEIVE_TIME (TBD23).
      option-len        4.
      receive-time      The receive-time.  An interval of seconds.

5.4.15.  OPTION_F_RECONFIGURE_DATA

   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 TBD24.







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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_F_RECONFIGURE_DATA   |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        reconfigure-time                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               .
      .                                                               .
      .                        reconfigure-key                        .
      .                           (variable)                          .
      .                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_RECONFIGURE_DATA (TBD24).
      option-len        4 + length of reconfigure-key.
      reconfigure-time  Time at which reconfigure-key was created.
                        This MUST be an absolute time (i.e. seconds
                        since midnight
                        January 1, 2000 UTC, modulo 2^32).
      reconfigure-key   The reconfigure-key.

5.4.16.  OPTION_F_RELATIONSHIP_NAME

   A name for this failover relationshiop.

   A UTF-8 encoded text string suitable for display to an end user,
   which MUST NOT be null-terminated.

   The code for this option is TBD25.





















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_F_RELATIONSHIP_NAME  |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               .
      .                                                               .
      .                       relationship-name                       .
      .                           (variable)                          .
      .                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      option-code       OPTION_F_RELATIONSHIP_NAME (TBD25).
      option-len        0 + length of relationship-name.
      relationship-name A UTF-8 encoded text string suitable for
                        display to an end user, which MUST NOT be
                        null-terminated.

5.4.17.  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 TBD26.


       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 (TBD26).
      option-len        1.
      server-flags      The server flags, see below:












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

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

      4 (B): SECONDARY (BACKUP)
             Indicates that the sending server is a secondary
             (or backup) server.
      5 (A): ACK_STARTUP
             Set to 1 to indicate that the OPTION_F_SERVER_FLAGS 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.
      0-3  : MBZ
             Must be zero

5.4.18.  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 TBD27.


       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 (TBD27).
      option-len        1.
      server-state      Failover endpoint state.








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      Value   Server State
      -----   ----------------------------------------------------------
      0       reserved
      1       STARTUP                Startup state (1)
      2       NORMAL                 Normal state
      3       COMMUNICATIONS-INTERRUPTED Communication interrupted
      4       PARTNER-DOWN           Partner down
      5       POTENTIAL-CONFLICT     Synchronizing
      6       RECOVER                Recovering bindings from partner
      7       SHUTDOWN               Shutting down for an long period.
      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 server-flags options (see
   Section 8.3.

5.4.19.  OPTION_F_START_TIME_OF_STATE

   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 or IA_PD message, the state to which it refers is the binding-
   status value in the IA_NA or IA_PD option.  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 TBD28.


       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 (TBD28).
      option-len        4.
      start-time-of-state  The start-time-of-state. This MUST be an
                           absolute time (i.e. seconds since midnight
                           January 1, 2000 UTC, modulo 2^32).




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5.4.20.  OPTION_F_STATE_EXPIRATION_TIME

   The state-expiration-time is 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).

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

   The code for this option is TBD29.


       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 (TBD29).
      option-len        4.
      state-expiration-time  The state-expiration-time.  This MUST be an
                             absolute time (i.e. seconds since midnight
                             January 1, 2000 UTC, modulo 2^32).

5.5.  Status Codes

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

   AddressInUseByOtherClient (TBD30)
      The one client on one server has leased resources that are in
      conflict with the resources that this client has leased on another
      server.

   ConfigurationConflict (TBD31)
      The configuration implied by the information in a BNDUPD (e.g. the
      IPV6 address or prefix address) is in direct conflict with the
      information known to the receiving server.

   MissingBindingInformation (TBD32)
      There is insufficient information in a BNDUPD to effectively
      process it.

   OutdatedBindingInformation (TBD33)
      Returned when the information in a server's binding database
      conflicts with the information found in an incoming BNDUPD, and




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      the server believes that the information in its binding database
      more accurately reflects reality.

   ServerShuttingDown (TBD34)
      Returned when the server is undergoing an operator directed or
      otherwise planned shutdown.

6.  Connection Management

6.1.  Creating Connections

   Every primary server implementing the failover protocol MUST attempt
   to connect to all of its configured partners periodically, where the
   period is implementation dependent and SHOULD be configurable.  In
   the event that a connection has been rejected by a CONNECTACK message
   with a reject-reason 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 connection attempts from the primary server.

   When a primary server attempts to connect with a secondary server, it
   MUST do so 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 "DHCPv6
   server".  The message that is sent over the newly established
   connection is a CONNECT message, instead of an ACTIVELEASEQUERY
   message.

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

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

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



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   When the CONNECT and CONNECTACK 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.

   o  OPTION_F_MCLT containing the configured MCLT.

   o  OPTION_F_RECEIVE_TIME containing the 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 is
      not ok.

   o  OPTION_F_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.

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 accept the connection.  The
   processing is performed as follows:

   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 - Compare the MCLT received with the configured
      MCLT, and if they are different the server MUST alert operational
      staff of this difference.  Use the MCLT supplied by the primary
      server until something explicitly alters the MCLT defined on the
      secondary server.

   o  OPTION_F_RECEIVE_TIME - Remember the receive-time as the
      FO_RECEIVE_TIME when implementing the Unreachability Detection
      algorithm described in Section 6.6.

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



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   A CONNECT message SHOULD always be followed by a CONNECTACK message,
   either to accept the connection or to reject the connection by
   including an OPTION_STATUS_CODE option with an error reject.  In
   order to reject the connection attempt, simply send a CONNECTACK
   message with the OPTION_STATUS_CODE with the correct status.  If
   accepting the connection attempt, then send a CONNECTACK message with
   the following information:

   o  OPTION_F_PROTOCOL_VERSION containing the protocol version being
      used by the secondary server.

   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.

   o  OPTION_F_RECEIVE_TIME containing the 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 is
      not ok.

   o  OPTION_F_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.

   After sending a CONNECTACK 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 CONNECTACK message

   A server receiving a CONNECTACK message must process the information
   in the message and decide whether or not 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.

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

   o  OPTION_F_RECEIVE_TIME - Remember the receive-time as the
      FO_RECEIVE_TIME when implementing the Unreachability Detection
      algorithm described in Section 6.6.





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

   After receiving a CONNECTACK 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 DHCPv6
   prefixes that are configured on the DHCPv6 server where the endpoint
   appears.  A DHCPv6 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 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 end-
   points may reside in the same server process.

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

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

6.3.  Sending a STATE message

   A server MUST send a STATE message to its failover partner whenever
   the state of the failover endpoint changes.  Sending the occasional
   duplicate STATE message will cause no problems, and not updating the
   failover partner with information about a failover endpoint state




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   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 the 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 the failover endpoint.

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

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

6.4.  Receiving a STATE message

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

   o  OPTION_F_SERVER_STATE - If this represents a change in state for
      the failover partner, react according to the direction 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 by code implementing other
      parts of this document.

   o  OPTION_F_START_TIME_OF_STATE - Remember this information in an
      appropriate data area.

   o  OPTION_F_PARTNER_DOWN_TIME - Remember this information in an
      appropriate data area if the value of the OPTION_F_SERVER_STATE is
      PARTNER-DOWN.





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   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_RECEIVE_TIMER  timer     counts down to time connection
                                  assumed dead due to lack of packets

      FO_RECEIVE_TIME   60        maximum time server will consider
                                  connection still up with no packets

      FO_CONTACT_PER_RECEIVE_TIME number of CONTACT messages to send
                        4         during partner's FO_RECEIVE_TIME
                                  period

      FO_SEND_TIMER     timer     counts down to time to send next
                                  CONTACT message

      FO_SEND_TIME      15        maximum time to wait between sending
                                  CONTACT packets if no other traffic
                                  Created from partner's FO_RECEIVE_TIME
                                  divided by FO_CONTACT_PER_RECEIVE_TIME

      FO_SKEW_AVG       10        Number of time-skew values to include
                                  in the moving average time-skew
                                  calculatin

6.6.  Unreachability detection

   Each partner MUST maintain an FO_SEND_TIMER for each failover
   connection.  The FO_SEND_TIMER is reset to FO_SEND_TIME every time
   any message is transmitted, and counts down once per second.  If the
   timer reaches zero, a CONTACT message is transmitted and timer 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_RECEIVE_TIME
   divided by FO_CONTACT_PER_RECEIVE_TIME.  When a CONNECT or CONNECTACK
   message is received, the OPTION_F_RECEIVE_TIME option is checked, and
   if it appears then the value in that option is used to calculate the
   FO_SEND_TIME by dividing the value received by the configured
   FO_CONTACT_PER_RECEIVE_TIME.



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   Each partner MUST maintain an FO_RECEIVE_TIMER for each failover
   connection.  This timer is initialized to FO_RECEIVE_TIME and counts
   down once per second.  It is reset to FO_RECEIVE_TIME whenever a
   packet is received.  If it ever reaches zero, the connection is
   considered dead.  In addition, the FO_RECEIVE_TIME MUST be sent to
   the failover partner on every CONNECT or CONNECTACK messages, in the
   OPTION_F_RECEIVE_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.  Although a
   simple approach would be to require both partners to use synchronized
   time, e.g. by using the Network Time Protocol, such a service may not
   always be available.  Therefore a mechanism to measure and track
   relative time differences between servers is necessary.  To do so,
   each message contains the time of the transmission in the time
   context of the transmitter in the sent-time field of the message
   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
   local timestamp.

   To account for packet delay variation (jitter), the measured
   difference is not used directly, but rather the moving average of the
   last FO_SKEW_AVG packets time difference is calculated.  This
   averaged value is referred to as the time skew.  Note that the time
   skew algorithm allows cooperation between servers with completely
   desynchronized clocks as well as those whose desynchronization itself
   is not constant.

7.2.  Information model

   In most DHCPv6 servers a resource (an IPv6 address or a prefix) can
   take on several different binding-status values, sometimes also
   called lease states.  While no two DHCPv6 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 DHCPv6 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



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   correspond with any actual implementation of the DHCPv6 protocol in a
   DHCPv6 server, but rather that the binding-status values defined in
   this document should be convertable back and forth between those
   defined below and those in use by many DHCPv6 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 value.

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

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

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

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

   FREE  -- Is used when a DHCPv6 server needs to communicate that a
      resource is unused by any client, but it was not just released,
      expired or reset by a network administrator.  When the partner
      acks the BNDUPD 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 resource MAY be allocated to a client by the secondary
      server.  Client identification MAY appear and indicates the last
      client to have used this resource as a hint.

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





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   ABANDONED  -- indicates that a lease is considered unusable by the
      DHCPv6 system.  The primary reason for entering such state is
      reception of DECLINE message for the lease.  Client identification
      MAY appear.

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

   The lease state machine is presented in Figure 1.  Most states are
   stationary, i.e. the lease stays in a given state until external
   event triggers transition to another state.  The only transitive
   state is FREE*. Once it is reached, the state machine immediately
   transitions to either FREE or FREE_BACKUP state.




































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

                             FREE* transition

                       Figure 1: Lease State Machine

   Transitions between states are results of the following events:

      1.  Primary server allocates a lease.

      2.  Secondary server allocates a lease.

      3.  Client sends RELEASE and the lease is released.

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





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

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

      7.  Pool rebalance event occurs (POOLREQ/POOLRESP messages are
      exchanged).  Addresses (or prefixes) belonging to the primary
      server can be assigned to the secondary server pool (transition
      from FREE to FREE_BACKUP) or vice versa.

      8.  The lease has expired.

      9.  DECLINE message is received or a lease is deemed unusable for
      other reasons.

      10.  An administrative action is taken to recover an abandoned
      lease back to usable state.  This transition MAY occur due to an
      implementation specific handling on ABANDONED resource.  One
      possible example of such use is a Neighbor Discovery or ICMPv6
      Echo check if the address is still in use.

   The resource that is no longer in use (due to expiration or release),
   becomes FREE*. Depending of what allocation algorithm is used, the
   resource that is no longer is use, returns to the primary (FREE) or
   secondary pool (FREE_BACKUP).  The conditions for specific
   transitions are depicted in Figure 2.

   +----------------+---------+-----------+
   | \Resource owner|         |           |
   |  \----------\  | Primary | Secondary |
   |Algorithm     \ |         |           |
   +----------------+---------+-----------+
   | Proportional   | FREE    |FREE_BACKUP|
   | Independent    | FREE    |    FREE   |
   +----------------+---------+-----------+

                     Figure 2: 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 DHCPv6 protocol [RFC3315].

   expiration-time




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      The greatest lifteime that this server has ever acked to its
      failover partner in a BNDACK.

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

   partner-lifetime
      The time that we will send (or have sent) the partner, which will
      be the time after which the partner can consider the IPv6 address
      expired.

   client-last-transaction-time
      The time when the this server most recently intereacted with the
      client associated with this IPv6 address.

   partner-raw-clt-time
      The time when the partner most recently interacted with the client
      associated with this IPv6 address.  This time remains exactly as
      it was received by this server, and MUST NOT be adjusted to be in
      the time context of this server.

   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

   Each server updates its failover partner about recent changes in
   lease states using the BNDUPD message.  Every BNDUPD message contains
   information about one or more client bindings.  All information about
   a particular client binding is contained in a single
   OPTION_CLIENT_DATA option (see [RFC5007] Section 4.1.2.2).

   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_LQ_BASE_TIME containing the absolute time that the
      information was placed into this OPTION_CLIENT_DATA option.  (see
      [RFC7653] Section 6.3.1);





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   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]*;

   o  OPTION_IA_NA 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 associated with this client;

      *  IAID - Identity Association used by the client, while obtaining
         a given lease.  (Note1: one client may use many IAIDs
         simultaneously.  Note2: IAID for IA, TA and PD are orthogonal
         number spaces.)*;

      *  T1 time sent to client*;

      *  T2 time sent to client*;

      *  Inside of the IA_NA-options or IA_PD-option sections:

         +  OPTION_IAADDR for an IPv6 address or an OPTION_IAPREFIX for
            a IPv6 prefix;

            -  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;

               o  OPTION_F_START_TIME_OF_STATE containing the start-
                  time-of-state;

               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 this option*;

               o  OPTION_F_PARTNER_LIFETIME (absolute) containing
                  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  DHCP_O_CLIENT_FQDN containing the the FQDN information
                  associated with this resource and client*;

   Note that additonal data MAY be included beyond that listed above.
   The IAddr_options or IAprefix-options area are the places where
   additional information should be included.

   Items marked with a single asterisk (*) MUST appear only if the
   resource is associated with a client.  Otherwise it MUST NOT appear.

   Items marked with a double asterisk (**) MUST appear only if the
   value in the OPTION_F_BINDING_STATUS is associated with a timeout,
   otherwise it MUST NOT appear.

   The OPTION_CLT_TIME MUST, if it appears, be the time that the server
   last interacted with the DHCPv6 client.  It MUST NOT be, for
   instance, the time that the lease on an IPv6 address expired.  If
   there has been no interaction with the DHCPv6 client in question (or
   there is no DHCPv6 client presently associated with this resource),
   then there will be no OPTION_CLT_TIME option in the
   OPTION_CLIENT_DATA option

   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 Dynamic DNS.  Another reason the partner may be
   interested in keeping additional data is a better support for
   leasequery [RFC5007], bulk leasequery [RFC5460] or active leasequery
   [RFC7653], some of which features queries based on Relay-ID, by link
   address and by Remote-ID.

7.5.  Receiving Binding Updates

7.5.1.  Correcting Time Skew

   Unless otherwise specified, all of the times discussed below are
   corrected to be in the time context of the receiving server, as
   follows:

   1.  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 used to affect the
       time correction by being included in the moving average of the
       last FO_SKEW_AVG differences.  This is called the time-
       correction.




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   2.  Any OPTION_LQ_BASE_TIME options in the BNDUPD message MUST be
       corrected with the time-correction.  The result is called the
       corrected-base-time.

   3.  Any relative time values received in the BNDUPD MUST be added to
       or subtracted from the corrected-base-time.

   4.  Any absolute time values received in the BNDUPD MUST be corrected
       with the time-correction

   When all of this is done to an incoming time, that time can be
   before, after, or essentially the same as another time.  Any time
   which 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.  Processing Binding Updates

   When a BNDUPD is received each OPTION_CLIENT_DATA option is processed
   separately, and each must be independently accepted or rejected.

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

   The server receiving a BNDUPD update from its partner must evaluate
   the received information in each OPTION_CLIENT_DATA option to see if
   it is consistent with the server's already known state, and if it is
   not, decide which information - that previously known or that just
   received - is "better".  If the information in the BNDUPD is
   "better", the receiving server will accept the information in the
   BNDUPD.  If the information in the server's binding database is
   "better", the server will reject the information in the BNDUPD.

   A server receving a BNDUPD message MUST respond to the sender of that
   message with a BNDACK message which contains the same transaction-id
   as the BNDUPD message.  This BNDACK message MUST contain one or more
   OPTION_CLIENT_DATA options, each of which corresponds to one of the
   OPTION_CLIENT_DATA options in the BNDUPD message.

   Each OPTION_CLIENT_DATA in the BNDACK which 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.





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

7.5.3.  Accept or Reject?

   The first task in processing the information in an OPTION_CLIENT_DATA
   option is extract the client information and resource information out
   of the OPTION_CLIENT_DATA option, and to access the resource (IPv6
   address or prefix) information in the server's binding database.

   If the resource specified in the OPTION_CLIENT_DATA is not a resource
   associated with the failover endpoint which received the
   OPTION_CLIENT_DATA option, then reject it with reject-reason
   "ConfigurationConflict".

   In general, acceptance or rejection is based around the comparison of
   two different time values, one from the OPTION_CLIENT_DATA and one
   from receiving server's binding database associated with the resource
   found in the OPTION_CLIENT_DATA.  The time for the OPTION_CLIENT_DATA
   is the OPTION_CLT_TIME if one appears, and the
   OPTION_F_START_TIME_OF_STATE if one does not.  The time for the
   resource in the server's binding database is the client-last-
   transaction-time, if one appears, and the start-time-of-state if one
   does not.

   The basic approach is to compare these times, and if the one from the
   OPTION_CLIENT_DATA is clearly later, then accept the information in
   the OPTION_CLIENT_DATA.  If the one from the server's binding
   database is clearly later, then reject the information in the
   OPTION_CLIENT_DATA.  The challenge comes when they are essentially
   the same (i.e., +/- 5 seconds).  The table below (Figure 3) contains
   the rules for dealing with these situations.

                          binding-status in received OPTION_CLIENT_DATA
   binding-status
   in receiving                                      FREE        RESET
   server           ACTIVE   EXPIRED   RELEASED   FREE_BACKUP  ABANDONED

   ACTIVE           accept(4) time(2)   time(1)    time(2)      accept
   EXPIRED          time(1)   accept    accept     accept       accept
   RELEASED         time(1)   time(1)   accept     accept       accept
   FREE/FREE_BACKUP accept    accept    accept     accept       accept
   RESET            time(3)   accept    accept     accept       accept
   ABANDONED        reject    reject    reject     reject       accept

                       Figure 3: Conflict Resolution




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   time(1): If the time value in the OPTION_CLIENT_DATA is later than
   the time value in the server's binding database, accept it, else
   reject it.

   time(2): If the current time is later than the receiving server's
   state-expiration-time, accept it, else reject it.

   time(3): If the OPTION_CLT_TIME value in the OPTION_CLIENT_DATA is
   later than the start-time-of-state in the receiving server's binding,
   accept it, else reject it.

   (1,2,3): If rejecting, use reject reason
   "OutdatedBindingInformation".

   (4): If the client in an OPTION_CLIENT_DATA option and in a receiving
   server's binding differ, then if the receiving server is a secondary
   accept it, else reject it with a reject reason of
   "AddressInUseByOtherClient".

   The lease update may be accepted or rejected.  Rejection SHOULD NOT
   change the flag in a lease that says that it should be transmitted to
   the failover partner.  If this flag is set, then it should be
   transmitted, but if it is not already set, the rejection of a lease
   state update SHOULD NOT trigger an automatic update of the failover
   partner sending the rejected update.  The potential for update storms
   is too great, and in the unusual case where the servers simply can't
   agree, that disagreement is better than an update storm.

7.5.4.  Accepting Updates

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

   The information contained in the 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 IA_NA or IA_PD options in the OPTION_CLIENT_DATA
       option and for each of the OPTION_IADDR or OPTION_IAPREFIX
       options in the IA_* options:



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       1.  OPTION_F_BINDING_STATUS is stored as the binding-status

       2.  OPTION_F_PARTNER_LIFETIME is stored in the expiration-time

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

       4.  OPTION_F_CLT_TIME (which MUST NOT be converted with the
           corrected-base-time, but MUST be converted with the raw value
           from the OPTION_LQ_BASE_TIME) is stored in the partner-raw-
           clt-time

       5.  OPTION_F_PARTNER_RAW_CLT_TIME (which MUST NOT be corrected
           with the time-correction) replaces the client-last-
           transaction-time if it is later than the current client-last-
           transaction-time.

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

7.6.  Sending Binding Acks

   A server MUST respond to every BNDUPD message with a BNDACK message.
   The BNDACK message MUST contain an OPTION_CLIENT_DATA option
   corresponding to every OPTION_CLIENT_DATA option in the BNDUPD
   message.  The BNDACK message MUST have the same transaction-id as the
   BNDUPD message to which it is a response.  Each 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_IA_NA 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 associated with this client;

      *  Inside of the IA_NA-options or IA_PD-option sections:

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



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                  If the information in the corresponding
                  OPTION_CLIENT_DATA in the BNDACK was accepted, and no
                  status message was required (which is the usual case),
                  no OPTION_STATUS_CODE option appears.

               o  OPTION_F_BINDING_STATUS containing the binding-status
                  received in the BNDUPD;

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

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

7.7.  Receiving Binding Acks

   When a BNDACK is received each OPTION_CLIENT_DATA option is processed
   separately, and each can either represent an ACK or a NAK.  If a
   particular OPTION_CLIENT_DATA option does not contain an
   OPTION_STATUS_CODE option, or if there is an OPTION_STATUS_CODE
   option which contains a success code, then the OPTION_CLIENT_DATA
   option represents an acknowledgement (ACK) that the BNDUPD was a
   success.

   Alternatively, the appearance of an OPTION_STATUS_CODE representing
   an error in an OPTION_CLIENT_DATA option indicates a NAK of the
   BNDUPD represented by the OPTION_CLIENT_DATA.

   The information contained in the BNDACK in an OPTION_CLIENT_DATA that
   represents an ACK 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 IA_NA or IA_PD options in the OPTION_CLIENT_DATA
       option and for each of the OPTION_IADDR or OPTION_IAPREFIX
       options:

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

       2.  The time partner-lifetime is set to 0.








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7.8.  Acknowledging Reception

   Upon acceptance of a binding update, the server MUST notify its
   partner that it updated its binding database by sending a BNDACK.  A
   server MUST NOT send the BNDACK before its binding database is
   updated.

7.9.  BNDUPD/BNDACK Data Flow

   The following diagram shows the relationship of the times described
   in Section 7.3 with the options used to transmit them.  It also
   relates the times on one failover partner to the other failover
   partner.






































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   ----------------------- 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      (uncorrected)
                                               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

   ----------------------- BNDACK ------------------------------

     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
                          STATE_EXPIRATION_TIME state-expiration-time

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


                 Figure 4: BNDUPD and BNDACK Time Handling

8.  Endpoint States

8.1.  State Machine Operation

   Each server (or, more accurately, failover endpoint) can take on a
   variety of failover states.  These states play a crucial role in
   determining the actions that a server will perform when processing a
   request from a DHCPv6 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:



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   o  Responsiveness -- the server is either responsive to DHCPv6 client
      requests or it is not.

   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 message.

   o  MCLT -- ensure that valid lifetimes are not beyond what the
      partner has acked plus the MCLT (or not).

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

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

   The following state transition diagram gives a condensed view of the
   state machine.  If there is a difference between the words describing
   a particular state and the diagram below, the words should be
   considered authoritative.

   In the diagram below, the word (responsive) or (unresponsive) appers
   in the states, and refers to whether the server in this state is
   allowed to respond to client DHCPv6 requests.

   In the state transition diagram below, the "+" or "-" in the upper
   right corner of each state is a notation about whether communication
   is ongoing with the other server.







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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    + +<--------------+          |
   |    +--+----------+-->+  (responsive)   +-------External Command-->+
   |       ^          ^   +--------+--------+                          |
   |       |          |            |             |                     |
   |   Wait for   Comm. OK  Comm. Failed         |                     |
   |    Other      Other           |             |             External
   |    State:     State:          |             |             Command
   | RECOVER-DONE  NORMAL     Start Safe      Comm. OK            or
   |       |     COMM. INT.  Period Timer    Other State:        Safe
   |    Comm. OK.     |            V          All Others        Period
   |   Other State:   |  +---------+--------+    |            expiration
   |     RECOVER      +--+ COMMUNICATIONS - +----+                     |
   |       +-------------+   INTERRUPTED    |                          |
   RECOVER               |  (responsive)    +------------------------->+
   RECOVER-WAIT--------->+------------------+

                 Figure 5: 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 packet 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 packet received from partner: None until packet
      received.

8.3.  STARTUP State

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

   The STARTUP state is not shown with any specific state transitions in
   the state machine diagram (Figure 5) because the processing during
   the STARTUP state can cause the server to transition to any of the




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   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 DHCPv6 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 message and the previously
   recorded failover state MUST be placed in the server-state option.

8.3.2.  Transition Out of STARTUP State

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

   Step 1:

   If there is any record in stable storage of a previous failover state
   for this server, set PREVIOUS-STATE to the last recorded value in
   stable storage, 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 which
   already have lease information to synchronize themselves prior to
   operating.

   In some cases, an existing server will be commissioned as a failover
   server and brought back into operation where 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 DHCPv6 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 goes 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 was "OK", then set the previous state to the state



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   that is the result of the communications failed state transition when
   in that state (if such transition exists -- some states don't have a
   communications 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 be created to a heavily loaded partner
   across a slow network.

   Step 4:

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

   Step 5:

   Wait for "communications OK".

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

   If the partner is in PARTNER-DOWN state, and if the time at which it
   entered PARTNER-DOWN state (as received in the 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 the PREVIOUS-STATE.








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

8.4.1.  Operation in PARTNER-DOWN State

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

   It will allow renewal of all outstanding leases on all resources.

   For those resources for which the server is using proportional
   allocation (i.e. prefixes), it will allocate resources 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 IPv6
   addresses from the set of all available pools.  Server MUST fully
   deplete its own pool, before starting allocations from its downed
   partner's pool.

   IPv6 addresses available for independent allocation by the other
   server (at entry to PARTNER-DOWN state) 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 resource to a
   DHCPv6 client different from that to which it was allocated at the
   entrance to 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 lease times
   while in PARTNER-DOWN state.





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   In the unlikely case when there are two servers operating in a
   PARTNER-DOWN state, there is a chance of duplicate leases for the
   same prefix to be assigned.  This leads to a POTENTIAL-CONFLICT
   (unresponsive) state when they re-establish contact.  The duplicate
   lease issue can be postponed to a large extent by the server granting
   new leases first 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 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 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 server-flags option of the STATE
   message.

   If the STARTUP bit is not set in the 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, RECOVER-WAIT ] state stay in
      PARTNER-DOWN state

   o  If the partner is in: [ RECOVER-DONE ] state transition into
      NORMAL state

8.5.  RECOVER State

   This state indicates that the server has no information in its stable
   storage or that it is re-integrating 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.






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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 to POTENTIAL-CONFLICT state
   itself.

   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, while its partner does have a record of
   communicating with it, it MUST send an UPDREQALL message, otherwise
   it MUST send an UPDREQ message.

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

   If communications fails during the reception of the results of the
   UPDREQ or UPDREQALL message, the server will remain in RECOVER state,
   and will re-issue the UPDREQ or UPDREQALL when communications are re-
   established.

   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.


















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                   A                                        B
                 Server                                  Server

                   |                                        |
                RECOVER                               PARTNER-DOWN
                   |                                        |
                   | >--UPDREQ-------------------->         |
                   |                                        |
                   |        <---------------------BNDUPD--< |
                   | >--BNDACK-------------------->         |
                  ...                                      ...
                   |                                        |
                   |        <---------------------BNDUPD--< |
                   | >--BNDACK-------------------->         |
                   |                                        |
                   |        <--------------------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 6: Transition out of RECOVER state

   If at any time while a server is in RECOVER state communications
   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
   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



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   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 to RECOVER-WAIT state the server MUST start a timer whose
   expiration is set to a time equal to the time the server went down
   (if known) or the time the server started (if the down-time is
   unknown) plus the maximum-client-lead-time.  When this timer expires,
   the server will transition into RECOVER-DONE state.

   This is to allow any IPv6 addresses that were allocated by this
   server prior to 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 communications fails while a server is in RECOVER-WAIT state, it
   has no effect on the operation of this state.  The server SHOULD
   continue to operate its timer, and if the timer expires during the
   period where communications with the other server have failed, then
   the server SHOULD transition to RECOVER-DONE state.  This is rare --
   failover state transitions are not usually made while communications
   are interrupted, but in this case there is no reason to inhibit this
   transition.

8.7.  RECOVER-DONE State

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

8.7.1.  Operation in RECOVER-DONE State

   A server in RECOVER-DONE state SHOULD be unresponsive, but MAY
   respond to RENEW requests but MUST only change the state of resources
   that appear in the RENEW request.  It MUST NOT allocate any
   additional resources when in RECOVER-DONE state.





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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, then it will
   transition into NORMAL state.

   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 bindings 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 resources
   (prefixes) for allocation using the POOLREQ message.

8.8.1.  Operation in NORMAL State

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

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

   Lease time calculations
      As discussed in Section 4.4, the lease interval given to a DHCPv6
      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 DHCPv6 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 DHCPv6 client request attempts to update
      its partner with the new binding information.  See Section 4.3.




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   Reallocation of resources 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 BNDACK is received for this message,
      the resource cannot be allocated to another client.  It cannot be
      allocated to the same client again if a BNDUPD 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 7.5.4).  It records
   these in its binding database in stable storage and then sends a
   corresponding BNDACK message to its partner server (see Section 7.6).

8.8.2.  Transition Out of NORMAL State

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

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

   If the communications is determined to not be "ok" (as defined in
   Section 6.6), then 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 for the
   partner to transition from POTENTIAL-CONFLICT into NORMAL state, but
   not for the partner to transition from NORMAL into POTENTIAL-CONFLICT
   state.

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







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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 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 duplicate
   resource allocation 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 some alarm condition to alert
   administrative staff to a potential problem in the DHCP subsystem.

8.9.1.  Operation in COMMUNICATIONS-INTERRUPTED State

   In this state a server MUST respond to all DHCPv6 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 DHCPv6 client's current lease on a resource 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
   latest of: 1) the partner lifetime already acknowledged by the other
   server, or 2) now, or 3) the partner lifetime received from the
   partner server.

   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-client-lease-time, which would be unusual).

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







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8.9.2.  Transition Out of COMMUNICATIONS-INTERRUPTED State

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

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

   If communications is restored with the other server, 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 the 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.

   The following figure illustrates the transition from NORMAL to
   COMMUNICATIONS-INTERRUPTED state and then back to NORMAL state again.

























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      Primary                                Secondary
       Server                                  Server

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

    Figure 7: Transition from NORMAL to COMMUNICATIONS-INTERRUPTED and
                                   back

8.10.  POTENTIAL-CONFLICT State

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




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   It is a goal of this protocol 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 which 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
   DHCPv6 requests.

8.10.2.  Transition Out of POTENTIAL-CONFLICT State

   If communications fails with the partner 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 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--< |
         | >--BNDACK-------------------->         |
        ...                                      ...
         |                                        |
         |        <---------------------BNDUPD--< |
         | >--BNDACK-------------------->         |
         |                                        |
         |        <--------------------UPDDONE--< |
   CONFLICT-DONE                                  |
         | >--STATE--(CONFLICT-DONE)---->         |
         |        <---------------------UPDREQ--< |
         |                                        |
         | >--BNDUPD-------------------->         |
         |        <---------------------BNDACK--< |
        ...                                      ...
         | >--BNDUPD-------------------->         |
         |        <---------------------BNDACK--< |
         |                                        |
         | >--UPDDONE------------------->         |
         |                                     NORMAL
         |        <------------STATE--(NORMAL)--< |
      NORMAL                                      |
         | >--STATE--(NORMAL)----------->         |
         |                                        |
         |        <--------------------POOLREQ--< |
         | >------POOLRESP-------------->         |
         |                                        |

              Figure 8: Transition out of POTENTIAL-CONFLICT

8.11.  RESOLUTION-INTERRUPTED State

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

   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 DHCPv6



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   client requests and there will be no DHCPv6 service available.  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
   DHCPv6 subsystem.

8.11.1.  Operation in RESOLUTION-INTERRUPTED State

   In this state a server MUST respond to all DHCPv6 client requests.
   When allocating new resources, each server SHOULD allocate from its
   own pool (if that can be determined), where the primary SHOULD
   allocate only FREE resources, and the secondary SHOULD allocate only
   FREE_BACKUP resources.  When responding to renewal requests, each
   server will allow continued renewal of a DHCPv6 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 maximum client lead time (MCLT) beyond the latest of: 1) the
   partner lifetime already acknowledged by the other server or 2) now
   or 3) partner lifetime received from the partner server.

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

   If communications is restored with the other server, 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 re-integrate with each other, the primary server
   has received all of the updates from the secondary server.  It makes
   a transition into CONFLICT-DONE state in order that it may be totally
   responsive to the client load.  There is no operational difference
   between CONFLICT-DONE and NORMAL for primary as in both states it
   responds to all clients' requests.  The distinction between CONFLICT-
   DONE and NORMAL states will 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
   DHCPv6 clients (similar to the situation in COMMUNICATIONS-
   INTERRUPTED state).

   If communications fails, remain in CONFLICT-DONE state.  If
   communications becomes OK, remain in CONFLICT-DONE state until the
   conditions for transition out become satisfied.

8.12.2.  Transition Out of CONFLICT-DONE State

   If communications fails with the partner 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.  Dynamic DNS Considerations

   DHCPv6 servers (and clients) can use DNS Dynamic Updates as described
   in RFC 2136 [RFC2136] to maintain DNS name-mappings as they maintain
   DHCPv6 leases.  Many different administrative models for DHCP-DNS
   integration are possible.  Descriptions of several of these models,
   and guidelines that DHCPv6 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
   dynamic DNS (DDNS) updates that are not part of non-failover
   environments.  This section describes these issues, and defines the
   information which failover partners should exchange in order to
   ensure consistent behavior.  The presence of this section should not
   be interpreted as requiring an implementation of the DHCPv6 failover
   protocol to also support DDNS updates.

   The purpose of this discussion is to clarify the areas where the
   failover and DHCP-DDNS protocols intersect for the benefit of
   implementations which support both protocols, not to introduce a new
   requirement into the DHCPv6 failover protocol.  Thus, a DHCPv6 server
   which implements the failover protocol MAY also support dynamic DNS
   updates, but if it does support dynamic 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 DHCPv6 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 which is utilizing the DDNS protocol to update a DNS server
   should not be a partner with a server which is not utilizing the DDNS
   protocol to update a DNS server.  However, a server which is not able
   to support DDNS or is not configured to support DDNS SHOULD output a
   warning message when it receives BNDUPD messages which indicate that
   its failover partner is configured to support the DDNS protocol to
   update a DNS server.  An implementation MAY consider this an error
   and refuse to operate, or it MAY choose to operate anyway, having
   warned the administrator of the problem in some way.

9.1.  Relationship between failover and dynamic DNS update

   The failover protocol describes the conditions under which each
   failover server may renew a lease to its current DHCPv6 client, and
   describes the conditions under which it may grant a lease to a new
   DHCPv6 client.  An analogous set of conditions determines when a
   failover server should initiate a DDNS 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 which they obtained from one failover partner
   even if they can only communicate with the other partner; allowing
   the secondary DHCPv6 server to grant new leases even if it is unable
   to communicate with the primary server.  The desired external DDNS
   behavior for DHCPv6 failover servers is similar to that described
   above for the failover protocol itself:

   1.  Allow timely DDNS updates from the server which grants a lease to
       a client.  Recognize that there is often a DDNS update lifecycle
       which 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 leased resource 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 DDNS update 'lifecycle' even if the
       other server originally granted the lease.

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

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





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   While DHCPv6 servers configured for DDNS typically perform these
   operations on both the AAAA and the PTR resource records, 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 DDNS Information

   In order for either server to be able to complete a DDNS update, or
   to remove DNS records which were added by its partner, both servers
   need to know the FQDN associated with the lease-client binding.  In
   addition, to properly handle DDNS 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
       resource.  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 and DDNS 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 resources 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 DDNS update related to the lease.

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





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

   The FQDN option contains the FQDN that will be associated with the
   AAAA RR (if the server is performing an AAAA RR update for the
   client).  The PTR RR can be generated automatically from the IPv6
   address or prefix value.  The FQDN may be composed in any of several
   ways, depending on server configuration and the information provided
   by the client in its DHCP messages.  The client may supply a hostname
   which 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 DDNS update at the time it sends the first BNDUPD 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 which use some or all of
   the information which 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 which 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 DDNS 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
   which the client includes in its DHCPv6 requests.

9.3.  Adding RRs to the DNS

   A failover server which is going to perform DDNS updates SHOULD
   initiate the DDNS update when it grants a new lease to a client.  The
   server which did not grant the lease SHOULD NOT initiate a DDNS
   update when it receives the BNDUPD after the lease has been granted.
   The failover protocol ensures that only one of the partners will



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   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 DDNS updates when it
   renews leases.  In two cases, however, a failover server MAY initiate
   a DDNS update when it renews a lease to its existing client:

   1.  When the lease was granted before the server was configured to
       perform DDNS updates, the server MAY be configured to perform
       updates when it next renews existing leases.  The server which
       granted the lease is the server which should initiate the DDNS
       update.

   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 DDNS update.  It MAY initiate this update
       immediately upon entry to PARTNER-DOWN state, it may perform this
       in the background, or it MAY initiate this update upon next
       hearing from the DHCPv6 client.

9.4.  Deleting RRs from the DNS

   The failover server which makes a resource FREE* SHOULD initiate any
   DDNS deletes, if it has recorded that DNS records were added on
   behalf of the client.

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

   It is at this point that it should initiate the DDNS operations to
   delete RRs from the DDNS.  Its partner SHOULD NOT initiate DDNS
   deletes for DNS records related to the lease binding as part of
   sending the BNDACK 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 resource FREE*. The server making the



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   resource FREE* may be doing so while it is in NORMAL communication
   with its partner, or it may be in PARTNER-DOWN state.  If a server is
   in PARTNER-DOWN state, it may be performing DDNS deletes for RRs
   which its partner added originally.  This allows a single remaining
   partner server to assume responsibility for all of the DDNS activity
   which the two servers were undertaking.

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

9.5.  Name Assignment with No Update of DNS

   In some cases, a DHCPv6 server is configured to return a name to the
   DHCPv6 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 to which the DHCPv6 client begins to interact.

10.  Security Considerations

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

   The use of TCP introduces some additional concerns.  Attacks that
   attempt to exhaust the DHCPv6 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.

   When operating in secure mode, TLS [RFC5246] 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.






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   Authentication for DHCPv6 messages [RFC3315] MUST NOT be used to
   attempt to secure transmission of the messages described in this
   document.

11.  IANA Considerations

   IANA is requested to assign values for the following new DHCPv6
   Message types in the registry maintained in
   http://www.iana.org/assignments/dhcpv6-parameters:

   o  BNDUPD (TBD1)

   o  BNDACK (TBD2)

   o  POOLREQ (TBD3)

   o  POOLRESP (TBD4)

   o  UPDREQ (TBD5)

   o  UPDREQALL (TBD6)

   o  UPDDONE (TBD7)

   o  CONNECT (TBD8)

   o  CONNECTACK (TBD9)

   o  DISCONNECT (TBD10)

   o  STATE (TBD11)

   o  CONTACT (TBD12)

   IANA is requested to assign values for the following new DHCPv6
   Option codes in the registry maintained in
   http://www.iana.org/assignments/dhcpv6-parameters:

      OPTION_F_BINDING_STATUS (TBD13)

      OPTION_F_DNS_REMOVAL_INFO (TBD14)

      OPTION_F_FAILOVER_EXPIRE_TIME (TBD15)

      OPTION_F_MAX_UNACKED_BNDUPD (TBD16)

      OPTION_F_MCLT (TBD17)




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      OPTION_F_PARTNER_LIFETIME (TBD18)

      OPTION_F_PARTNER_LIFETIME_SENT (TBD19)

      OPTION_F_PARTNER_DOWN_TIME (TBD20)

      OPTION_F_PARTNER_RAW_CLT_TIME (TBD21)

      OPTION_F_PROTOCOL_VERSION (TBD22)

      OPTION_F_RECEIVE_TIME (TBD23)

      OPTION_F_RECONFIGURE_DATA (TBD24)

      OPTION_F_RELATIONSHIP_NAME (TBD25)

      OPTION_F_SERVER_FLAGS (TBD26)

      OPTION_F_SERVER_STATE (TBD27)

      OPTION_F_START_TIME_OF_STATE (TBD28)

      OPTION_F_STATE_EXPIRATION_TIME (TBD29)

   IANA is requested to assign values for the following new DHCPv6
   Status codes in the registry maintained in
   http://www.iana.org/assignments/dhcpv6-parameters:

      AddressInUseByOtherClient (TBD30)

      ConfigurationConflict (TBD31)

      MissingBindingInformation (TBD32)

      OutdatedBindingInformation (TBD33)

      ServerShuttingDown (TBD34)

12.  Acknowledgements

   This document extensively uses concepts, definitions and other parts
   of an effort to document failover for DHCPv4.  Authors would like to
   thank Shawn Routher, Greg Rabil, Bernie Volz and Marcin Siodelski for
   their significant involvement and contributions.  Authors would like
   to thank VithalPrasad Gaitonde, Krzysztof Gierlowski, Krzysztof
   Nowicki and Michal Hoeft for their insightful comments.





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   This work has been partially supported by Department of Computer
   Communications (a division of Gdansk University of Technology) and
   the Polish Ministry of Science and Higher Education under the
   European Regional Development Fund, Grant No.
   POIG.01.01.02-00-045/09-00 (Future Internet Engineering Project).

13.  References

13.1.  Normative References

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

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

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

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

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

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

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

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



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Internet-Draft          DHCPv6 Failover Protocol            October 2015


   [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>.

   [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>.

13.2.  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>.

Authors' Addresses

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

   Phone: +1 650 423 1345
   Email: tomasz.mrugalski@gmail.com


   Kim Kinnear
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, Massachusetts  01719
   USA

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






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