Dynamic Host Configuration (DHC)                            T. Mrugalski
Internet-Draft                                                       ISC
Intended status: Standards Track                              K. Kinnear
Expires: April 25, 2013 January 16, 2014                                          Cisco
                                                        October 22, 2012
                                                           July 15, 2013

                         DHCPv6 Failover Design
                draft-ietf-dhc-dhcpv6-failover-design-02
                draft-ietf-dhc-dhcpv6-failover-design-03

Abstract

   DHCPv6 defined in [RFC3315] does not offer server redundancy.  This
   document defines a design 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.  This is a DHCPv6 Failover design document, it is
   not protocol specification document.  It is a second document in a
   planned series of three documents.  DHCPv6 failover requirements are
   specified in [I-D.ietf-dhc-dhcpv6-failover-requirements].  A protocol
   specification document is planned to follow this document.

Status of this This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 25, 2013. January 16, 2014.

Copyright Notice

   Copyright (c) 2012 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .  4   3
   2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4   3
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Additional  Design Requirements . . . . . . . . . . . . . . . . . . .   6
     3.2.  Features out of Scope: Load Balancing . . . . . . . . . .   6
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Failover State Machine Overview . . . . . . . . . . . . .   8
     4.2.  Messages  . . . . . . . . . . . . . . . . . . . . . . . . .  9  10
   5.  Connection Management . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Creating Connections  . . . . . . . . . . . . . . . . . . .  11
     5.2.  Endpoint Identification . . . . . . . . . . . . . . . . . 12  13
   6.  Resource Allocation . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Proportional Allocation . . . . . . . . . . . . . . . . .  14
     6.2.  Independent Allocation  . . . . . . . . . . . . . . . . . .  16
     6.3.  Choosing Allocation Algorithm . . . . . . . . . . . . . . 16  17
   7.  Information model . . . . . . . . . . . . . . . . . . . . . . 17  18
   8.  Failover Mechanisms . . . . . . . . . . . . . . . . . . . . . 21  22
     8.1.  Time Skew . . . . . . . . . . . . . . . . . . . . . . . . 21  22
     8.2.  Time expression . . . . . . . . . . . . . . . . . . . . . 22  23
     8.3.  Lazy updates  . . . . . . . . . . . . . . . . . . . . . . . 22  23
     8.4.  MCLT concept  . . . . . . . . . . . . . . . . . . . . . . . 22  23
       8.4.1.  MCLT example  . . . . . . . . . . . . . . . . . . . . . 24  25
     8.5.  Unreachability detection  . . . . . . . . . . . . . . . . . 25  26
     8.6.  Re-allocating Leases  . . . . . . . . . . . . . . . . . . . 25  26
     8.7.  Sending Binding Update  . . . . . . . . . . . . . . . . . . 26  27
     8.8.  Receiving Binding Update  . . . . . . . . . . . . . . . . . 28  29
     8.9.  Conflict Resolution . . . . . . . . . . . . . . . . . . . 28  30
     8.10. Acknowledging Reception . . . . . . . . . . . . . . . . . 30  32
   9.  Endpoint States . . . . . . . . . . . . . . . . . . . . . . . 30  32
     9.1.  State Machine Operation . . . . . . . . . . . . . . . . . 30  32
     9.2.  State Machine Initialization  . . . . . . . . . . . . . . . 33  35
     9.3.  STARTUP State . . . . . . . . . . . . . . . . . . . . . . 33  35
       9.3.1.  Operation in STARTUP State  . . . . . . . . . . . . . . 34  36
       9.3.2.  Transition Out of STARTUP State . . . . . . . . . . . 34  36
     9.4.  PARTNER-DOWN State  . . . . . . . . . . . . . . . . . . . . 35  38
       9.4.1.  Operation in PARTNER-DOWN State . . . . . . . . . . . 35  38
       9.4.2.  Transition Out of PARTNER-DOWN State  . . . . . . . . . 36  39
     9.5.  RECOVER State . . . . . . . . . . . . . . . . . . . . . . 37  40
       9.5.1.  Operation in RECOVER State  . . . . . . . . . . . . . . 37  40
       9.5.2.  Transition Out of RECOVER State . . . . . . . . . . . 37  40

     9.6.  RECOVER-WAIT State  . . . . . . . . . . . . . . . . . . . . 39  41
       9.6.1.  Operation in RECOVER-WAIT State . . . . . . . . . . . 40  41
       9.6.2.  Transition Out of RECOVER-WAIT State  . . . . . . . . . 40  42
     9.7.  RECOVER-DONE State  . . . . . . . . . . . . . . . . . . . . 40  42
       9.7.1.  Operation in RECOVER-DONE State . . . . . . . . . . . 41  42
       9.7.2.  Transition Out of RECOVER-DONE State  . . . . . . . . . 41  42
     9.8.  NORMAL State  . . . . . . . . . . . . . . . . . . . . . . . 41  43
       9.8.1.  Operation in NORMAL State . . . . . . . . . . . . . . 41  43
       9.8.2.  Transition Out of NORMAL State  . . . . . . . . . . . . 42  44
     9.9.  COMMUNICATIONS-INTERRUPTED State  . . . . . . . . . . . . . 43  44
       9.9.1.  Operation in COMMUNICATIONS-INTERRUPTED State . . . . 43  45
       9.9.2.  Transition Out of COMMUNICATIONS-INTERRUPTED State  . . 44  45
     9.10. POTENTIAL-CONFLICT State  . . . . . . . . . . . . . . . . . 45  47
       9.10.1.  Operation in POTENTIAL-CONFLICT State  . . . . . . . . 46  47
       9.10.2.  Transition Out of POTENTIAL-CONFLICT State . . . . . . 46  47
     9.11. RESOLUTION-INTERRUPTED State  . . . . . . . . . . . . . . . 47  49
       9.11.1.  Operation in RESOLUTION-INTERRUPTED State  . . . . . . 48  49
       9.11.2.  Transition Out of RESOLUTION-INTERRUPTED State . . . . 48  49
     9.12. CONFLICT-DONE State . . . . . . . . . . . . . . . . . . . 48  49
       9.12.1.  Operation in CONFLICT-DONE State . . . . . . . . . . . 48  50
       9.12.2.  Transition Out of CONFLICT-DONE State  . . . . . . . . 49  50
   10. Proposed extensions . . . . . . . . . . . . . . . . . . . . . 49  50
     10.1.  Active-active mode . . . . . . . . . . . . . . . . . . . . 49  50
   11. Dynamic DNS Considerations  . . . . . . . . . . . . . . . . . . 50  51
     11.1.  Relationship between failover and dynamic DNS update . . . 50  51
     11.2.  Exchanging DDNS Information  . . . . . . . . . . . . . . . 51  52
     11.3.  Adding RRs to the DNS  . . . . . . . . . . . . . . . . . . 53  54
     11.4.  Deleting RRs from the DNS  . . . . . . . . . . . . . . . . 54  55
     11.5.  Name Assignment with No Update of DNS  . . . . . . . . . . 54  55
   12. Reservations and failover . . . . . . . . . . . . . . . . . . 55  56
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 56  57
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 56  58
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 56  58
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 57  58
     16.1.  Normative References . . . . . . . . . . . . . . . . . . . 57  58
     16.2.  Informative References . . . . . . . . . . . . . . . . . . 57  58
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 58  59

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
   [I-D.ietf-dhc-dhcpv6-failover-requirements].

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

   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.  Note that a single prefix can only be
      associated with a single failover relationship.  This failover
      endpoint can take actions and hold unique states.  Typically,
      there is a one failover endpoint per partner (server), although
      there may be more.  'Server' and 'failover endpoint' are
      synonymous only if the server participates in only one failover
      relationship.  However, for the sake of simplicity 'Server' is
      used throughout the document to refer to a failover endpoint
      unless to do so would be confusing.

   o  Failover transmission communication - all messages exchanged between partners.

   o  Independent Allocation - a prefix an allocation algorithm to split 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).  See Section 6.2
      for details.

   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 simply partner.

   o  Primary Server - First out of two DHCPv6 servers that participate
      in a failover relationship.  In active-passive mode this is the
      server that handles most of the client traffic.  Its failover
      partner is referred to as secondary server.

   o  Proportional Allocation - a prefix an allocation algorithm that splits the
      available resources (addresses or prefixes) between the primary
      and secondary servers that and maintains proportions between available
      resources on both.  It is particularly well suited for more
      limited resources.  See Section 6.1 for details.

   o  Resource - Any type of resource that is assignable using managed by DHCPv6.
      Currently there are two types of such resources defined: a non-
      temporary IPv6 address and an IPv6 prefix.  Due to the nature of
      temporary addresses, they are not covered by the failover
      mechanism.  Other resource types may be defined in the future.

   o  Responsive - A server that is responsive, will respond to DHCPv6
      client requests.

   o  Secondary Server - Second of out two DHCPv6 servers that
      participate in a failover relationship.  Its failover partner is
      referred to as primary server.  In active-passive mode this server
      typically does not handle client traffic and acts as a backup.

   o  Server - 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 design 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 in the event of failure by one or the other
   of the two servers.

   This protocol defines 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 does receive clients' requests, but
   does not respond to them and only maintains a copy of lease database
   and is ready to take over incoming queries in case of primary server
   failure).  Active-active mode (i.e. both servers actively handling
   clients' requests) is currently not supported for the sake of
   simplicity.  Such a mode may is likely to be defined as an exension at a
   later time. time and will probably be based on
   [I-D.ietf-dhc-dhcpv6-load-balancing].

   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, required as well as requirements to handle time
   skew between failover partners (See Section 8.1), 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 design attempts to fulfill all DHCPv6 failover requirements
   defined in [I-D.ietf-dhc-dhcpv6-failover-requirements].

3.1.  Additional  Design Requirements

   The following requirements are not related to failover mechanism in
   general, but rather to this particular design.

   1.  Minimize Asymmetry - while there are two distinct roles in
       failover (primary and secondary server), the differences between
       those two roles should be as small as possible.  This will yield
       a simpler design as well as a simpler implementation of that
       design.

3.2.  Features out of Scope: Load Balancing

   While it is tempting to extend DHCPv6 failover mechanism to also
   offer load balancing, as DHCPv4 failover did, this design does not do
   that.  Here is the reasoning for this decision.  In general case (not
   related to failover) load balancing solutions are used when each
   server is not able to handle total incoming traffic.  However, by the
   very definition, DHCPv6 failover is supposed to assume service
   availability despite failure of one server.  That leads to conclusion
   that each server must be able to handle whole traffic.  Therefore in
   properly provisioned setup, load balancing is not needed.

   It is likely that active-active mode that is essentially a load
   balancing will be defined as an extension in the near future.

4.  Protocol Overview

   The DHCPv6 Failover Protocol is defined as a communication between
   failover partners with all associated algorithms and mechanisms.
   Failover communication is conducted over a TCP connection established
   between the partners.  The protocol reuses the framing format
   specified in Section 5.1 of DHCPv6 Bulk Leasequery [RFC5460], but
   uses different message types.  New failover-specific message types
   are listed in Section 4.2.  All information is sent over the
   connection as typical DHCPv6 messages that convey DHCPv6 options,
   following format defined in Section 22.1 of [RFC3315].

   After initialization, the primary server establishes a TCP connection
   with its partner.  The primary server sends a CONNECT message with
   initial parameters.  Secondary server responds with CONNECTACK.

   Depending on the failover state of each partner, they MUST initiate
   one of

   If the binding update procedures.  Each primary server MAY send an UPDREQ cannot immediately establish a connection with
   its partner, it will continue to attempt to establish a connection.
   See Section 5.1 for details.

   Depending on the failover state of each partner, they MUST initiate
   one of the binding update procedures.  Each server MAY send an UPDREQ
   message to request its partner to send all updates that have not been
   sent yet (this case applies when the partner has an existing database
   and wants to update it).  Alternatively, a server MAY choose to send
   an UPDREQALL message to request a full lease database transmission
   including all leases (this case applies in case of booting up new
   server after installation, corruption or complete loss of database,
   or other catastrophic failure).

   Servers exchange lease information by using BNDUPD messages.
   Depending on the local and remote state of a lease, a server may
   either accept or reject the update.  Reception of lease update
   information is confirmed by responding with a BNDACK message with
   appropriate status.  The majority of the messages sent over a
   failover TCP connection consists of BNDUPD and BNDACK messages.

   A subset of available resources (addresses or prefixes) is reserved
   for secondary server use.  This is required for handling a case where
   both servers are able to communicate with clients, but unable to
   communicate with each other.  After the initial connection is
   established, the secondary server requests a pool of available
   addresses by sending a POOLREQ message.  The primary server assigns
   addresses to the secondary by sending a series of BNDUPD messages.
   When this process is complete, the primary server sends a POOLRESP
   message to the secondary server.  The secondary server may initiate
   such pool request at any time when in communication with primary
   server.

   Failover servers use a lazy update mechanism to update their failover
   partner about changes to their lease state database.  After a server
   performs any modifications to its lease state database (assign a new
   lease, extend an existing one, extend, release or expire a existing lease), it sends its
   response to the client's request first (performing the "regular"
   DHCPv6 operation) and then informs its failover partner using a
   BNDUPD message.  This BNDUPD message SHOULD be sent soon after the
   response is sent to the DHCPv6 client, but there is no specific
   requirement of a minimum time in which to do so.

   The major problem with lazy update mechanism is the case when the
   server crashes after sending a response to client, but before sending
   the lazy update to its partner (or when communication between
   partners is interrupted).  To solve this problem, the concept known
   as the Maximum Client Lead Time (initially designed for DHCPv4
   failover) is used.  The MCLT is the maximum amount of time that one
   server can extend a lease for a client's binding beyond the time
   known by its failover partner.  See Section 8.4 for detailed
   desciption how the MCLT affects assigned lease times.

   Servers verify each others availability by periodically exchanging
   CONTACT messages.  See Section 8.5 for discussion about detecting a
   partner's unreachability.

   A server that is being shut down transmits a DISCONNECT message,
   closes the connection with its failover partner and stops operation.
   A Server SHOULD transmit any pending lease updates before
   transmitting DISCONNECT message.

4.1.  Failover State Machine Overview

   The following section provides a simplified description of all
   states.  For the sake of clarity and simplicity, it omits important
   details.  For complete description, see Section 9.  In case of a
   disagreement between the simplified and complete description, please
   follow Section 9.

   Each server MUST be in one of the well defines states.  In each state
   a server may be either responsive (responds to clients' queries) or
   unresponsive (clients' queries are ignored).

   A server starts its operation in short-lived STARTUP state.  A server
   determines its partner reachability and state and sets its own state
   based on that determination.  It frequently typically returns back to the state
   it was in before shutdown. shutdown, though the details can be complicated.
   See Section 9.3.2.

   During typical operation when servers maintain communication, both
   are in NORMAL state.  In that state only the primary responds to
   clients' requests.  A secondary server in unresponsive to DHCPv6
   clients. is unresponsive.

   If a server discovers that its partner is no longer reachable, it
   goes to COMMUNICATIONS-INTERRUPTED state.  A server must be extra
   cautious as it can't distingush if its partner is down or just
   communication between servers is interrupted.  Since communication
   between partners is not possible, a server must act on the assumtion
   that its partner is up.  A failover server must follow a defined
   procedure, in particular, it MUST NOT extend any lease more than the
   MCLT beyond its partner's knowledge of the lease expiration time.
   This imposes an additional burden on the server, in that clients will
   return to the server for lease renewals more frequently than they
   would otherwise.  Therefore it is not recommended to operate for
   prolonged periods in this state.  Once communication is
   reestablished, a server may go into NORMAL, POTENTIAL-CONFLICT or
   PARTNER-DOWN state.  It may also stay in COMMUNICATIONS-INTERRUPTED
   state if certain conditions are met.

   Once a server is switched into PARTNER-DOWN (when auto-partner-down
   is used or as a result of administrative action), it can extend
   leases, regardless of the original server that initially granted the
   lease.  In that state server handles leases from its own pool, but
   once its own pool is depleted is also able to serve pool from its
   downed partner.  MCLT restrictions no longer apply.  Operation in
   this mode is less demanding for the server that remains operational,
   than in COMMUNICATIONS-INTERRUPTED state, but PARTNER-DOWN does not
   offer any kind of redundancy.

   When  Even when in PARTNER-DOWN state, a
   failover server does not have an intact lease state database (e.g. due continues to attempt to connect with its failover
   partner.

   A server switches into RECOVER state when any of a variety of
   conditions are encountered:

   o  When a backup server contacts its failover partner for the first time run or catastrophic failure) or detects
      time.

   o  When either server discovers that its failover partner has
      contacted it before but it has no local record of this contact.
      If the record of previous contact is held in the lease-state
      database, then this situation implies that the server has lost its
      lease state database.

   o  When its failover partner is in PARTNER-DOWN state and additional conditions are met, it
   switches to RECOVER state.  In

   Any of these conditions signal that state the server acknowledges
   that content of its database is doubtful and it needs to refresh its
   lease-state database from its partner.  Once this operation is
   complete, it switches to RECOVER-WAIT and later to RECOVER-DONE.  See
   Section 9.6.2.

   Once servers reestablish connection, they discover each others'
   state.  Depending on the conditions, they may return to NORMAL or
   move to POTENTINAL-CONFLICT if the partner is in a state that doesn't
   allow a simple re-integration of the server's lease state databases.
   It is a goal of this protocol to minimize the possibility that
   POTENTIAL-CONFLICT state is ever entered.  Servers running in
   POTENTIAL-CONFLICT do not respond to clients' requests and work only
   on resolving potential conflicts.  Once outstanding lease updates are
   exchanged, servers move to CONFLICT-DONE or NORMAL states.

   Servers that are recovering from potential conflicts and loose
   communication, switch to RESOLUTION-INTERRUPTED.

   A Server server that is being shut down sends a DISCONNECT message.  See
   Section 4.2.  A server that receives a DISCONNECT message moves into
   COMMUNICATIONS-INTERRUPTED state.

4.2.  Messages

   The failover protocol is centered around the message exchanges used
   by one server to update its partner and respond to received updates.
   The following list enumerates these messages.
   It should be noted that no specific formats or message type values
   are assigned at in this stage. document.  Appropriate implementation details
   will be specified in a separate protocol specification document.  The
   following list enumerates these messages:

   o  BNDUPD - The binding update message 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.

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

   o  POOLREQ - The Pool Request message is used by one server
      (typically secondary) to request allocation of resources
      (addresses or prefixes) from its partner.  The partner responds
      with POOLRSP. POOLRESP.

   o  POOLRSP  POOLRESP - The Pool Response message is used by one server
      (typically primary) to repond to its partner's request for
      resources allocation.  One POOLRSP POOLRESP message may contain more than
      one pool.

   o  UPDREQ - The update request message is used by one server to
      request that its partner send all binding database changes that
      has not been sent and confirmed already.  Requested partner is
      expected to respond with zero or more BNDUPD messages, followed by
      UPDDONE that signals end of updates.

   o  UPDREQALL - The update request all is used by one server to
      request that all binding database information be sent in order to
      recover from a total loss of its binding database by the
      requesting server.  Requested server responds with zero or more
      BNDUPD messages, followed by UPDDONE that signal end of updates.

   o  UPDDONE - The update done message is used by the responding server
      to indicate that all requested updates have been sent by the
      responding server and acked by the requesting server.

   o  CONNECT - The connect message is used by the primary server to
      establish a high level connection with the other server, and to
      transmit several important configuration data items between the
      servers.  The partner is expected to confirm by responding with
      CONNECTACK message.

   o  CONNECTACK - The connect acknowledgement message is used by the
      secondary server to respond to a CONNECT message from the primary
      server.

   o  DISCONNECT - The disconnect message is used by either server when
      closing a connection and shutting down.  No response is required
      for this message.

   o  STATE - The state message 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 current state, e.g. after
      connection is established in COMMUNICATIONS-INTERRUPTED or
      PARTNER-DOWN states.

   o  CONTACT - The contact message is used by either server to ensure
      that the other server continues to see the connection as opera-
      tional.  It MUST be transmitted periodically over every esta-
      blished connection if other message traffic is not flowing, and it
      MAY be sent at any time.

5.  Connection Management

5.1.  Creating Connections

   Every primary server implementing the failover protocol SHOULD
   attempt to connect to all of its 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 SHOULD continue to attempt to connect
   periodically.

   Every secondary server implementing the failover protocol SHOULD
   listen for connection attempts from the primary server.

   When a connection attempt succeeds, the primary server which has
   initiated the connection attempt MUST send a CONNECT message down the
   connection.

   When a connection attempt is received, the only information that the
   receiving 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 seconary server, it should just await the CONNECT
   message to determine which relationship this connection is to serve.

   If it has no secondary relationships with the connecting server, it
   SHOULD drop the connection.  The goal is to limit the resources
   expended dealing with attempts to create a spurious failover
   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 simply listens for connection
   attempts from the primary server.

   Once a connection is established, the primary server MUST send a
   CONNECT message across the connection.  A secondary server MUST wait
   for the CONNECT message from a primary server.  If the secondary
   server doesn't receive a CONNECT message from the primary server in
   an installation dependent amount of time, it MAY drop the connection.

   Every CONNECT message includes a TLS-request option, and if the
   CONNECTACK message does not reject the CONNECT message and the TLS-
   reply option says TLS MUST be used, then the servers will immediately
   enter into TLS negotiation.

   Once TLS negotiation is complete, the primary server MUST resend the
   CONNECT message on the newly secured TLS connection and then wait for
   the CONNECTACK message in response.  The TLS-request and TLS-reply
   options MUST NOT appear in either this second CONNECT or its
   associated CONNECTACK message as they had in the first messages.

   The second message sent over a new connection (either a bare TCP
   connection or a connection utilizing TLS) is a STATE message.  Upon
   the receipt of this message, the receiver can consider communications
   up.

5.2.  Endpoint Identification

   The proper operation of the failover protocol requires more than the
   transmission of messages between one server and the other.  Each
   endpoint might seem to be a single DHCPv6 server, but in fact there
   are situations where additional flexibility in configuration is
   useful.  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.

   There is typically little need for addtional additional relationships between
   any two servers but there MAY be more than one failover relationship
   between two servers -- however each MUST have a unique relationship
   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.  Conversely, on a particular server, any given prefix 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
   IP address of the partner, the relationship-name, and the role of the
   receiving server.

6.  Resource Allocation

   Currently there are two allocation algorithms defined for resources
   (addresses or prefixes).  Additional allocation schemes may be
   defined as future extensions.

   1.  Proportional Allocation - This allocation algorithm is a direct
       application of the algorithm defined in [dhcpv4-failover] to
       DHCPv6.  Available  Remaining available resources are split between the
       primary and secondary servers. servers in a configured proportion.
       Released resources are always returned to the primary server.
       Primary and secondary servers may initiate a rebalancing
       procedure when disparity between resources available to each
       server reaches a preconfigured threshold.  Only resources that
       are not leased to any clients are "owned" by one of the servers.
       This algorithm is particularly well suited for scenarios where
       amount of available resources is limited, as may be the case with
       prefix delegation.  See Section 6.1 for details.

   2.  Independent Allocation - This allocation algorithm assumes that
       available resources are split between primary and secondary
       servers as well.  In this case, however, resources are assigned
       to a specific server for all time, regardless if they are
       available or currently used.  This algorithm is much simpler than
       proportional allocation, because resource imbalance doesn't have
       to be checked and there is no rebalancing for independent
       allocation.  This algorithm is particularly well suited for
       scenarios where the there is an abundance of available resources
       which is typically the case for DHCPv6 address allocation.  See
       Section 6.2 for details.

6.1.  Proportional Allocation

   In this allocation scheme, each server has its own pool of available
   resources.  Note that a resource is not "owned" by  Remaining available resources are split between the
   primary and secondary servers in a configured proportion.  Note that
   a resource is not "owned" by a particular server throughout its
   entire lifetime.  Only a resource 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 resource is as follows: initially a resource 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 resources which they own to
   clients, in which case they cease to own them.  When the client
   releases the resource or the lease on it expires, it will again
   become available and will be owned by the primary.

   A resource will not become owned by the server which allocated it
   initially when it is released or the lease expires because, in
   general, that server will have had to replenish its pool of available
   resources well in advance of any likely lease expirations.  Thus,
   having a particular resource cycle back to the secondary might well
   put the secondary more out of balance with respect to the primary
   instead of enhancing the balance of available addresses or prefixes
   between them.

   Pools governed by proportional allocation are used for allocation
   when the server is in all states, except PARTNER-DOWN.  In PARTNER-
   DOWN state the healthy partner can allocate from either pool (both
   its own and its partner's).  This allocation and maintenance of these
   address pools is an area of some sensitivity, 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 by the POOLRESP message where the primary tells the
   secondary how many resources it allocated to the secondary.  Then,
   the primary sends the allocated resources to the secondary via BNDUPD
   messages.  The POOLREQ/POOLRESP message is a trigger to the primary
   to perform a scan of its database and to ensure that the secondary
   has enough resources (based on some configured ratio).

   The primary server SHOULD examine some or all of its database from
   time to time to determine if resources should be shifted between the
   primary and secondary (in either direction).  The POOLREQ/POOLRESP
   message exchange allows the secondary server to explicitly request
   that the primary server examine the entirety of its database to
   ensure that the secondary has the approprite resources available.

   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 in such a way that each
   knows the type and number of resources on every network segment
   participating in the failover protocol.  The primary server is
   responsible for allocating the secondary server the correct
   proportion of available resources of each kind, and the secondary
   server is responsible for being MUST be configured in such a way that it can tell the kind of
   every resource based solely on the IP or prefix address itself.

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

   Available resources can be delegated back to the primary server in
   certain cases.  BNDUPD will contain state FREE for leases that were
   previously in FREE_BACKUP state.

   The POOLREQ/POOLRESP message exchange initiated by the secondary is
   valid at any time, 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
   resources from any of the prefixes.

   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
   resources as it needs (but that it doesn't have more than it needs).

   The primary server SHOULD examine the balance of available resources
   between the primary and secondary for a particular prefix whenever
   the number of available resources for either the primary or secondary
   changes by more than a configured limit.  The primary server SHOULD
   adjust the available resource balance as required to ensure the
   configured resource 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 resource to
   the secondary using a BNDUPD with the state BACKUP.  The primary
   server can attempt to take an available resource away from the
   secondary by sending a BNDUPD with the state FREE.  If the secondary
   accepts the BNDUPD, then it the resource is now available to the PRIMARY primary
   and not available to the secondary.  Of course, the secondary MUST
   reject that BNDUPD if it has already used that resource for a DHCP
   client.

6.2.  Independent Allocation

   In this allocation scheme, available resources are permanently (until
   server configuration changes) split between servers.  Available
   resources are split between the primary and secondary servers as part
   of initial connection establishment.  Once resources are allocated to
   each server, there is no need to reassign them.  The resource
   allocation is algorithmic in nature, and does not require a message
   exchange for each resources allocated.  This algorithm is simpler
   than proportional allocation since it requires similar initial
   communication, but does not require a rebalancing mechanism.  It
   assumes that the pool assigned to each server will never deplete.
   That is often a reasonable assumption for IPv6 addresses (e.g.
   servers are often assigned a /64 pool that contains many more
   addresses than existing electronic devices on Earth).  This
   allocation mechanism SHOULD be used for IPv6 addresses, unless the
   configured address pool is small or is otherwise administratively
   limited.

   Once each server is assigned a resource pool during initial
   connection establishment, it may allocate assigned resources to
   clients.  Once a client release releases a resource or its lease is expired,
   the returned resource returns to pool for the server that leased it.
   Resources never changes servers.

   Resources using 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 resources that were assigned to its downed
   partner and later released by a client unless it is in PARTNER-DOWN
   state.  Server SHOULD use its own pool first before starting new
   assignements from its downed partner's pool.  As the assumption is
   that independent allocation should be used only when available
   resources are vast and not expected to be fully used at any given
   time, it is very unlikely that the server will ever need to use its
   downed partner pools.  This makes a recovery even after prolonged
   down-time much easier.

6.3.  Choosing Allocation Algorithm

   All implementations MUST support proportional allocation algorithm
   and SHOULD support independent allocation.  If the implementation
   implement
   implements both and let lets the user configure it, choose between them, the default
   algorithm used SHOULD be proportional allocation algorithm.

   Proportional allocation mechanism is more flexible as it can
   dynamically rebalance available resources between servers.  That
   balance includes additional burden for the servers and generates more
   traffic between servers.  Proportional algorithm can be considered as
   more efficient at managing available resources more efficiently than resources, compared to
   idenpendent.  That is important aspect when working in a network that
   is nearing address and/or prefix depletion.

   Independent allocation can be used when the number of available
   resources are large and there is no realistic danger of running out
   of resources.  Use of the independent allocation makes communication
   between partners simpler.  It also makes recovery easier and
   potential conflict less likely to appear.

   Typically indepentent independent allocation is used for IPv6 addresses, because
   even for /64 pools a server will never run out of addresses to
   assign, so there is no need to rebalance.  For the prefix delegation
   mechanism, available resources are typically much smaller, so there
   is a danger of running out of addresses. prefixes.  Therefore typically
   proportional allocation will be used for prefix delegations.
   Independent allocation still may be used, but the implication must be
   well understood.  For example in a network that delegates /64
   prefixes out out /48 prefix (so there can be up to 65536 prefixes
   delegated) and a 1000 requesting routers, it is safe to use
   independent allocation.

   It should be stressed out that independent allocation algorithm
   SHOULD NOT be used when number of resources is limited and there is a
   realistic danger of depleting resources.  If this recommendation is
   violated, it may lead to a case, when one server denies clients due
   to pool depletion despite the fact the the other partner still have
   many resources available.

   With independent allocation it is very unlikely to remaining healthy
   server to allocate resources from its unavailable partner's pool.
   That makes recovery easier and any potential conflicts are less
   likely to appear.

7.  Information model

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

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

   The lease binding-status values defined for the failover protocol are
   listed below.  Unless otherwise noted below, there MAY be client
   information associated with each of these binding-status 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 DHCP 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 if proportional algorithm is used.  Client identification
      MAY appear.

   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.

   ABANDONED  -- indicates that a lease is considered unusable by the
      DHCP system.  The primary reason for entering such state is
      reception of DECLINE message for said lease.  Client
      identification MUST NOT appear.

   RESET  -- indicates that this resource was previously abandoned, but
      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 has been presented in Figure 1.  Most states
   are stationary, i.e. the lease stays in a given state untile until exernal
   event triggers transition to another state.  The only transitive
   state is FREE*. One it is reached, the the state machine immediately
   transitions to either FREE or FREE_BACKUP state.

                               +---------+
                /------------->|  ACTIVE |<--------------\
                |              +---------+               |
                |                |  |  |                 |
                |       /--(8)--/  (3)  \--(9)-\         |
                |      |            |           |        |
                |      V            V           V        |
                |  +-------+   +--------+   +---------+  |
                |  |EXPIRED|   |RELEASED|   |ABANDONED|  |
                |  +-------+   +--------+   +---------+  |
                |      |            |            |       |
                |      |            |           (10)     |
                |      |            |            V       |
                |      |            |       +---------+  |
                |      |            |       |  RESET  |  |
                |      |            |       +---------+  |
                |      |            |            |       |
                |       \--(4)--\  (4)  /--(4)--/        |
                |                |  |  |                 |
               (1)               V  V  V                (2)
                |              /---------\               |
                |              |  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.

      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. 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/POOLRSP (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 is 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 ICMP 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 primary (FREE) or
   secondary pool (FREE_BACKUP).  The conditions for specific
   transitions are depicted in Figure 2.

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

                     Figure 2: FREE* State Transitions

   In case of servers operating in active-passive mode, while a majority
   of the resources are owned by the primary server, the secondary
   server will need a portion of the resources to serve new clients
   while operating in COMMUNICATION-INTERRUPTED state and also in
   PARTNER-DOWN state before it can take over the entire address pool
   (after the expiry of MCLT).

   The secondary server connot simply take over the entire resource pool
   immediately, since we have to handle the case where both servers are
   able to communicate with DHCP clients, but unable to communicate with
   each other.

   The size of the resource pool allocated to the secondary is specified
   as a percentage of the currently available resources.  Thus, as the
   number of available resources changes on the primary server, the
   number of resources available to the secondary server MUST also
   change, although the frequency of the changes made to the secondary
   server's pool of address resources SHOULD be low enough to not use
   significant processing power or network bandwidth.

   The required size of this private pool allocated to the secondary
   server is based only on the arrival rate of new DHCP clients and the
   length of expected downtime of the primary server, and is not
   directly influenced by the total number of DHCP clients supported by
   the server pair.

8.  Failover Mechanisms

   This section lays out an overview of the communication between
   partners and other mechanisms required for failover operation.  As
   this is a design document, not a protocol specification, high level
   ideas are presented without implementation specific details (e.g. on-
   wire protocol formats).  Specific protocol details are out of the
   scope of this document, and may be specified in a separate draft.

8.1.  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 NTP, such a service may not always be available
   in some scenarios that failover expects to cover.  Therefore a
   mechanism to measure and track relative time differences between
   servers is necessary.  To do so, each message MUST contain
   information about the time of the transmission in the time context of
   the transmitter.  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
   last TIME_SKEW_PKTS_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 clients with completely
   desynchronized clocks as well as those whose desynchronization itself
   is not constant.

8.2.  Time expression

   Timestamps are expressed as number of seconds since midnight (UTC),
   January 1, 2000, modulo 2^32.  Note: that is the same approach as
   used in creation of DUID-LLT (see Section 9.2 of [RFC3315]).

   Time differences are expressed in seconds and are signed.

8.3.  Lazy updates

   Lazy update refers to the requirement placed on a server implementing
   a failover protocol to update its failover partner whenever the
   binding database changes.  A failover protocol which didn't support
   lazy update would require the failover partner update to complete
   before a DHCPv6 server could respond to a DHCPv6 client request.
   Such approach is often referred to as 'lockstep' and is the opposite
   of lazy updates.  The lazy update mechanism allows a server to
   allocate a new or extend an existing lease 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 client with a particular lease time and before updating
   its partner, the partner will believe that a lease has expired even
   though the client still retains a valid lease on that address or
   prefix.

8.4.  MCLT concept
   In order to handle problem introduced by lazy updates (see
   Section 8.3), 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 typically updates its
   partner with a potential expiration time which is longer than the
   lease time previously given to the client and which is longer than
   the lease time that the server has been configured to give a client.
   This allows that server to give a longer 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 exceed the MCLT beyond the potential
   expiration time acknowledged by its partner.

   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 under any circumstances be greater by more than the maximum
   client lead time (MCLT) greater than MCLT beyond the potential
   expiration time known to a 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
   other server in the pair to be able to make certain assumptions in
   the absence of an ability to communicate between servers.

   The different times are:

   desired valid lifetime:
      The desired valid 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 valid lifetime:
      The actual valid lifetime is the lease interval that a DHCPv6
      server gives out to a DHCPv6 client.  It may be shorter than the
      desired valid lifetime (as explained below).

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

   acknowledged potential valid lifetime:
      The acknowledged potential valid lifetime is the potential lease
      interval the partner server has most recently acknowledged in a
      BNDACK message.

8.4.1.  MCLT example

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

   When a server makes an offer for a new lease on an IP address to a
   DHCPv6 client, it determines the desired valid lifetime (in this
   case, 3 days).  It then examines the acknowledged potential valid
   lifetime (which in this case is zero) and determines the remainder of
   the time left to run, which is also zero.  To this it  It adds the MCLT. MCLT to this
   value.  Since the actual valid lifetime cannot be allowed to exceed
   the remainder of the current acknowledged potential valid lifetime
   plus the MCLT, the offer made to the client is for the remainder of
   the current acknowledged potential valid lifetime (i.e., (i.e. zero) plus
   the MCLT.  Thus, the actual valid lifetime is 1 hour.

   Once the server has sent the REPLY to the DHCPv6 client, it will
   update its failover partner with the lease information.  However, the
   desired potential valid lifetime will be composed of one half of the
   current actual valid lifetime added to the desired valid lifetime.
   Thus, the failover partner is updated with a BNDUPD with a potential
   valid lifetime of 3 days + 1/2 hour.

   When the primary server receives a BNDACK to its update of the
   secondary server's (partner's) potential valid lifetime, it records
   that as the acknowledged potential valid 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.  Thus, the primary server in this case can be sure that the
   secondary server has recorded the potential 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 valid lifetime, which is still 3 days.  It
   then compares this with the original acknowledged potential valid
   lifetime (3 days + 1/2 hour) and adjusts for the time passed since
   the secondary was last updated (1/2 hour).  Thus the time remaining
   of the acknowledged potential valid interval is 3 days.  Adding the
   MCLT to this yields 3 days plus 1 hour, which is more than the
   desired valid lifetime of 3 days.  So the client is renewed for the
   desired valid 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 potential valid lifetime as the T1 fraction of
   the actual client valid lifetime (1/2 of 3 days this time = 1.5
   days).  To this it will add the desired client valid lifetime of 3
   days, yielding a total desired potential valid 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 valid lifetime so as
   to be able to always offer the client the desired client valid
   lifetime.

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

8.5.  Unreachability detection

   Each partner maintains an MUST maintain a FO_SEND timer for each partner failover
   connection.  The FO_SEND timer is reset every time any message is
   transmitted.  If the timer reaches the FO_SEND_MAX value, a CONTACT
   message is transmitted and timer is reset.  The CONTACT message may
   be transmitted at any time.  Implementation MAY use additional
   mechanisms to detect partner unreachability.

   Implementors are advised to keep in mind that the timer based CONTACT
   message mechanism is not perfect and may not detect some failures.
   In particular, if the partner is using one interface to reach clients
   ("downlink") and another to reach its partner ("uplink"), it is
   possible that communication with the clients will break, yet the
   mechanism will still claim full reachability.  For that reason it is
   beneficial to share the same interface for client traffic and
   communication with the failover partner.  That approach may have
   drawbacks in some network topologies.

8.6.  Re-allocating Leases
   When in PARTNER-DOWN state there is a waiting period after which a
   resource can be re-allocated to another client.  For resources which
   are available when the server enters PARTNER-DOWN state, the period
   is the MCLT from the entry into PARTNER-DOWN state.  For resources
   which are not available when the server enters PARTNER-DOWN state,
   the period is the MCLT after the later of the following times: the
   potential valid lifetime, the most recently transmitted potential
   valid lifetime, the most recently received acknowledged potential
   valid lifetime, and the most recently transmitted acknowledged
   potential valid lifetime.  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 resource from one
   client to another without first notifying its partner (through a
   BNDUPD message) and receiving acknowledgement (through a BNDACK mes-
   sage) that its partner is aware that that first client is not using
   the resource.

   This could be modeled in the following way.  Though this specific
   implementation is in no way required, it may serve to better illus-
   trate the concept.

   An "available" resource on a server may be allocated to any client.
   A resource which was 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 resource was EXPIRED or RELEASED through a BNDUPD.  When
   the sending server received the BNDACK for that resource 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 resource 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 PARTNER-DOWN
   state, for instance.

8.7.  Sending Binding Update
   This and the following section is written as though every BNDUPD
   message contains only a single binding update transaction in order to
   reduce the complexity of the discussion.  Note that while a server
   MAY generate BNDUPD messages with multiple binding update
   transactions, every server MUST be able to process a BNDUPD message
   which contains multiple binding update transactions and generate the
   corresponding BNDACK messages with status for multiple binding update
   transactions.

   Each server updates its failover partner about recent changes in
   lease states.  Each update MUST include at least the following
   information:

   1.   resource type - non-temporary address or a prefix.  Resource
        type can be indicated by the container that conveys the actual
        resource (e.g. an IA_NA option indicates non-temporary IPv6
        address).
        address);

   2.   resource information - the actual address or prefix.  That is
        conveyed using the appropriate option, e.g. an IAADDR for an
        address or an IAPREFIX for prefix. a prefix;

   3.   valid life time requested by client client*;

   4.   valid life time sent to client client*;

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

   6.   Next Expected Client Transmission - time interval since Client
        Last Transmission Time, when a response from a client is
        expected.
        expected*;

   7.   potential valid life time - a lifetime that the server is
        willing to set if there were no MCLT/failover restrictions
        imposed.
        imposed*;

   8.   preferred life time sent to client - the actual value sent back
        to the client client*;

   9.   CLTT - Client Last Transaction Time, a timestamp of the last
        received transmission from a client client*;

   10.  Client DUID DUID*.

   Items marked with asterisk MUST appear only if the lease is/was
   associated with a client.  Otherwise it MUST NOT appear, e.g. for
   updates from FREE to FREE_BACKUP state.  Server MUST reject updates
   that does not include any of the aforementioned information.

   The BNDUPD message MAY contain additional information related to the
   updated lease.  The additional information MAY include, but is not
   limited to:

   1.  assigned FQDN name, defined in [RFC4704] [RFC4704];

   2.  Options Requested by the client, i.e. content of the ORO ORO;

   3.  Remote-ID, defined in [RFC4649] [RFC4649];

   4.  Relay-ID, defined in [RFC5460], section 5.4.1 5.4.1;

   5.  Link-layer address
       [I-D.ietf-dhc-dhcpv6-client-link-layer-addr-opt] [RFC6939];

   6.  Any other options the updating partner deems useful.

   Receiving partner MAY store received additional information, but it
   MAY choose to ignore them as well.  Some information may be useful,
   so it is a good idea to keep or update them. it.  One reason is FQDN
   information.  A server SHOULD be prepared to clean up DNS information
   once the lease expires or is released.  See Section Section 11 for
   detailed discussion about Dynamic DNS.  Another reason the partner
   may be interested in keepin keeping additional data is a better support for
   leasequery [RFC5007] or bulk leasequery [RFC5460], which features
   queries based on Relay-ID, by link address and by Remote-ID.

8.8.  Receiving Binding Update

   When a server receives a BNDUPD message, it needs to decide how to
   process the binding update transaction it contains and whether that
   transaction represents a conflict of any sort.  The conflict
   resolution process MUST be used on the receipt of every BNDUPD
   message, not just those that are received while in POTENTIAL-CONFLICT
   state, in order to increase the robustness of the protocol.

   There are three sorts of conflicts:

   1.  Two clients, one resource - This is the duplicate resource
       allocation conflict.  There two different clients each allocated
       the same resource.  See Section 8.9.

   2.  Two resources, one client conflict - This conflict exists when a
       client on one server is associated with a one resource, and on
       the other server with a different resource in the same or related
       subnet.  This does not refer to the case where a single client
       has resources in multiple different subnets or administrative
       domains,
       domains (i.e. a mobile client that changed its location), but
       rather the case where on the same subnet the client has a lease
       on one IP address in one server and on a different IP address on
       the other server.

       This conflict may or may not be a problem for a given DHCP server
       implementation and policy.  If implementations and policies
       allow, both resources can be assigned to a given client.  In the
       event that a DHCP server requires that a DHCP client have only
       one outstanding lease of a given type, the conflict MUST be
       resolved by accepting the lease which has the latest CLTT.

       It should be further clarified that DHCPv6 protocol makes
       assignments based on (client DUID, resource type, iaid) triplet.
       The possibility of using different IAIDs was omitted in this
       paragraph for clarity.  If one client is assigned multiple
       resources of the same type, but with different IAIDs, there is no
       conflict.  Also, iaid values for different resource types are
       orthogonal, i.e. IA_NA with iaid=1 is different than IA_PD with
       iaid=1 and there is no conflict.

   3.  binding-status conflict - This is normal conflict, where one
       server is updating the other with newer information.  See
       Section 8.9 for details of how to resolve these conflicts.

8.9.  Conflict Resolution

   The server receiving a lease update from its partner must evaluate
   the received lease information to see if it is consistent with
   already known state and decide which information - the previously
   known or that just received - is "better".  The server should take
   into consideration the following aspects: if the lease is already
   assigned to a specific client, who had contact with client recently,
   start time of the lease, etc.

   When analyzing a BNDUPD message from a partner server, if there is
   insufficient information in the BNDUPD to process it, then reject the
   BNDUPD with reject-reason 3: "Missing binding information".

   If the resource in the BNDUPD is not a resource associated with the
   failover endpoint which received the BNDUPD message, then reject it
   with reject-reason 1: "Illegal IP address or prefix (not part of any
   address or prefix pool)".

   Every BNDUPD message SHOULD contain a client-last-transaction-time
   option, which MUST, if it appears, be the time that the server last
   interacted with the DHCP client.  It MUST NOT be, for instance, the
   time that the lease on an IP address expired.  If there has been no
   interaction with the DHCP client in question (or there is no DHCP
   client presently associated with this resource), then there will be
   no client-last-transaction-time option in the BNDUPD message.

   The list in Figure 3 presents the conflict resolution outcome.  To
   "accept" BNDUPD means to update the server's bindings database with
   the information contained in the BNDUDP and once the update is
   complete, send a BNDACK message corresponding to the BNDUPD message.
   To "reject" a BNDUPD means to lease the server's binding database
   unchangeg and to respond to the BNDUPD with BNDACK with a rejest-
   reason option included.

   When interpreting the information in the following table (Figure 3),
   for those rules that are listed with "time" -- if a BNDUPD doesn't
   have a client-last-transaction-time value, then it MUST NOT be
   considered later than the client-last-transaction-time in the
   receiving server's binding.  If the BNDUPD contains a client-last-
   transaction-time value and the receiving server's binding does not,
   then the client-last-transaction-time value in the BNDUPD MUST be
   considered later than the server's.

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

   ACTIVE           accept(5) 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(4) reject(4) reject(4)  reject(4)    accept

                       Figure 3: Conflict Resolution

   time(1): If the client-last-transaction-time in the BNDUPD is later
   than the client-last-transaction-time in the receiving server's
   binding, accept it, else reject it.

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

   time(3): If the client-last-transaction-time in the BNDUPD 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 "Outdated binding
   information".

   (4): Use reject reason "Less critical binding information".

   (5): If the clients in a BNDUPD message 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 "Fatal conflict exists:
   address in use by other client".

   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.

8.10.  Acknowledging Reception

   Upon acceptance of a binding lease, server must notify its partner
   that it updated its database.  Server SHOULD NOT send BNDACK before
   its database is updated.  BNDACK MUST contain at lease minimum set of
   information required to unabiguously identify BNDUDP.

9.  Endpoint States

9.1.  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:

   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 allocation 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).

   Several events can cause the transition from one failover state to
   another.

   o  Change in communications status (OK or not OK). OK);

   o  Change in partner's failover state. state;

   o  Explicit administrative action;

   o  Receipt of particular messages. messages;

   o  Expiration of timers.

   Whenever either of the last two of the above state variables changes
   state, the state machine is invoked, which may then trigger a change
   in the current failove 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. words should be
   considered authoritative.

   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.

       +---------------+  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----->+   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 |  Changed|
   +---Other State:-+   RECOVER  |   Secondary   |       V       V   | |
   |    |           |     DONE   |    resolve    |  ++----------+---++ |
   | All Others:  POTENT.  |     |   conflict    |  |CONFLICT-DONE-|+| |
   | Wait for    CONFLICT-    CONFLICT--|-----+      | ----+    see (9.10)        |  | (responsive)   | |
   | Other State:          V            V        |  +------+---------+ |
   | NORMAL or RECOVER    ++------------+---+    | Other State: NORMAL |
   |    |       DONE      |     NORMAL    + +<--------------+          |
   |    +--+----------+-->+   (balanced)    +-------External Command--->+ 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 4: Failover Endpoint State Machine

9.2.  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.

9.3.  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 4) because the processing during
   the STARTUP state can cause the server to transition to any of the
   other states, so that specific state transition arcs would only
   obscure other information.

9.3.1.  Operation in STARTUP State

   The server MUST NOT be responsive 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.

9.3.2.  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 DHCP server nonetheless.  To properly handle
   this situation, a server SHOULD be configurable in such a way as to
   move directly into PARTNER-DOWN state after the startup period
   expires if it has been unable to contact its partner during the
   startup period.

   Step 2:

   If

   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 failoed,
   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
   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:

   Attempt to create a TCP connection to the failover partner.

   Step 5:

   Wait for "communications OK".

   When and if communications become "okay", 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 the server SHOULD transition to the
   PREVIOUS-STATE.

9.4.  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 the 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.

9.4.1.  Operation in PARTNER-DOWN State

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

   It will allow renewal of all outstanding leases on IP addresses. addresses or
   prefixes.  For those IP addresses resources for which the server is using
   proportional allocation, it will allocate IP addresses resources from its own
   pool, and after a fixed period of time (the MCLT interval) has
   elapsed from entry into PARTNER-DOWN state, it will may allocate IP
   addresses from the set of all available IP addresses. pools.  Server SHOULD fully
   deplete its own pool, before starting allocations from its downed
   partner.

   Any IP address resource tagged as available for allocation by the other server
   (at entry to PARTNER-DOWN state) MUST NOT be allocated to a new
   client until the maximum-client-lead-time MCLT beyond the entry into PARTNER-DOWN state has
   elapsed.

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

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

   In the unlikely case, when there are two servers operating in a
   PARTNER-DOWN state, there is a chance of duplicate leases 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.

9.4.2.  Transition Out of PARTNER-DOWN State

   When a server in PARTNER-DOWN state succeeds in establishing a con-
   nection 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.  Essentially, 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)

   If the partner is in:

   NORMAL, COMMUNICATIONS-INTERRUPTED, PARTNER-DOWN, POTENTIAL-CONFLICT,
   RESOLUTION-INTERRUPTED, or CONFLICT-DONE state

   transition to POTENTIAL-CONFLICT state

   If the partner is in:

   RECOVER, RECOVER-WAIT state

   stay in PARTNER-DOWN state

   If the partner is in:

   RECOVER-DONE state

   transition into NORMAL state

9.5.  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.

9.5.1.  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.

9.5.2.  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.

                   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 5: 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.

9.6.  RECOVER-WAIT State

   This state indicates that the server has done 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
   processing that this server might have done prior to losing its
   stable storage will not cause future difficulties.

9.6.1.  Operation in RECOVER-WAIT State

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

9.6.2.  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 IP 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 this is the first time this server has run failover -- as
   determined by the information received from the partner, not
   necessarily only as determined by this server's stable storage (as
   that may have been lost), then the waiting time discussed above may
   be skipped, and the server may MAY transition immediately to RECOVER-DONE
   state.

   If the server has never before run failover, then there is no need to
   wait in this state -- but, again, 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 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 the
   timer.

9.7.  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.

9.7.1.  Operation in RECOVER-DONE State

   A server in RECOVER-DONE state MUST respond only to DHCPREQUEST/
   RENEWAL RENEW, REBIND,
   CONFIRM and DHCPREQUEST/REBINDING DHCP INFORMATION-REQUEST client messages.

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

9.8.  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 IP addresses resources
   (addresses and/or prefixes) for allocation using the POOLREQ message.

9.8.1.  Operation in NORMAL State

   Primary server is responsive in NORMAL state.  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 8.4, the lease interval given to a DHCP
      client can never be more than the MCLT greater than the most
      recently received potential- expiration-time potential-expiration-time from the failover
      partner or the current time, whichever is later.

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

   Lazy update of partner server
      After sending an REPLY that includes lease update to a client, the
      server servicing a DHCP client request attempts to update its
      partner with the new binding information.  Server transmits both
      desired valid lifetime and actual valid lifetime.

   Reallocation of IP addresses resources between clients
      Whenever a client binding is released or expires, a BNDUPD mes-
      sage 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 IP address 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 8.6. 8.6 for details.

   In normal NORMAL state, each server receives binding updates from its
   partner server in BNDUPD messages.  It records these in its client
   binding database in stable storage and then sends a corresponding
   BNDACK message to its partner server.

9.8.2.  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.  Using the
   command in this case would be appropriate if the polling interval and
   timeout were long.

   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 con-
   nection between the server and also capable of sending a CONTACT mes-
   sage every tSend seconds, periodically, but was (for some reason) incapable of pro-
   cessing BNDUPD messages.

   If the communications is determined to not be "ok" (as defined in
   Section 8.5), 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 tran-
   sition 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.

9.9.  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 IP
   address
   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., a "safe period"
   has been configured, see section 10), TODO), then a timer MUST be started
   for the length of the configured safe 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.

9.9.1.  Operation in COMMUNICATIONS-INTERRUPTED State

   In this state a server MUST respond to all DHCP client requests.
   When allocating new leases, each server allocates from its own pool,
   where the primary MUST allocate only FREE resources (addresses or
   prefixes), and the secondary MUST allocate only FREE_BACKUP resources
   (addresses or prefixes).  When responding to RENEW messages, each
   server will allow continued renewal of a DHCP client's current lease
   on an IP address or prefix irrespective 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 potential valid lifetime already acknowledged by the other
   server, or 2) the actual valid lifetime sent to the DHCPv6 client, or
   3) the potential valid lifetime received from the partner server.

   However, since the server cannot communicate with its partner in this
   state, the acknowledged potential valid lifetime will not be updated
   in any new bindings.  This is likely to eventually cause the actual
   valid lifetimes to be the current time plus the MCLT (unless this is
   greater than the desired-client-lease- time). desired-client-lease-time).

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

9.9.2.  Transition Out of COMMUNICATIONS-INTERRUPTED State

   If the safe period 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.

      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-(2)-------------->         |
         |                                        |
         | >--BNDUPD-(#1)--------------->         |
         |        <---------------------BNDACK--< |
         |                                        |
         |        <--------------------POOLREQ--< |
         | >--POOLRESP-(0)-------------->         |
         |                                        |
         | >--BNDUPD-(#2)--------------->         |
         |        <---------------------BNDACK--< |
         |                                        |

    Figure 6: Transition from NORMAL to COMMUNICATIONS-INTERRUPTED and
          back (example with 2 addresses allocated to secondary)

9.10.  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.

   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 of which it is
   currently unaware 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.

9.10.1.  Operation in POTENTIAL-CONFLICT State

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

9.10.2.  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.

       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-(n)---------->         |
         |              addresses                 |

              Figure 7: Transition out of POTENTIAL-CONFLICT

9.11.  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.

   If the servers remained in POTENTIAL-CONFLICT while communications
   was interrupted, neither server would be responsive to DHCP client
   requests, and if one server had crashed, then there might be no
   server able to process DHCP requests.

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

9.11.1.  Operation in RESOLUTION-INTERRUPTED State

   In this state a server MUST respond to all DHCP client requests.
   When allocating new resources (addresses or prefixes), 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 BACKUP resources.  When responding to renewal
   requests, each server will allow continued renewal of a DHCP client's
   current lease independent of whether that lease was given out by the
   receiving server or not, although the renewal period MUST NOT exceed
   the maximum client lead time (MCLT) beyond the latest of: 1) the
   potential valid lifetime already acknowledged by the other server or
   2) the lease-expiration-time or 3) potential valid lifetime received
   from the partner server.

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

9.11.2.  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.

9.12.  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 make a
   transition into CONFLICT-DONE state in order that it may be totally
   responsive to the client load, load.  There is no operational difference
   between CONFLICT-DONE and NORMAL for primary as opposed in both states it
   responds to all clients' requests.  The distinction between CONFLICT-
   DONE and NORMAL state where it
   would states will be in a "balanced" responsive state, running the more apparent when load balancing
   algorithm.
   extension will be defined.

9.12.1.  Operation in CONFLICT-DONE State

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

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

9.12.2.  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.

10.  Proposed extensions

   The following section discusses possible extensions to the proposed
   failover mechanism.  Listed extensions must be sufficiently simple to
   not further complicate failover protocol.  Any proposals that are
   considered complex will be defined as stand-alone extensions in
   separate documents.

10.1.  Active-active mode

   A very simple way to achieve active-active mode is to remove the
   restriction that seconary server MUST NOT respond to SOLICIT and
   REQUEST messages.  Instead it could respond, but MUST have lower
   preference than primary server.  Clients discovering available
   servers will receive ADVERTISE messages from both servers, but are
   expected to select the primary server as it has higher preference
   value configured.  The following REQUEST message will be directed to
   primary server.

   Discussion: Do DHCPv6 clients actually do this?  DHCPv4 clients were
   rumored to wait for a "while" to accept the best offer, but to a
   first approximation, they all take the first offer they receive that
   is even acceptable.

   The benefit of this approach, compared to the "basic" active--passive
   solution is that there is no delay between primary failure and the
   moment when secondary starts serving requests.

   Discussion: The possibility of setting both servers preference to an
   equal value could theoretically work as a crude attempt to provide
   load balancing.  It wouldn't do much good on its own, as one (faster)
   server could be chosen more frequently (assuming that with equal
   preference sets clients will pick first responding server, which is
   not mandated by DHCPv6).  We could design a simple mechanism of
   dynamically updating preference depending on usage of available
   resources.  This concept hasn't been investigated in detail yet.

11.  Dynamic DNS Considerations

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

   The nature of the failover protocol introduces some issues concerning
   dynamic DNS 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.

   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 user of the problem in some way.

11.1.  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 DHCP client, and
   describes the conditions under which it may grant a lease to a new
   DHCP client.  An analogous set of conditions determines when a
   failover server should initiate a 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 DHCP server to grant new leases even if it is unable to
   communicate with the primary server.  The desired external DDNS
   behavior for DHCP 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.

   While DHCP 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 DHCP 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.

11.2.  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 informatin and the FQDN with
   the current DDNS information it has associated with the lease
   binding, and update its notion of the DDNS status accordingly.

   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 IP
   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 DHCP requests.

11.3.  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
   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 DHCP client.

11.4.  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
   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.

11.5.  Name Assignment with No Update of DNS
   In some cases, a DHCP 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.

12.  Reservations and failover

   Some DHCP servers support a capability to offer specific
   preconfigured resources to DHCP clients.  These are real DHCP
   clients, they do the entire DHCP protocol, but these servers always
   offer the client a specific pre-configured resource, one they offer
   that resource to no other clients.  Such a capability has several
   names, but it is sometimes called a "reservation", in that the
   resource is reserved for a particular DHCP client.

   In a situation where there are two DHCP servers serving the same
   subnet without using failover, the two DHCP server's need to have
   disjoint resource pools, but identical reservations for the DHCP
   clients.

   In a failover context, both servers need to be configured with the
   proper reservations in an identical manner, but if we stop there
   problems can occur around the edge conditions where reservations are
   made for resource that has already been leased to a different client.
   Different servers handle this conflict in different ways, but the
   goal of the failover protocol is to allow correct operation with any
   server's approach to the normal processing of the DHCP protocol.

   The general solution with regards to reservations is as follows.
   Whenever a reserved resource becomes FREE (i.e., when first
   configured or whenever a client frees it or it expires or is reset),
   the primary server MUST show that resource as FREE (and thus
   available for its own allocation) and it MUST send it to the
   secondary server in a BNDUPD with a flag set showing that it is
   reserved and with a status of BACKUP.

   Note that this implies that a reserved resource goes through the
   normal state changes from FREE to ACTIVE (and possibly back to FREE).
   The failover protocol supports this approach to reservations, i.e.,
   where the resource undergoes the normal state changes of any
   resource, but it can only be offered to the client for which it is
   reserved.

   From the above, it follows that a reservation soley on the secondary
   will not necessarily allow the secondary to offer that address to
   client to whom it is reserved.  The reservation must also appear on
   the primary as well for the secondary to be able to offer the
   resource to the client to which is is reserved.

   When the reservation on a resource is cancelled, if the resource is
   currently FREE and the server is the primary, or BACKUP and the
   server is the secondary, the server MUST send a BNDUPD to the other
   server with the binding-status FREE and an indication that the
   resource is no longer reserved.

13.  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.

   As traffic exchange between clients and server is not encrypted, an
   attacker than penetrated the network and is able to intercept
   traffic, will not gain anything any additional information by also sniffing
   communication between partners.

   An attacker that can is able to impersonate one partner can efficiently
   perform a denial of service attack on the remaining uncompromised
   server.  Several techniques may be used: pretending that conflict
   resolution is required, requesting rebalance, claming that a valid
   lease was released or declined etc.  For that reason the
   communication between servers SHOULD support failover connections
   over TLS, as explained in Section Section 5.1.  Such secure
   connection SHOULD be optional and configurable by the administrator.

   A server MUST NOT operate in PARTNER-DOWN if its partner is up.
   Network administrator is expected to switch remaining active server
   to PARTNER-DOWN state only if he or she is sure that the other server
   is indeed down.  Failing to obey this requirement will result in both
   servers likely assigning duplicate leases to different clients.
   Implementors should take that into consideration if they decide to
   implement timer-based transition to PARTNER-DOWN state.

   Running a network protected by DHCPv6 failover requires more
   resources than running without it.  In particular some of the
   resources are allocated to the secondary server and they are not
   usable in a normal (i.e. non failures) operation.  While limiting
   this pool may be preferable from resource utilisation perspective, it
   must be reasonably large pool, so the secondary may take over once
   primary becomes unavailable.

   TODO: Security considerations section contains loose notes and will
   be transformed into consistent text once the core design solidifies.

14.  IANA Considerations

   IANA is not requested to perform any actions at this time.

15.  Acknowledgements

   This document extensively uses concepts, definitions and other parts
   of [dhcpv4-failover] document.  Authors would like to thank Shawn
   Routher, Greg Rabil, and Bernie Volz for their significant
   involvement and contributions.  Authors would like to thank Marcin
   Siodelski for his thorough review and VithalPrasad Gaitonde for his
   insightful comments.

   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).

16.  References

16.1.  Normative References

   [I-D.ietf-dhc-dhcpv6-client-link-layer-addr-opt]
              Halwasia, G., Systems, C., and W. Dec, "Client Link-layer
              Address Option in DHCPv6",
              draft-ietf-dhc-dhcpv6-client-link-layer-addr-opt-03 (work
              in progress), October 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC4703]  Stapp, M. and B. Volz, "Resolution of Fully Qualified
              Domain Name (FQDN) Conflicts among Dynamic Host
              Configuration Protocol (DHCP) Clients", RFC 4703, October
              2006.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, October 2006.

   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, May 2013.

16.2.  Informative References

   [I-D.ietf-dhc-dhcpv6-failover-requirements]
              Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
              Requirements",
              draft-ietf-dhc-dhcpv6-failover-requirements-02 draft-ietf-dhc-dhcpv6-failover-
              requirements-06 (work in progress), July 2013.

   [I-D.ietf-dhc-dhcpv6-load-balancing]
              Kostur, A., "DHC Load Balancing Algorithm for DHCPv6",
              draft-ietf-dhc-dhcpv6-load-balancing-00 (work in
              progress), September December 2012.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC4649]  Volz, B., "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6) Relay Agent Remote-ID Option", RFC 4649, August
              2006.

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, September 2007.

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, February
              2009.

   [dhcpv4-failover]
              Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S.,
              Rabil, G., Dooley, M., and A. Kapur, "DHCP Failover
              Protocol", draft-ietf-dhc-failover-12 (work in progress),
              March 2003.

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 Ave.
   Boxborough, Massachusetts  01719
   USA

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