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RFC 7031 - DHCPv6 Failover Requirements

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Internet Engineering Task Force (IETF)                      T. Mrugalski
Request for Comments: 7031                                           ISC
Category: Informational                                       K. Kinnear
ISSN: 2070-1721                                                    Cisco
                                                          September 2013

                      DHCPv6 Failover Requirements


   The DHCPv6 protocol, defined in RFC 3315, 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.  RFC 3315 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 .............................................8
      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 ..........................9
   5. Principles of DHCPv6 Failover ...................................9
      5.1. Failure Modes ..............................................9
           5.1.1. Server Failure .....................................10
           5.1.2. Network Partition ..................................10
      5.2. Synchronization Mechanisms ................................11
           5.2.1. Lockstep ...........................................11
           5.2.2. Lazy Updates .......................................12
   6. DHCPv4 and DHCPv6 Failover Comparison ..........................12
   7. DHCPv6 Failover Requirements ...................................13
      7.1. Features out of Scope .....................................14
   8. Security Considerations ........................................15
   9. Acknowledgements ...............................................15
   10. References ....................................................16
      10.1. Normative References .....................................16
      10.2. Informative References ...................................16

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 the
   standardization process but has several successful, operationally
   proven vendor-specific implementations.  For a discussion 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" is a DHCPv6 server,
   unless explicitly noted otherwise.  A "relay" is a DHCPv6 relay.

   Binding (or client binding):  a group of server data records
      containing the information the server has about the addresses in
      an IA (Identity Association, 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.

   DNS Update:  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

   Failover:  the 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

   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

   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

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

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.

   Requirements document:
      It begins with this document specifying the requirements for
      DHCPv6 failover.

   Design document:
      A later document is expected to address the design of the DHCPv6
      failover protocol.

   Protocol document:
      If sufficient interest exists, a later document is expected to
      address the protocol details required to implement the DHCPv6
      failover protocol itself.

   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 in which 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 (Preboot
   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 a client's request.  The client should
   choose the one with the 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.

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 misconception 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 it did not eliminate the
   underlying issue.

   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.

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 [LOAD-BALANCING].

4.  Failover Scenarios

   The following sections provide 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 the
   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
   data centers.  In such a scenario, one server is located in one data
   center, and its failover partner is located in another remote data
   center.  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.

   Note that in a load-balancing situation that 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 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 time.

   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
   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 describes high-level concepts and
   issues that are important to DHCPv6 failover.  These issues form a
   basis for later documents that will deal with the solutions to these

   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 arises.  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 the 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.  This requirements document
   does not make any claims whether those two failures are
   distinguishable by a server.

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

   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
   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 (see Sections 5.2.1 and 5.2.2).  These concepts are
   related to distributed databases, so some familiarity with
   distributed database technology is useful to better understand this

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

   This approach has the benefit of a minimal impact on server response
   times; it is thus much better from a performance perspective.
   However, it makes the lease databases loosely synchronized between
   partners.  This makes the synchronization more complex (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 the 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 a failed partner, and must be able to resume
   operation after the 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], which is a crucial element of the DHCPv6
   protocol.  IPv6 also introduced the concept of deprecated addresses
   with separate preferred and valid lifetimes, both configured via
   DHCPv6.  Negative response (NACK) in DHCPv4 has been replaced with
   the ability in DHCPv6 to provide a corrected response in a REPLY
   message, which 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.
   Such additional features will be considered optional parts of
   failover and will be 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 lease.

   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
   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 interoperable 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 the 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 - This 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 the complexity of the
       core protocol.  See [LOAD-BALANCING].

   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
   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 the
   remote Certificate Authority is unreachable due to a network
   partition event).

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

9.  Acknowledgements

   This document extensively uses concepts, definitions and other parts
   of [DHCPV4-FAILOVER].  Thanks to Bernie Volz and Shawn Routhier for
   their frequent reviews and substantial contributions.  The 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, Stephen Farrell, Michal Hoeft, and Krzysztof Gierlowski for
   their comments and feedback.

   This work has been partially supported by the 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).

10.  References

10.1.  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)
              Option", RFC 4704, October 2006.

10.2.  Informative References

              Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S.,
              Rabil, G., Dooley, M., and A. Kapur, "DHCP Failover
              Protocol", Work in Progress, March 2003.

              Kostur, A., "DHC Load Balancing Algorithm for DHCPv6",
              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.

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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