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Versions: (draft-mrugalski-dhc-dhcpv6-failover-design) 00 01 02 03 04 draft-ietf-dhc-dhcpv6-failover-protocol

Dynamic Host Configuration (DHC)                            T. Mrugalski
Internet-Draft                                                       ISC
Intended status: Standards Track                              K. Kinnear
Expires: January 5, 2013                                           Cisco
                                                            July 4, 2012


                         DHCPv6 Failover Design
                draft-ietf-dhc-dhcpv6-failover-design-00

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 Memo

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

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

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

   This Internet-Draft will expire on January 5, 2013.

Copyright Notice

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

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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   2.  Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Additional Requirements  . . . . . . . . . . . . . . . . .  5
     3.2.  Features out of Scope: Load Balancing  . . . . . . . . . .  6
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Failover Machine Sate Overview . . . . . . . . . . . . . .  7
   5.  Connection Management  . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Creating Connections . . . . . . . . . . . . . . . . . . .  9
     5.2.  Endpoint Identification  . . . . . . . . . . . . . . . . . 10
   6.  Resource Allocation  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Proportional Allocation  . . . . . . . . . . . . . . . . . 12
     6.2.  Independent Allocation . . . . . . . . . . . . . . . . . . 13
     6.3.  Determining Allocation Approach  . . . . . . . . . . . . . 13
       6.3.1.  IPv6 Addresses . . . . . . . . . . . . . . . . . . . . 13
       6.3.2.  IPv6 Prefixes  . . . . . . . . . . . . . . . . . . . . 13
   7.  Information model  . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Failover Mechanisms  . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Time Skew  . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.2.  Time expression  . . . . . . . . . . . . . . . . . . . . . 14
     8.3.  Lazy updates . . . . . . . . . . . . . . . . . . . . . . . 15
     8.4.  MCLT concept . . . . . . . . . . . . . . . . . . . . . . . 15
       8.4.1.  MCLT example . . . . . . . . . . . . . . . . . . . . . 16
     8.5.  Unreachability detection . . . . . . . . . . . . . . . . . 18
     8.6.  Re-allocating Leases . . . . . . . . . . . . . . . . . . . 18
     8.7.  Sending Data . . . . . . . . . . . . . . . . . . . . . . . 18
       8.7.1.  Required Data  . . . . . . . . . . . . . . . . . . . . 19
       8.7.2.  Optional Data  . . . . . . . . . . . . . . . . . . . . 19
     8.8.  Receiving Data . . . . . . . . . . . . . . . . . . . . . . 19
       8.8.1.  Conflict Resolution  . . . . . . . . . . . . . . . . . 19
       8.8.2.  Acknowledging Reception  . . . . . . . . . . . . . . . 19
   9.  Endpoint States  . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  State Machine Operation  . . . . . . . . . . . . . . . . . 19
     9.2.  State Machine Initialization . . . . . . . . . . . . . . . 22
     9.3.  STARTUP State  . . . . . . . . . . . . . . . . . . . . . . 22
       9.3.1.  Operation in STARTUP State . . . . . . . . . . . . . . 22
       9.3.2.  Transition Out of STARTUP State  . . . . . . . . . . . 22
     9.4.  PARTNER-DOWN State . . . . . . . . . . . . . . . . . . . . 24
       9.4.1.  Operation in PARTNER-DOWN State  . . . . . . . . . . . 24



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       9.4.2.  Transition Out of PARTNER-DOWN State . . . . . . . . . 25
     9.5.  RECOVER State  . . . . . . . . . . . . . . . . . . . . . . 25
       9.5.1.  Operation in RECOVER State . . . . . . . . . . . . . . 26
       9.5.2.  Transition Out of RECOVER State  . . . . . . . . . . . 26
     9.6.  RECOVER-WAIT State . . . . . . . . . . . . . . . . . . . . 27
       9.6.1.  Operation in RECOVER-WAIT State  . . . . . . . . . . . 28
       9.6.2.  Transition Out of RECOVER-WAIT State . . . . . . . . . 28
     9.7.  RECOVER-DONE State . . . . . . . . . . . . . . . . . . . . 28
       9.7.1.  Operation in RECOVER-DONE State  . . . . . . . . . . . 29
       9.7.2.  Transition Out of RECOVER-DONE State . . . . . . . . . 29
     9.8.  NORMAL State . . . . . . . . . . . . . . . . . . . . . . . 29
       9.8.1.  Operation in NORMAL State  . . . . . . . . . . . . . . 29
       9.8.2.  Transition Out of NORMAL State . . . . . . . . . . . . 30
     9.9.  COMMUNICATIONS-INTERRUPTED State . . . . . . . . . . . . . 31
       9.9.1.  Operation in COMMUNICATIONS-INTERRUPTED State  . . . . 31
       9.9.2.  Transition Out of COMMUNICATIONS-INTERRUPTED State . . 32
     9.10. POTENTIAL-CONFLICT State . . . . . . . . . . . . . . . . . 33
       9.10.1. Operation in POTENTIAL-CONFLICT State  . . . . . . . . 34
       9.10.2. Transition Out of POTENTIAL-CONFLICT State . . . . . . 34
     9.11. RESOLUTION-INTERRUPTED State . . . . . . . . . . . . . . . 35
       9.11.1. Operation in RESOLUTION-INTERRUPTED State  . . . . . . 36
       9.11.2. Transition Out of RESOLUTION-INTERRUPTED State . . . . 36
     9.12. CONFLICT-DONE State  . . . . . . . . . . . . . . . . . . . 36
       9.12.1. Operation in CONFLICT-DONE State . . . . . . . . . . . 37
       9.12.2. Transition Out of CONFLICT-DONE State  . . . . . . . . 37
     9.13. PAUSED State . . . . . . . . . . . . . . . . . . . . . . . 37
       9.13.1. Operation in PAUSED State  . . . . . . . . . . . . . . 37
       9.13.2. Transition Out of PAUSED State . . . . . . . . . . . . 38
     9.14. SHUTDOWN State . . . . . . . . . . . . . . . . . . . . . . 38
       9.14.1. Operation in SHUTDOWN State  . . . . . . . . . . . . . 38
       9.14.2. Transition Out of SHUTDOWN State . . . . . . . . . . . 38
   10. Proposed extensions  . . . . . . . . . . . . . . . . . . . . . 38
     10.1. Active-active mode . . . . . . . . . . . . . . . . . . . . 39
   11. Dynamic DNS Considerations . . . . . . . . . . . . . . . . . . 39
   12. Reservations and failover  . . . . . . . . . . . . . . . . . . 39
   13. Protocol entities  . . . . . . . . . . . . . . . . . . . . . . 39
     13.1. Failover Protocol  . . . . . . . . . . . . . . . . . . . . 40
     13.2. Protocol constants . . . . . . . . . . . . . . . . . . . . 40
   14. Open questions . . . . . . . . . . . . . . . . . . . . . . . . 40
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 41
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     18.2. Informative References . . . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42





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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  Failover endpoint - The failover protocol allows for there to be a
      unique failover 'endpoint' per partner per role per relationship
      (where role is primary or secondary and the relationship is
      defined by the relationship-name).  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 - all messages exchanged between partners.

   o  Independent Allocation - a prefix allocation algorithm to split
      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  Primary Server

   o  Proportional Allocation - a prefix allocation algorithm to split
      the available free leases between the primary and secondary
      servers that is particularly well suited for more limited
      resources.  See Section 6.1 for details.

   o  Resource - an IPv6 address or a IPv6 prefix.

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

   o  Secondary Server





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   o  Server - A DHCPv6 server that implements DHCPv6 failover.
      'Server' and 'failover endpoint' as synonymous only if 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 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 mode may be defined as an exension at a later time.

   The failover protocol is designed to provide lease stability for
   leases with lease times beyond a short period.  Due to the additional
   overhead required, 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 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



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       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

   It may be tempting to extend DHCPv6 failover mechanism to also offer
   load balancing, as DHCPv4 failover did.  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.


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.  Additional failover-specific message
   types will be defined.  All information is sent over the connection
   as typical 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 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 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 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 BNDACK message with appropriate
   status.  The majority of the messages sent over a failover TCP



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   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 initial connection is
   established, the secondary server requests a pool of available
   addresses by sending a POOLREQ message.  The primary server assigns a
   pool to the secondary by transmitting a POOLRESP message and then
   sending a series of BNDUPD messages.  The secondary server may
   initiate such pool request at any time when maintaining 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, release or expire a 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 response to client, but before sending
   the lazy update to its partner (or when communication between
   partners is interrupted).  To solve this problem, concept known as
   the Maximum Client Lead Time (MCLT) (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 MCLT affects assigned lease times.

   Servers verify each others availability by periodically exchanging
   CONTACT messages.  See Section 8.5 for discussion about detecting
   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 Machine Sate Overview

   The following section provides 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 simplified and complete description, please follow Section 9.



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   Each server may 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 reachibility and state and usually returns
   back to the state it was in before shutdown.

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

   If a server discovers that its partner is no longer reachable, it
   goes to COMMUNICATIONS-INTERRUPTED state.  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 if its partner is up, it follows defined procedure.  In
   particular, not extend any lease beyond its partner knowledge by at
   most MCLT.  That imposes additional burden on the server.  Therefore
   it is not recommended to operate for prolonged periods in this state.
   Once communication is reestablished, server may go into NORMAL,
   POTENTIAL-CONFLICT or PARTNER-DOWN state.  It may also stay in
   COMMUNICATIONS-INTERRUPTED 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 is
   albo 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 server loses its database (e.g. due to first time run or
   catastrophic failure) or detects that is partner is in PARTNER-DOWN
   state and additional conditions are met, it switches to RECOVER
   state.  In that state server acknowledges that content of its
   database is doubtful and needs to refresh its database from its
   partner.  Once this operation is done, it switches to RECOVER-WAIT
   and later to RECOVER-DONE.

   Once servers reestablish connection, they discover each others'
   state.  Depending on the conditions, they may return to NORMAL or
   move to POTENTINAL-CONFLICT in case of unexpected partner's state.
   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 on



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   resolving potential conflicts.  Once outstanding lease updates are
   exchanged, servers move to CONFLICT-DONE or NORMAL states.

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

   Server that is being shut down, switches briefly to SHUTDOWN state
   and communicates its state to its partner before actual termination.


5.  Connection Management

5.1.  Creating Connections

   Every 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.

   When a connection attempt succeeds, if the server generating the
   connection attempt is a primary server for that relationship, then it
   MUST send a CONNECT message down the connection.  If it is not a
   primary server for the relationship, then it MUST just drop the
   connection and wait for the primary server to connect to it.

   When a connection attempt is received, the only information that the
   receiving server has is the IP address of the partner initiating a
   connection.  It also knows whether it has the primary role for any
   failover relationships with the connecting server.  If it has any
   relationships for which it is a primary server, it should initiate a
   connection of its own to the partner server, one for each primary
   relationship it has with that server.

   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.

   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 attempts to create a connection to
   the server which is primary in the relationship, but that connection



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   is only used to stimulate the primary server into recognizing that
   the secondary server is ready for operation.  The reason behind this
   is that the secondary server has no way to communicate to the primary
   server which relationship a connection is designed to serve.

   A server which has multiple secondary relationships with a primary
   server SHOULD only send one stimulus connection attempt to 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
   and send another stimulus connection attempt to the primary server.

   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.

   A secondary server MUST NOT respond to the closing of a TCP
   connection with a blind attempt to reconnect -- there may be another
   TCP connection to the same failover partner already in use.

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



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   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 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 algorithm defined in [dhcpv4-failover] to DHCPv6.
       Available resources are split between primary and secondary
       server.  Released resources are always returned to 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 for
       prefix delegation.  See Section 6.1 for details.



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   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 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.

   So, 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.

   TODO: Need to rework this v4-specific vocabulary to v6, once we
   decide how things will look like in v6.

   When they are used, these proportional pools are used for allocation
   when in every state but PARTNER-DOWN state.  In PARTNER-DOWN state a
   failover server can allocate from either pool.  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



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   available addresses between the two servers.

   TODO: Reuse rest of the description from section 5.4 from
   [dhcpv4-failover] here.

6.2.  Independent Allocation

   In this allocation scheme, available resources are 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.  This algorithm is simpler than proportional allocation since
   it requires no less initial communicagtion and does not require a
   rebalancing mechanism, but 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 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 a resource or its lease is expired,
   the returned resource returns to pool for the same server.  Resources
   never changes servers.

   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.

6.3.  Determining Allocation Approach

6.3.1.  IPv6 Addresses

6.3.2.  IPv6 Prefixes


7.  Information model

   TODO: Describe information model here.  In particular, we need to
   describe lease lifecycle here.

   TODO: In case of Active-Passive model, while majority of addresses
   are owned by the primary server, secondary server will need a portion
   of addresses to serve new clients while operating in communication-



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   interrupted state as also in partner down state before it can take
   over the entire address pool (expiry of MCLT).  The concept of a
   percentage of pool reserved for secondary should be described here.


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.
   lack of on-wire formats).  Implementation details will 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 become unavailable in
   some scenarios that failover expects to cover, e.g. network
   partition.  Therefore a mechanism to measure and track relative time
   differences between servers is necessary.  To do so, each message
   MUST contain FO_TIMESTAMP option that contains the timestamp 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 event 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.




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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.  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 more than the maximum
   client lead time (MCLT) greater than the potential expiration time
   known to a server's partner.

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



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   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 adds the MCLT.
   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., zero) plus the
   MCLT.  Thus, the actual valid lifetime is 1 hour.




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   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 remaining 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.






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8.5.  Unreachability detection

   Each partner maintains an FO_SEND timer for each partner 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.

   Discussion: Perhaps it would be more reasonable to use echo-reply
   approach, rather than periodic transmissions?

8.6.  Re-allocating Leases

   TODO: Describe controlled re-allocation of released/expired leases to
   different clients.

8.7.  Sending Data

   Each server updates its failover partner about recent changes in
   lease states.  Each update must include following information:

   1.  resource type - non-temporary address or a prefix

   2.  resource information - actual address or prefix

   3.  valid life time requested by client

   4.  IAID - Identity Association used by client, while obtaining this
       lease.  (Note1: one client may use many IAID simulatenously.
       Note2: IAID for IA, TA and PD are orthogonal number spaces.)

   5.  valid life time sent to client

   6.  potential valid life time

   7.  preferred life time sent to client

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

   9.  assigned FQDN names, if any (optional)

   Discussion: Do we need T1 as well?  Something like next expected
   client transmission?

   Q: Maybe we could reuse IA_NA and IA_PD options here?  Yes.

   Q: Do we care about preferred lifetime? (presumably no).  Certainly



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   not what was requested by the client.

   Q: Do we care about IAID? (presumably yes) Yes.

8.7.1.  Required Data

8.7.2.  Optional Data

8.8.  Receiving Data

8.8.1.  Conflict Resolution

   TODO: This is just a loose collection of notes.  This section will
   probably need to be rewritten as a a flowchart of some kind.

   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 - previously known
   or just received - is "better".  The server should take into
   consideration the following aspects: if the lease is already assigned
   to specific client, who had contact with client recently, start time
   of the lease, etc.

   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.

   Discussion: There will definitely be different types of update
   rejections.  For example, this will allow a server to treat
   differently a case when receiving a new lease that it previously
   haven't seen than a case when partner sents old version of a lease
   for which a newer state is known.

8.8.2.  Acknowledging Reception


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



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   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 on receipt of a SOLICIT message.

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

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

   o  Current failover state.

   o  Communications status (OK or not OK).

   o  Partner's failover state (if known).

   Whenever the 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 is 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.

   A transition into SHUTDOWN or PAUSED state is not represented in the
   following figure, since other than sending that state to its partner,
   the remaining actions involved look just like the server halting in
   its otherwise current state, which then becomes the previous state
   upon server restart.



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      +---------------+  V  +--------------+
      |    RECOVER -|+|  |  |   STARTUP  - |
      |(unresponsive) |  +->+(unresponsive)|
      +------+--------+     +--------------+
      +-Comm. OK             +-----------------+
      |     Other State:     |  PARTNER DOWN - +<----------------------+
      |    RESOLUTION-INTER. | (responsive)    |                       ^
     All     POTENTIAL-      +----+------------+                       |
    Others   CONFLICT------------ | --------+                          |
      |      CONFLICT-DONE     Comm. OK     |     +--------------+     |
   UPDREQ or                 Other State:   |  +--+ RESOLUTION - |     |
   UPDREQALL                  |       |     |  |  | INTERRUPTED  |     |
   Rcv UPDDONE             RECOVER    All   |  |  | (responsive) |     |
      |  +---------------+    |      Others |  |  +------------+-+     |
      +->+RECOVER-WAIT +-| RECOVER    |     |  |         ^     |       |
         |(unresponsive) |  WAIT or   |     |  Comm.     |    Ext.     |
         +-----------+---+  DONE      |     |  OK     Comm.   Cmd----->+
  Comm.---+     Wait MCLT     |       V     V  V     Failed            |
  Changed |          V    +---+   +---+-----+--+-+       |             |
   |  +---+----------++   |       |  POTENTIAL + +-------+             |
   |  |RECOVER-DONE +-|  Wait     |  CONFLICT    +------+              |
   +->+(unresponsive) |  for      |(unresponsive)|   Primary           |
      +------+--------+  Other  +>+----+--------++   resolve     Comm. |
       Comm. OK          State: |      |        ^    conflict  Changed |
  +---Other State:-+   RECOVER  |   Secondary   |       V       V   |  |
  |    |           |     DONE   |    resolve    |   ++----------+---++ |
  | All Others:  POTENT.  |     |   conflict    |   |CONFLICT-DONE-|+| |
  | Wait for    CONFLICT- | ----+    see (9.10) |   | (responsive)   | |
  | Other State:          V            V        |   +------+---------+ |
  | NORMAL or RECOVER    ++------------+---+      Other State: NORMAL  |
  |    |       DONE      |     NORMAL    + +<--------------+           |
  |    +--+----------+-->+   (balanced)    +-------External Command--->+
  |       ^          ^   +--------+--------+       or Other State:     |
  |       |          |            |             |  SHUTDOWN            |
  |   Wait for   Comm. OK  Comm. Failed or      |                      |
  |    Other      Other    Other State: PAUSED  |               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 1: Failover Endpoint State Machine




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9.2.  State Machine Initialization

   TODO

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 1) 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 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



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   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 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 (if such transition exists --
   some states don't have a communications failed state transition,
   since they allow both commun- ications 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 go transition to the
   PREVIOUS-STATE.



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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.

9.4.1.  Operation in PARTNER-DOWN State

   The server MUST be responsive in PARTNER-DOWN state.

   It will allow renewal of all outstanding leases on IP addresses.  For
   those IP addresses for which the server is using proportional
   allocation, it will allocate IP addresses from its own pool, and
   after a fixed period of time (the MCLT interval) has elapsed from
   entry into PARTNER-DOWN state, it will allocate IP addresses from the
   set of all available IP addresses.

   Any IP address 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 beyond the entry into
   PARTNER-DOWN state has elapsed.

   A server in PARTNER-DOWN state MUST NOT allocate an IP address 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
   beyond the maximum of the following times: client expiration time,
   most recently transmitted potential-expiration-time, most recently
   received ack of potential-expiration-time from the partner, and most
   recently acked potential-expiration-time to the partner.  If this
   time would be earlier than the current time plus the 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 tmes
   while in PARTNER-DOWN state.

   In the unlikely case, when there are two servers operating in a
   PARTNER-DOWN state, there is a change od 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 giving new leases from its own pool.
   Therefore the server operating in PARTNER-DOWN state MUST use its own
   pool first for new leases before assigning any leases from its downed
   partner pool.






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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.  THIS NEEDS TO BE MOVED

   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, SHUTDOWN, PAUSED 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.






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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.


















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                   A                                        B
                 Server                                  Server

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

                 Figure 2: 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



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   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 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.






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9.7.1.  Operation in RECOVER-DONE State

   A server in RECOVER-DONE state MUST respond only to DHCPREQUEST/
   RENEWAL and DHCPREQUEST/REBINDING DHCP 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 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 for
   allocation using the POOLREQ message.

9.8.1.  Operation in NORMAL State

   When in NORMAL state a 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 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



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      desired valid lifetime and actual valid lifetime.

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

   In 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, 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 any messages from its partner
   where the PARTNER has changed into SHUTDOWN state, the server should
   transition into PARTNER-DOWN state.





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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 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), 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 lease, each server allocates from its own pool,
   where the primary MUST allocate only FREE resources (addresses or
   prefixes), and the secondary MUST allocate only 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 lease- expiration-time , 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).

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







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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.

   o  SHUTDOWN: Transition into PARTNER-DOWN state.

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























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      Primary                                Secondary
       Server                                  Server

       NORMAL                                  NORMAL
         | >--CONTACT------------------->         |
         |        <--------------------CONTACT--< |
         |         [TCP connection broken]        |
    COMMUNICATIONS          :              COMMUNICATIONS
      INTERRUPTED           :                INTERRUPTED
         |      [attempt new TCP connection]      |
         |         [connection succeeds]          |
         |                                        |
         | >--CONNECT------------------->         |
         |        <-----------------CONNECTACK--< |
         |                                     NORMAL
         |        <-------------------STATE-----< |
       NORMAL                                     |
         | >--STATE--------------------->         |
         |
         | >--BNDUPD-------------------->         |
         |        <---------------------BNDACK--< |
         |                                        |
         |        <---------------------BNDUPD--< |
         | >------BNDACK---------------->         |
        ...                                      ...
         |                                        |
         |        <--------------------POOLREQ--< |
         | >--POOLRESP-(2)-------------->         |
   t>      |                                        |
         | >--BNDUPD-(#1)--------------->         |
         |        <---------------------BNDACK--< |
         |                                        |
         |        <--------------------POOLREQ--< |
         | >--POOLRESP-(0)-------------->         |
         |                                        |
         | >--BNDUPD-(#2)--------------->         |
         |        <---------------------BNDACK--< |
         |                                        |

    Figure 3: 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



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   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.















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       Primary                                Secondary
       Server                                  Server

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

              Figure 4: 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.



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   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 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 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, as opposed to NORMAL state where it
   would be in a "balanced" responsive state, running the load balancing
   algorithm.

   TODO: We do not support load balancing, so CONFLICT-DONE is actually
   equal to NORMAL.  Need to remove CONFLICT-DONE and replace all its
   references to NORMAL.





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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.

9.13.  PAUSED State

   TODO: Remove PAUSED state completely

   This state exists to allow one server to inform another that it will
   be out of service for what is predicted to be a relatively short
   time, and to allow the other server to transition to COMMUNICATIONS-
   INTERRUPTED state immediately and to begin servicing all DHCP clients
   with no interruption in service to new DHCP clients.

   A server which is aware that it is shutting down temporarily SHOULD
   send a STATE message with the server-state option containing PAUSED
   state and close the TCP connection.

   While a server may or may not transition internally into PAUSED
   state, the 'previous' state determined when it is restarted MUST be
   the state the server was in prior to receiving the command to shut-
   down and restart and which precedes its entry into the PAUSED state.
   See Section 9.3.2 concerning the use of the previous state upon
   server restart.

   When entering PAUSED state, the server MUST store the previous state
   in stable storage, and use that state as the previous state when it
   is restarted.

9.13.1.  Operation in PAUSED State

   Server MUST NOT perform any operation while in PAUSED state.




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9.13.2.  Transition Out of PAUSED State

   A server makes a transition out of PAUSED state by being restarted.
   At that time, the previous state MUST be the state the server was in
   prior to entering the PAUSED state.

9.14.  SHUTDOWN State

   This state exists to allow one server to inform another that it will
   be out of service for what is predicted to be a relatively long time,
   and to allow the other server to transition immediately to PARTNER-
   DOWN state, and take over completely for the server going down.

   When entering SHUTDOWN state, the server MUST record the previous
   state in stable storage for use when the server is restarted.  It
   also MUST record the current time as the last time operational.

   A server which is aware that it is shutting down SHOULD send a STATE
   message with the server-state field containing SHUTDOWN.

9.14.1.  Operation in SHUTDOWN State

   A server in SHUTDOWN state MUST NOT respond to any DHCP client input.

   If a server receives any message indicating that the partner has
   moved to PARTNER-DOWN state while it is in SHUTDOWN state then it
   MUST record RECOVER state as the previous state to be used when it is
   restarted.

   A server SHOULD wait for a few seconds after informing the partner of
   entry into SHUTDOWN state (if communications are okay) to determine
   if the partner entered PARTNER-DOWN state.

9.14.2.  Transition Out of SHUTDOWN State

   A server makes a transition out of SHUTDOWN state by being restarted.


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.






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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

   TODO: Descibe DNS Updates challenges in failover environment.  It is
   nicely described in Section 5.12 of [dhcpv4-failover].


12.  Reservations and failover

   TODO: Describe how lease reservation works with failover.  See
   Section 5.13 in [dhcpv4-failover].


13.  Protocol entities

   Discussion: It is unclear if following sections belong to design or
   protocol draft.  It is currently kept here as a scratchbook with list
   of things that will have to be defined eventually.  Whether or not it
   will stay in this document or will be moved to the protocol spec



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   document is TBD.

13.1.  Failover Protocol

   This section enumerates list of options that will be defined in
   failover protocol specification.  Rough description of purpose and
   content for each option is specified.  Exact on wire format will be
   defined in protocol specification.

   1.  OPTION_FO_TIMESTAMP - convey information about timestamp.  It is
       used by time skew measurement algorithm (see Section 8.1).

13.2.  Protocol constants

   This section enumerates various constants that have to be defined in
   actual protocol specification.

   1.  TIME_SKEW_PKTS_AVG - number of packets that are used to calculate
       average time skew between partners.  See (see Section 8.1).


14.  Open questions

   This is scratchbook.  This section will be removed once questions are
   answered.

   Q: Do we want to support temporary addresses?  I think not.  They are
   short-lived by definition, so clients should not mind getting new
   temporary addresses.

   Q: Do we want to support CGA-registered addresses?  There is
   currently work in DHC WG about this, but I haven't looked at it yet.
   If that is complicated, we may not define it here, but rather as an
   extension.  [If it moves forward, we need to support it.]


15.  Security Considerations

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


16.  IANA Considerations

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





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17.  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
   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).


18.  References

18.1.  Normative References

   [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.

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

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

18.2.  Informative References

   [I-D.ietf-dhc-dhcpv6-failover-requirements]
              Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
              Requirements",
              draft-ietf-dhc-dhcpv6-failover-requirements-00 (work in
              progress), October 2011.

   [I-D.ietf-dhc-dhcpv6-redundancy-consider]
              Tremblay, J., Brzozowski, J., Chen, J., and T. Mrugalski,
              "DHCPv6 Redundancy Deployment Considerations",



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              draft-ietf-dhc-dhcpv6-redundancy-consider-02 (work in
              progress), October 2011.

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

   [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

















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