Dynamic Host Configuration (DHC) T. Mrugalski
Internet-Draft ISC
Intended status: Standards Track K. Kinnear
Expires: January 16, 2014 Cisco
July 15, 2013
DHCPv6 Failover Design
draft-ietf-dhc-dhcpv6-failover-design-03
Abstract
DHCPv6 defined in [RFC3315] does not offer server redundancy. This
document defines a design for DHCPv6 failover, a mechanism for
running two servers on the same network with capability for either
server to take over clients' leases in case of server failure or
network partition. This is a DHCPv6 Failover design document, it is
not protocol specification document. It is a second document in a
planned series of three documents. DHCPv6 failover requirements are
specified in [I-D.ietf-dhc-dhcpv6-failover-requirements]. A protocol
specification document is planned to follow this document.
Status of This 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
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on January 16, 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
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
2. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Design Requirements . . . . . . . . . . . . . . . . . . . 6
3.2. Features out of Scope: Load Balancing . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Failover State Machine Overview . . . . . . . . . . . . . 8
4.2. Messages . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Connection Management . . . . . . . . . . . . . . . . . . . . 11
5.1. Creating Connections . . . . . . . . . . . . . . . . . . 11
5.2. Endpoint Identification . . . . . . . . . . . . . . . . . 13
6. Resource Allocation . . . . . . . . . . . . . . . . . . . . . 13
6.1. Proportional Allocation . . . . . . . . . . . . . . . . . 14
6.2. Independent Allocation . . . . . . . . . . . . . . . . . 16
6.3. Choosing Allocation Algorithm . . . . . . . . . . . . . . 17
7. Information model . . . . . . . . . . . . . . . . . . . . . . 18
8. Failover Mechanisms . . . . . . . . . . . . . . . . . . . . . 22
8.1. Time Skew . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2. Time expression . . . . . . . . . . . . . . . . . . . . . 23
8.3. Lazy updates . . . . . . . . . . . . . . . . . . . . . . 23
8.4. MCLT concept . . . . . . . . . . . . . . . . . . . . . . 23
8.4.1. MCLT example . . . . . . . . . . . . . . . . . . . . 25
8.5. Unreachability detection . . . . . . . . . . . . . . . . 26
8.6. Re-allocating Leases . . . . . . . . . . . . . . . . . . 26
8.7. Sending Binding Update . . . . . . . . . . . . . . . . . 27
8.8. Receiving Binding Update . . . . . . . . . . . . . . . . 29
8.9. Conflict Resolution . . . . . . . . . . . . . . . . . . . 30
8.10. Acknowledging Reception . . . . . . . . . . . . . . . . . 32
9. Endpoint States . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. State Machine Operation . . . . . . . . . . . . . . . . . 32
9.2. State Machine Initialization . . . . . . . . . . . . . . 35
9.3. STARTUP State . . . . . . . . . . . . . . . . . . . . . . 35
9.3.1. Operation in STARTUP State . . . . . . . . . . . . . 36
9.3.2. Transition Out of STARTUP State . . . . . . . . . . . 36
9.4. PARTNER-DOWN State . . . . . . . . . . . . . . . . . . . 38
9.4.1. Operation in PARTNER-DOWN State . . . . . . . . . . . 38
9.4.2. Transition Out of PARTNER-DOWN State . . . . . . . . 39
9.5. RECOVER State . . . . . . . . . . . . . . . . . . . . . . 40
9.5.1. Operation in RECOVER State . . . . . . . . . . . . . 40
9.5.2. Transition Out of RECOVER State . . . . . . . . . . . 40
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9.6. RECOVER-WAIT State . . . . . . . . . . . . . . . . . . . 41
9.6.1. Operation in RECOVER-WAIT State . . . . . . . . . . . 41
9.6.2. Transition Out of RECOVER-WAIT State . . . . . . . . 42
9.7. RECOVER-DONE State . . . . . . . . . . . . . . . . . . . 42
9.7.1. Operation in RECOVER-DONE State . . . . . . . . . . . 42
9.7.2. Transition Out of RECOVER-DONE State . . . . . . . . 42
9.8. NORMAL State . . . . . . . . . . . . . . . . . . . . . . 43
9.8.1. Operation in NORMAL State . . . . . . . . . . . . . . 43
9.8.2. Transition Out of NORMAL State . . . . . . . . . . . 44
9.9. COMMUNICATIONS-INTERRUPTED State . . . . . . . . . . . . 44
9.9.1. Operation in COMMUNICATIONS-INTERRUPTED State . . . . 45
9.9.2. Transition Out of COMMUNICATIONS-INTERRUPTED State . 45
9.10. POTENTIAL-CONFLICT State . . . . . . . . . . . . . . . . 47
9.10.1. Operation in POTENTIAL-CONFLICT State . . . . . . . 47
9.10.2. Transition Out of POTENTIAL-CONFLICT State . . . . . 47
9.11. RESOLUTION-INTERRUPTED State . . . . . . . . . . . . . . 49
9.11.1. Operation in RESOLUTION-INTERRUPTED State . . . . . 49
9.11.2. Transition Out of RESOLUTION-INTERRUPTED State . . . 49
9.12. CONFLICT-DONE State . . . . . . . . . . . . . . . . . . . 49
9.12.1. Operation in CONFLICT-DONE State . . . . . . . . . . 50
9.12.2. Transition Out of CONFLICT-DONE State . . . . . . . 50
10. Proposed extensions . . . . . . . . . . . . . . . . . . . . . 50
10.1. Active-active mode . . . . . . . . . . . . . . . . . . . 50
11. Dynamic DNS Considerations . . . . . . . . . . . . . . . . . 51
11.1. Relationship between failover and dynamic DNS update . . 51
11.2. Exchanging DDNS Information . . . . . . . . . . . . . . 52
11.3. Adding RRs to the DNS . . . . . . . . . . . . . . . . . 54
11.4. Deleting RRs from the DNS . . . . . . . . . . . . . . . 55
11.5. Name Assignment with No Update of DNS . . . . . . . . . 55
12. Reservations and failover . . . . . . . . . . . . . . . . . . 56
13. Security Considerations . . . . . . . . . . . . . . . . . . . 57
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 58
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 58
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 58
16.1. Normative References . . . . . . . . . . . . . . . . . . 58
16.2. Informative References . . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 59
1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Glossary
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This is a supplemental glossary that should be combined with
definitions in Section 3 of
[I-D.ietf-dhc-dhcpv6-failover-requirements].
o auto-partner-down - a capability where a failover server will move
from COMMUNICATIONS-INTERRUPTED state to PARTNER-DOWN state
automatically, without operator intervention.
o Failover endpoint - The failover protocol allows for there to be a
unique failover 'endpoint' for each failover relationship in which
a failover server participates. The failover relationship is
defined by a relationship name, and includes the failover partner
IP address, the role this server takes with respect to that
partner (primary or secondary), and the prefixes associated with
that relationship. Note that a single prefix can only be
associated with a single failover relationship. This failover
endpoint can take actions and hold unique states. Typically,
there is one failover endpoint per partner (server), although
there may be more. 'Server' and 'failover endpoint' are
synonymous only if the server participates in only one failover
relationship. However, for the sake of simplicity 'Server' is
used throughout the document to refer to a failover endpoint
unless to do so would be confusing.
o Failover communication - all messages exchanged between partners.
o Independent Allocation - an allocation algorithm that splits the
available pool of resources between the primary and secondary
servers that is particularly well suited for vast pools (i.e. when
available resources are not expected to deplete). See Section 6.2
for details.
o Partner - name of the other DHCPv6 server that participates in
failover relationship. When the role (primary or secondary) is
not important, the other server is referred to as a "failover
partner" or simply partner.
o Primary Server - First out of two DHCPv6 servers that participate
in a failover relationship. In active-passive mode this is the
server that handles most of the client traffic. Its failover
partner is referred to as secondary server.
o Proportional Allocation - an allocation algorithm that splits the
available resources (addresses or prefixes) between the primary
and secondary servers and maintains proportions between available
resources on both. It is particularly well suited for more
limited resources. See Section 6.1 for details.
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o Resource - Any type of resource that is managed by DHCPv6.
Currently there are two types of such resources defined: a non-
temporary IPv6 address and an IPv6 prefix. Due to the nature of
temporary addresses, they are not covered by the failover
mechanism. Other resource types may be defined in the future.
o Responsive - A server that is responsive, will respond to DHCPv6
client requests.
o Secondary Server - Second of out two DHCPv6 servers that
participate in a failover relationship. Its failover partner is
referred to as primary server. In active-passive mode this server
typically does not handle client traffic and acts as a backup.
o Server - A DHCPv6 server that implements DHCPv6 failover.
'Server' and 'failover endpoint' are synonymous only if the server
participates in only one failover relationship.
o Unresponsive - A server that is unresponsive will not respond to
DHCPv6 client requests.
3. Introduction
The failover protocol design provides a means for cooperating DHCPv6
servers to work together to provide a DHCPv6 service with
availability that is increased beyond that which could be provided by
a single DHCPv6 server operating alone. It is designed to protect
DHCPv6 clients against server unreachability, including server
failure and network partition. It is possible to deploy exactly two
servers that are able to continue providing a lease on an IPv6
address [RFC3315] or on an IPv6 prefix [RFC3633] without the DHCPv6
client experiencing lease expiration or a reassignment of a lease to
a different IPv6 address in the event of failure by one or the other
of the two servers.
This protocol defines active-passive mode, sometimes also called a
hot standby model. This means that during normal operation one
server is active (i.e. actively responds to clients' requests) while
the second is passive (i.e. it does receive clients' requests, but
does not respond to them and only maintains a copy of lease database
and is ready to take over incoming queries in case of primary server
failure). Active-active mode (i.e. both servers actively handling
clients' requests) is currently not supported for the sake of
simplicity. Such a mode is likely to be defined as an exension at a
later time and will probably be based on
[I-D.ietf-dhc-dhcpv6-load-balancing].
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The failover protocol is designed to provide lease stability for
leases with lease times beyond a short period. Due in part to the
additional overhead required as well as requirements to handle time
skew between failover partners (See Section 8.1), failover is not
suitable for leases shorter than 30 seconds. The DHCPv6 Failover
protocol MUST NOT be used for leases shorter than 30 seconds.
This design attempts to fulfill all DHCPv6 failover requirements
defined in [I-D.ietf-dhc-dhcpv6-failover-requirements].
3.1. Design Requirements
The following requirements are not related to failover mechanism in
general, but rather to this particular design.
1. Minimize Asymmetry - while there are two distinct roles in
failover (primary and secondary server), the differences between
those two roles should be as small as possible. This will yield
a simpler design as well as a simpler implementation of that
design.
3.2. Features out of Scope: Load Balancing
While it is tempting to extend DHCPv6 failover mechanism to also
offer load balancing, as DHCPv4 failover did, this design does not do
that. Here is the reasoning for this decision. In general case (not
related to failover) load balancing solutions are used when each
server is not able to handle total incoming traffic. However, by the
very definition, DHCPv6 failover is supposed to assume service
availability despite failure of one server. That leads to conclusion
that each server must be able to handle whole traffic. Therefore in
properly provisioned setup, load balancing is not needed.
It is likely that active-active mode that is essentially a load
balancing will be defined as an extension in the near future.
4. Protocol Overview
The DHCPv6 Failover Protocol is defined as a communication between
failover partners with all associated algorithms and mechanisms.
Failover communication is conducted over a TCP connection established
between the partners. The protocol reuses the framing format
specified in Section 5.1 of DHCPv6 Bulk Leasequery [RFC5460], but
uses different message types. New failover-specific message types
are listed in Section 4.2. All information is sent over the
connection as typical DHCPv6 messages that convey DHCPv6 options,
following format defined in Section 22.1 of [RFC3315].
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After initialization, the primary server establishes a TCP connection
with its partner. The primary server sends a CONNECT message with
initial parameters. Secondary server responds with CONNECTACK.
If the primary server cannot immediately establish a connection with
its partner, it will continue to attempt to establish a connection.
See Section 5.1 for details.
Depending on the failover state of each partner, they MUST initiate
one of the binding update procedures. Each server MAY send an UPDREQ
message to request its partner to send all updates that have not been
sent yet (this case applies when the partner has an existing database
and wants to update it). Alternatively, a server MAY choose to send
an UPDREQALL message to request a full lease database transmission
including all leases (this case applies in case of booting up new
server after installation, corruption or complete loss of database,
or other catastrophic failure).
Servers exchange lease information by using BNDUPD messages.
Depending on the local and remote state of a lease, a server may
either accept or reject the update. Reception of lease update
information is confirmed by responding with a BNDACK message with
appropriate status. The majority of the messages sent over a
failover TCP connection consists of BNDUPD and BNDACK messages.
A subset of available resources (addresses or prefixes) is reserved
for secondary server use. This is required for handling a case where
both servers are able to communicate with clients, but unable to
communicate with each other. After the initial connection is
established, the secondary server requests a pool of available
addresses by sending a POOLREQ message. The primary server assigns
addresses to the secondary by sending a series of BNDUPD messages.
When this process is complete, the primary server sends a POOLRESP
message to the secondary server. The secondary server may initiate
such pool request at any time when in communication with primary
server.
Failover servers use a lazy update mechanism to update their failover
partner about changes to their lease state database. After a server
performs any modifications to its lease state database (assign a new
lease, extend, release or expire existing lease), it sends its
response to the client's request first (performing the "regular"
DHCPv6 operation) and then informs its failover partner using a
BNDUPD message. This BNDUPD message SHOULD be sent soon after the
response is sent to the DHCPv6 client, but there is no specific
requirement of a minimum time in which to do so.
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The major problem with lazy update mechanism is the case when the
server crashes after sending a response to client, but before sending
the lazy update to its partner (or when communication between
partners is interrupted). To solve this problem, the concept known
as the Maximum Client Lead Time (initially designed for DHCPv4
failover) is used. The MCLT is the maximum amount of time that one
server can extend a lease for a client's binding beyond the time
known by its failover partner. See Section 8.4 for detailed
desciption how the MCLT affects assigned lease times.
Servers verify each others availability by periodically exchanging
CONTACT messages. See Section 8.5 for discussion about detecting a
partner's unreachability.
A server that is being shut down transmits a DISCONNECT message,
closes the connection with its failover partner and stops operation.
A Server SHOULD transmit any pending lease updates before
transmitting DISCONNECT message.
4.1. Failover State Machine Overview
The following section provides a simplified description of all
states. For the sake of clarity and simplicity, it omits important
details. For complete description, see Section 9. In case of a
disagreement between the simplified and complete description, please
follow Section 9.
Each server MUST be in one of the well defines states. In each state
a server may be either responsive (responds to clients' queries) or
unresponsive (clients' queries are ignored).
A server starts its operation in short-lived STARTUP state. A server
determines its partner reachability and state and sets its own state
based on that determination. It typically returns back to the state
it was in before shutdown, though the details can be complicated.
See Section 9.3.2.
During typical operation when servers maintain communication, both
are in NORMAL state. In that state only the primary responds to
clients' requests. A secondary server is unresponsive.
If a server discovers that its partner is no longer reachable, it
goes to COMMUNICATIONS-INTERRUPTED state. A server must be extra
cautious as it can't distingush if its partner is down or just
communication between servers is interrupted. Since communication
between partners is not possible, a server must act on the assumtion
that its partner is up. A failover server must follow a defined
procedure, in particular, it MUST NOT extend any lease more than the
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MCLT beyond its partner's knowledge of the lease expiration time.
This imposes an additional burden on the server, in that clients will
return to the server for lease renewals more frequently than they
would otherwise. Therefore it is not recommended to operate for
prolonged periods in this state. Once communication is
reestablished, a server may go into NORMAL, POTENTIAL-CONFLICT or
PARTNER-DOWN state. It may also stay in COMMUNICATIONS-INTERRUPTED
state if certain conditions are met.
Once a server is switched into PARTNER-DOWN (when auto-partner-down
is used or as a result of administrative action), it can extend
leases, regardless of the original server that initially granted the
lease. In that state server handles leases from its own pool, but
once its own pool is depleted is also able to serve pool from its
downed partner. MCLT restrictions no longer apply. Operation in
this mode is less demanding for the server that remains operational,
than in COMMUNICATIONS-INTERRUPTED state, but PARTNER-DOWN does not
offer any kind of redundancy. Even when in PARTNER-DOWN state, a
failover server continues to attempt to connect with its failover
partner.
A server switches into RECOVER state when any of a variety of
conditions are encountered:
o When a backup server contacts its failover partner for the first
time.
o When either server discovers that its failover partner has
contacted it before but it has no local record of this contact.
If the record of previous contact is held in the lease-state
database, then this situation implies that the server has lost its
lease state database.
o When its failover partner is in PARTNER-DOWN state.
Any of these conditions signal that the server needs to refresh its
lease-state database from its partner. Once this operation is
complete, it switches to RECOVER-WAIT and later to RECOVER-DONE. See
Section 9.6.2.
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Once servers reestablish connection, they discover each others'
state. Depending on the conditions, they may return to NORMAL or
move to POTENTINAL-CONFLICT if the partner is in a state that doesn't
allow a simple re-integration of the server's lease state databases.
It is a goal of this protocol to minimize the possibility that
POTENTIAL-CONFLICT state is ever entered. Servers running in
POTENTIAL-CONFLICT do not respond to clients' requests and work only
on resolving potential conflicts. Once outstanding lease updates are
exchanged, servers move to CONFLICT-DONE or NORMAL states.
Servers that are recovering from potential conflicts and loose
communication, switch to RESOLUTION-INTERRUPTED.
A server that is being shut down sends a DISCONNECT message. See
Section 4.2. A server that receives a DISCONNECT message moves into
COMMUNICATIONS-INTERRUPTED state.
4.2. Messages
The failover protocol is centered around the message exchanges used
by one server to update its partner and respond to received updates.
It should be noted that no specific formats or message type values
are assigned in this document. Appropriate implementation details
will be specified in a separate protocol specification document. The
following list enumerates these messages:
o BNDUPD - The binding update message is used to send the binding
lease changes to the partner. One message may contain one or more
lease updates. The partner is expected to respond with a BNDACK
message.
o BNDACK - The binding acknowledgement is used for confirmation of
the received BNDUPD message. It may contain a positive or
negative response (e.g. due to detected lease conflict).
o POOLREQ - The Pool Request message is used by one server
(typically secondary) to request allocation of resources
(addresses or prefixes) from its partner. The partner responds
with POOLRESP.
o POOLRESP - The Pool Response message is used by one server
(typically primary) to repond to its partner's request for
resources allocation. One POOLRESP message may contain more than
one pool.
o UPDREQ - The update request message is used by one server to
request that its partner send all binding database changes that
has not been sent and confirmed already. Requested partner is
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expected to respond with zero or more BNDUPD messages, followed by
UPDDONE that signals end of updates.
o UPDREQALL - The update request all is used by one server to
request that all binding database information be sent in order to
recover from a total loss of its binding database by the
requesting server. Requested server responds with zero or more
BNDUPD messages, followed by UPDDONE that signal end of updates.
o UPDDONE - The update done message is used by the responding server
to indicate that all requested updates have been sent by the
responding server and acked by the requesting server.
o CONNECT - The connect message is used by the primary server to
establish a high level connection with the other server, and to
transmit several important configuration data items between the
servers. The partner is expected to confirm by responding with
CONNECTACK message.
o CONNECTACK - The connect acknowledgement message is used by the
secondary server to respond to a CONNECT message from the primary
server.
o DISCONNECT - The disconnect message is used by either server when
closing a connection and shutting down. No response is required
for this message.
o STATE - The state message is used by either server to inform its
partner about a change of failover state. In some cases it may be
used to also inform the partner about current state, e.g. after
connection is established in COMMUNICATIONS-INTERRUPTED or
PARTNER-DOWN states.
o CONTACT - The contact message is used by either server to ensure
that the other server continues to see the connection as opera-
tional. It MUST be transmitted periodically over every esta-
blished connection if other message traffic is not flowing, and it
MAY be sent at any time.
5. Connection Management
5.1. Creating Connections
Every primary server implementing the failover protocol SHOULD
attempt to connect to all of its partners periodically, where the
period is implementation dependent and SHOULD be configurable. In
the event that a connection has been rejected by a CONNECTACK message
with a reject-reason option contained in it or a DISCONNECT message,
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a server SHOULD reduce the frequency with which it attempts to
connect to that server but it SHOULD continue to attempt to connect
periodically.
Every secondary server implementing the failover protocol SHOULD
listen for connection attempts from the primary server.
When a connection attempt succeeds, the primary server which has
initiated the connection attempt MUST send a CONNECT message down the
connection.
When a connection attempt is received, the only information that the
receiving server has is the IP address of the partner initiating a
connection. If it has any relationships with the connecting server
for which it is a seconary server, it should just await the CONNECT
message to determine which relationship this connection is to serve.
If it has no secondary relationships with the connecting server, it
SHOULD drop the connection. The goal is to limit the resources
expended dealing with attempts to create a spurious failover
connection.
To summarize -- a primary server MUST use a connection that it has
initiated in order to send a CONNECT message. Every server that is a
secondary server in a relationship simply listens for connection
attempts from the primary server.
Once a connection is established, the primary server MUST send a
CONNECT message across the connection. A secondary server MUST wait
for the CONNECT message from a primary server. If the secondary
server doesn't receive a CONNECT message from the primary server in
an installation dependent amount of time, it MAY drop the connection.
Every CONNECT message includes a TLS-request option, and if the
CONNECTACK message does not reject the CONNECT message and the TLS-
reply option says TLS MUST be used, then the servers will immediately
enter into TLS negotiation.
Once TLS negotiation is complete, the primary server MUST resend the
CONNECT message on the newly secured TLS connection and then wait for
the CONNECTACK message in response. The TLS-request and TLS-reply
options MUST NOT appear in either this second CONNECT or its
associated CONNECTACK message as they had in the first messages.
The second message sent over a new connection (either a bare TCP
connection or a connection utilizing TLS) is a STATE message. Upon
the receipt of this message, the receiver can consider communications
up.
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5.2. Endpoint Identification
The proper operation of the failover protocol requires more than the
transmission of messages between one server and the other. Each
endpoint might seem to be a single DHCPv6 server, but in fact there
are situations where additional flexibility in configuration is
useful. A failover endpoint is always associated with a set of
DHCPv6 prefixes that are configured on the DHCPv6 server where the
endpoint appears. A DHCPv6 prefix MUST NOT be associated with more
than one failover endpoint.
The failover protocol SHOULD be configured with one failover
relationship between each pair of failover servers. In this case
there is one failover endpoint for that relationship on each failover
partner. This failover relationship MUST have a unique name.
There is typically little need for additional relationships between
any two servers but there MAY be more than one failover relationship
between two servers -- however each MUST have a unique relationship
name.
Any failover endpoint can take actions and hold unique states.
This document frequently describes the behavior of the protocol in
terms of primary and secondary servers, not primary and secondary
failover endpoints. However, it is important to remember that every
'server' described in this document is in reality a failover endpoint
that resides in a particular process, and that several failover end-
points may reside in the same server process.
It is not the case that there is a unique failover endpoint for each
prefix that participates in a failover relationship. On one server,
there is (typically) one failover endpoint per partner, regardless of
how many prefixes are managed by that combination of partner and
role. Conversely, on a particular server, any given prefix will be
associated with exactly one failover endpoint.
When a connection is received from the partner, the unique failover
endpoint to which the message is directed is determined solely by the
IP address of the partner, the relationship-name, and the role of the
receiving server.
6. Resource Allocation
Currently there are two allocation algorithms defined for resources
(addresses or prefixes). Additional allocation schemes may be
defined as future extensions.
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1. Proportional Allocation - This allocation algorithm is a direct
application of the algorithm defined in [dhcpv4-failover] to
DHCPv6. Remaining available resources are split between the
primary and secondary servers in a configured proportion.
Released resources are always returned to the primary server.
Primary and secondary servers may initiate a rebalancing
procedure when disparity between resources available to each
server reaches a preconfigured threshold. Only resources that
are not leased to any clients are "owned" by one of the servers.
This algorithm is particularly well suited for scenarios where
amount of available resources is limited, as may be the case with
prefix delegation. See Section 6.1 for details.
2. Independent Allocation - This allocation algorithm assumes that
available resources are split between primary and secondary
servers as well. In this case, however, resources are assigned
to a specific server for all time, regardless if they are
available or currently used. This algorithm is much simpler than
proportional allocation, because resource imbalance doesn't have
to be checked and there is no rebalancing for independent
allocation. This algorithm is particularly well suited for
scenarios where the there is an abundance of available resources
which is typically the case for DHCPv6 address allocation. See
Section 6.2 for details.
6.1. Proportional Allocation
In this allocation scheme, each server has its own pool of available
resources. Remaining available resources are split between the
primary and secondary servers in a configured proportion. Note that
a resource is not "owned" by a particular server throughout its
entire lifetime. Only a resource which is available is "owned" by a
particular server -- once it has been leased to a client, it is not
owned by either failover partner. When it finally becomes available
again, it will be owned initially by the primary server, and it may
or may not be allocated to the secondary server by the primary
server.
The flow of a resource is as follows: initially a resource is owned
by the primary server. It may be allocated to the secondary server
if it is available, and then it is owned by the secondary server.
Either server can allocate available resources which they own to
clients, in which case they cease to own them. When the client
releases the resource or the lease on it expires, it will again
become available and will be owned by the primary.
A resource will not become owned by the server which allocated it
initially when it is released or the lease expires because, in
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general, that server will have had to replenish its pool of available
resources well in advance of any likely lease expirations. Thus,
having a particular resource cycle back to the secondary might well
put the secondary more out of balance with respect to the primary
instead of enhancing the balance of available addresses or prefixes
between them.
Pools governed by proportional allocation are used for allocation
when the server is in all states, except PARTNER-DOWN. In PARTNER-
DOWN state the healthy partner can allocate from either pool (both
its own and its partner's). This allocation and maintenance of these
address pools is an area of some sensitivity, since the goal is to
maintain a more or less constant ratio of available addresses between
the two servers.
The initial allocation when the servers first integrate is triggered
by the POOLREQ message from the secondary to the primary. This is
followed by the POOLRESP message where the primary tells the
secondary how many resources it allocated to the secondary. Then,
the primary sends the allocated resources to the secondary via BNDUPD
messages. The POOLREQ/POOLRESP message is a trigger to the primary
to perform a scan of its database and to ensure that the secondary
has enough resources (based on some configured ratio).
The primary server SHOULD examine some or all of its database from
time to time to determine if resources should be shifted between the
primary and secondary (in either direction). The POOLREQ/POOLRESP
message exchange allows the secondary server to explicitly request
that the primary server examine the entirety of its database to
ensure that the secondary has the approprite resources available.
Servers frequently have several kinds of resources available on a
particular network segment. The failover protocol assumes that both
primary and secondary servers are configured in such a way that each
knows the type and number of resources on every network segment
participating in the failover protocol. The primary server is
responsible for allocating the secondary server the correct
proportion of available resources of each kind, and the secondary
server MUST be configured in such a way that it can tell the kind of
every resource based solely on the IP or prefix address itself.
The resources are delegated to the secondary using the BNDUPD message
with a state of FREE_BACKUP, which indicates the resource is now
available for allocation by the secondary. Once the message is sent,
the primary MUST NOT use these resources for allocation to DHCPv6
clients.
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Available resources can be delegated back to the primary server in
certain cases. BNDUPD will contain state FREE for leases that were
previously in FREE_BACKUP state.
The POOLREQ/POOLRESP message exchange initiated by the secondary is
valid at any time both partners remain in contact, and the primary
server SHOULD, whenever it receives the POOLREQ message, scan its
database of prefixes and determine if the secondary needs more
resources from any of the prefixes.
In order to support a reasonably dynamic balance of the resources
between the failover partners, the primary server needs to do
additional work to ensure that the secondary server has as many
resources as it needs (but that it doesn't have more than it needs).
The primary server SHOULD examine the balance of available resources
between the primary and secondary for a particular prefix whenever
the number of available resources for either the primary or secondary
changes by more than a configured limit. The primary server SHOULD
adjust the available resource balance as required to ensure the
configured resource balance, excepting that the primary server SHOULD
employ some threshold mechanism to such a balance adjustment in order
to minimize the overhead of maintaining this balance.
An example of a threshold approach is: do not attempt to re-balance
the prefixes on the primary and secondary until the out of balance
value exceeds a configured value.
The primary server can, at any time, send an available resource to
the secondary using a BNDUPD with the state BACKUP. The primary
server can attempt to take an available resource away from the
secondary by sending a BNDUPD with the state FREE. If the secondary
accepts the BNDUPD, then the resource is now available to the primary
and not available to the secondary. Of course, the secondary MUST
reject that BNDUPD if it has already used that resource for a DHCP
client.
6.2. Independent Allocation
In this allocation scheme, available resources are permanently (until
server configuration changes) split between servers. Available
resources are split between the primary and secondary servers as part
of initial connection establishment. Once resources are allocated to
each server, there is no need to reassign them. The resource
allocation is algorithmic in nature, and does not require a message
exchange for each resources allocated. This algorithm is simpler
than proportional allocation since it requires similar initial
communication, but does not require a rebalancing mechanism. It
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assumes that the pool assigned to each server will never deplete.
That is often a reasonable assumption for IPv6 addresses (e.g.
servers are often assigned a /64 pool that contains many more
addresses than existing electronic devices on Earth). This
allocation mechanism SHOULD be used for IPv6 addresses, unless the
configured address pool is small or is otherwise administratively
limited.
Once each server is assigned a resource pool during initial
connection establishment, it may allocate assigned resources to
clients. Once a client releases a resource or its lease is expired,
the returned resource returns to pool for the server that leased it.
Resources never changes servers.
Resources using the independent allocation approach are ignored when
a server processes a POOLREQ message.
During COMMUNICATION-INTERRUPTED events, a partner MAY continue
extending existing leases when requested by clients. A healthy
partner MUST NOT lease resources that were assigned to its downed
partner and later released by a client unless it is in PARTNER-DOWN
state. Server SHOULD use its own pool first before starting new
assignements from its downed partner's pool. As the assumption is
that independent allocation should be used only when available
resources are vast and not expected to be fully used at any given
time, it is very unlikely that the server will ever need to use its
downed partner pools. This makes a recovery even after prolonged
down-time much easier.
6.3. Choosing Allocation Algorithm
All implementations MUST support proportional allocation algorithm
and SHOULD support independent allocation. If the implementation
implements both and lets the user choose between them, the default
algorithm used SHOULD be proportional allocation algorithm.
Proportional allocation mechanism is more flexible as it can
dynamically rebalance available resources between servers. That
balance includes additional burden for the servers and generates more
traffic between servers. Proportional algorithm can be considered
more efficient at managing available resources, compared to
idenpendent. That is important aspect when working in a network that
is nearing address and/or prefix depletion.
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Independent allocation can be used when the number of available
resources are large and there is no realistic danger of running out
of resources. Use of the independent allocation makes communication
between partners simpler. It also makes recovery easier and
potential conflict less likely to appear.
Typically independent allocation is used for IPv6 addresses, because
even for /64 pools a server will never run out of addresses to
assign, so there is no need to rebalance. For the prefix delegation
mechanism, available resources are typically much smaller, so there
is a danger of running out of prefixes. Therefore typically
proportional allocation will be used for prefix delegations.
Independent allocation still may be used, but the implication must be
well understood. For example in a network that delegates /64
prefixes out out /48 prefix (so there can be up to 65536 prefixes
delegated) and a 1000 requesting routers, it is safe to use
independent allocation.
It should be stressed out that independent allocation algorithm
SHOULD NOT be used when number of resources is limited and there is a
realistic danger of depleting resources. If this recommendation is
violated, it may lead to a case, when one server denies clients due
to pool depletion despite the fact the the other partner still have
many resources available.
With independent allocation it is very unlikely to remaining healthy
server to allocate resources from its unavailable partner's pool.
That makes recovery easier and any potential conflicts are less
likely to appear.
7. Information model
In most DHCP servers a resource (an IP address or a prefix) can take
on several different binding-status values, sometimes also called
lease states. While no two DHCP server implementations probably have
exactly the same possible binding-status values, [RFC3315] enforces
some commonality among the general semantics of the binding-status
values used by various DHCP server implementations.
In order to transmit binding database updates between one server and
another using the failover protocol, some common denominator binding-
status values must be defined. It is not expected that these values
correspond with any actual implementation of the DHCP protocol in a
DHCP server, but rather that the binding-status values defined in
this document should be a common denominator of those in use by many
DHCP server implementations.
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The lease binding-status values defined for the failover protocol are
listed below. Unless otherwise noted below, there MAY be client
information associated with each of these binding-status value.
ACTIVE -- The lease is assigned to a client. Client identification
data MUST appear.
EXPIRED -- indicates that a client's binding on a given lease has
expired. When the partner acks the BNDUPD of an expired lease,
the server sets its internal state to FREE*. Client identification
SHOULD appear.
RELEASED -- indicates that a client sent in RELEASE message. When
the partner acks the BNDUPD of a released lease, the server sets
its internal state to FREE*. Client identification SHOULD appear.
FREE* -- Once a lease is expired or released, its state becomes
FREE*. Depending on which algorithm and which pool was used to
allocate a given lease, FREE* may either mean FREE or FREE_BACKUP.
Implementations do not have to implement this FREE* state, but may
choose to switch to the destination state directly. For a clarity
of representation, this transitional FREE* state is treated as a
separate state.
FREE -- Is used when a DHCP server needs to communicate that a
resource is unused by any client, but it was not just released,
expired or reset by a network administrator. When the partner
acks the BNDUPD of a FREE lease, the server marks the lease as
available for assignment by the primary server. Note that on a
secondary server running in PARTNER-DOWN state, after waiting the
MCLT, the resource MAY be allocated to a client by the secondary
server if proportional algorithm is used. Client identification
MAY appear.
FREE_BACKUP -- indicates that this resource can be allocated by the
secondary server to a client at any time. Note that the primary
server running in PARTNER-DOWN state, after waiting the MCLT, the
resource MAY be allocated to a client by the primary server if
proportional algorithm was used. Client identification MAY
appear.
ABANDONED -- indicates that a lease is considered unusable by the
DHCP system. The primary reason for entering such state is
reception of DECLINE message for said lease. Client
identification MUST NOT appear.
RESET -- indicates that this resource was previously abandoned, but
was made available by operator command. This is a distinct state
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so that the reason that the resource became FREE can be
determined. Client identification MAY appear.
The lease state machine has been presented in Figure 1. Most states
are stationary, i.e. the lease stays in a given state until exernal
event triggers transition to another state. The only transitive
state is FREE*. One it is reached, the the state machine immediately
transitions to either FREE or FREE_BACKUP state.
+---------+
/------------->| ACTIVE |<--------------\
| +---------+ |
| | | | |
| /--(8)--/ (3) \--(9)-\ |
| | | | |
| V V V |
| +-------+ +--------+ +---------+ |
| |EXPIRED| |RELEASED| |ABANDONED| |
| +-------+ +--------+ +---------+ |
| | | | |
| | | (10) |
| | | V |
| | | +---------+ |
| | | | RESET | |
| | | +---------+ |
| | | | |
| \--(4)--\ (4) /--(4)--/ |
| | | | |
(1) V V V (2)
| /---------\ |
| | FREE* | |
| \---------/ |
| | | |
| /-(5)--/ \-(6)-\ |
| | | |
| V V |
| +-------+ +-----------+ |
\----| FREE |<--(7)-->|FREE_BACKUP|-----/
+-------+ +-----------+
FREE* transition
Figure 1: Lease State Machine
Transitions between states are results of the following events:
1. Primary server allocates a lease.
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2. Secondary server allocates a lease.
3. Client sends RELEASE and the lease is released.
4. Partner acknowledges state change. This transition MAY also
occur if the server is in PARTNER-DOWN state and the MCLT has
passed since the entry in RELEASED, EXPIRED, or RESET states.
5. The lease belongs to a pool that is governed by the
proportional allocation, or independent allocation is used and
this lease belongs to primary server pool.
6. The lease belongs to a pool that is governed by the
independent allocation and the lease belongs to the secondary
server.
7. Pool rebalance event occurs (POOLREQ/POOLRESP messages are
exchanged). Addresses (or prefixes) belonging to the primary
server can be assigned to the secondary server pool (transition
from FREE to FREE_BACKUP) or vice versa.
8. The lease has expired.
9. DECLINE message is received or a lease is deemed unusable for
other reasons.
10. An administrative action is taken to recover an abandoned
lease back to usable state. This transition MAY occur due to an
implementation specific handling on ABANDONED resource. One
possible example of such use is a Neighbor Discovery or ICMP Echo
check if the address is still in use.
The resource that is no longer in use (due to expiration or release),
becomes FREE*. Depending of what allocation algorithm is used, the
resource that is no longer is use, returns to primary (FREE) or
secondary pool (FREE_BACKUP). The conditions for specific
transitions are depicted in Figure 2.
+---------------+---------+-----------+
| \ Pool owner| | |
| \-------\ | Primary | Secondary |
|Algorithm \ | | |
+---------------+---------+-----------+
| Proportional | FREE | FREE |
| Independent | FREE |FREE_BACKUP|
+---------------+---------+-----------+
Figure 2: FREE* State Transitions
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In case of servers operating in active-passive mode, while a majority
of the resources are owned by the primary server, the secondary
server will need a portion of the resources to serve new clients
while operating in COMMUNICATION-INTERRUPTED state and also in
PARTNER-DOWN state before it can take over the entire address pool
(after the expiry of MCLT).
The secondary server connot simply take over the entire resource pool
immediately, since we have to handle the case where both servers are
able to communicate with DHCP clients, but unable to communicate with
each other.
The size of the resource pool allocated to the secondary is specified
as a percentage of the currently available resources. Thus, as the
number of available resources changes on the primary server, the
number of resources available to the secondary server MUST also
change, although the frequency of the changes made to the secondary
server's pool of address resources SHOULD be low enough to not use
significant processing power or network bandwidth.
The required size of this private pool allocated to the secondary
server is based only on the arrival rate of new DHCP clients and the
length of expected downtime of the primary server, and is not
directly influenced by the total number of DHCP clients supported by
the server pair.
8. Failover Mechanisms
This section lays out an overview of the communication between
partners and other mechanisms required for failover operation. As
this is a design document, not a protocol specification, high level
ideas are presented without implementation specific details (e.g. on-
wire protocol formats).
8.1. Time Skew
Partners exchange information about known lease states. To reliably
compare a known lease state with an update received from a partner,
servers must be able to reliably compare the times stored in the
known lease state with the times received in the update. Although a
simple approach would be to require both partners to use synchronized
time, e.g. by using NTP, such a service may not always be available
in some scenarios that failover expects to cover. Therefore a
mechanism to measure and track relative time differences between
servers is necessary. To do so, each message MUST contain
information about the time of the transmission in the time context of
the transmitter. The transmitting server MUST set this as close to
the actual transmission as possible. The receiving partner MUST
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store its own timestamp of reception as close to the actual reception
as possible. The received timestamp information is then compared
with local timestamp.
To account for packet delay variation (jitter), the measured
difference is not used directly, but rather the moving average of
last TIME_SKEW_PKTS_AVG packets time difference is calculated. This
averaged value is referred to as the time skew. Note that the time
skew algorithm allows cooperation between clients with completely
desynchronized clocks as well as those whose desynchronization itself
is not constant.
8.2. Time expression
Timestamps are expressed as number of seconds since midnight (UTC),
January 1, 2000, modulo 2^32. Note: that is the same approach as
used in creation of DUID-LLT (see Section 9.2 of [RFC3315]).
Time differences are expressed in seconds and are signed.
8.3. Lazy updates
Lazy update refers to the requirement placed on a server implementing
a failover protocol to update its failover partner whenever the
binding database changes. A failover protocol which didn't support
lazy update would require the failover partner update to complete
before a DHCPv6 server could respond to a DHCPv6 client request.
Such approach is often referred to as 'lockstep' and is the opposite
of lazy updates. The lazy update mechanism allows a server to
allocate a new or extend an existing lease and then update its
failover partner as time permits.
Although the lazy update mechanism does not introduce additional
delays in server response times, it introduces other difficulties.
The key problem with lazy update is that when a server fails after
updating a client with a particular lease time and before updating
its partner, the partner will believe that a lease has expired even
though the client still retains a valid lease on that address or
prefix.
8.4. MCLT concept
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In order to handle problem introduced by lazy updates (see
Section 8.3), a period of time known as the "Maximum Client Lead
Time" (MCLT) is defined and must be known to both the primary and
secondary servers. Proper use of this time interval places an upper
bound on the difference allowed between the lease time provided to a
DHCPv6 client by a server and the lease time known by that server's
failover partner.
The MCLT is typically much less than the lease time that a server has
been configured to offer a client, and so some strategy must exist to
allow a server to offer the configured lease time to a client.
During a lazy update the updating server typically updates its
partner with a potential expiration time which is longer than the
lease time previously given to the client and which is longer than
the lease time that the server has been configured to give a client.
This allows that server to give a longer lease time to the client the
next time the client renews its lease, since the time that it will
give to the client will not exceed the MCLT beyond the potential
expiration time acknowledged by its partner.
The fundamental relationship on which much of the correctness of this
protocol depends is that the lease expiration time known to a DHCPv6
client MUST NOT be greater by more than the MCLT beyond the potential
expiration time known to that server's failover partner.
The remainder of this section makes the above fundamental
relationship more explicit.
This protocol requires a DHCPv6 server to deal with several different
lease intervals and places specific restrictions on their
relationships. The purpose of these restrictions is to allow the
other server in the pair to be able to make certain assumptions in
the absence of an ability to communicate between servers.
The different times are:
desired valid lifetime:
The desired valid lifetime is the lease interval that a DHCPv6
server would like to give to a DHCPv6 client in the absence of any
restrictions imposed by the failover protocol. Its determination
is outside of the scope of this protocol. Typically this is the
result of external configuration of a DHCPv6 server.
actual valid lifetime:
The actual valid lifetime is the lease interval that a DHCPv6
server gives out to a DHCPv6 client. It may be shorter than the
desired valid lifetime (as explained below).
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potential valid lifetime:
The potential valid lifetime is the potential lease expiration
interval the local server tells to its partner in a BNDUPD
message.
acknowledged potential valid lifetime:
The acknowledged potential valid lifetime is the potential lease
interval the partner server has most recently acknowledged in a
BNDACK message.
8.4.1. MCLT example
The following example demonstrates the MCLT concept in practice. The
values used are arbitrarily chosen are and not a recommendation for
actual values. The MCLT in this case is 1 hour. The desired valid
lifetime is 3 days, and its renewal time is half the valid lifetime.
When a server makes an offer for a new lease on an IP address to a
DHCPv6 client, it determines the desired valid lifetime (in this
case, 3 days). It then examines the acknowledged potential valid
lifetime (which in this case is zero) and determines the remainder of
the time left to run, which is also zero. It adds the MCLT to this
value. Since the actual valid lifetime cannot be allowed to exceed
the remainder of the current acknowledged potential valid lifetime
plus the MCLT, the offer made to the client is for the remainder of
the current acknowledged potential valid lifetime (i.e. zero) plus
the MCLT. Thus, the actual valid lifetime is 1 hour.
Once the server has sent the REPLY to the DHCPv6 client, it will
update its failover partner with the lease information. However, the
desired potential valid lifetime will be composed of one half of the
current actual valid lifetime added to the desired valid lifetime.
Thus, the failover partner is updated with a BNDUPD with a potential
valid lifetime of 3 days + 1/2 hour.
When the primary server receives a BNDACK to its update of the
secondary server's (partner's) potential valid lifetime, it records
that as the acknowledged potential valid lifetime. A server MUST NOT
send a BNDACK in response to a BNDUPD message until it is sure that
the information in the BNDUPD message has been updated in its lease
database. Thus, the primary server in this case can be sure that the
secondary server has recorded the potential lease interval in its
stable storage when the primary server receives a BNDACK message from
the secondary server.
When the DHCPv6 client attempts to renew at T1 (approximately one
half an hour from the start of the lease), the primary server again
determines the desired valid lifetime, which is still 3 days. It
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then compares this with the original acknowledged potential valid
lifetime (3 days + 1/2 hour) and adjusts for the time passed since
the secondary was last updated (1/2 hour). Thus the time remaining
of the acknowledged potential valid interval is 3 days. Adding the
MCLT to this yields 3 days plus 1 hour, which is more than the
desired valid lifetime of 3 days. So the client is renewed for the
desired valid lifetime -- 3 days.
When the primary DHCPv6 server updates the secondary DHCPv6 server
after the DHCPv6 client's renewal REPLY is complete, it will
calculate the desired potential valid lifetime as the T1 fraction of
the actual client valid lifetime (1/2 of 3 days this time = 1.5
days). To this it will add the desired client valid lifetime of 3
days, yielding a total desired potential valid lifetime of 4.5 days.
In this way, the primary attempts to have the secondary always "lead"
the client in its understanding of the client's valid lifetime so as
to be able to always offer the client the desired client valid
lifetime.
Once the initial actual client valid lifetime of the MCLT is past,
the protocol operates effectively like the DHCPv6 protocol does today
in its behavior concerning valid lifetimes. However, the guarantee
that the actual client valid lifetime will never exceed the remaining
acknowledged partner server potential valid lifetime by more than the
MCLT allows full recovery from a variety of failures.
8.5. Unreachability detection
Each partner MUST maintain a FO_SEND timer for each failover
connection. The FO_SEND timer is reset every time any message is
transmitted. If the timer reaches the FO_SEND_MAX value, a CONTACT
message is transmitted and timer is reset. The CONTACT message may
be transmitted at any time. Implementation MAY use additional
mechanisms to detect partner unreachability.
Implementors are advised to keep in mind that the timer based CONTACT
message mechanism is not perfect and may not detect some failures.
In particular, if the partner is using one interface to reach clients
("downlink") and another to reach its partner ("uplink"), it is
possible that communication with the clients will break, yet the
mechanism will still claim full reachability. For that reason it is
beneficial to share the same interface for client traffic and
communication with the failover partner. That approach may have
drawbacks in some network topologies.
8.6. Re-allocating Leases
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When in PARTNER-DOWN state there is a waiting period after which a
resource can be re-allocated to another client. For resources which
are available when the server enters PARTNER-DOWN state, the period
is the MCLT from the entry into PARTNER-DOWN state. For resources
which are not available when the server enters PARTNER-DOWN state,
the period is the MCLT after the later of the following times: the
potential valid lifetime, the most recently transmitted potential
valid lifetime, the most recently received acknowledged potential
valid lifetime, and the most recently transmitted acknowledged
potential valid lifetime. If this time would be earlier than the
current time plus the MCLT, then the time the server entered PARTNER-
DOWN state plus the maximum-client-lead-time is used.
In any other state, a server cannot reallocate a resource from one
client to another without first notifying its partner (through a
BNDUPD message) and receiving acknowledgement (through a BNDACK mes-
sage) that its partner is aware that that first client is not using
the resource.
This could be modeled in the following way. Though this specific
implementation is in no way required, it may serve to better illus-
trate the concept.
An "available" resource on a server may be allocated to any client.
A resource which was leased to a client and which expired or was
released by that client would take on a new state, EXPIRED or
RELEASED respectively. The partner server would then be notified
that this resource was EXPIRED or RELEASED through a BNDUPD. When
the sending server received the BNDACK for that resource showing it
was FREE, it would move the resource from EXPIRED or RELEASED to
FREE, and it would be available for allocation by the primary server
to any clients.
A server MAY reallocate a resource in the EXPIRED or RELEASED state
to the same client with no restrictions provided it has not sent a
BNDUPD message to its partner. This situation would exist if the
lease expired or was released after the transition into PARTNER-DOWN
state, for instance.
8.7. Sending Binding Update
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This and the following section is written as though every BNDUPD
message contains only a single binding update transaction in order to
reduce the complexity of the discussion. Note that while a server
MAY generate BNDUPD messages with multiple binding update
transactions, every server MUST be able to process a BNDUPD message
which contains multiple binding update transactions and generate the
corresponding BNDACK messages with status for multiple binding update
transactions.
Each server updates its failover partner about recent changes in
lease states. Each update MUST include at least the following
information:
1. resource type - non-temporary address or a prefix. Resource
type can be indicated by the container that conveys the actual
resource (e.g. an IA_NA option indicates non-temporary IPv6
address);
2. resource information - the actual address or prefix. That is
conveyed using the appropriate option, e.g. an IAADDR for an
address or an IAPREFIX for a prefix;
3. valid life time requested by client*;
4. valid life time sent to client*;
5. IAID - Identity Association used by the client, while obtaining
a given lease. (Note1: one client may use many IAIDs
simulatenously. Note2: IAID for IA, TA and PD are orthogonal
number spaces.)*;
6. Next Expected Client Transmission - time interval since Client
Last Transmission Time, when a response from a client is
expected*;
7. potential valid life time - a lifetime that the server is
willing to set if there were no MCLT/failover restrictions
imposed*;
8. preferred life time sent to client - the actual value sent back
to the client*;
9. CLTT - Client Last Transaction Time, a timestamp of the last
received transmission from a client*;
10. Client DUID*.
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Items marked with asterisk MUST appear only if the lease is/was
associated with a client. Otherwise it MUST NOT appear, e.g. for
updates from FREE to FREE_BACKUP state. Server MUST reject updates
that does not include any of the aforementioned information.
The BNDUPD message MAY contain additional information related to the
updated lease. The additional information MAY include, but is not
limited to:
1. assigned FQDN name, defined in [RFC4704];
2. Options Requested by the client, i.e. content of the ORO;
3. Remote-ID, defined in [RFC4649];
4. Relay-ID, defined in [RFC5460], section 5.4.1;
5. Link-layer address [RFC6939];
6. Any other options the updating partner deems useful.
Receiving partner MAY store received additional information, but it
MAY choose to ignore them as well. Some information may be useful,
so it is a good idea to keep or update it. One reason is FQDN
information. A server SHOULD be prepared to clean up DNS information
once the lease expires or is released. See Section Section 11 for
detailed discussion about Dynamic DNS. Another reason the partner
may be interested in keeping additional data is a better support for
leasequery [RFC5007] or bulk leasequery [RFC5460], which features
queries based on Relay-ID, by link address and by Remote-ID.
8.8. Receiving Binding Update
When a server receives a BNDUPD message, it needs to decide how to
process the binding update transaction it contains and whether that
transaction represents a conflict of any sort. The conflict
resolution process MUST be used on the receipt of every BNDUPD
message, not just those that are received while in POTENTIAL-CONFLICT
state, in order to increase the robustness of the protocol.
There are three sorts of conflicts:
1. Two clients, one resource - This is the duplicate resource
allocation conflict. There two different clients each allocated
the same resource. See Section 8.9.
2. Two resources, one client conflict - This conflict exists when a
client on one server is associated with a one resource, and on
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the other server with a different resource in the same or related
subnet. This does not refer to the case where a single client
has resources in multiple different subnets or administrative
domains (i.e. a mobile client that changed its location), but
rather the case where on the same subnet the client has a lease
on one IP address in one server and on a different IP address on
the other server.
This conflict may or may not be a problem for a given DHCP server
implementation and policy. If implementations and policies
allow, both resources can be assigned to a given client. In the
event that a DHCP server requires that a DHCP client have only
one outstanding lease of a given type, the conflict MUST be
resolved by accepting the lease which has the latest CLTT.
It should be further clarified that DHCPv6 protocol makes
assignments based on (client DUID, resource type, iaid) triplet.
The possibility of using different IAIDs was omitted in this
paragraph for clarity. If one client is assigned multiple
resources of the same type, but with different IAIDs, there is no
conflict. Also, iaid values for different resource types are
orthogonal, i.e. IA_NA with iaid=1 is different than IA_PD with
iaid=1 and there is no conflict.
3. binding-status conflict - This is normal conflict, where one
server is updating the other with newer information. See
Section 8.9 for details of how to resolve these conflicts.
8.9. Conflict Resolution
The server receiving a lease update from its partner must evaluate
the received lease information to see if it is consistent with
already known state and decide which information - the previously
known or that just received - is "better". The server should take
into consideration the following aspects: if the lease is already
assigned to a specific client, who had contact with client recently,
start time of the lease, etc.
When analyzing a BNDUPD message from a partner server, if there is
insufficient information in the BNDUPD to process it, then reject the
BNDUPD with reject-reason "Missing binding information".
If the resource in the BNDUPD is not a resource associated with the
failover endpoint which received the BNDUPD message, then reject it
with reject-reason "Illegal IP address or prefix (not part of any
address or prefix pool)".
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Every BNDUPD message SHOULD contain a client-last-transaction-time
option, which MUST, if it appears, be the time that the server last
interacted with the DHCP client. It MUST NOT be, for instance, the
time that the lease on an IP address expired. If there has been no
interaction with the DHCP client in question (or there is no DHCP
client presently associated with this resource), then there will be
no client-last-transaction-time option in the BNDUPD message.
The list in Figure 3 presents the conflict resolution outcome. To
"accept" BNDUPD means to update the server's bindings database with
the information contained in the BNDUDP and once the update is
complete, send a BNDACK message corresponding to the BNDUPD message.
To "reject" a BNDUPD means to lease the server's binding database
unchangeg and to respond to the BNDUPD with BNDACK with a rejest-
reason option included.
When interpreting the information in the following table (Figure 3),
for those rules that are listed with "time" -- if a BNDUPD doesn't
have a client-last-transaction-time value, then it MUST NOT be
considered later than the client-last-transaction-time in the
receiving server's binding. If the BNDUPD contains a client-last-
transaction-time value and the receiving server's binding does not,
then the client-last-transaction-time value in the BNDUPD MUST be
considered later than the server's.
binding-status in received BNDUPD.
binding-status
in receiving FREE RESET
server ACTIVE EXPIRED RELEASED FREE_BACKUP ABANDONED
ACTIVE accept(5) time(2) time(1) time(2) accept
EXPIRED time(1) accept accept accept accept
RELEASED time(1) time(1) accept accept accept
FREE/FREE_BACKUP accept accept accept accept accept
RESET time(3) accept accept accept accept
ABANDONED reject(4) reject(4) reject(4) reject(4) accept
Figure 3: Conflict Resolution
time(1): If the client-last-transaction-time in the BNDUPD is later
than the client-last-transaction-time in the receiving server's
binding, accept it, else reject it.
time(2): If the current time is later than the receiving server's
lease-expiration-time, accept it, else reject it.
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time(3): If the client-last-transaction-time in the BNDUPD is later
than the start-time-of-state in the receiving server's binding,
accept it, else reject it.
(1,2,3): If rejecting, use reject reason "Outdated binding
information".
(4): Use reject reason "Less critical binding information".
(5): If the clients in a BNDUPD message and in a receiving server's
binding differ, then if the receiving server is a secondary accept
it, else reject it with a reject reason of "Fatal conflict exists:
address in use by other client".
The lease update may be accepted or rejected. Rejection SHOULD NOT
change the flag in a lease that says that it should be transmitted to
the failover partner. If this flag is set, then it should be
transmitted, but if it is not already set, the rejection of a lease
state update SHOULD NOT trigger an automatic update of the failover
partner sending the rejected update. The potential for update storms
is too great, and in the unusual case where the servers simply can't
agree, that disagreement is better than an update storm.
8.10. Acknowledging Reception
Upon acceptance of a binding lease, server must notify its partner
that it updated its database. Server SHOULD NOT send BNDACK before
its database is updated. BNDACK MUST contain at lease minimum set of
information required to unabiguously identify BNDUDP.
9. Endpoint States
9.1. State Machine Operation
Each server (or, more accurately, failover endpoint) can take on a
variety of failover states. These states play a crucial role in
determining the actions that a server will perform when processing a
request from a DHCPv6 client as well as dealing with changing
external conditions (e.g., loss of connection to a failover partner).
The failover state in which a server is running controls the
following behaviors:
o Responsiveness -- the server is either responsive to DHCPv6 client
requests or it is not.
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o Allocation Pool -- which pool of addresses (or prefixes) can be
used for advertisement on receipt of a SOLICIT or allocation on
receipt of a REQUEST message.
o MCLT -- ensure that valid lifetimes are not beyond what the
partner has acked plus the MCLT (or not).
A server will transition from one failover state to another based on
the specific values held by the following state variables:
o Current failover state.
o Communications status (OK or not OK).
o Partner's failover state (if known).
Several events can cause the transition from one failover state to
another.
o Change in communications status (OK or not OK);
o Change in partner's failover state;
o Explicit administrative action;
o Receipt of particular messages;
o Expiration of timers.
Whenever either of the last two of the above state variables changes
state, the state machine is invoked, which may then trigger a change
in the current failove state. Thus, whenever the communications
status changes, the state machine processing is invoked. This may or
may not result in a change in the current failover state.
Whenever a server transitions to a new failover state, the new state
MUST be communicated to its failover partner in a STATE message if
the communications status is OK. In addition, whenever a server
makes a transition into a new state, it MUST record the new state,
its current understanding of its partner's state, and the time at
which it entered the new state in stable storage.
The following state transition diagram gives a condensed view of the
state machine. If there is a difference between the words describing
a particular state and the diagram below, the words should be
considered authoritative.
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In the state transition diagram below, the "+" or "-" in the upper
right corner of each state is a notation about whether communication
is ongoing with the other server.
+---------------+ V +--------------+
| RECOVER -|+| | | STARTUP - |
|(unresponsive) | +->+(unresponsive)|
+------+--------+ +--------------+
+-Comm. OK +-----------------+
| Other State: | PARTNER DOWN - +<---------------------+
| RESOLUTION-INTER. | (responsive) | ^
All POTENTIAL- +----+------------+ |
Others CONFLICT------------ | --------+ |
| CONFLICT-DONE Comm. OK | +--------------+ |
UPDREQ or Other State: | +--+ RESOLUTION - | |
UPDREQALL | | | | | INTERRUPTED | |
Rcv UPDDONE RECOVER All | | | (responsive) | |
| +---------------+ | Others | | +------------+-+ |
+->+RECOVER-WAIT +-| RECOVER | | | ^ | |
|(unresponsive) | WAIT or | | Comm. | Ext. |
+-----------+---+ DONE | | OK Comm. Cmd---->+
Comm.---+ Wait MCLT | V V V Failed |
Changed | V +---+ +---+-----+--+-+ | |
| +---+----------++ | | POTENTIAL + +-------+ |
| |RECOVER-DONE +-| Wait | CONFLICT +------+ |
+->+(unresponsive) | for |(unresponsive)| Primary |
+------+--------+ Other +>+----+--------++ resolve Comm. |
Comm. OK State: | | ^ conflict Changed|
+---Other State:-+ RECOVER | Secondary | V V | |
| | | DONE | resolve | ++----------+---++ |
| All Others: POTENT. | | conflict | |CONFLICT-DONE-|+| |
| Wait for CONFLICT--|-----+ | | | (responsive) | |
| Other State: V V | +------+---------+ |
| NORMAL or RECOVER ++------------+---+ | Other State: NORMAL |
| | DONE | NORMAL + +<--------------+ |
| +--+----------+-->+ (balanced) +-------External Command-->+
| ^ ^ +--------+--------+ |
| | | | | |
| Wait for Comm. OK Comm. Failed | |
| Other Other | | External
| State: State: | | Command
| RECOVER-DONE NORMAL Start Safe Comm. OK or
| | COMM. INT. Period Timer Other State: Safe
| Comm. OK. | V All Others Period
| Other State: | +---------+--------+ | expiration
| RECOVER +--+ COMMUNICATIONS - +----+ |
| +-------------+ INTERRUPTED | |
RECOVER | (responsive) +------------------------->+
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RECOVER-WAIT--------->+------------------+
Figure 4: Failover Endpoint State Machine
9.2. State Machine Initialization
The state machine is characterized by storage (in stable storage) of
at least the following information:
o Current failover state.
o Previous failover state.
o Start time of current failover state.
o Partner's failover state.
o Start time of partner's failover state.
o Time most recent packet received from partner.
The state machine is initialized by reading these data items from
stable storage and restoring their values from the information saved.
If there is no information in stable storage concerning these items,
then they should be initialized as follows:
o Current failover state: Primary: PARTNER-DOWN, Secondary: RECOVER
o Previous failover state: None.
o Start time of current failover state: Current time.
o Partner's failover state: None until reception of STATE message.
o Start time of partner's failover state: None until reception of
STATE message.
o Time most recent packet received from partner: None until packet
received.
9.3. STARTUP State
The STARTUP state affords an opportunity for a server to probe its
partner server, before starting to service DHCP clients. When in the
STARTUP state, a server attempts to learn its partner's state and
determine (using that information if it is available) what state it
should enter.
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The STARTUP state is not shown with any specific state transitions in
the state machine diagram (Figure 4) because the processing during
the STARTUP state can cause the server to transition to any of the
other states, so that specific state transition arcs would only
obscure other information.
9.3.1. Operation in STARTUP State
The server MUST NOT be responsive in STARTUP state.
Whenever a STATE message is sent to the partner while in STARTUP
state the STARTUP flag MUST be set in the message and the previously
recorded failover state MUST be placed in the server-state option.
9.3.2. Transition Out of STARTUP State
The following algorithm is followed every time the server initializes
itself, and enters STARTUP state.
Step 1:
If there is any record in stable storage of a previous failover state
for this server, set PREVIOUS-STATE to the last recorded value in
stable storage, and go to Step 2.
If there is no record of any previous failover state in stable
storage for this server, then set the PREVIOUS-STATE to RECOVER and
set the TIME-OF-FAILURE to 0. This will allow two servers which
already have lease information to synchronize themselves prior to
operating.
In some cases, an existing server will be commissioned as a failover
server and brought back into operation where its partner is not yet
available. In this case, the newly commissioned failover server will
not operate until its partner comes online -- but it has operational
responsibilities as a DHCP server nonetheless. To properly handle
this situation, a server SHOULD be configurable in such a way as to
move directly into PARTNER-DOWN state after the startup period
expires if it has been unable to contact its partner during the
startup period.
Step 2:
Implementations will differ in the ways that they deal with the state
machine for failover endpoint states. In many cases, state
transitions will occur when communications goes from "OK" to failoed,
or from failed to "OK", and some implementations will implement a
portion of their state machine processing based on these changes.
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In these cases, during startup, if the previous state is one where
communications was "OK", then set the previous state to the state
that is the result of the communications failed state transition when
in that state (if such transition exists -- some states don't have a
communications failed state transition, since they allow both
communications OK and failed).
Step 3:
Start the STARTUP state timer. The time that a server remains in the
STARTUP state (absent any communications with its partner) is
implementation dependent but SHOULD be short. It SHOULD be long
enough for a TCP connection to be created to a heavily loaded partner
across a slow network.
Step 4:
Attempt to create a TCP connection to the failover partner.
Step 5:
Wait for "communications OK".
When and if communications become "okay", clear the STARTUP flag, and
set the current state to the PREVIOUS-STATE.
If the partner is in PARTNER-DOWN state, and if the time at which it
entered PARTNER-DOWN state (as received in the start-time-of-state
option in the STATE message) is later than the last recorded time of
operation of this server, then set CURRENT-STATE to RECOVER. If the
time at which it entered PARTNER-DOWN state is earlier than the last
recorded time of operation of this server, then set CURRENT-STATE to
POTENTIAL-CONFLICT.
Then, transition to the current state and take the "communications
OK" state transition based on the current state of this server and
the partner.
Step 6:
If the startup time expires the server SHOULD transition to the
PREVIOUS-STATE.
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9.4. PARTNER-DOWN State
PARTNER-DOWN state is a state either server can enter. When in this
state, the server assumes that it is the only server operating and
serving the client base. If one server is in PARTNER-DOWN state, the
other server MUST NOT be operating.
A server can enter PARTNER-DOWN state either as a result of operator
intervention (when an operator determines that the server's partner
is, indeed, down), or as a result of the auto-partner-down capability
where PARTNER-DOWN state is entered automatically after a server has
been in COMMUNICATIONS-INTERRUPTED state for a pre-determined period
of time.
9.4.1. Operation in PARTNER-DOWN State
The server MUST be responsive in PARTNER-DOWN state, regardess if it
is primary or secondary.
It will allow renewal of all outstanding leases on addresses or
prefixes. For those resources for which the server is using
proportional allocation, it will allocate resources from its own
pool, and after a fixed period of time (the MCLT interval) has
elapsed from entry into PARTNER-DOWN state, it may allocate IP
addresses from the set of all available pools. Server SHOULD fully
deplete its own pool, before starting allocations from its downed
partner.
Any resource tagged as available for allocation by the other server
(at entry to PARTNER-DOWN state) MUST NOT be allocated to a new
client until the MCLT beyond the entry into PARTNER-DOWN state has
elapsed.
A server in PARTNER-DOWN state MUST NOT allocate a resource to a DHCP
client different from that to which it was allocated at the entrance
to PARTNER-DOWN state until the MCLT beyond the maximum of the
following times: client expiration time, most recently transmitted
potential-expiration-time, most recently received ack of potential-
expiration-time from the partner, and most recently acked potential-
expiration-time to the partner. If this time would be earlier than
the current time plus the maximum-client-lead-time, then the time the
server entered PARTNER-DOWN state plus the maximum-client-lead-time
is used.
The server is not restricted by the MCLT when offering lease times
while in PARTNER-DOWN state.
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In the unlikely case, when there are two servers operating in a
PARTNER-DOWN state, there is a chance of duplicate leases assigned.
This leads to a POTENTIAL-CONFLICT (unresponsive) state when they re-
establish contact. The duplicate lease issue can be postponed to a
large extent by the server granting new leases first from its own
pool. Therefore the server operating in PARTNER-DOWN state MUST use
its own pool first for new leases before assigning any leases from
its downed partner pool.
9.4.2. Transition Out of PARTNER-DOWN State
When a server in PARTNER-DOWN state succeeds in establishing a con-
nection to its partner, its actions are conditional on the state and
flags received in the STATE message from the other server as part of
the process of establishing the connection.
If the STARTUP bit is set in the server-flags option of a received
STATE message, a server in PARTNER-DOWN state MUST NOT take any state
transitions based on reestablishing communications. Essentially, if
a server is in PARTNER-DOWN state, it ignores all STATE messages from
its partner that have the STARTUP bit set in the server-flags option
of the STATE message.
If the STARTUP bit is not set in the server-flags option of a STATE
message received from its partner, then a server in PARTNER-DOWN
state takes the following actions based on the state of the partner
as received in a STATE message (either immediately after establishing
communications or at any time later when a new state is received)
If the partner is in:
NORMAL, COMMUNICATIONS-INTERRUPTED, PARTNER-DOWN, POTENTIAL-CONFLICT,
RESOLUTION-INTERRUPTED, or CONFLICT-DONE state
transition to POTENTIAL-CONFLICT state
If the partner is in:
RECOVER, RECOVER-WAIT state
stay in PARTNER-DOWN state
If the partner is in:
RECOVER-DONE state
transition into NORMAL state
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9.5. RECOVER State
This state indicates that the server has no information in its stable
storage or that it is re-integrating with a server in PARTNER-DOWN
state after it has been down. A server in this state MUST attempt to
refresh its stable storage from the other server.
9.5.1. Operation in RECOVER State
The server MUST NOT be responsive in RECOVER state.
A server in RECOVER state will attempt to reestablish communications
with the other server.
9.5.2. Transition Out of RECOVER State
If the other server is in POTENTIAL-CONFLICT, RESOLUTION-INTERRUPTED,
or CONFLICT-DONE state when communications are reestablished, then
the server in RECOVER state will move to POTENTIAL-CONFLICT state
itself.
If the other server is in any other state, then the server in RECOVER
state will request an update of missing binding information by
sending an UPDREQ message. If the server has determined that it has
lost its stable storage because it has no record of ever having
talked to its partner, while its partner does have a record of
communicating with it, it MUST send an UPDREQALL message, otherwise
it MUST send an UPDREQ message.
It will wait for an UPDDONE message, and upon receipt of that message
it will transition to RECOVER-WAIT state.
If communications fails during the reception of the results of the
UPDREQ or UPDREQALL message, the server will remain in RECOVER state,
and will re-issue the UPDREQ or UPDREQALL when communications are re-
established.
If an UPDDONE message isn't received within an implementation
dependent amount of time, and no BNDUPD messages are being received,
the connection SHOULD be dropped.
A B
Server Server
| |
RECOVER PARTNER-DOWN
| |
| >--UPDREQ--------------------> |
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| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
... ...
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
| |
| <--------------------UPDDONE--< |
| |
RECOVER-WAIT |
| |
| >--STATE-(RECOVER-WAIT)------> |
| |
| |
Wait MCLT from last known |
time of failover operation |
| |
RECOVER-DONE |
| |
| >--STATE-(RECOVER-DONE)------> |
| NORMAL
| <-------------(NORMAL)-STATE--< |
NORMAL |
| >---- State-(NORMAL)---------------> |
| |
| |
Figure 5: Transition out of RECOVER state
If, at any time while a server is in RECOVER state communications
fails, the server will stay in RECOVER state. When communications
are restored, it will restart the process of transitioning out of
RECOVER state.
9.6. RECOVER-WAIT State
This state indicates that the server has done an UPDREQ or UPDREQALL
and has received the UPDDONE message indicating that it has received
all outstanding binding update information. In the RECOVER-WAIT
state the server will wait for the MCLT in order to ensure that any
processing that this server might have done prior to losing its
stable storage will not cause future difficulties.
9.6.1. Operation in RECOVER-WAIT State
The server MUST NOT be responsive in RECOVER-WAIT state.
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9.6.2. Transition Out of RECOVER-WAIT State
Upon entry to RECOVER-WAIT state the server MUST start a timer whose
expiration is set to a time equal to the time the server went down
(if known) or the time the server started (if the down-time is
unknown) plus the maximum-client-lead-time. When this timer expires,
the server will transition into RECOVER-DONE state.
This is to allow any IP addresses that were allocated by this server
prior to loss of its client binding information in stable storage to
contact the other server or to time out.
If this is the first time this server has run failover -- as
determined by the information received from the partner, not
necessarily only as determined by this server's stable storage (as
that may have been lost), then the waiting time discussed above may
be skipped, and the server MAY transition immediately to RECOVER-DONE
state.
If the server has never before run failover, then there is no need to
wait in this state -- but, again, to determine if this server has run
failover it is vital that the information provided by the partner be
utilized, since the stable storage of this server may have been lost.
If communications fails while a server is in RECOVER-WAIT state, it
has no effect on the operation of this state. The server SHOULD
continue to operate its timer, and the timer expires during the
period where communications with the other server have failed, then
the server SHOULD transition to RECOVER-DONE state. This is rare --
failover state transitions are not usually made while communications
are interrupted, but in this case there is no reason to inhibit the
timer.
9.7. RECOVER-DONE State
This state exists to allow an interlocked transition for one server
from RECOVER state and another server from PARTNER-DOWN or
COMMUNICATIONS-INTERRUPTED state into NORMAL state.
9.7.1. Operation in RECOVER-DONE State
A server in RECOVER-DONE state MUST respond only to RENEW, REBIND,
CONFIRM and INFORMATION-REQUEST client messages.
9.7.2. Transition Out of RECOVER-DONE State
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When a server in RECOVER-DONE state determines that its partner
server has entered NORMAL or RECOVER-DONE state, then it will
transition into NORMAL state.
If communication fails while in RECOVER-DONE state, a server will
stay in RECOVER-DONE state.
9.8. NORMAL State
NORMAL state is the state used by a server when it is communicating
with the other server, and any required resynchronization has been
performed. While some bindings database synchronization is performed
in NORMAL state, potential conflicts are resolved prior to entry into
NORMAL state as is binding database data loss.
When entering NORMAL state, a server will send to the other server
all currently unacknowledged binding updates as BNDUPD messages.
When the above process is complete, if the server entering NORMAL
state is a secondary server, then it will request resources
(addresses and/or prefixes) for allocation using the POOLREQ message.
9.8.1. Operation in NORMAL State
Primary server is responsive in NORMAL state. Secondary is
unresponsive in NORMAL state.
When in NORMAL state a primary server will operate in the following
manner:
Lease time calculations
As discussed in Section 8.4, the lease interval given to a DHCP
client can never be more than the MCLT greater than the most
recently received potential-expiration-time from the failover
partner or the current time, whichever is later.
As long as a server adheres to this constraint, the specifics of
the lease interval that it gives to a DHCP client or the value of
the potential-expiration-time sent to its failover partner are
implementation dependent.
Lazy update of partner server
After sending an REPLY that includes lease update to a client, the
server servicing a DHCP client request attempts to update its
partner with the new binding information. Server transmits both
desired valid lifetime and actual valid lifetime.
Reallocation of resources between clients
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Whenever a client binding is released or expires, a BNDUPD message
must be sent to the partner, setting the binding state to RELEASED
or EXPIRED. However, until a BNDACK is received for this message,
the resource cannot be allocated to another client. It cannot be
allocated to the same client again if a BNDUPD was sent, otherwise
it can. See Section 8.6 for details.
In NORMAL state, each server receives binding updates from its
partner server in BNDUPD messages. It records these in its client
binding database in stable storage and then sends a corresponding
BNDACK message to its partner server.
9.8.2. Transition Out of NORMAL State
If an external command is received by a server in NORMAL state
informing it that its partner is down, then transition into PARTNER-
DOWN state. Generally, this would be an unusual situation, where
some external agency knew the partner server was down. Using the
command in this case would be appropriate if the polling interval and
timeout were long.
If a server in NORMAL state fails to receive acks to messages sent to
its partner for an implementation dependent period of time, it MAY
move into COMMUNICATIONS-INTERRUPTED state. This situation might
occur if the partner server was capable of maintaining the TCP con-
nection between the server and also capable of sending a CONTACT mes-
sage periodically, but was (for some reason) incapable of pro-
cessing BNDUPD messages.
If the communications is determined to not be "ok" (as defined in
Section 8.5), then transition into COMMUNICATIONS-INTERRUPTED state.
If a server in NORMAL state receives any messages from its partner
where the partner has changed state from that expected by the server
in NORMAL state, then the server should transition into
COMMUNICATIONS-INTERRUPTED state and take the appropriate state tran-
sition from there. For example, it would be expected for the partner
to transition from POTENTIAL-CONFLICT into NORMAL state, but not for
the partner to transition from NORMAL into POTENTIAL-CONFLICT state.
If a server in NORMAL state receives a DISCONNECT message from its
partner, the server should transition into COMMUNICATIONS-INTERRUPTED
state.
9.9. COMMUNICATIONS-INTERRUPTED State
A server goes into COMMUNICATIONS-INTERRUPTED state whenever it is
unable to communicate with its partner. Primary and secondary
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servers cycle automatically (without administrative intervention)
between NORMAL and COMMUNICATIONS-INTERRUPTED state as the network
connection between them fails and recovers, or as the partner server
cycles between operational and non-operational. No duplicate
resource allocation can occur while the servers cycle between these
states.
When a server enters COMMUNICATIONS-INTERRUPTED state, if it has been
configured to support an automatic transition out of COMMUNICATIONS-
INTERRUPTED state and into PARTNER-DOWN state (i.e., a "safe period"
has been configured, see section TODO), then a timer MUST be started
for the length of the configured safe period.
A server transitioning into the COMMUNICATIONS-INTERRUPTED state from
the NORMAL state SHOULD raise some alarm condition to alert
administrative staff to a potential problem in the DHCP subsystem.
9.9.1. Operation in COMMUNICATIONS-INTERRUPTED State
In this state a server MUST respond to all DHCP client requests.
When allocating new leases, each server allocates from its own pool,
where the primary MUST allocate only FREE resources (addresses or
prefixes), and the secondary MUST allocate only FREE_BACKUP resources
(addresses or prefixes). When responding to RENEW messages, each
server will allow continued renewal of a DHCP client's current lease
on an IP address or prefix irrespective of whether that lease was
given out by the receiving server or not, although the renewal period
MUST NOT exceed the maximum client lead time (MCLT) beyond the latest
of: 1) the potential valid lifetime already acknowledged by the other
server, or 2) the actual valid lifetime sent to the DHCPv6 client, or
3) the potential valid lifetime received from the partner server.
However, since the server cannot communicate with its partner in this
state, the acknowledged potential valid lifetime will not be updated
in any new bindings. This is likely to eventually cause the actual
valid lifetimes to be the current time plus the MCLT (unless this is
greater than the desired-client-lease-time).
The server should continue to try to establish a connection with its
partner.
9.9.2. Transition Out of COMMUNICATIONS-INTERRUPTED State
If the safe period timer expires while a server is in the
COMMUNICATIONS-INTERRUPTED state, it will transition immediately into
PARTNER-DOWN state.
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If an external command is received by a server in COMMUNICATIONS-
INTERRUPTED state informing it that its partner is down, it will
transition immediately into PARTNER-DOWN state.
If communications is restored with the other server, then the server
in COMMUNICATIONS-INTERRUPTED state will transition into another
state based on the state of the partner:
o NORMAL or COMMUNICATIONS-INTERRUPTED: Transition into the NORMAL
state.
o RECOVER: Stay in COMMUNICATIONS-INTERRUPTED state.
o RECOVER-DONE: Transition into NORMAL state.
o PARTNER-DOWN, POTENTIAL-CONFLICT, CONFLICT-DONE, or RESOLUTION-
INTERRUPTED: Transition into POTENTIAL-CONFLICT state.
The following figure illustrates the transition from NORMAL to
COMMUNICATIONS-INTERRUPTED state and then back to NORMAL state again.
Primary Secondary
Server Server
NORMAL NORMAL
| >--CONTACT-------------------> |
| <--------------------CONTACT--< |
| [TCP connection broken] |
COMMUNICATIONS : COMMUNICATIONS
INTERRUPTED : INTERRUPTED
| [attempt new TCP connection] |
| [connection succeeds] |
| |
| >--CONNECT-------------------> |
| <-----------------CONNECTACK--< |
| NORMAL
| <-------------------STATE-----< |
NORMAL |
| >--STATE---------------------> |
|
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
| |
| <---------------------BNDUPD--< |
| >------BNDACK----------------> |
... ...
| |
| <--------------------POOLREQ--< |
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| >--POOLRESP-(2)--------------> |
| |
| >--BNDUPD-(#1)---------------> |
| <---------------------BNDACK--< |
| |
| <--------------------POOLREQ--< |
| >--POOLRESP-(0)--------------> |
| |
| >--BNDUPD-(#2)---------------> |
| <---------------------BNDACK--< |
| |
Figure 6: Transition from NORMAL to COMMUNICATIONS-INTERRUPTED and
back (example with 2 addresses allocated to secondary)
9.10. POTENTIAL-CONFLICT State
This state indicates that the two servers are attempting to
reintegrate with each other, but at least one of them was running in
a state that did not guarantee automatic reintegration would be
possible. In POTENTIAL-CONFLICT state the servers may determine that
the same resource has been offered and accepted by two different
clients.
It is a goal of this protocol to minimize the possibility that
POTENTIAL-CONFLICT state is ever entered.
When a primary server enters POTENTIAL-CONFLICT state it should
request that the secondary send it all updates of which it is
currently unaware by sending an UPDREQ message to the secondary
server.
A secondary server entering POTENTIAL-CONFLICT state will wait for
the primary to send it an UPDREQ message.
9.10.1. Operation in POTENTIAL-CONFLICT State
Any server in POTENTIAL-CONFLICT state MUST NOT process any incoming
DHCP requests.
9.10.2. Transition Out of POTENTIAL-CONFLICT State
If communications fails with the partner while in POTENTIAL-CONFLICT
state, then the server will transition to RESOLUTION-INTERRUPTED
state.
Whenever either server receives an UPDDONE message from its partner
while in POTENTIAL-CONFLICT state, it MUST transition to a new state.
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The primary MUST transition to CONFLICT-DONE state, and the secondary
MUST transition to NORMAL state. This will cause the primary server
to leave POTENTIAL-CONFLICT state prior to the secondary, since the
primary sends an UPDREQ message and receives an UPDDONE before the
secondary sends an UPDREQ message and receives its UPDDONE message.
When a secondary server receives an indication that the primary
server has made a transition from POTENTIAL-CONFLICT to CONFLICT-DONE
state, it SHOULD send an UPDREQ message to the primary server.
Primary Secondary
Server Server
| |
POTENTIAL-CONFLICT POTENTIAL-CONFLICT
| |
| >--UPDREQ--------------------> |
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
... ...
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
| |
| <--------------------UPDDONE--< |
CONFLICT-DONE |
| >--STATE--(CONFLICT-DONE)----> |
| <---------------------UPDREQ--< |
| |
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
... ...
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
| |
| >--UPDDONE-------------------> |
| NORMAL
| <------------STATE--(NORMAL)--< |
NORMAL |
| >--STATE--(NORMAL)-----------> |
| |
| <--------------------POOLREQ--< |
| >------POOLRESP-(n)----------> |
| addresses |
Figure 7: Transition out of POTENTIAL-CONFLICT
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9.11. RESOLUTION-INTERRUPTED State
This state indicates that the two servers were attempting to
reintegrate with each other in POTENTIAL-CONFLICT state, but
communications failed prior to completion of re-integration.
If the servers remained in POTENTIAL-CONFLICT while communications
was interrupted, neither server would be responsive to DHCP client
requests, and if one server had crashed, then there might be no
server able to process DHCP requests.
When a server enters RESOLUTION-INTERRUPTED state it SHOULD raise an
alarm condition to alert administrative staff of a problem in the
DHCP subsystem.
9.11.1. Operation in RESOLUTION-INTERRUPTED State
In this state a server MUST respond to all DHCP client requests.
When allocating new resources (addresses or prefixes), each server
SHOULD allocate from its own pool (if that can be determined), where
the primary SHOULD allocate only FREE resources, and the secondary
SHOULD allocate only BACKUP resources. When responding to renewal
requests, each server will allow continued renewal of a DHCP client's
current lease independent of whether that lease was given out by the
receiving server or not, although the renewal period MUST NOT exceed
the maximum client lead time (MCLT) beyond the latest of: 1) the
potential valid lifetime already acknowledged by the other server or
2) the lease-expiration-time or 3) potential valid lifetime received
from the partner server.
However, since the server cannot communicate with its partner in this
state, the acknowledged potential valid lifetime will not be updated
in any new bindings.
9.11.2. Transition Out of RESOLUTION-INTERRUPTED State
If an external command is received by a server in RESOLUTION-
INTERRUPTED state informing it that its partner is down, it will
transition immediately into PARTNER-DOWN state.
If communications is restored with the other server, then the server
in RESOLUTION-INTERRUPTED state will transition into POTENTIAL-
CONFLICT state.
9.12. CONFLICT-DONE State
This state indicates that during the process where the two servers
are attempting to re-integrate with each other, the primary server
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has received all of the updates from the secondary server. It make a
transition into CONFLICT-DONE state in order that it may be totally
responsive to the client load. There is no operational difference
between CONFLICT-DONE and NORMAL for primary as in both states it
responds to all clients' requests. The distinction between CONFLICT-
DONE and NORMAL states will be more apparent when load balancing
extension will be defined.
9.12.1. Operation in CONFLICT-DONE State
A primary server in CONFLICT-DONE state is fully responsive to all
DHCP clients (similar to the situation in COMMUNICATIONS-INTERRUPTED
state).
If communications fails, remain in CONFLICT-DONE state. If
communications becomes OK, remain in CONFLICT-DONE state until the
conditions for transition out become satisfied.
9.12.2. Transition Out of CONFLICT-DONE State
If communications fails with the partner while in CONFLICT-DONE
state, then the server will remain in CONFLICT-DONE state.
When a primary server determines that the secondary server has made a
transition into NORMAL state, the primary server will also transition
into NORMAL state.
10. Proposed extensions
The following section discusses possible extensions to the proposed
failover mechanism. Listed extensions must be sufficiently simple to
not further complicate failover protocol. Any proposals that are
considered complex will be defined as stand-alone extensions in
separate documents.
10.1. Active-active mode
A very simple way to achieve active-active mode is to remove the
restriction that seconary server MUST NOT respond to SOLICIT and
REQUEST messages. Instead it could respond, but MUST have lower
preference than primary server. Clients discovering available
servers will receive ADVERTISE messages from both servers, but are
expected to select the primary server as it has higher preference
value configured. The following REQUEST message will be directed to
primary server.
Discussion: Do DHCPv6 clients actually do this? DHCPv4 clients were
rumored to wait for a "while" to accept the best offer, but to a
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first approximation, they all take the first offer they receive that
is even acceptable.
The benefit of this approach, compared to the "basic" active--passive
solution is that there is no delay between primary failure and the
moment when secondary starts serving requests.
11. Dynamic DNS Considerations
DHCP servers (and clients) can use DNS Dynamic Updates as described
in RFC 2136 [RFC2136] to maintain DNS name-mappings as they maintain
DHCP leases. Many different administrative models for DHCP-DNS
integration are possible. Descriptions of several of these models,
and guidelines that DHCP servers and clients should follow in
carrying them out, are laid out in RFC 4704 [RFC4704].
The nature of the failover protocol introduces some issues concerning
dynamic DNS updates that are not part of non-failover environments.
This section describes these issues, and defines the information
which failover partners should exchange in order to ensure consistent
behavior. The presence of this section should not be interpreted as
requiring an implementation of the DHCPv6 failover protocol to also
support DDNS updates.
The purpose of this discussion is to clarify the areas where the
failover and DHCP-DDNS protocols intersect for the benefit of
implementations which support both protocols, not to introduce a new
requirement into the DHCPv6 failover protocol. Thus, a DHCPv6 server
which implements the failover protocol MAY also support dynamic DNS
updates, but if it does support dynamic DNS updates it SHOULD utilize
the techniques described here in order to correctly distribute them
between the failover partners. See RFC 4704 [RFC4704] as well as RFC
4703 [RFC4703] for information on how DHCPv6 servers deal with
potential conflicts when updating DNS even without failover.
From the standpoint of the failover protocol, there is no reason why
a server which is utilizing the DDNS protocol to update a DNS server
should not be a partner with a server which is not utilizing the DDNS
protocol to update a DNS server. However, a server which is not able
to support DDNS or is not configured to support DDNS SHOULD output a
warning message when it receives BNDUPD messages which indicate that
its failover partner is configured to support the DDNS protocol to
update a DNS server. An implementation MAY consider this an error
and refuse to operate, or it MAY choose to operate anyway, having
warned the user of the problem in some way.
11.1. Relationship between failover and dynamic DNS update
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The failover protocol describes the conditions under which each
failover server may renew a lease to its current DHCP client, and
describes the conditions under which it may grant a lease to a new
DHCP client. An analogous set of conditions determines when a
failover server should initiate a DDNS update, and when it should
attempt to remove records from the DNS. The failover protocol's
conditions are based on the desired external behavior: avoiding
duplicate address and prefix assignments; allowing clients to
continue using leases which they obtained from one failover partner
even if they can only communicate with the other partner; allowing
the secondary DHCP server to grant new leases even if it is unable to
communicate with the primary server. The desired external DDNS
behavior for DHCP failover servers is similar to that described above
for the failover protocol itself:
1. Allow timely DDNS updates from the server which grants a lease to
a client. Recognize that there is often a DDNS update lifecycle
which parallels the DHCP lease lifecycle. This is likely to
include the addition of records when the lease is granted, and
the removal of DNS records when the leased resource is
subsequently made available for allocation to a different client.
2. Communicate enough information between the two failover servers
to allow one to complete the DDNS update 'lifecycle' even if the
other server originally granted the lease.
3. Avoid redundant or overlapping DDNS updates, where both failover
servers are attempting to perform DDNS updates for the same
lease-client binding.
4. Avoid situations where one partner is attempting to add RRs
related to a lease binding while the other partner is attempting
to remove RRs related to the same lease binding.
While DHCP servers configured for DDNS typically perform these
operations on both the AAAA and the PTR resource records, this is not
required. It is entirely possible that a DHCP server could be
configured to only update the DNS with PTR records, and the DHCPv6
clients could be responsible for updating the DNS with their own AAAA
records. In this case, the discussions here would apply only to the
PTR records.
11.2. Exchanging DDNS Information
In order for either server to be able to complete a DDNS update, or
to remove DNS records which were added by its partner, both servers
need to know the FQDN associated with the lease-client binding. In
addition, to properly handle DDNS updates, additional information is
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required. All of the following information needs to be transmitted
between the failover partners:
1. The FQDN that the client requested be associated with the
resource. If the client doesn't request a particular FQDN and
one is synthesized by the failover server or if the failover
server is configured to replace a client requested FQDN with a
different FQDN, then the server generated value would be used.
2. The FQDN that was actually placed in the DNS for this lease. It
may differ from the client requested FQDN due to some form of
disambiguation or other DHCP server configuration (as described
above).
3. The status of and DDNS operations in progress or completed.
4. Information sufficient to allow the failover partner to remove
the FQDN from the DNS should that become necessary.
These data items are the minimum necessary set to reliably allow two
failover partners to successfully share the responsibility to keep
the DNS up to date with the resources allocated to clients.
This information would typically be included in BNDUPD messages sent
from one failover partner to the other. Failover servers MAY choose
not to include this information in BNDUPD messages if there has been
no change in the status of any DDNS update related to the lease.
The partner server receiving BNDUPD messages containing the DDNS
information SHOULD compare the status informatin and the FQDN with
the current DDNS information it has associated with the lease
binding, and update its notion of the DDNS status accordingly.
Some implementations will instead choose to send a BNDUPD without
waiting for the DDNS update to complete, and then will send a second
BNDUPD once the DDNS update is complete. Other implementations will
delay sending the partner a BNDUPD until the DDNS update has been
acknowledged by the DNS server, or until some time-limit has elapsed,
in order to avoid sending a second BNDUPD.
The FQDN option contains the FQDN that will be associated with the
AAAA RR (if the server is performing an AAAA RR update for the
client). The PTR RR can be generated automatically from the IP
address or prefix value. The FQDN may be composed in any of several
ways, depending on server configuration and the information provided
by the client in its DHCP messages. The client may supply a hostname
which it would like the server to use in forming the FQDN, or it may
supply the entire FQDN. The server may be configured to attempt to
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use the information the client supplies, it may be configured with an
FQDN to use for the client, or it may be configured to synthesize an
FQDN.
Since the server interacting with the client may not have completed
the DDNS update at the time it sends the first BNDUPD about the lease
binding, there may be cases where the FQDN in later BNDUPD messages
does not match the FQDN included in earlier messages. For example,
the responsive server may be configured to handle situations where
two or more DHCP client FQDNs are identical by modifying the most-
specific label in the FQDNs of some of the clients in an attempt to
generate unique FQDNs for them (a process sometimes called
"disambiguation"). Alternatively, at sites which use some or all of
the information which clients supply to form the FQDN, it's possible
that a client's configuration may be changed so that it begins to
supply new data. The server interacting with the client may react by
removing the DNS records which it originally added for the client,
and replacing them with records that refer to the client's new FQDN.
In such cases, the server SHOULD include the actual FQDN that was
used in subsequent DDNS options in any BNDUPD messages exchanged
between the failover partners. This server SHOULD include relevant
information in its BNDUPD messages. This information may be
necessary in order to allow the non-responsive partner to detect
client configuration changes that change the hostname or FQDN data
which the client includes in its DHCP requests.
11.3. Adding RRs to the DNS
A failover server which is going to perform DDNS updates SHOULD
initiate the DDNS update when it grants a new lease to a client. The
server which did not grant the lease SHOULD NOT initiate a DDNS
update when it receives the BNDUPD after the lease has been granted.
The failover protocol ensures that only one of the partners will
grant a lease to any individual client, so it follows that this
requirement will prevent both partners from initiating updates
simultaneously. The server initiating the update SHOULD follow the
protocol in RFC 4704 [RFC4704]. The server may be configured to
perform a AAAA RR update on behalf of its clients, or not.
Ordinarily, a failover server will not initiate DDNS updates when it
renews leases. In two cases, however, a failover server MAY initiate
a DDNS update when it renews a lease to its existing client:
1. When the lease was granted before the server was configured to
perform DDNS updates, the server MAY be configured to perform
updates when it next renews existing leases. The server which
granted the lease is the server which should initiate the DDNS
update.
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2. If a server is in PARTNER-DOWN state, it can conclude that its
partner is no longer attempting to perform an update for the
existing client. If the remaining server has not recorded that
an update for the binding has been successfully completed, the
server MAY initiate a DDNS update. It MAY initiate this update
immediately upon entry to PARTNER-DOWN state, it may perform this
in the background, or it MAY initiate this update upon next
hearing from the DHCP client.
11.4. Deleting RRs from the DNS
The failover server which makes a resource FREE SHOULD initiate any
DDNS deletes, if it has recorded that DNS records were added on
behalf of the client.
A server not in PARTNER-DOWN state "makes a resource FREE" when it
initiates a BNDUPD with a binding-status of FREE, FREE_BACKUP,
EXPIRED, or RELEASED. Its partner confirms this status by acking
that BNDUPD, and upon receipt of the BNDACK the server has "made the
resource FREE". Conversely, a server in PARTNER-DOWN state "makes a
resource FREE" when it sets the binding-status to FREE, since in
PARTNER-DOWN state no communications is required with the partner.
It is at this point that it should initiate the DDNS operations to
delete RRs from the DDNS. Its partner SHOULD NOT initiate DDNS
deletes for DNS records related to the lease binding as part of
sending the BNDACK message. The partner MAY have issued BNDUPD
messages with a binding-status of FREE, EXPIRED, or RELEASED
previously, but the other server will have rejected these BNDUPD
messages.
The failover protocol ensures that only one of the two partner
servers will be able to make a resource FREE. The server making the
resource FREE may be doing so while it is in NORMAL communication
with its partner, or it may be in PARTNER-DOWN state. If a server is
in PARTNER-DOWN state, it may be performing DDNS deletes for RRs
which its partner added originally. This allows a single remaining
partner server to assume responsibility for all of the DDNS activity
which the two servers were undertaking.
Another implication of this approach is that no DDNS RR deletes will
be performed while either server is in COMMUNICATIONS-INTERRUPTED
state, since no resource are moved into the FREE state during that
period.
11.5. Name Assignment with No Update of DNS
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In some cases, a DHCP server is configured to return a name to the
DHCPv6 client but not enter that name into the DNS. This is
typically a name that it has discovered or generated from information
it has received from the client. In this case this name information
SHOULD be communicated to the failover partner, if only to ensure
that they will return the same name in the event the partner becomes
the server to which the DHCPv6 client begins to interact.
12. Reservations and failover
Some DHCP servers support a capability to offer specific
preconfigured resources to DHCP clients. These are real DHCP
clients, they do the entire DHCP protocol, but these servers always
offer the client a specific pre-configured resource, one they offer
that resource to no other clients. Such a capability has several
names, but it is sometimes called a "reservation", in that the
resource is reserved for a particular DHCP client.
In a situation where there are two DHCP servers serving the same
subnet without using failover, the two DHCP server's need to have
disjoint resource pools, but identical reservations for the DHCP
clients.
In a failover context, both servers need to be configured with the
proper reservations in an identical manner, but if we stop there
problems can occur around the edge conditions where reservations are
made for resource that has already been leased to a different client.
Different servers handle this conflict in different ways, but the
goal of the failover protocol is to allow correct operation with any
server's approach to the normal processing of the DHCP protocol.
The general solution with regards to reservations is as follows.
Whenever a reserved resource becomes FREE (i.e., when first
configured or whenever a client frees it or it expires or is reset),
the primary server MUST show that resource as FREE (and thus
available for its own allocation) and it MUST send it to the
secondary server in a BNDUPD with a flag set showing that it is
reserved and with a status of BACKUP.
Note that this implies that a reserved resource goes through the
normal state changes from FREE to ACTIVE (and possibly back to FREE).
The failover protocol supports this approach to reservations, i.e.,
where the resource undergoes the normal state changes of any
resource, but it can only be offered to the client for which it is
reserved.
From the above, it follows that a reservation soley on the secondary
will not necessarily allow the secondary to offer that address to
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client to whom it is reserved. The reservation must also appear on
the primary as well for the secondary to be able to offer the
resource to the client to which is is reserved.
When the reservation on a resource is cancelled, if the resource is
currently FREE and the server is the primary, or BACKUP and the
server is the secondary, the server MUST send a BNDUPD to the other
server with the binding-status FREE and an indication that the
resource is no longer reserved.
13. Security Considerations
DHCPv6 failover is an extension of a standard DHCPv6 protocol, so all
security considerations from [RFC3315], Section 23 and [RFC3633],
Section 15 related to the server apply.
As traffic exchange between clients and server is not encrypted, an
attacker than penetrated the network and is able to intercept
traffic, will not gain any additional information by also sniffing
communication between partners.
An attacker that is able to impersonate one partner can efficiently
perform a denial of service attack on the remaining uncompromised
server. Several techniques may be used: pretending that conflict
resolution is required, requesting rebalance, claming that a valid
lease was released or declined etc. For that reason the
communication between servers SHOULD support failover connections
over TLS, as explained in Section Section 5.1. Such secure
connection SHOULD be optional and configurable by the administrator.
A server MUST NOT operate in PARTNER-DOWN if its partner is up.
Network administrator is expected to switch remaining active server
to PARTNER-DOWN state only if he or she is sure that the other server
is indeed down. Failing to obey this requirement will result in both
servers likely assigning duplicate leases to different clients.
Implementors should take that into consideration if they decide to
implement timer-based transition to PARTNER-DOWN state.
Running a network protected by DHCPv6 failover requires more
resources than running without it. In particular some of the
resources are allocated to the secondary server and they are not
usable in a normal (i.e. non failures) operation. While limiting
this pool may be preferable from resource utilisation perspective, it
must be reasonably large pool, so the secondary may take over once
primary becomes unavailable.
TODO: Security considerations section contains loose notes and will
be transformed into consistent text once the core design solidifies.
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14. IANA Considerations
IANA is not requested to perform any actions at this time.
15. Acknowledgements
This document extensively uses concepts, definitions and other parts
of [dhcpv4-failover] document. Authors would like to thank Shawn
Routher, Greg Rabil, and Bernie Volz for their significant
involvement and contributions. Authors would like to thank Marcin
Siodelski for his thorough review and VithalPrasad Gaitonde for his
insightful comments.
This work has been partially supported by Department of Computer
Communications (a division of Gdansk University of Technology) and
the Polish Ministry of Science and Higher Education under the
European Regional Development Fund, Grant No. POIG.01.01.02-00-045/
09-00 (Future Internet Engineering Project).
16. References
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC4703] Stapp, M. and B. Volz, "Resolution of Fully Qualified
Domain Name (FQDN) Conflicts among Dynamic Host
Configuration Protocol (DHCP) Clients", RFC 4703, October
2006.
[RFC4704] Volz, B., "The Dynamic Host Configuration Protocol for
IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
Option", RFC 4704, October 2006.
[RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
Address Option in DHCPv6", RFC 6939, May 2013.
16.2. Informative References
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Internet-Draft DHCPv6 Failover Design July 2013
[I-D.ietf-dhc-dhcpv6-failover-requirements]
Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
Requirements", draft-ietf-dhc-dhcpv6-failover-
requirements-06 (work in progress), July 2013.
[I-D.ietf-dhc-dhcpv6-load-balancing]
Kostur, A., "DHC Load Balancing Algorithm for DHCPv6",
draft-ietf-dhc-dhcpv6-load-balancing-00 (work in
progress), December 2012.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC4649] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Remote-ID Option", RFC 4649, August
2006.
[RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
"DHCPv6 Leasequery", RFC 5007, September 2007.
[RFC5460] Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, February
2009.
[dhcpv4-failover]
Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S.,
Rabil, G., Dooley, M., and A. Kapur, "DHCP Failover
Protocol", draft-ietf-dhc-failover-12 (work in progress),
March 2003.
Authors' Addresses
Tomasz Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 423 1345
Email: tomasz.mrugalski@gmail.com
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Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, Massachusetts 01719
USA
Phone: +1 (978) 936-0000
Email: kkinnear@cisco.com
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