RFC 8156
DHCPv6 Failover Protocol
Pages: 96
Proposed Standard
Proposed Standard
Part 1 of 5 – Pages 1 to 18
Internet Engineering Task Force (IETF) T. Mrugalski Request for Comments: 8156 ISC Category: Standards Track K. Kinnear ISSN: 2070-1721 Cisco June 2017 DHCPv6 Failover ProtocolAbstract
DHCPv6 as defined in "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" (RFC 3315) does not offer server redundancy. This document defines a protocol implementation to provide DHCPv6 failover, a mechanism for running two servers with the capability for either server to take over clients' leases in case of server failure or network partition. It meets the requirements for DHCPv6 failover detailed in "DHCPv6 Failover Requirements" (RFC 7031). Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8156. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................5 2. Requirements Language ...........................................5 3. Glossary ........................................................6 4. Failover Concepts and Mechanisms ...............................10 4.1. Required Server Configuration .............................10 4.2. IPv6 Address and Delegable Prefix Allocation ..............10 4.2.1. Independent Allocation .............................10 4.2.1.1. Independent Allocation Algorithm ..........11 4.2.2. Proportional Allocation ............................11 4.2.2.1. Reallocating Leases .......................13 4.3. Lazy Updates ..............................................14 4.4. Maximum Client Lead Time (MCLT) ...........................14 4.4.1. MCLT Example .......................................16 5. Message and Option Definitions .................................19 5.1. Message Framing for TCP ...................................19 5.2. Failover Message Format ...................................19 5.3. Messages ..................................................20 5.3.1. BNDUPD .............................................20 5.3.2. BNDREPLY ...........................................20 5.3.3. POOLREQ ............................................20 5.3.4. POOLRESP ...........................................21 5.3.5. UPDREQ .............................................21 5.3.6. UPDREQALL ..........................................21 5.3.7. UPDDONE ............................................21 5.3.8. CONNECT ............................................21 5.3.9. CONNECTREPLY .......................................22 5.3.10. DISCONNECT ........................................22 5.3.11. STATE .............................................22 5.3.12. CONTACT ...........................................22 5.4. Transaction-id ............................................22 5.5. Options ...................................................23 5.5.1. OPTION_F_BINDING_STATUS ............................23 5.5.2. OPTION_F_CONNECT_FLAGS .............................24 5.5.3. OPTION_F_DNS_REMOVAL_INFO ..........................25 5.5.3.1. OPTION_F_DNS_HOST_NAME ....................26 5.5.3.2. OPTION_F_DNS_ZONE_NAME ....................26 5.5.3.3. OPTION_F_DNS_FLAGS ........................27 5.5.4. OPTION_F_EXPIRATION_TIME ...........................28 5.5.5. OPTION_F_MAX_UNACKED_BNDUPD ........................29 5.5.6. OPTION_F_MCLT ......................................29 5.5.7. OPTION_F_PARTNER_LIFETIME ..........................30 5.5.8. OPTION_F_PARTNER_LIFETIME_SENT .....................30 5.5.9. OPTION_F_PARTNER_DOWN_TIME .........................31 5.5.10. OPTION_F_PARTNER_RAW_CLT_TIME .....................32 5.5.11. OPTION_F_PROTOCOL_VERSION .........................32
5.5.12. OPTION_F_KEEPALIVE_TIME ...........................33
5.5.13. OPTION_F_RECONFIGURE_DATA .........................34
5.5.14. OPTION_F_RELATIONSHIP_NAME ........................35
5.5.15. OPTION_F_SERVER_FLAGS .............................36
5.5.16. OPTION_F_SERVER_STATE .............................37
5.5.17. OPTION_F_START_TIME_OF_STATE ......................38
5.5.18. OPTION_F_STATE_EXPIRATION_TIME ....................38
5.6. Status Codes ..............................................39
6. Connection Management ..........................................40
6.1. Creating Connections ......................................40
6.1.1. Sending a CONNECT Message ..........................41
6.1.2. Receiving a CONNECT Message ........................42
6.1.3. Receiving a CONNECTREPLY Message ...................43
6.2. Endpoint Identification ...................................44
6.3. Sending a STATE Message ...................................45
6.4. Receiving a STATE Message .................................46
6.5. Connection Maintenance Parameters .........................46
6.6. Unreachability Detection ..................................47
7. Binding Updates and Acks .......................................47
7.1. Time Skew .................................................47
7.2. Information Model .........................................48
7.3. Times Required for Exchanging Binding Updates .............52
7.4. Sending Binding Updates ...................................53
7.5. Receiving Binding Updates .................................56
7.5.1. Monitoring Time Skew ...............................56
7.5.2. Acknowledging Reception ............................56
7.5.3. Processing Binding Updates .........................57
7.5.4. Accept or Reject? ..................................57
7.5.5. Accepting Updates ..................................59
7.6. Sending Binding Replies ...................................61
7.7. Receiving Binding Acks ....................................63
7.8. BNDUPD/BNDREPLY Data Flow .................................65
8. Endpoint States ................................................66
8.1. State Machine Operation ...................................66
8.2. State Machine Initialization ..............................69
8.3. STARTUP State .............................................70
8.3.1. Operation in STARTUP State .........................70
8.3.2. Transition out of STARTUP State ....................70
8.4. PARTNER-DOWN State ........................................72
8.4.1. Operation in PARTNER-DOWN State ....................72
8.4.2. Transition out of PARTNER-DOWN State ...............73
8.5. RECOVER State .............................................74
8.5.1. Operation in RECOVER State .........................74
8.5.2. Transition out of RECOVER State ....................74
8.6. RECOVER-WAIT State ........................................76
8.6.1. Operation in RECOVER-WAIT State ....................76
8.6.2. Transition out of RECOVER-WAIT State ...............76
8.7. RECOVER-DONE State ........................................77
8.7.1. Operation in RECOVER-DONE State ....................77
8.7.2. Transition out of RECOVER-DONE State ...............77
8.8. NORMAL State ..............................................77
8.8.1. Operation in NORMAL State ..........................78
8.8.2. Transition out of NORMAL State .....................78
8.9. COMMUNICATIONS-INTERRUPTED State ..........................79
8.9.1. Operation in COMMUNICATIONS-INTERRUPTED State ......80
8.9.2. Transition out of COMMUNICATIONS-INTERRUPTED
State ..............................................80
8.10. POTENTIAL-CONFLICT State .................................82
8.10.1. Operation in POTENTIAL-CONFLICT State .............82
8.10.2. Transition out of POTENTIAL-CONFLICT State ........82
8.11. RESOLUTION-INTERRUPTED State .............................83
8.11.1. Operation in RESOLUTION-INTERRUPTED State .........84
8.11.2. Transition out of RESOLUTION-INTERRUPTED State ....84
8.12. CONFLICT-DONE State ......................................84
8.12.1. Operation in CONFLICT-DONE State ..................85
8.12.2. Transition out of CONFLICT-DONE State .............85
9. DNS Update Considerations ......................................85
9.1. Relationship between Failover and DNS Update ..............86
9.2. Exchanging DNS Update Information .........................87
9.3. Adding RRs to the DNS .....................................89
9.4. Deleting RRs from the DNS .................................90
9.5. Name Assignment with No Update of DNS .....................91
10. Security Considerations .......................................91
11. IANA Considerations ...........................................92
12. References ....................................................94
12.1. Normative References .....................................94
12.2. Informative References ...................................96
Acknowledgements ..................................................96
Authors' Addresses ................................................96
1. Introduction
This document defines a DHCPv6 failover protocol, which is a mechanism for running two DHCPv6 servers [RFC3315] with the capability for either server to take over clients' leases in case of server failover or network partition. For a general overview of DHCPv6 failover problems, use cases, benefits, and shortcomings, see [RFC7031]. The failover protocol provides a means for cooperating DHCP servers to work together to provide a service to DHCP clients with availability that is increased beyond the availability that could be provided by a single DHCP server operating alone. It is designed to protect DHCP 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 for an IPv6 address [RFC3315] or on an IPv6 prefix [RFC3633] without the DHCP client experiencing lease expiration or a reassignment of a lease to a different IPv6 address or prefix in the event of failure by one or the other of the two servers. The failover protocol defines an active-passive mode, sometimes also called a "hot standby" model. This means that during normal operation one server is active (i.e., it actively responds to clients' requests) while the second is passive (i.e., it receives clients' requests but responds only to those specifically directed to it). The secondary server maintains a copy of the binding database and is ready to take over all incoming queries in case the primary server fails. The failover protocol is designed to provide lease stability for leases with valid lifetimes beyond a short period. The DHCPv6 failover protocol MUST NOT be used for new leases shorter than 30 seconds. Leases reaching the end of their lifetimes are not affected by this restriction. The failover protocol fulfills all DHCPv6 failover requirements defined in [RFC7031].2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
3. Glossary
This is a supplemental glossary that should be used in combination with the definitions in Section 2 of RFC 7031 [RFC7031]. o Absolute Time "Absolute time" refers to the time in seconds since midnight January 1, 2000 UTC, modulo 2^32. o Address Lease "Address lease" refers to a lease involving an IPv6 address. Typically used when it is necessary to distinguish the lease for an IPv6 address from a lease for a DHCP prefix. See the definitions for "delegated prefix" and "prefix lease" below. o auto-partner-down "auto-partner-down" refers to a capability where a failover server will move from COMMUNICATIONS-INTERRUPTED state to PARTNER-DOWN state automatically, without operator intervention. o Available (Lease or Prefix) A lease or delegable prefix is available if it could be allocated for use by a DHCP client. It is available on the main server when it is in the FREE state and available on the secondary server when it is in the FREE-BACKUP state. The term "available" is sometimes used when it would be awkward to say "FREE on the primary server and FREE-BACKUP on the secondary server". o Binding-Status A lease can hold a variety of states (see Section 5.5.1 for a list); these are also referred to as the "binding-status" of the lease. o Delegable Prefix "Delegable prefix" refers to a prefix from which other prefixes may be delegated, using the mechanisms described in [RFC3633]. A prefix that has been delegated is known as a "delegated prefix" or a "prefix lease".
o Delegated Prefix
A delegated prefix is a prefix that has been delegated to a DHCP
client as described in [RFC3633]. Depending on the context, a
delegated prefix may also be described as a "prefix lease" when it
is necessary to distinguish it from an "address lease".
o DHCP Prefix
A DHCP prefix is part of the IPv6 address space configured to be
managed by a DHCP server.
o Failover Endpoint
The failover protocol permits 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 from which addresses can be leased, as well as
prefixes from which other prefixes can be delegated (delegable
prefixes), that are associated with that relationship.
The failover endpoint can take actions and hold unique states.
Typically, there is one failover endpoint per partner (server),
although there may be more.
o Failover Communication
"Failover communication" refers to all messages exchanged between
partners.
o Independent Allocation
"Independent allocation" refers to an allocation algorithm that
splits the available pool of address leases between the primary
and secondary servers. It is used for IPv6 address allocations.
See Section 4.2.1.
o Lease
A lease is an association of a DHCP client with an IPv6 address or
delegated prefix. This might refer to either an existing
association or a potential association.
o MCLT (Maximum Client Lead Time)
The fundamental relationship on which much of the correctness of
the failover protocol depends is that the lease expiration time
known to a DHCP client MUST NOT be greater by more than the MCLT
beyond the later of the partner lifetime acknowledged by that
server's failover partner or the current time (i.e., now). See
Section 4.4.
o Partner
The other DHCP server that participates in a failover relationship
is referred to as the "partner". When the role (primary or
secondary) is not important, the other server is referred to as a
"failover partner" or sometimes simply "partner".
o Prefix Lease
A prefix lease is a lease involving a prefix that is delegated or
could be delegated, as opposed to a lease for a single IPv6
address. A prefix lease can also be described as a "delegated
prefix".
o Primary Server
The primary server is the first of the two DHCP servers that
participate in a failover relationship. When both servers are
operating, this server handles most of the client traffic. Its
failover partner is referred to as the "secondary server".
o Proportional Allocation
"Proportional allocation" is an allocation algorithm that splits
the delegable prefixes between the primary and secondary servers
and maintains a more or less fixed proportion of the delegable
prefixes between both servers. See Section 4.2.2.
o Renew Responsive
A server that is "renew responsive" will respond to valid DHCP
client messages that are directed to it by having an
OPTION_SERVERID option in the message that contains the DHCP
Unique Identifier (DUID) of the renew responsive server. See
[RFC3315].
o Responsive
A server that is responsive will respond to all valid DHCP client
messages.
o Secondary Server
The secondary server is the second of the two DHCP servers that
participate in a failover relationship. Its failover partner is
referred to as the "primary server" (as defined above). When both
servers are operating, this server (the secondary) typically does
not handle client traffic and acts as a backup to the primary
server. However, it will respond to RENEW requests directed
specifically to it.
o Server
"Server" refers to a DHCP server that implements DHCPv6 failover.
"Server" and "failover endpoint" are synonymous only if the server
participates in only one failover relationship.
o State
The term "state" is used in two ways in this document. A failover
endpoint is always in some state, and there are a series of states
that a failover endpoint can move through. See Section 8 for
details of the failover endpoint states. A lease also has a
state, and this is sometimes referred to as a "binding-status".
See Section 5.5.1 for a list of the states a lease can hold.
o Unresponsive
A server that is unresponsive will not respond to DHCP client
messages.
4. Failover Concepts and Mechanisms
The following concepts and mechanisms are necessary for the operation of the failover protocol. They are not currently employed by DHCPv6 [RFC3315]. The failover protocol provides support for all of these concepts and mechanisms.4.1. Required Server Configuration
Servers frequently have several kinds of leases available on a particular network segment. The failover protocol assumes that both the primary server and the secondary server are configured identically with regard to the prefixes and links involved in DHCP. For delegable prefixes (involved in proportional allocation), the primary server is responsible for allocating to the secondary server the correct proportion of the available delegable prefixes. IPv6 addresses (involved in independent allocation) are allocated to the primary and secondary servers algorithmically and do not require an explicit message transfer to be distributed.4.2. IPv6 Address and Delegable Prefix Allocation
Currently, there are two allocation algorithms defined: one for address leases and one for prefix leases.4.2.1. Independent Allocation
In this allocation scheme, which is used for allocating individual IPv6 addresses, available IPv6 addresses are permanently (until server configuration changes) split between servers. Available IPv6 addresses are split between the primary and secondary servers as part of initial connection establishment. Once IPv6 addresses are allocated to each server, there is no need to reassign them. The IPv6 address allocation is algorithmic in nature and does not require a message exchange for each server to know which IPv6 addresses it has been allocated. This algorithm is simpler than proportional allocation, since it does not require a rebalancing mechanism. It also assumes that the pool assigned to each server will never be depleted. Once each server is assigned a pool of IPv6 addresses during initial connection establishment, it may allocate its assigned IPv6 addresses to clients. Once a client releases a lease or its lease on an IPv6 address expires, the returned IPv6 address returns to the pool for the server that leased it. A lease on an IPv6 address can be renewed by a responsive server or by a renew responsive server. When an IPv6 address goes PENDING-FREE (see Section 7.2), it is owned by whichever server it is allocated to by the independent allocation algorithm.
IPv6 addresses, which use the independent allocation approach, will be ignored when a server processes a POOLREQ message. During COMMUNICATIONS-INTERRUPTED events, a partner MAY continue extending existing address leases as requested by clients. An operational partner MUST NOT lease IPv6 addresses that were assigned to its downed partner and later expired or that were released or declined by a client. When it is in PARTNER-DOWN state, a server MUST allocate new leases from its own pool. It MUST NOT use its partner's pool to allocate new leases.4.2.1.1. Independent Allocation Algorithm
For each address that can be allocated, the primary server MUST allocate only IPv6 addresses when the low-order bit (i.e., bit 127) is equal to 1, and the secondary server MUST allocate only the IPv6 addresses when the low-order bit (i.e., bit 127) is equal to 0.4.2.2. Proportional Allocation
In this allocation scheme, each server has its own pool of prefixes available for delegation, known as "delegable prefixes". These delegable prefixes may be prefixes from which other prefixes can be delegated, or they may be prefixes that are the correct size for delegation but are not, at present, delegated to a particular client. Remaining delegable prefixes are split between the primary and secondary servers in a configured proportion. Note that a delegated prefix (also known as a "prefix lease") is not "owned" by a particular server. Only a delegable prefix that is available is owned by a particular server -- once it has been delegated (leased) to a client, it becomes a prefix lease and is not owned by either failover partner. When it finally becomes available again, it will be initially owned by the primary server, and it may or may not be allocated to the secondary server by the primary server. The flow of a delegable prefix is as follows: initially, the delegable prefix is part of a set of delegable prefixes, all of which are initially owned by the primary server. A delegable prefix may be allocated to the secondary server, and it is then owned by the secondary server. Either server can allocate and delegate prefixes out of the delegable prefixes that they own. Once these prefixes are delegated (leased) to clients, the servers cease to own them, and they are owned by the clients to which they have been delegated (leased). When the client releases the delegated prefix or the lease on it expires, the prefix will again become available, will again be a delegable prefix, and will be owned by the primary.
A server delegates prefixes only from its own pool of delegable prefixes in all states except for PARTNER-DOWN. In PARTNER-DOWN state, the operational partner can delegate prefixes from either pool (both its own, and its partner's after some time constraints have elapsed). The operational partner SHOULD allocate from its own pool before using its partner's pool. The allocation and maintenance of these pools of delegable prefixes are important, since the goal is to maintain a more or less constant ratio of delegable prefixes between the two servers. Each server knows which delegable prefixes are in its own pool as well as which are in its partner's pool, so that it can allocate delegable prefixes from its partner's pool without communication with its partner if that becomes necessary. The initial allocation of delegable prefixes from the primary to the secondary when the servers first integrate is triggered by the POOLREQ message from the secondary to the primary. This is followed (at some point) by the POOLRESP message, where the primary tells the secondary that it received and processed the POOLREQ message. The primary sends the allocated delegable prefixes to the secondary as prefix leases via BNDUPD messages. The POOLRESP message may be sent before, during, or at the completion of the BNDUPD message exchanges that were triggered by the POOLREQ message. The POOLREQ/POOLRESP message exchange is a trigger to the primary to perform a scan of its database and to ensure that the secondary has enough delegable prefixes (based on some configured ratio). The delegable prefixes are sent to the secondary as prefix leases using the BNDUPD message containing an OPTION_IAPREFIX with a state of FREE-BACKUP, which indicates that the prefix lease is now available for allocation by the secondary. Once the message is sent, the primary MUST NOT use these prefixes for allocation to DHCP clients (except when the server is in PARTNER-DOWN 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 delegable prefixes and determine if the secondary needs more delegable prefixes from any of the delegable prefixes that it currently owns. In order to support a reasonably dynamic balance of the leases between the failover partners, the primary server needs to do additional work to ensure that the secondary server has as many delegable prefixes as it needs (but that it doesn't have more than it needs).
The primary server SHOULD examine the balance of delegable prefixes between the primary and secondary for a particular prefix whenever the number of possibly delegable prefixes for either the primary or secondary changes by more than a predetermined amount. Typically, this comparison would not involve actually comparing the count of existing instances of delegable prefixes but would instead involve determining the number of prefixes that could be delegated given the address ranges of the delegable prefixes allocated to each server. The primary server SHOULD adjust the delegable prefix balance as required to ensure the configured delegable prefix balance, except that the primary server SHOULD employ some threshold mechanism to such a balance adjustment in order to minimize the overhead of maintaining this balance. The primary server can, at any time, send an available delegable prefix to the secondary using a BNDUPD message with the state FREE-BACKUP. The primary server can attempt to take an available delegable prefix away from the secondary by sending a BNDUPD message with the state FREE. If the secondary accepts the BNDUPD message, then the lease is now available to the primary and not available to the secondary. Of course, the secondary MUST reject that BNDUPD message if it has already allocated that lease to a DHCP client.4.2.2.1. Reallocating Leases
When the server is in PARTNER-DOWN state, there is a waiting period after which a delegated prefix can be reallocated to another client. For delegable prefixes that are "available" when the server enters PARTNER-DOWN state, the period is the MCLT from the entry into PARTNER-DOWN state. For delegated prefixes that are not available when the server enters PARTNER-DOWN state, the period is the MCLT after the later of the following times: the acked-partner-lifetime, the partner-lifetime (if any), the expiration-time, or the entry into PARTNER-DOWN time. In any other state, a server cannot reallocate a delegated prefix from one client to another without first notifying its partner (through a BNDUPD message) and receiving acknowledgement (through a BNDREPLY message) that its partner is aware that the first client is not using the lease. Specifically, an "available" delegable prefix on a server may be allocated to any client. A prefix that was delegated (leased) to a client and that 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 delegated prefix was EXPIRED or RELEASED through a BNDUPD message. When the sending server received the BNDREPLY message for that delegated prefix showing that it was
FREE, it would move the lease from EXPIRED or RELEASED to FREE, and the prefix would be available for allocation by the primary server to any clients. A server MAY reallocate a delegated prefix in the EXPIRED or RELEASED state to the same client with no restrictions, provided it has not sent a BNDUPD message regarding the delegated prefix to its partner. This situation would exist if the prefix lease expired or was released after the transition into PARTNER-DOWN state, for instance.4.3. Lazy Updates
[RFC7031] includes the requirement that failover must not introduce significant performance impact on server response times (see Sections 7 and 5.2.2 of [RFC7031]). In order to realize this requirement, a server implementing the failover protocol must be able to respond to a DHCP client without waiting to update its failover partner whenever the binding database changes. The "lazy update" mechanism allows a server to allocate a new lease or extend an existing lease, respond to the DHCP client, 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 DHCP client with a particular valid lifetime but before updating its failover partner, the failover partner will eventually believe that the client's lease has expired -- even though the DHCP client still retains a valid lease on that address or prefix. It is also possible that the failover partner will have no record at all of the lease being assigned to the DHCP client. Both of these issues are dealt with by using the MCLT when allocating or extending leases (see Section 4.4).4.4. Maximum Client Lead Time (MCLT)
In order to handle problems introduced by lazy updates (see Section 4.3), a period of time known as the "Maximum Client Lead Time" (MCLT) is defined and must be known to both the primary server and the secondary server. Proper use of this time interval places an upper bound on the difference allowed between the valid lifetime provided to a DHCP client by a server and the valid lifetime known by that server's failover partner. The MCLT is typically much less than the valid lifetime that a server has been configured to offer a client, and so some strategy must exist to allow a server to offer the configured valid lifetime to a client. During a lazy update, the updating server updates its
failover partner with a partner lifetime that is longer than the valid lifetime previously given to the DHCP client and that is longer than the valid lifetime that the server has been configured to give a client. This allows the server to give the configured valid lifetime to the client the next time the client renews its lease, since the time that it will give to the client will not be longer than the MCLT beyond the partner lifetime acknowledged by its partner or the current time. The fundamental relationship on which the failover protocol depends is as follows: the lease expiration time known to a DHCP client MUST NOT be greater by more than the MCLT beyond the later of the partner lifetime acknowledged by that server's failover partner or the current time. The remainder of this section makes the above fundamental relationship more explicit. The failover protocol requires a DHCP server to deal with several different lease intervals and places specific restrictions on their relationships. The purpose of these restrictions is to allow the partner to be able to make certain assumptions in the absence of an ability to communicate between servers. In the following explanation, all of the lifetimes are "valid" lifetimes, in the context of [RFC3315]. The different times are as follows: desired lifetime: The desired lifetime is the lease interval that a DHCP server would like to give to a DHCP client in the absence of any restrictions imposed by the failover protocol. Its determination is outside of the scope of the failover protocol. Typically, this is the result of external configuration of a DHCP server. actual lifetime: The actual lifetime is the lease interval that a DHCP server gives out to a DHCP client. It may be shorter than the desired lifetime (as explained below). partner lifetime: The partner lifetime is the lease expiration interval the local server sends to its partner in a BNDUPD message.
acknowledged partner lifetime:
The acknowledged partner lifetime is the partner lifetime the
partner server has most recently acknowledged in a BNDREPLY
message.
4.4.1. MCLT Example
The following example demonstrates the MCLT concept in practice. The
values used are arbitrarily chosen and are not a recommendation for
actual values. The MCLT in this case is 1 hour. The desired
lifetime is 3 days, and its renewal time is half the lifetime.
When a server makes an offer for a new lease on an IPv6 address to a
DHCP client, it determines the desired lifetime (in this case,
3 days). It then examines the acknowledged partner 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 lifetime cannot be allowed to exceed the remainder of the
current acknowledged partner lifetime plus the MCLT, the offer made
to the client is for the remainder of the current acknowledged
partner lifetime (i.e., zero) plus the MCLT. Thus, the actual
lifetime is 1 hour (the MCLT).
Once the server has sent the REPLY to the DHCP client, it will update
its failover partner with the lease information using a BNDUPD
message. The partner lifetime will be composed of the T1 fraction
(1/2) of the actual lifetime added to the desired lifetime. Thus,
the failover partner is updated using a BNDUPD message with a partner
lifetime of 1/2 hour + 3 days.
When the primary server receives a BNDREPLY to its update of the
secondary server's (partner's) partner lifetime, it records that as
the acknowledged partner lifetime. A server MUST NOT send a BNDREPLY
message in response to a BNDUPD message until it is sure that the
information in the BNDUPD message has been updated in its lease
database. See Section 7.5.2. Thus, the primary server in this case
can be sure that the secondary server has recorded the partner
lifetime in its stable storage when the primary server receives a
BNDREPLY message from the secondary server.
When the DHCP client attempts to renew at T1 (approximately 1/2 hour
from the start of the lease), the primary server again determines the
desired lifetime, which is still 3 days. It then compares this with
the original acknowledged partner lifetime (1/2 hour + 3 days) and
adjusts for the time passed since the secondary was last updated
(1/2 hour). Thus, the remaining time for the acknowledged partner
interval is 3 days. Adding the MCLT to this yields 3 days plus 1 hour, which is more than the desired lifetime of 3 days. So, the client may have its lease renewed for the desired lifetime -- 3 days. When the primary DHCP server updates the secondary DHCP server after the DHCP client's renewal REPLY is complete, it will calculate the partner lifetime as the T1 fraction of the actual client lifetime (1/2 of 3 days = 1.5 days). To this it will add the desired lifetime of 3 days, yielding a total partner 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 lifetime so as to be able to always offer the client the desired lifetime. Once the initial actual client lifetime of the MCLT has passed, the failover protocol operates effectively like DHCP does today in its behavior concerning lifetimes. However, the guarantee that the actual client lifetime will never exceed the partner server's remaining acknowledged partner lifetime by more than the MCLT allows full recovery from a variety of DHCP server failures.
Fundamental relationship: lease time = floor( desired lifetime, acked-partner-lifetime + MCLT ) Initial conditions: MCLT = 1h, desired lifetime = 3d DHCPv6 Primary Secondary time Client Server Server | >-SOLICIT------> | | | acknowledged partner lifetime = 0 | | lease time = floor( 3d, 0 + 1h ) = 1h | | <-----ADVERTISE-< | | | lease-time = 1h | | | >-REQUEST------> | | t | <---------REPLY-< | | | lease-time = 1h | | | | >-BNDUPD------> | | | partner-lifetime = 1/2h + 3d | | <----BNDREPLY-< | | | partner-lifetime = 1/2h + 3d |acknowledged partner lifetime = 1/2h + 3d | 1/2h passes ... ... ... t+1/2h | >-RENEW--------> | | | acknowledged partner lifetime = 3d | | lease time = floor( 3d, 3d + 1h ) = 3d | | <---------REPLY-< | | | lease-time = 3d | | | | >-BNDUPD-------> | | | partner-lifetime = 1.5d + 3d | | <----BNDREPLY-< | | | partner-lifetime = 1.5d + 3d |acknowledged partner lifetime = 1.5d + 3d | 1.5d passes ... ... ... | | | t+1.5d + 1/2h | >-RENEW--------> | | | acknowledged partner lifetime = 3d | | lease time = floor( 3d, 3d + 1h ) = 3d | | <---------REPLY-< | | | lease-time = 3d | | | | >-BNDUPD-------> | | | partner-lifetime = 1.5d + 3d | | <----BNDREPLY-< | | | partner-lifetime = 1.5d + 3d |acknowledged partner lifetime = 1.5d + 3d | Figure 1: MCLT Example
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