Dynamic Host Configuration (DHC) T. Mrugalski, Ed.
Internet-Draft ISC
Intended status: Informational K. Kinnear
Expires: December 28, 2011 Cisco
June 26, 2011
DHCPv6 Failover Requirements
draft-mrugalski-dhc-dhcpv6-failover-requirements-00
Abstract
The DHCPv6 protocol, defined in [RFC3315] allows for multiple servers
to operate on a single network, however it does not define any way to
decide which server responds to which client queries. 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
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 December 28, 2011.
Copyright Notice
Copyright (c) 2011 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
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Scope of work . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Failover alternatives . . . . . . . . . . . . . . . . . . 6
4.1.1. Short-lived addresses . . . . . . . . . . . . . . . . 6
5. Failover Scenarios . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Hot Standby Model . . . . . . . . . . . . . . . . . . . . 6
5.2. Geographically Distributed Failover . . . . . . . . . . . 7
5.3. Load balancing . . . . . . . . . . . . . . . . . . . . . . 7
5.4. 1-to-1, m-to-1 and m-to-m models . . . . . . . . . . . . . 7
5.5. Split prefixes . . . . . . . . . . . . . . . . . . . . . . 7
5.6. Long lived connections . . . . . . . . . . . . . . . . . . 8
6. Principles of DHCPv6 Failover . . . . . . . . . . . . . . . . 8
6.1. Failure modes . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1. Server Failure . . . . . . . . . . . . . . . . . . . . 8
6.1.2. Network partition . . . . . . . . . . . . . . . . . . 9
6.2. Synchronization mechanisms . . . . . . . . . . . . . . . . 9
6.2.1. Lockstep . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.2. Lazy updates . . . . . . . . . . . . . . . . . . . . . 10
7. DHCPv4 and DHCPv6 Failover Comparison . . . . . . . . . . . . 10
8. DHCPv6 Failover Requirements . . . . . . . . . . . . . . . . . 11
9. Related work . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. DHCPv4 failover concepts . . . . . . . . . . . . . . . . . 13
9.1.1. Goals of DHCPv4 Failover . . . . . . . . . . . . . . . 13
9.1.2. Goals lead to Concepts . . . . . . . . . . . . . . . . 14
9.1.3. Use of the MCLT in practice . . . . . . . . . . . . . 15
9.1.4. Network Partition Events . . . . . . . . . . . . . . . 16
9.1.5. Conflict Resolution . . . . . . . . . . . . . . . . . 16
9.1.6. Load Balancing . . . . . . . . . . . . . . . . . . . . 16
9.2. DHCPv6 Redundancy Considerations . . . . . . . . . . . . . 17
10. DHCP Best Practices . . . . . . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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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. Introduction
The DHCPv6 protocol, defined in [RFC3315] allows for multiple servers
to be operating on a single network, however it does not define any
way to decide which server responds to which client queries. 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
protocol.
3. 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 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.
A binding (or, client binding) is a group of server data records
containing the information the server has about the addresses in
an IA 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].
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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 zone name. 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.
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.
4. 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.
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Additional documents describing extensions may also be defined.
DHCPv6 Failover requirements are presented in Section 8.
4.1. Failover alternatives
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.
4.1.1. 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 a scenario where
hosts require an address during boot. Address and possibly other
configuration parameters are used during boot process and are
discarded afterwards. Any lack of available DHCPv6 service at this
time may render the devices unbootable.
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 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.
5. 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 inside or
outside of scope for DHCPv6 failover protocol as defined by this
document. They are enumerated here to provide a common basis for
discussion.
5.1. Hot Standby Model
In the simplest case, there are two partners that are connected to
the same network. Only one of 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
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the network is functional. This assumption may affect its operation.
5.2. 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.
5.3. 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 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.
5.4. 1-to-1, m-to-1 and m-to-m models
A failover relationship for a specific network is provided by two
failover partners. Those partners communicate with each other. 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-m - assumes that each partner participates in
multiple failover relationships.
5.5. 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 [I-D.ietf-dhc-dhcpv6-redundancy-consider].
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5.6. Long lived connections
Certain nodes may maintain long lived connections. Since the IPv6
address space is large, techniques exist (e.g.
[I-D.ietf-dhc-dhcpv6-redundancy-consider]) 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.
6. 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.
6.1. Failure modes
This section documents failure modes. Each failure mode is listed as
either an in-scope or out-of-scope requirement for the failover
protocol.
6.1.1. 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
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
partners.
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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
increased.
6.1.2. 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 partner also detected the network
partition event and follows procedures specified for such a
situation.
It should be noted that recovery after 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 5.2).
6.2. Synchronization mechanisms
Partners must exchange information about changes made to the lease
database. There are two types of sychronization methods that may be
used.
6.2.1. Lockstep
When a server receives a REQUEST message from a client it consults
its lease database and assigns requested addresses and/or prefixes.
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To make sure that its partner maintains a consistent database, it
then sends information about new or just updated lease to the partner
and waits for the partner's response. After the response from
partner is received the REPLY message is transmitted to the client.
This approach has the benefit of having a completely consistent lease
database between partners at all times. Unfortunately, there is a
large 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.
6.2.2. 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 varation 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
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 server
crashes after sending REPLY to the client, but before sending update
information to its partner).
7. 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
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protocol. IPv6 also introduced the concept of deprecated addresses
with separate prefered 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. And, it may
also be possible and reasonable to use a much longer MCLT value -- as
reusing an address for a different client is generally not a
requirement (at least over the near term) as close to 2^64 addresses
may be available.
However, DHCPv6 Prefix Delegation is similar to IPv4 addressing and
therefore techniques for pool balancing and rebalancing and shorter
MCLT times will be needed.
8. 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 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 specific lease; this
approach is often called 1-to-1 model.
2. For each prefix or address pool, server MUST NOT participate in
more than one failover relationship.
3. Server MAY participate in multiple relationships only if those
relationships cover different prefix or address pools.
4. One partner MUST be able to continue serving leases offered by
the other partner. This property is also sometimes called
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'lease stability'. The failure of either failover partner
SHOULD pose 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. Long-lived connections
MUST NOT be disturbed.
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 partner MUST be done using some form of
lazy updates (see Section 6.2.2).
7. The pair of failover servers MUST be able to recover from server
down failure (see Section 6.1.1).
8. The pair of failover servers MUST be able to recover from a
network partition event (see Section 6.1.2).
9. The design MUST allow secure communication.
10. The definition of extensions to this core protocol SHOULD be
allowed, when possible.
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
sometime called lease stability.
The core protocol MUST be limited to the 1-to-1 model (see
Section 5.4). In addition, the core protocol MUST restrict each
address or prefix pool to participate in at most one failover
relationship. (Note: these are different statements!) If there is
sufficient community support for failover servers to participate in
more than one failover relationship (thus providing support for a
form of m-to-1 deployment), this capability SHALL be defined as an
extension to the core failover protocol.
Despite the lack of standardization of DHCPv4 failover, the
coexistence of DHCPv4 and DHCPv6 failover MAY be taken into
consideration. In particular, certain features that are common for
both IPv4 and IPv6, like DNS Update mechanism SHOULD be taken into
consideration.
A Load Balancing capability is considered an extension and MAY be
defined in a separate document. It MUST NOT be part of the core
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protocol, but rather defined as an extension. The primary reason for
this the desire to limit core protocol complexity.
Following features and capabilities are outside of scope of this
document:
m-to-m model (see section Section 5.4)
servers participating in multiple failover relationships
load balancing
9. Related work
This section describes related work. Readers may benefit from
familiarizing themselves with these approaches, their advantages and
limitations.
9.1. DHCPv4 failover concepts
9.1.1. Goals of DHCPv4 Failover
1. Provide a high availability DHCP service by leveraging the hooks
built into DHCPv4 [RFC2131] and its usual implementation to
support multiple servers able to respond to client requests.
These hooks are:
(a) The broadcast of DHCPDISCOVER requests.
(b) The transition from unicast for DHCPREQUEST/RENEW to
broadcast for DHCPREQUEST/REBIND.
(c) The usual implementation of DHCPv4 relay agents to allow
forwarding of DHCPv4 requests to multiple different DHCPv4
servers.
2. Produce a minimal impact on performance of the DHCPv4 server.
3. Prevent duplicate IP address allocation even in the event of a
network partition.
4. Allow multiple failover relationships per server.
5. Standardize only the minimum necessary to provide a high
availability DHCP service. In particular, avoid standardizing
the interchange of configuration information.
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9.1.2. Goals lead to Concepts
The goal to have a minimal performance impact on the operation of the
DHCPv4 servers participating in failover is the driving force behind
the design of the DHCPv4 failover protocol.
The steps in this chain of reasoning are as follows:
1. To avoid the major performance impact that a lockstep update of a
failover partner would inflict, use a lazy update approach (see
Section 6.2.2).
2. When using lazy update, there is always the problem that the
failover server could crash after it has responded to some number
of DHCPv4 clients and before it has updated its partner with the
lease information it provided to those clients.
3. Thus, when one failover server cannot communicate with another
failover server, it cannot know what that other failover server
has told any of its DHCPv4 clients.
This creates an obvious problem.
The central concept in the DHCPv4 failover design is to place a limit
on the uncertainty described in point #3, above. The DHCPv4 failover
protocol is designed to ensure that every failover server knows at
all times, not exactly what its failover partner has told to the
DHCPv4 clients with which it is communicating, but rather the limits
of what its failover partner could have told any DHCPv4 clients with
which it was communicating.
This is done by ensuring that no DHCPv4 server participating in a
failover relationship will ever offer a lease time to any DHCPv4
client that is more than an agreed-upon value beyond that known by
its failover partner.
This agreed-upon value is called the "Maximum Client Lead Time", and
abbreviated MCLT.
Thus, every DHCPv4 failover partner needs to know what its partner
knows about every lease in the server, and it needs to ensure that it
will never provide a lease time to any DHCPv4 client that is beyond
what its partner believes the current lease time to be plus the MCLT.
Given this fundamental guarantee, if one failover server cannot
communicate with its failover partner, then it knows the limits of
what any DHCPv4 client of that missing partner might have for a lease
time. If this failover server waits until it believes a lease has
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expired and then also waits until the MCLT has passed, it knows that
the lease is sure to have expired (or the DHCPv4 client will have
tried to renew the lease and communicated with the remaining DHCPv4
server). (We will deal with network partition events below.)
In order to allow a remaining failover server to provide service to
newly arrived DHCPv4 clients, while waiting out the MCLT beyond the
lease expiration (if any), the protocol provides for allocation of
some percentage of the available leases to each failover partner.
A failover server which cannot commuicate with its partner must
therefore wait out the MCLT beyond the lease expiration (if any) of
IP addresses before it can allocate them to DHCPv4 clients. This
could impact the server's ability to provide available IP addresses
to newly arrived DHCPv4 clients. To prevent this impact the DHCPv4
failover protocol divides the allocation of the available leases
between each failover partner. The protocol supports periodic
rebalancing of the allocation of these available leases.
9.1.3. Use of the MCLT in practice
From the above discussion, it should be clear how to avoid re-using
an IP address before it has expired. The MCLT is central to the
operation of the protocol. One server cannot offer a lease to a
DHCPv4 client that is more than the MCLT beyond the current lease
time for this client that is known by the failover partner. From
this standpoint, it would be good for the MCLT to be as long as
possible. However, in a failure situation, waiting the MCLT beyond
the current lease time in order to reuse a leased lease would suggest
that the MCLT should be as short as possible.
This tension is resolved by anticipating the need to extend lease
times when communicating with the failover partner. The first lease
offered to a DHCPv4 client can be only as long as the MCLT. However,
when the failover server updates its partner, it updates the partner
with the desired lease time plus the MCLT. Thus, when the client
returns with a renewal request at halfway through the MCLT, the
failover server can extend its lease for only the lease time known by
the partner plus the MCLT. But the partner now knows the desired
lease time, so that the server can extend the lease for as long as it
was configured since it had pre-updated the failover partner with
this time.
Using this approach, one can keep the MCLT relatively short, say 1
hour, and still offer leases of any desired extent to clients -- once
the failover partner has been updated with the desired lease time.
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9.1.4. Network Partition Events
It is clear that when one failover server finds that it is unable to
communicate with its failover partner, it is impossible for that
server to tell if its failover partner is down or if the
communication path to that failover partner is broken, a situation
known as "network partition" (see Section 6.1.2). The DHCPv4
failover protocol distinguishes between these different situations by
having different failover states to represent "communications-
interrupted" situations and a "partner-down" situations. The
expectation is that (at least in some situations) it requires an
operator action to distinguish between a communications-interrupted
and partner-down event. In particular, the DHCPv4 failover protocol
does not conflate these two situations.
Correct handling of network partition events requires that a failover
server unable to communicate with its failover partner (but not yet
informed that its failover partner is down), must not re-allocate an
IP address from one DHCPv4 client to another. Available addresses
may be allocated to any DHCPv4 client.
After a failover server has been informed that its partner is down,
it can reallocate an IP address from one DHCPv4 client to another
once it has waited the MCLT beyond the lease expiration of that IP
address. This need to be informed by an external entity that the
failover partner is down is the only impact of correctly handling
network partition events. Of course, specific implementations can
assume that an unreachable failover partner is down after a shorter
or longer time, thus limiting the support for a network partition
event.
9.1.5. Conflict Resolution
Whenever one failover server receives an update from its failover
partner, it needs to decide if the update it has received is "better"
than the information it has in its own database concerning the DHCPv4
client or the lease on the IPv4 address. The DHCPv4 failover
protocol does not mandate the details of this decision, but this
activity must be part of any DHCPv4 implementation. In most cases,
comparing the times associated with the failover update with the
times held in the server's own database will allow this decision to
be made.
9.1.6. Load Balancing
The DHCPv4 Load Balancing protocol [RFC3074] integrates with the
DHCPv4 failover protocol by defining the way that each server decides
which DHCPv4 clients to process. Use of load balancing with the
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DHCPv4 failover protocol is an optional extension to the failover
protocol. Both a simple active -- passive relationship without load
balancing is defined as well as a more complex active -- active
relationship.
9.2. DHCPv6 Redundancy Considerations
[I-D.ietf-dhc-dhcpv6-redundancy-consider] specifies an interim
architecture to provide a semi-redundant DHCPv6 solution before the
availability of vendor or standard based solutions. The proposed
architecture may be used in wide range of networks. Two notable
deployment models are discussed: service provider and enterprise
network environments. The described architecture leverages only
existing and implemented DHCPv6 standards.
10. DHCP Best Practices
TODO: Qin Wu provided this best practices link. Describe it briefly.
http://technet.microsoft.com/en-us/library/cc780311(WS.10).aspx
11. Security Considerations
TBD...
12. IANA Considerations
IANA is not requested to perform any actions at this time.
13. Acknowledgements
This document extensively uses concepts, definitions and other parts
of [dhcpv4-failover] document. Authors would like to thank Shawn
Routhier, Qin Wu, Jean-Francois Tremblay, Frank Sweetser, Jiang
Sheng, Yu Fu, Greg Gabil and Bernie Volz for their comments and
feedback.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3074] Volz, B., Gonczi, S., Lemon, T., and R. Stevens, "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
[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.
14.2. Informative References
[I-D.ietf-dhc-dhcpv6-redundancy-consider]
Brzozowski, J., Tremblay, J., Chen, J., and T. Mrugalski,
"DHCPv6 Redundancy Deployment Considerations",
draft-ietf-dhc-dhcpv6-redundancy-consider-00 (work in
progress), April 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 (editor)
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 423 1345
Email: tomasz.mrugalski@gmail.com
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Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, Massachusetts 01719
USA
Phone: +1 (978) 936-0000
Email: kkinear@cisco.com
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