Dynamic Host Configuration (DHC)                            T. Mrugalski
Internet-Draft                                                       ISC
Intended status: Informational                                K. Kinnear
Expires: January 20, 2014                                          Cisco
                                                           July 19, 2013

                      DHCPv6 Failover Requirements


   The DHCPv6 protocol, defined in [RFC3315] allows for multiple servers
   to operate on a single network, however it does not define any way
   the servers could share information about currently active clients
   and their leases.  Some sites are interested in running multiple
   servers in such a way as to provide increased availability in case of
   server failure.  In order for this to work reliably, the cooperating
   primary and secondary servers must maintain a consistent database of
   the lease information.  [RFC3315] allows for but does not define any
   redundancy or failover mechanisms.  This document outlines
   requirements for DHCPv6 failover, enumerates related problems, and
   discusses the proposed scope of work to be conducted.  This document
   does not define a DHCPv6 failover protocol.

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
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   This Internet-Draft will expire on January 20, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Scope of work  . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Alternatives to Failover . . . . . . . . . . . . . . . . .  5
       3.1.1.  Short-lived addresses  . . . . . . . . . . . . . . . .  5
       3.1.2.  Redundant servers  . . . . . . . . . . . . . . . . . .  6
       3.1.3.  Distributed databases  . . . . . . . . . . . . . . . .  6
       3.1.4.  Load Balancing . . . . . . . . . . . . . . . . . . . .  7
   4.  Failover Scenarios . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Hot Standby Model  . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Geographically Distributed Failover  . . . . . . . . . . .  7
     4.3.  Load balancing . . . . . . . . . . . . . . . . . . . . . .  7
     4.4.  1-to-1, m-to-1 and m-to-n models . . . . . . . . . . . . .  8
     4.5.  Split prefixes . . . . . . . . . . . . . . . . . . . . . .  8
     4.6.  Long lived connections . . . . . . . . . . . . . . . . . .  8
     4.7.  Partial server communication loss  . . . . . . . . . . . .  8
   5.  Principles of DHCPv6 Failover  . . . . . . . . . . . . . . . .  9
     5.1.  Failure modes  . . . . . . . . . . . . . . . . . . . . . .  9
       5.1.1.  Server Failure . . . . . . . . . . . . . . . . . . . .  9
       5.1.2.  Network partition  . . . . . . . . . . . . . . . . . . 10
     5.2.  Synchronization mechanisms . . . . . . . . . . . . . . . . 11
       5.2.1.  Lockstep . . . . . . . . . . . . . . . . . . . . . . . 11
       5.2.2.  Lazy updates . . . . . . . . . . . . . . . . . . . . . 11
   6.  DHCPv4 and DHCPv6 Failover Comparison  . . . . . . . . . . . . 12
   7.  DHCPv6 Failover Requirements . . . . . . . . . . . . . . . . . 12
     7.1.  Features out of scope  . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   The DHCPv6 protocol, defined in [RFC3315] allows for multiple servers
   to be operating on a single network, however it does not define how
   the servers can share the same address and prefix delegation pools
   and allow a client to seamlessly extend its existing leases when the
   original server is down.  [RFC3315] provides for these capabilities,
   but does not document how the servers cooperate and communicate to
   provide this capability.  Some sites are interested in running
   multiple servers in such a way as to provide redundancy in case of
   server failure.  In order for this to work reliably, the cooperating
   primary and secondary servers must maintain a consistent database of
   the lease information.

   This document discusses failover implementations scenarios, failure
   modes, and synchronization approaches to provide background to the
   list of requirements for a DHCPv6 failover protocol.  It then defines
   a minimum set of requirements that failover must provide to be
   useful, while acknowledging that additional features may be specified
   as extensions.  This document does not define a DHCPv6 failover

   The failover model, to which these requirements apply, will initially
   be a pairwise "hot standby" model (see Section 4.1) with a primary
   server used in normal operation switching over to a backup secondary
   server in the event of failure.  Optionally, a secondary server may
   provide failover service for multiple primary servers.  However the
   requirements will not preclude a future load-balancing extension
   where there is a symmetric failover relationship.

   The DHCPv6 failover concept borrows heavily from its DHCPv4
   counterpart [dhcpv4-failover] that never completed standardization
   process, but has several successful, operationally proven vendor-
   specific implementations.  For a dicussion about commonalities and
   differences, see Section 6.

2.  Definitions

   This section defines terms that are relevant to DHCPv6 failover.

   Definitions from [RFC3315] are included by reference.  In particular,
   client means any device e.g., end user host, CPE (Customer Premises
   Equipment) or other router that implements client functionality of
   the DHCPv6 protocol.  A server means a DHCPv6 server, unless
   explicitly noted otherwise.  A relay is a DHCPv6 relay.

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      A binding (or client binding) is a group of server data records
      containing the information the server has about the addresses in
      an IA (Identity Assocation, see Section 10 of [RFC3315]) or
      configuration information explicitly assigned to the client.
      Configuration information that has been returned to a client
      through a policy - for example, the information returned to all
      clients on the same link - does not require a binding.

      DDNS - an abbreviation for "Dynamic DNS", which refers to the
      capability to update a DNS server's name database using the on-
      the-wire protocol defined in [RFC2136].  Clients and servers can
      negotiate the scope of such updates as defined in [RFC4704].

      Failover - an ability of one partner to continue offering services
      provided by another partner, with minimal or no impact on clients.

      FQDN - a fully qualified domain name.  A fully qualified domain
      name generally is a host name with at least one domain label under
      the top-level domain.  For example "dhcp.example.org" is a fully
      qualified domain name.

      High Availability - a desired property of DHCPv6 servers to
      continue providing services despite experiencing unwanted events
      such as server crashes, link failures, or network partitions.

      Load Balancing - the ability for two or more servers to each
      process some portion of the client request traffic in a conflict-
      free fashion.

      Lease - an IPv6 address, an IPv6 prefix or other resource that was
      assigned ("leased") by a server to a specific client.  A lease may
      include additional information, like associated fully qualified
      domain name (FQDN) and/or information about associated DNS
      updates.  A client obtains a lease for a specified period of time
      (valid lifetime).

      Partner - A "partner", for the purpose of this document, refers to
      a failover server, typically the other failover server in a
      failover relationship.

      Stable Storage - each DHCP server is required to keep its lease
      database in some form of storage (known as "stable storage") that
      will be consistent throughout reboots, crashes and power failures.

      Partner Failure - A power outage, unexpected shutdown, crash or
      other type of failure that renders a partner unable to continue
      its operation.

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3.  Scope of work

   In order to fit within the IETF process effectively and efficiently,
   the standardization effort for DHCPv6 failover is expected to proceed
   with the creation of documents of increasing specificity.  It begins
   with this document specifying the requirements for DHCPv6 failover
   ("requirements document").  Later documents are expected to address
   the design of the DHCPv6 failover protocol ("design document"), and
   if sufficient interest exists, the protocol details required to
   implement the DHCPv6 failover protocol itself ("protocol document").
   The goal of this partitioning is, in part, to ease the validation,
   review, and approval of the DHCPv6 failover protocol by presenting it
   in comprehensible parts to the larger community.

   Additional documents describing extensions may also be defined.

   DHCPv6 Failover requirements are presented in Section 7.

3.1.  Alternatives to Failover

   There are many scenarios when it seems that a failover capability
   would be useful.  However, there are often much simpler approaches
   that will meet the required goals.  This section documents examples
   where failover is not really needed.

3.1.1.  Short-lived addresses

   There are cases when IPv6 addresses are used only for a short time,
   but there is a need to have high degree of confidence that those
   addresses will be served.  A notable example is PXE: Pre eXecution
   Environment [RFC5970].  This is a mechanism for obtaining
   configuration early in the process of bootstrapping over the network.

   The PXE BIOS acquires an address in order to load the operating
   system image and continue booting.  Address and possibly other
   configuration parameters are used during the boot process and are
   discarded thereafter.  Any lack of available DHCPv6 service at this
   time will prevent such devices from booting.

   Instead of deploying failover, it is better to use the much simpler
   preference mechanism, defined in [RFC3315].  For example, consider
   two or more servers with each having a distinct preference set (e.g.,
   10 and 20).  Both will answer to a client's request.  The client
   should choose the one with larger preference value.  In case of
   failure of the most preferred server, the next server will keep
   responding to clients' queries.  This approach is simple to deploy,
   but does not offer lease stability, i.e., in case of server failure,
   clients' addresses and prefixes will change.

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3.1.2.  Redundant servers

   In some cases the desire to deploy failover is motivated by high
   availability, i.e., to continue providing services despite server
   failure.  If there are no additional requirements, that goal may be
   fulfilled with simply deploying two or more independent servers on
   the same link.

   There are several well-documented approaches showing how such a
   deployment could work.  They are discussed in detail in [RFC6853].
   Each of those approaches is simpler to deploy and maintain than full

3.1.3.  Distributed databases

   Some servers may allow their lease database to be stored in external
   databases.  Another possible alternative to failover is to configure
   two servers to connect to the same distributed database.

   Care should be taken to understand how inconsistencies are solved in
   such database backends and how such conflict resolutions affect
   DHCPv6 server operation.

   It is also essential to use only a database that provides equivalent
   reliability and failover capability.  Otherwise the single point of
   failure is only moved to a different location (database rather than
   DHCPv6 server).  Such a configuration does not improve redundancy,
   but significantly complicates deployment.

   A common miscoception regarding database-based redundancy is the
   assumption that a conflict resolution after recovering from a network
   partition is not necessary.  To explain that fallacy, let's consider
   an example where there is a very small pool with only one address.
   There are two servers, each connected to a co-located database node
   (i.e., running on the same hardware).  Network partition occurs.
   Each server is operating, but has lost connection to its partner.
   Two clients request an address, one from each server.  Each server
   consults its database and discovers that only one address is
   available, so it is assigned to the client.  Unfortunately, each
   server assigned the same address to a different client.  Making the
   scenario more realistic (millions of addresses rather than one) just
   decreased failure probability, but did not eliminate the underlying

   Any solution that involves a distributed database implementation of
   DHCPv6 failover must take into account the requirements for security.
   See Section 8 for additional information.

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3.1.4.  Load Balancing

   Sometimes the desire to deploy more than one server is based on the
   assumption that they will share the client traffic.  Administrators
   that are interested in such a capability are advised to deploy a load
   balancing mechanism, defined in [I-D.ietf-dhc-dhcpv6-load-balancing].

4.  Failover Scenarios

   The following section provides several examples of deployment
   scenarios and use cases that may be associated with capabilities
   commonly referred to as failover.  These scenarios may be in or out
   of scope for the DHCPv6 failover protocol to which this document's
   requirements apply; they are enumerated here to provide a common
   basis for discussion.

4.1.  Hot Standby Model

   In the simplest case, there are two partners that are connected to
   the same network.  Only one of the partners ("primary") provides
   services to clients.  In case of its failure, the second partner
   ("secondary") continues handling services previously handled by first
   partner.  As both servers are connected to the same network, a
   partner that fails to communicate with its partner while also
   receiving requests from clients may assume with high probability that
   its partner is down and the network is functional.  This assumption
   may affect its operation.

4.2.  Geographically Distributed Failover

   Servers may be physically located in separate locations.  A common
   example of such a topology is where a service provider has at least a
   regional high performance network between geographically distributed
   datacenters.  In such a scenario, one server is located in one
   datacenter and its failover partner is located in another remote
   datacenter.  In this scenario, when one partner finds that it cannot
   communicate with the other partner, it does not necessarily mean that
   the other partner is down.

4.3.  Load balancing

   A desire to have more than one server in a network may also be
   created by the desire to have incoming traffic be handled by several
   servers.  This decreases the load each server must endure when all
   servers are operational.  Although such a capability does not,
   strictly, require failover - it is clear that failover makes such an
   architecture more straightforward.

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   Note that in a load balancing situation which includes failover, each
   individual server must be able to handle the full load normally
   handled by both servers working together, or there is not a true
   increase in availability.

4.4.  1-to-1, m-to-1 and m-to-n models

   A failover relationship for a specific network is provided by two
   failover partners.  Those partners communicate with each other and
   back up all pools.  This scenario is sometimes referred to as the
   1-to-1 model and is considered relatively simple.  In larger networks
   one server may be participating in several failover relationships,
   i.e., it provides failover for several address or prefix pools, each
   served by separate partners.  Such a scenario can be referred to as
   m-to-1.  The most complex scenario - m-to-n - assumes that each
   partner participates in multiple failover relationships.

4.5.  Split prefixes

   Due to the extensive IPv6 address space, it is possible to provide
   semi-redundant service by splitting the available pool of addressees
   into two or more non-overlapping pools, with each server handling its
   own smaller pool.  Several versions of such a scenario are discussed
   in [RFC6853].

4.6.  Long lived connections

   Certain nodes may maintain long lived connections.  Since the IPv6
   address space is large, techniques exist (e.g., [RFC6853]) that use
   the easy availability of IPv6 addresses in order to provide increased
   DHCPv6 availability.  However, these approaches do not generally
   provide for stable IPv6 addresses for DHCPv6 clients should the
   server with which the client is interacting become unavailable.

   The obvious benefit of stable addresses is the ability to update DNS
   infrequently.  While the DNS can be updated every time an IPv6
   address changes, it introduces delays and (depending on DNS
   configuration) old entries may be cached for prolonged periods of

   The other benefit of having a stable address is that many monitoring
   solutions provide statistics on a per IP basis, so IP changes make
   measuring characteristics of a given box more difficult.

4.7.  Partial server communication loss

   There is a scenario where the DHCPv6 server may be configured to
   serve clients on one network adapter and communicate with a partner

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   server (server to server traffic) on a different network adapter.  In
   this scenario, if the server loses connectivity on the network
   adapter used to communicate with the clients because of network
   adapter (hardware) failure, there is no intimation of the loss of
   service to the partner in the DHCPv6 failover protocol.  Since the
   servers are able to communicate with each other, the partner remains
   ignorant of the loss of service to clients.

5.  Principles of DHCPv6 Failover

   This section describes important issues that will affect any DHCPv6
   failover protocol.  This section is not intended to define
   implementation details, but rather high level concepts and issues
   that are important to DHCPv6 failover.  These issues form a basis for
   later documents which deal with the solutions to these issues.

   The general failover concept assumes that there are backup servers
   that can provide service in case of a primary server failure.  In
   theory there could be more than one backup server that could take up
   the role if such a need arise.  However, having more than two servers
   introduces a very difficult issue of synchronizing between partners.
   In the case of just a pair of cooperating servers, the notification
   and processes can result in only one of two states: fully successful
   (got response from a partner) and total failure (no response, failure
   event occurred).  Were there more than two partners participating in
   a relationship, there would be intermediate, inconsistent states
   where some partners had updated their state and some had not.  This
   would greatly complicate protocol design, and would give little
   advantage in return.  Therefore an approach that assumes a pair of
   cooperating servers was chosen.

5.1.  Failure modes

   This section documents failure modes.

5.1.1.  Server Failure

   Servers may become unresponsive due to a software crash, hardware
   failure, power outage or any number of other reasons.  The failover
   partner will detect such event due to lack of responses from the down
   partner.  In this failure mode, the assumption is that the server is
   the only equipment that is off-line and all other network equipment
   is operating normally.  In particular, communication between other
   nodes is not interrupted.

   When working under the assumption that this is the only type of
   failure that can happen, the server may safely assume that its

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   partner unreachability means that it is down, so other nodes (clients
   in particular) are not able to reach it either and no services are

   It should be noted that recovery after the failed server is brought
   back on-line is straightforward, due to the fact that it just needs
   to download current information from the lease database of the
   healthy partner and there is no conflict resolution required.

   This is by far the most common failure mode between two failover

   When the two servers are located physically close to each other,
   possibly in the same room, the probability that a failure to
   communicate between failover partners is due to server failure is

5.1.2.  Network partition

   Another possible cause of partner unreachability is a failure in the
   network that connects the two servers.  This may be caused by failure
   of any kind of network equipment: router, switch, physical cables, or
   optic fibers.  As a result of such a failure the network is split
   into two or more disjoint sections (partitions) that are not able to
   communicate with each other.  Such an event is called network
   partition.  If failover partners are located in different partitions,
   they won't be able to communicate with each other.  Nevertheless,
   each partner may still be able to serve clients that happen to be
   part of the same partition.

   If this failure mode is taken into consideration, a server can't
   assume that partner unreachability automatically means that its
   partner is down.  They must consider the fact that the partner may
   continue operating and interacting with a subset of the clients.  The
   only valid assumption is that the partner also detected the network
   partition event and follows procedures specified for such a

   It should be noted that recovery after a partitioned network is
   rejoined is significantly more complicated than recovery from a
   server failure event.  As both servers may have kept serving clients,
   they have two separate lease databases, and they need to agree on the
   state of each lease (or follow any other algorithm to bring their
   lease databases into agreement).

   This failure mode is more likely (though still rare) in the situation
   where two servers are in physically distant locations with multiple
   network elements between them.  This is the case in geographically

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   distributed failover (see Section 4.2).

5.2.  Synchronization mechanisms

   Partners must exchange information about changes made to the lease
   database.  There are at least two types of synchronization methods
   that may be used.  These concepts are related to distributed
   databases, so some familiarity with distributed database technology
   is useful to better understand this topic.

5.2.1.  Lockstep

   When a server receives a REQUEST message from a client it consults
   its lease database and assigns requested addresses and/or prefixes.
   To make sure that its partner maintains a consistent database, it
   then sends information about a new or just updated lease to the
   partner and waits for the partner's response.  After the response
   from its partner is received the REPLY message is transmitted to the

   This approach has the benefit of having a completely consistent lease
   database between partners at all times.  Unfortunately, there is
   typically a significant performance penalty for this approach as each
   response sent to a client is delayed by the total sum of the delays
   caused by two transmissions between partners and the processing by
   the second partner.  The second partner is expected to update its own
   copy of the lease database in permanent storage, so this delay is not
   negligible, even in fast networks.

   Due to the advent of fast SSD (solid state disk) and battery backed
   RAM (random access memory) disk technology, this write performance
   penalty can be limited to some degree.

5.2.2.  Lazy updates

   Another approach to synchronizing the lease databases is to transmit
   the REPLY message to the client before completing the update to the
   partner.  The server sends the REPLY to the client immediately after
   assigning appropriate addresses and/or prefixes and initiates the
   partner update at a later time, depending on the algorithm chosen.
   Another variation of this approach is to initiate transmission to the
   partner, but not wait for its response before sending the REPLY to
   the client.

   This approach has benefit of a minimal impact on server response
   times, thus it is much better from a performance perspective.
   However, it makes the lease databases loosely synchronized between
   partners.  This makes the synchronization more complex (and

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   particularly the re-integration after a network partition event), as
   there may be cases where one client has been given a lease on an
   address or prefix of which the partner is not aware (e.g., if the
   server crashes after sending REPLY to the client, but before sending
   update information to its partner).

6.  DHCPv4 and DHCPv6 Failover Comparison

   There are significant similarities between existing DHCPv4 and
   envisaged DHCPv6 failovers.  In particular both serve IP addresses to
   clients, require maintaining consistent databases among partners,
   need to perform consistent DNS Updates, must be able take over
   bindings offered by failed partner, must be able to resume operation
   after partner is recovered.  DNS conflict resolution works on the
   same principles in both DHCPv4 and DHCPv6.

   Nevertheless, there are significant differences.  IPv6 introduced
   prefix delegation [RFC3633] that is a crucial element of the DHCPv6
   protocol.  IPv6 also introduced the concept of deprecated addresses
   with separate preferred and valid lifetimes, both being configured
   via DHCPv6.  Negative response (NACK) in DHCPv4 has been replaced
   with the ability in DHCPv6 to provide corrected response in a REPLY
   message that differs from a REQUEST.

   Also, the typical large address space (close to 2^64 addresses on /64
   prefixes expected to be available on most networks) may make managing
   address assignment significantly different from DHCPv4 failover.  In
   DHCPv4 it was not possible to use a hash or calculated technique to
   divide the significantly more limited address space and therefore
   much of the protocol that deals with pool balancing and rebalancing
   might not be necessary and can be done mathematically.  Also, because
   of the much lower degree of contention for IP addresses, the DHCPv6
   failover protocol does not need to be tuned to support rapid
   reclamation of IPv6 addresses following the loss of a failover peer's

   However, DHCPv6 Prefix Delegation is similar to IPv4 addressing in
   terms of the number of available leases and therefore techniques for
   pool balancing and rebalancing and more rapid reclamation of prefixes
   allocated by a failed peer will be needed.

7.  DHCPv6 Failover Requirements

   This section summarizes the requirements for DHCPv6 failover.

   Certain capabilities may be useful in some, but not all scenarios.

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   Such additional features will be considered optional parts of
   failover, and will be split and defined in separate documents.  As
   such, this document can be considered an attempt to define
   requirements for the DHCPv6 failover "core" protocol.

   The core of the DHCPv6 failover protocol is expected to provide the
   following properties:

   1.   The number of supported partners must be exactly two, i.e.,
        there are at most two servers that are aware of a specific

   2.   For each prefix or address pool, a server must not participate
        in more than one failover relationship.

   3.   The defined protocol must support the m-to-1 model (i.e., one
        server may form more than one relationship), but an
        implementation may choose to implement only the 1-to-1 model
        (i.e., everything from one server is backed on another).

   4.   One partner must be able to continue serving leases offered by
        the other partner.  This property is also sometimes called
        "lease stability".  The failure of either failover partner
        should have minimal or no impact on client connectivity.  In
        particular, it must not force the client to change addresses
        and/or change prefixes delegated to it.  Lease stability has the
        aim of avoiding disturbance to long-lived connections.

   5.   Prefix delegation must be supported.

   6.   Use of the failover protocol must not introduce significant
        performance impact on server response times.  Therefore
        synchronization between partners must be done using some form of
        lazy updates (see Section 5.2.2).

   7.   The pair of failover servers must be able to recover from a
        server down failure (see Section 5.1.1).

   8.   The pair of failover servers must be able to recover from a
        network partition event (see Section 5.1.2).

   9.   The design must allow secure communication between the failover

   10.  The definition of extensions to this core protocol should be
        allowed, when possible.

   Depending on the specific nature of the failure, the recovery

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   procedures mentioned in points 7 and 8 may require manual

   High Availability is a property of the protocol that allows clients
   to receive DHCPv6 services despite the failure of individual DHCPv6
   servers.  In particular, it means the server that takes over
   providing service to clients from its failed partner, will continue
   serving the same addresses and/or prefixes.  This property is also
   called "lease stability".

   Although progress on a standardized inter-operable DHCPv4 failover
   protocol has stalled, vendor-specific DHCPv4 failover protocols have
   been deployed that meet these requirements to a large extent.
   Accordingly it would be appropriate to take into account the likely
   coexistence of DHCPv4 and DHCPv6 failover solutions.  In particular,
   certain features that are common to both IPv4 and IPv6
   implementations, such as DNS Update mechanism, should be taken into
   consideration to ensure compatible operation.

7.1.  Features out of scope

   The following features are explicitly out of scope.

   1.  Load Balancing - a capability is considered an extension and may
       be defined in a separate document.  It must not be part of the
       core protocol, but rather defined as an extension.  The primary
       reason for this the desire to limit core protocol complexity.
       Load Balancing is likely to be defined as an extension.  See

   2.  Configuration synchronization - two failover partners are
       expected to maintain the same configuration.  Mismatched
       configuration between partners is a frequent problem in failover
       solutions.  Unfortunately, that is an open-ended problem, since
       different servers have very different configuration data models.

   3.  m-to-n model (see Section 4.4)

   4.  Servers participating in multiple failover relationships for any
       given prefix or address pool.

8.  Security Considerations

   The design must provide a mechanism whereby each peer in a failover
   relationship can identify the other peer, authenticate that
   identification, and validate that the identified peer is the one with
   which communication is intended.  This mechanism should also

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   optionally provide support for confidentiality.

   The protocol specification, when it is written, should provide
   operational guidelines in the case of authentication mechanisms that
   require access to network servers that have the potential to be
   unreachable (e.g. what to do if a partner is reachable, but remote
   Certificate Authority is unreachable due to network partition event).

   The security considerations for the design itself will be discussed
   in the design document.

9.  IANA Considerations

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

10.  Acknowledgements

   This document extensively uses concepts, definitions and other parts
   of [dhcpv4-failover] document.  Thanks to Bernie Volz and Shawn
   Routhier for their frequent reviews and substantial contributions.
   Authors would also like to thank Qin Wu, Jean-Francois Tremblay,
   Frank Sweetser, Jiang Sheng, Yu Fu, Greg Rabil, Vithalprasad
   Gaitonde, Krzysztof Nowicki, Steinar Haug, Elwyn Davies, Ted Lemon,
   Benoit Claise and Stephen Farrell for their comments and feedback.

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

11.  References

11.1.  Normative References

   [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)

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              Option", RFC 4704, October 2006.

11.2.  Informative References

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

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

   [RFC5970]  Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6
              Options for Network Boot", RFC 5970, September 2010.

   [RFC6853]  Brzozowski, J., Tremblay, J., Chen, J., and T. Mrugalski,
              "DHCPv6 Redundancy Deployment Considerations", BCP 180,
              RFC 6853, February 2013.

              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

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

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

   Kim Kinnear
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, Massachusetts  01719

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

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