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Network Working Group Ralph Droms
INTERNET DRAFT Bucknell University
Kim Kinnear
Mark Stapp
Cisco Systems
Bernie Volz
IPWorks
Steve Gonczi
Network Engines
Greg Rabil
Mike Dooley
Arun Kapur
Lucent Technologies
July 2000
Expires January 2001
DHCP Failover Protocol
<draft-ietf-dhc-failover-07.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
DHCP [RFC 2131] allows for multiple servers to be operating on a
single network. Some sites are interested in running multiple
servers in such a way so as to provide redundancy in case of server
failure. In order for this to work reliably, the cooperating primary
and secondary servers must maintain a consistent database of the
lease information. This implies that servers will need to coordinate
any and all lease activity so that this information is synchronized
in case of failover.
This document defines a protocol to provide such synchronization
between two servers. One server is designated the "primary" server,
the other is the "secondary" server. This document also describes a
way to integrate the failover protocol with the DHCP load balancing
approach.
This document is a substantial reorganization as well as a technical
and editorial revision of draft-ietf-dhc-failover-05.txt.
Table of Contents
1. Introduction................................................. 4
2. Terminology.................................................. 5
2.1. Requirements terminology................................... 5
2.2. DHCP and failover terminology.............................. 5
3. Background and External Requirements......................... 9
3.1. Key aspects of the DHCP protocol........................... 9
3.2. BOOTP relay agent implementation........................... 11
3.3. What does it mean if a server can't communicate with its partner? 12
3.4. Challenging scenarios for a Failover protocol.............. 12
3.5. Using TCP to detect partner server failure................. 14
4. Design Goals................................................. 15
4.1. Design goals for this protocol............................. 15
4.2. Limitations of this protocol............................... 16
5. Protocol Overview............................................ 17
5.1. Messages and States........................................ 17
5.2. Fundamental guarantees..................................... 20
5.3. Load balancing............................................. 26
5.4. Operating in NORMAL state.................................. 27
5.5. Operating in COMMUNICATIONS-INTERRUPTED state.............. 27
5.6. Operating in PARTNER-DOWN state............................ 28
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5.7. Operating in RECOVER state................................. 28
5.8. Operating in STARTUP state................................. 28
5.9. Time synchronization between servers....................... 28
5.10. IP address binding-status................................. 29
5.11. DNS dynamic update considerations......................... 33
5.12. Reservations and failover................................. 37
5.13. Dynamic BOOTP and failover................................ 39
5.14. Guidelines for selecting MCLT............................. 39
6. Common Message Format........................................ 40
6.1. Message header format...................................... 40
6.2. Common option format....................................... 43
6.3. Batching multiple binding update transactions in one BNDUPD mes- 44
7. Protocol Messages............................................ 46
7.1. BNDUPD message [3]......................................... 46
7.2. BNDACK message [4]......................................... 56
7.3. UPDREQ message [9]......................................... 59
7.4. UPDREQALL message [7]...................................... 60
7.5. UPDDONE message [8]........................................ 61
7.6. POOLREQ message [1]........................................ 62
7.7. POOLRESP message [2]....................................... 63
7.8. CONNECT message [5]........................................ 64
7.9. CONNECTACK message [6]..................................... 68
7.10. STATE message [10]........................................ 71
7.11. CONTACT message [11]...................................... 72
7.12. DISCONNECT message [12]................................... 73
8. Connection Management........................................ 74
8.1. Connection granularity..................................... 74
8.2. Creating the TCP connection................................ 74
8.3. Using the TCP connection for determining communications status 76
8.4. Using the TCP connection for binding data.................. 78
8.5. Using the TCP connection for control messages.............. 78
8.6. Losing the TCP connection.................................. 78
9. Failover Endpoint States..................................... 79
9.1. Server Initialization...................................... 79
9.2. Server State Transitions................................... 79
9.3. STARTUP state.............................................. 82
9.4. PARTNER-DOWN state......................................... 84
9.5. RECOVER state.............................................. 86
9.6. NORMAL state............................................... 89
9.7. COMMUNICATIONS-INTERRUPTED State........................... 91
9.8. POTENTIAL-CONFLICT state................................... 95
9.9. RESOLUTION-INTERRUPTED state............................... 96
9.10. RECOVER-DONE state........................................ 97
9.11. PAUSED state.............................................. 98
9.12. SHUTDOWN state............................................ 98
10. Safe Period................................................. 99
11. Security.................................................... 101
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11.1. Simple shared secret...................................... 101
11.2. TLS....................................................... 102
12. Failover Options............................................ 103
12.1. addresses-transferred..................................... 103
12.2. assigned-IP-address....................................... 103
12.3. binding-status............................................ 104
12.4. client-identifier......................................... 104
12.5. client-hardware-address................................... 105
12.6. client-last-transaction-time.............................. 105
12.7. client-reply-options...................................... 105
12.8. client-request-options.................................... 106
12.9. DDNS...................................................... 107
12.10. delayed-service-parameter................................ 108
12.11. hash-bucket-assignment................................... 108
12.12. lease-expiration-time.................................... 108
12.13. max-unacked-bndupd....................................... 109
12.14. MCLT..................................................... 109
12.15. message.................................................. 109
12.16. message-digest........................................... 110
12.17. potential-expiration-time................................ 110
12.18. receive-timer............................................ 110
12.19. protocol-version......................................... 111
12.20. reject-reason............................................ 112
12.21. sending-server-IP-address................................ 113
12.22. server-flags............................................. 113
12.23. server-state............................................. 114
12.24. start-time-of-state...................................... 114
12.25. TLS-reply................................................ 115
12.26. TLS-request.............................................. 115
12.27. vendor-class-identifier.................................. 115
12.28. vendor-specific-options.................................. 116
13. IANA Considerations......................................... 116
14. Acknowledgments............................................. 116
15. References.................................................. 118
16. Author's information........................................ 119
17. Full Copyright Statement.................................... 120
1. Introduction
DHCP [RFC 2131] allows for multiple servers to be operating on a sin-
gle network. Some sites are interested in running multiple servers
in such a way so as to provide redundancy in case of server failure
since the DHCP subsystem is in many cases a critical part of the net-
work infrastructure.
This document defines a protocol to provide synchronization between
two servers in order that each can take over for the other should
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either one fail or become unreachable.
One server is designated the "primary" server, the other is the
"secondary" server, and most DHCP client requests are sent to each
server (see Section 3.1.1 for details).
In order to provide a high availability DHCP service, these
cooperating primary and secondary servers must maintain a consistent
database of lease information. This implies that servers will need
to coordinate all lease activity so that this information is syn-
chronized in case failover is required. The protocol messages and
processing techniques required to maintain a consistent database are
specified in the protocol described here.
The failover protocol also contains a way to integrate the DHCP load-
balancing algorithm described in [LOADB] with the failover protocol.
2. Terminology
This section discusses both the generic requirements terminology com-
mon to many IETF protocol specifications as well as specialized DHCP
and failover protocol specific terminology.
2.1. Requirements terminology
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 [RFC 2119].
2.2. DHCP and failover terminology
This document uses the following terms:
o "binding"
A binding is a collection of configuration parameters, includ-
ing at least an IP address, associated with or "bound to" a
DHCP client. Bindings are managed by DHCP servers.
o "binding database"
The collection of bindings managed by a primary and secondary.
o "binding update transaction"
A binding update transaction refers to the set of information
(contained in options) necessary to perform a binding update
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for a single IP address. It will be comprised of the
assigned-IP-address option and the binding-status option, along
other options as appropriate.
o "binding-status"
The binding-status is the status of an IP address with respect
to its association with a client. There are specific binding-
status values defined for use by the failover protocol, e.g.,
ACTIVE, FREE, RELEASED, ABANDONED, etc. These are designed to
map more or less directly onto the binding-status values used
internally in most DHCP server implementations. The term
binding-status refers to the concept also sometimes known as
"lease state" or "IP address state", but in this document the
term "state" is reserved for the failover state of a failover
endpoint, and binding-status is always used to refer to the
state associated with an IP address or lease.
o "DHCP client" or "client"
A DHCP client is an Internet host using DHCP to obtain confi-
guration parameters such as a network address. The term
"client" used within this document always means a DHCP client,
and never one of the two failover servers.
o "DHCP server" or "server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
o "DDNS"
An abbreviation for "Dynamic DNS", which refers to the capabil-
ity to update a DNS server's name (actually resource record)
database using an on-the-wire protocol defined in [RFC 2136].
o "DNS"
An abbreviation for "Domain Name System", a scheme where a cen-
tral name repository is used to map names to IP addresses and IP
addresses to names.
o "failover endpoint"
The failover protocol allows for there to be a unique failover
endpoint per partner per role (where role is primary or secon-
dary). This failover endpoint can take actions and hold unique
states. There are thus a maximum of two failover endpoints per
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server per partner (one for each partner as a primary and one
for that same partner as a secondary.)
o "FQDN"
An FQDN is a "fully qualified domain name". A fully qualified
domain name generally is a host name with at least one zone
name, for example "www.dhcp.org" is a fully qualified domain
name.
o "lazy update"
Lazy update refers to the requirement placed on a server imple-
menting a failover protocol to update its failover partner when-
ever the binding database changes. A failover protocol which
didn't support lazy update would require the failover partner
update to be complete before a DHCP server could respond to a
DHCP client request with a DHCPACK. A failover protocol which
does support lazy update places no such restriction on the
update of the failover partner server, and so a server can allo-
cate an IP address or extend a lease on an IP address and then
update its failover partner as time permits. A failover proto-
col which supports lazy update not only removes the requirement
to update the failover partner prior to responding to a DHCP
client with a DHCPACK, but also allows gathering up batches of
updates from one failover server to its partner.
o "MCLT"
The MCLT refers to maximum client lead time. This time is con-
figured on the primary server and transmitted from the primary
to the secondary server in the CONNECT message. It is the max-
imum amount of time that one server can extend a lease for a
client's binding beyond the time known by the partner server.
See section 5.2.1 for details.
o "partner"
A "partner", for the purposes of this document, refers to a
failover server, typically the other failover server. In many
(if not most) cases, the failover protocol is symmetric with
respect to the primary or secondary nature of the servers, and
so it is often appropriate to discuss "updating the partner
server", since it could be a primary server updating a secondary
server or a secondary server updating a primary server.
o "Primary server" or "Primary"
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A DHCP server configured to provide primary service to a set of
DHCP clients for a particular set of subnet address pools.
o "RR"
"RR" is an abbreviation for "resource record". All records in
the DNS are resource records. The resource records of most
relevance to this document are the "A" resource record, which
maps a DNS name to a particular IP address, the "PTR" resource
record, which allows a "reverse map", from the IP address back
to a DNS name, and the "KEY" resource record, which is used in
ways defined in [DDNS] to tag a DNS name with the identity of
the DHCP client with which it is associated.
o "Secondary server" or "Secondary"
A DHCP server configured to act as backup to a primary server
for a particular set of subnet address pools.
o "stable storage"
Every DHCP server is assumed to have some form of what is called
"stable storage". Stable storage is used to hold information
concerning IP address bindings (among other things) so that this
information is not lost in the event of a server failure which
requires restart of the server.
o "state"
In this document, the term "state" refers exclusively to the
state of a failover endpoint, for example: NORMAL,
COMMUNICATIONS-INTERRUPTED, PARTNER-DOWN. It is not used to
refer to any attributes of an IP address or a binding of an IP
address. See "binding-status".
o "subnet address pool"
A subnet address pool is the set of IP addresses which is asso-
ciated with a particular network number and subnet mask. In the
simple case, there is a single network number and subnet mask
and a set of IP addresses. In the more complex case (sometimes
called "secondary subnets", sometimes "superscopes"), several
(apparently unrelated) network number and subnet mask combina-
tions with their associated IP addresses may all be configured
together into one subnet address pool.
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3. Background and External Requirements
This section highlights key aspects of the DHCP protocol on which the
failover protocol depends. It also discusses the requirements that
the failover protocol places on other aspects of the network infras-
tructure, and some general issues surrounding server failure detec-
tion. Some failure scenarios that provide particular challenges to a
failover protocol are discussed. Finally, the challenges inherent in
using a TCP connection as a means to detect failure of a partner
server are elaborated.
3.1. Key aspects of the DHCP protocol
The failover protocol is designed to augment the DHCP protocol as
described in RFC 2131 [RFC 2131]. There are several key aspects of
the DHCP protocol which are required by the failover protocol in
order to successfully meet its design goals.
3.1.1. Broadcast behavior
There are two aspects of the broadcast behavior of the DHCP protocol
which are key to making the failover protocol operate successfully.
The first is simply that the DHCP protocol requires a DHCP client to
broadcast all DHCPDISCOVER and DHCPREQUEST/INIT-REBOOT messages.
Because of this requirement, a DHCP client who was communicating with
one server will automatically be able to communicate with another
server if one is available.
The second aspect of broadcast behavior is similar to the first, but
involves the distinction between a DHCPREQUEST/RENEW and
DHCPREQUEST/REBINDING. A DHCPREQUEST/RENEW is the message that a
DHCP client uses to extend its lease. It is unicast to the DHCP
server from which it acquired the lease. However, the DHCP protocol
(in a farsighted move), was explicitly designed so that in the event
that a DHCP client cannot contact the server from which it received a
lease on an IP address using a DHCPREQUEST/RENEW, the client is
required to broadcast its renewal using a DHCPREQUEST/REBINDING to
any available DHCP server. Since all DHCP clients were required to
implement this algorithm, the failover protocol can have a different
server from the one that initially granted a lease be the server to
renew a lease. Thus, one server can take over for another with no
interruption in the service as experienced by the DHCP client or its
associated applications software.
3.1.2. Client responsibility
In the DHCP protocol the DHCP clients are entrusted with a consider-
able responsibility. In particular, after they are granted a lease
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on an IP address, they are enjoined to only use that IP address while
their lease is valid. Every DHCP client is expected to stop using an
IP address if the expiration time on the lease has passed and if it
cannot get an extension on the lease for that IP address from some
DHCP server. Thus, the correct behavior of every DHCP client in this
regard is required to ensure the integrity of the DHCP service. On
the other hand, incorrect behavior by a client in this area will tend
to adversely affect at most one other DHCP client.
Furthermore, any DHCP client which sends in a DHCPREQUEST/RENEW or
DHCPREQUEST/REBINDING to a DHCP server (either unicast for a RENEW or
broadcast for a REBINDING) MUST still have time to run on the lease
for that IP address. The DHCP server sends the DHCPACK back unicast
to the IP address from which the RENEW or REBINDING originated.
Given the existing responsibility placed on the client to only use an
IP address when the lease is valid, and to only send in a RENEW or
REBINDING if the lease is valid, the failover protocol relies on DHCP
clients to perform responsibly and will, in the absence of conflict-
ing information, believe a DHCP client that is attempting to RENEW or
REBIND a lease on an IP address is the legitimate owner of that IP
address.
If clients do not follow these rules, it is possible for an address
to be in use by more than one client. For a single server, this hap-
pens because the server has leased the expired address to another
client and the original client is also attempting to use the address.
The server would NAK the renewal request. This is made slightly worse
in the failover protocol if the two servers are unable to communicate
with each other and one server leases an available address to a new
client while the other server receives a renewal from a different
client. In this case, both servers lease the same address to dif-
ferent clients for the MCLT time.
One troublesome issue is that of the DHCP client responsibility when
sending in DHCPREQUEST/INIT-REBOOT requests. While the original DHCP
RFC was written to require a DHCP client to have time left to run on
the lease for an IP address if the client is sending an INIT-REBOOT
request, it was sufficiently unclear that some client vendors didn't
realize this until recently. Since the INIT-REBOOT request was sent
with the IP address in the dhcp-requested-address option and not in
the ciaddr (for perfectly good reasons), the similarity to the RENEW
and REBINDING case was lost on many people.
At present, the failover protocol does not assume that a client send-
ing in an INIT-REBOOT request necessarily has a valid lease on the IP
address appearing in the dhcp-requested-address option in the INIT-
REBOOT request.
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The implications of this are as follows: Assume that there is a DHCP
client that gets a lease from one server while that server is unable
to communicate with its failover partner. Then, assume that after
that client reboots it is able only to communicate with the other
failover server. If the failover servers have not been able to com-
municate with each other during this process, then the DHCP client
will get a new IP address instead of being able to continue to use
its existing IP address. This will affect no applications on the DHCP
client, since it is rebooting. However, it will use up an additional
IP address in this marginal case.
3.1.3. Stable storage update before DHCPACK
The DHCP protocol allocates resources, and in order to operate
correctly it requires that a DHCP server update some form of stable
storage prior to sending a DHCPACK to a DHCP client in order to grant
that client a lease on an IP address.
One of the goals of the failover protocol is that it not add signifi-
cant additional time to this already time consuming requirement to
update stable storage prior to a DHCPACK. In particular, adding a
requirement to communicate with another server prior to sending a
DHCPACK would greatly simplify the failover protocol, but it would
unacceptably limit the potential scalability of any DHCP server which
employed the failover protocol.
3.2. BOOTP relay agent implementation
Many DHCP clients are not resident on the same network segment as a
DHCP server. In order to support this form of network architecture,
most contemporary routers implement something known as a BOOTP Relay
Agent. This capability inside of a router listens for all broadcasts
at the DHCP port, port 67, and will relay any broadcasts that it
receives on to a DHCP server. The IP address of the DHCP server must
have been previously configured into the router. As part of the
relay process, the relay agent will place the address of the inter-
face on which it received the broadcast into the giaddr field of the
DHCP packet.
Since the failover protocol requires two DHCP servers to receive any
broadcast DHCP messages, in order to work with DHCP clients which are
not local to the DHCP server, the BOOTP relay agent on the router
closest to the DHCP client must be configured to point at more than
one DHCP server.
Most BOOTP relay agent implementations allow this duplication of
packets.
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If this is not possible, an administrator might be able to configure
the relay agent with a subnet broadcast address, but in this case the
primary and secondary DHCP servers in a failover pair must both
reside on the same subnet.
3.3. What does it mean if a server can't communicate with its partner?
In any protocol designed to allow one server to take over some
responsibilities from a partner server in the event of "failure" of
that partner server, there is an inherent difficulty in determining
when that partner server has failed.
In fact, it is fundamentally impossible for one server to distinguish
a network communications failure from the outright failure of the
server to which it is trying to communicate. In the case where each
server is handing out resources (in this case IP addresses) to a
client community, mistaking an inability to communicate with a
partner server for failure of that partner server could easily cause
both servers to be handing out the same IP addresses to different
clients.
One way that this is sometimes handled is for there to be more than
two servers. In the case of an odd number of servers, the servers
that can still communicate with a majority of other servers will con-
sider themselves operational, and any server which can't communicate
to a majority of other servers must immediately cease operations.
While this technique works in some domains, having the only server to
which a DHCP client can communicate voluntarily shut itself down
seems like something worth avoiding.
The failover protocol will operate correctly while both servers are
unable to communicate, whether they are both running or not. At some
point there may be resource contention, and if one of the servers is
actually down, then the operator can inform the operational server
and the operational server will be able to use all of the failed
server's resources.
The protocol also allows detection of an orderly shutdown of a parti-
cipating server.
3.4. Challenging scenarios for a Failover protocol
There exist two failure scenarios which provide particular challenges
to the correctness guarantees of a failover protocol.
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3.4.1. Primary Server crash before "lazy" update:
In the case where the primary server sends a DHCPACK to a client for
a newly allocated IP address and then crashes prior to sending the
corresponding update to the secondary server, the secondary server
will have no record of the IP address allocation. When the secondary
server takes over, it may well try to allocate that IP address to a
different client. In the case where the first client to receive the
IP address is not on the net at the time (yet while there was still
time to run on its lease), an ICMP echo (i.e., ping) will not prevent
the secondary server from allocating that IP address to a different
client.
The failover protocol deals with this situation by having the primary
and secondary servers allocate addresses for new clients from dis-
joint address pools. See section 5.4 for details.
A more likely (in that DHCPRENEWs are presumably more common than
DHCPDISCOVERs) and more subtle version of this problem is where the
primary server crashes after extending a client's lease time, and
before updating the secondary with a new time using a lazy update.
After the secondary takes over, if the client is not connected to the
network the secondary will believe the client's lease has expired
when, in fact, it has not. In this case as well, the IP address
might be reallocated to a different client while the first client is
still using it.
This scenario is handled by the failover protocol through control of
the lease time and the use of the maximum client lead time (MCLT).
See section 5.2.1 for details.
3.4.2. Network partition where DHCP servers can't communicate but each
can talk to clients:
Several conditions are required for this situation to occur. First,
due to a network failure, the primary and secondary servers cannot
communicate. As well, some of the DHCP clients must be able to com-
municate with the primary server, and some of the clients must now
only be able to communicate with the secondary server. When this
condition occurs, both primary and secondary servers could attempt to
allocate IP addresses for new clients from the same pool of available
addresses. At some point, then, two clients will end up being allo-
cated the same IP address. This will cause problems when the network
failure that created this situation is corrected.
The failover protocol deals with this situation by having the primary
and secondary servers allocate addresses for new clients from dis-
joint address pools. See section 5.4 for details.
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3.5. Using TCP to detect partner server failure
There are several characteristics of TCP that are important to the
functioning of the failover protocol, which uses one TCP connection
for both bulk data transfer as well as to assess communications
integrity with the other server. Reliable and ordered message
delivery are chief among these important characteristics.
It would be nice to use the capabilities built in to TCP to allow it
to determine if communications integrity exists to the failover
partner but this strategy contains some problems which require
analysis. There exist three fundamental cases for an open TCP con-
nection that must be examined.
1. When no data is being sent then no messages are traveling
across the TCP connection.
2. When data is queued to be sent, and the receiver has not
blocked the sending of additional data, then messages are
flowing across the TCP connection containing the applications
data.
3. When data is queued to be sent, and the receiver has blocked
the transmission of additional data, then persist messages are
flowing from the receiver to the sender to ensure that the
sender doesn't miss the receiver opening the window for
further transmissions.
The first case can be turned into the second case by sending
application-level keep-alive messages periodically when there is no
other data queued to be sent. Note TCP keep-alive messages might be
used as well, but they present additional problems.
Thus, we can ensure that the TCP connection has messages flowing
periodically across the connection fairly easily. The question
remains as to what TCP will do if the other end of the connection
fails to respond (either because of network partition or because the
receiving server crashes). TCP will attempt to retransmit a message
with an exponential backoff, and will eventually timeout that
retransmission. However, the length of that timeout cannot, in gen-
eral, be set on a per-connection basis, and is frequently as long as
nine minutes, though in some cases it may be as short as two minutes.
On some systems it can be set system-wide, while on other systems it
cannot be changed at all.
A value for this timeout that would be appropriate for the failover
protocol, say less than 1 minute, could have unpleasant side-effects
on other applications running on the same server, assuming that it
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could be changed at all on the host operating system.
Nine minutes is a long time for the DHCP service to be unavailable to
any new clients that were being served by the server which has
crashed, when there is another server running that could respond to
them as soon as it determines that its partner is not operational.
The conclusion drawn from this analysis is that TCP provides very
useful support for the failover protocol in the areas of reliable and
ordered message delivery, but cannot by itself be relied upon to
detect partner server failure in a fashion acceptable to the needs of
the failover protocol. Additional failover protocol capabilities
have been created to support timely detection of partner server
failure. See section 8.3 for details on this mechanism.
4. Design Goals
This section lists the design goals and the limitations of the fail-
over protocol.
4.1. Design goals for this protocol
The following is a list of goals that are met by this protocol. They
are listed in priority order.
1. Implementations of this protocol must work with existing DHCP
client implementations based on the DHCP protocol [1].
2. Implementations of the protocol must work with existing BOOTP
relay agent implementations.
3. The protocol must provide failover redundancy between servers
that are not located on the same subnet.
4. Provide for continued service to DHCP clients through an
automated mechanism in the event of failure of the primary
server.
5. Avoid binding an IP address to a client while that binding is
currently valid for another client. In other words, do not
allocate the same IP address to two clients.
6. Minimize any need for manual administrative intervention.
7. Introduce no additional delays in server response time as a
result of the network communications required to implement the
failover protocol, i.e., don't require communications with the
partner between the receipt of a DHCPREQUEST and the
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corresponding DHCPACK.
8. Share IP address ranges between primary and secondary servers;
i.e., impose no requirement that the pool of available
addresses be manually or permanently divided between servers.
9. Continue to meet the goals and objectives of this protocol in
the event of server failure or network partition.
10. Provide graceful reintegration of full protocol service after
server failure or network partition.
11. Allow for one computer to act as a secondary server for multi-
ple primary servers. The protocol must allow failover primary
and secondary configuration choices to be made at a granular-
ity smaller than "all of the subnets served by a single
server", though individual implementations may not choose to
allow such flexibility.
12. Ensure that an existing client can keep its existing IP
address binding if it can communicate with either the primary
or secondary DHCP server implementing this protocol - not just
whichever server that originally offered it the binding.
13. Ensure that a new client can get an IP address from some
server. Ensure that in the face of partition, where servers
continue to run but cannot communicate with each other, the
above goals and requirements may be met. In addition, when
the partition condition is removed, allow graceful automatic
re-integration without requiring human intervention.
14. If either primary or secondary server loses all of the infor-
mation that it has stored in stable storage, ensure that it be
able to refresh its stable storage from the other server.
15. Support load balancing between the primary and secondary
servers, and allow configuration of the percentage of the
client population served by each with a moderately fine granu-
larity.
4.2. Limitations of this protocol
The following are explicit limitations of this protocol.
1. This protocol provides only one level of redundancy through a
single secondary server for each primary server.
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2. A subset of the address pool is reserved for secondary server
use. In order to handle the failure case where both servers
are able to communicate with DHCP clients, but unable to com-
municate with each other, a subset of the IP address pool must
be set aside as a private address pool for the secondary
server. The secondary can use these to service newly arrived
DHCP clients during such a period. The required size of this
private pool is based only on the arrival rate of new DHCP
clients and the length of expected downtime, and is not influ-
enced in any way by the total number of DHCP clients supported
by the server pair.
The failover protocol can be used in a mode where both the
primary and secondary servers can share the load between them
when both are operating. In this load balancing mode, the
addresses allocated by the primary server to the secondary
server are not unused, but are used instead to service the
portion of the client base to which the secondary server is
required to respond. See section 5.3 for more information on
load balancing.
3. The primary and secondary servers do not respond to client
requests at all while recovering from a failure that could
have resulted in duplicate IP assignments. (When synchroniz-
ing in POTENTIAL-CONFLICT state).
5. Protocol Overview
This section will discuss the failover protocol at a relatively high
level of detail. In the event that a description in this section
conflicts (or appears to conflict due to the overview nature of this
section) with information in later sections of this draft, the infor-
mation in the later sections should be considered authoritative.
5.1. Messages and States
This protocol is centered around the message exchange used by one
server to update the other server of binding database changes result-
ing from DHCP client activity:
o Communication of binding database changes
The binding update (BNDUPD) message is used to send the binding
database changes to the partner server, and the partner server
responds with a binding acknowledgement (BNDACK) message when it
has successfully committed those changes to its own stable
storage.
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All of the other messages involve ancillary issues:
o Management of available IP addresses
The pool request (POOLREQ) is used by the secondary server to
request an allocation of IP addresses from the primary server.
The pool response (POOLRESP) is used by the primary server to
inform the secondary server how many IP addresses were allocated
to the secondary server as the result of the pool request.
o Synchronization of the binding databases between the servers
after they've been out of communications
The update request (UPDREQ) message is used by one server to
request that its partner send it all binding database informa-
tion that it has not already seen. The update request all
(UPDREQALL) message 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.
The update done (UPDDONE) message is used by the responding
server to indicate that all requested updates have been sent the
responding server and acked by the requesting server.
o Connection establishment
The connect (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 connect acknowledgement message (CONNECTACK) is
used by the secondary server to respond to a CONNECT message
from the primary server. The disconnect (DISCONNECT) message is
used by either server when closing a connection.
o Server synchronization
The state change (STATE) message is used by either server to
inform the other server of a change of failover state.
o Connection integrity management
The contact (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.
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5.1.1. Failover endpoints
The proper operation of the failover protocol requires more than the
transmission of messages between one server and the other. Each end-
point might seem to be a single DHCP server, but in fact there are
many situations where additional flexibility in configuration is use-
ful.
For instance, there might be several servers which are each primary
for a distinct set of address pools, and one server which is secon-
dary for all of those address pools. The situation with the pri-
maries is straightforward, but the secondary will need to maintain a
separate failover state, partner state, and communications up/down
status for each of the separate primary servers for which it is act-
ing as a secondary.
The failover protocol calls for there to be a unique failover end-
point per partner per role (where role is primary or secondary).
This failover endpoint can take actions and hold unique states.
There are thus a maximum of two failover endpoints per partner (one
for the partner as a primary and one for that same partner as a
secondary.)
Thus, in the case where there are two primary servers A and B each
backed up by a single common secondary server C, there is one fail-
over endpoint on each of A and B, and two different failover end-
points on C. The two different failover endpoints on C each have
unique states and independent TCP connections.
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 many failover end-
points may reside in the same process.
It is not the case that there is a unique failover endpoint for each
subnet address pool that participates in a failover relationship. On
one server, there is one failover endpoint per partner per role,
regardless of how many subnet address pools are managed by that com-
bination of partner and role. Conversely, on a particular server,
any given subnet address pool 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 and the port to which the connection is
directed by the partner. See section 8.2.
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5.2. Fundamental guarantees
There a several fundamental restrictions this protocol places on what
one server can do in the absence of knowledge of the other server.
Operating within these restrictions allows certain guarantees to be
made to the partner server, and these are key to the correct opera-
tion of the protocol.
5.2.1. Control of lease time
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 IP address.
In order to handle this problem, a period of time known as the "Max-
imum 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 DHCP client by a server and the lease time known
by that server's partner. However, 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 PARTNER-DOWN state exists so that a server can be sure that its
partner is, indeed, down. Correct operation while in that state
requires (generally) that the server wait the MCLT after anything
that happened prior to its transition into PARTNER-DOWN state (or,
more accurately, when the other server went down if that is known).
Thus, the server MUST wait the MCLT after the partner server went
down before allocating any of the partner's addresses which were
available for allocation. In the event the partner was not in com-
munication prior to going down, it might have allocated one or more
of its FREE addresses to a DHCP client and been unable to inform the
server entering PARTNER-DOWN prior to going down itself. By waiting
the MCLT after the time the partner went down, the server in
PARTNER-DOWN state ensures that any clients which have a lease on one
of the partner's FREE addresses will either time out or contact the
server in PARTNER-DOWN by the time that period ends.
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In addition, once a server has transitioned to PARTNER-DOWN state, it
MUST NOT reallocate an IP address from one client to another client
until an additional MCLT interval after the lease by the original
client expires. (Actually, until the maximum client lead time after
what it believes to be the lease expiration time of the client.)
Some optimizations exist for this restriction, in that it only
applies to leases that were issued BEFORE entering PARTNER-DOWN. Once
a server has entered PARTNER-DOWN and it leases out an address, it
need not wait this time as long as it has never communicated with the
partner since the lease was given out.
The fundamental relationship on which much of the correctness of this
protocol depends is that the lease expiration time known to a DHCP
client MUST NOT be more than the maximum client lead time greater
than the potential expiration time known to a server's partner.
The remainder of this section makes the above fundamental relation-
ship more explicit.
This protocol requires a DHCP server to deal with several different
lease intervals and places specific restrictions on their relation-
ships. 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 lease times are:
o desired lease interval
The desired lease interval 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 determina-
tion is outside of the scope of this protocol. Typically this is
the result of external configuration of a DHCP server.
o actual lease interval
The actual lease internal is the lease interval that a DHCP
server gives out to a DHCP client in the dhcp-lease-time option
of a DHCPACK packet. It may be shorter than the desired client
lease interval (as explained below).
o potential lease interval
The potential lease interval is the lease expiration interval
the local server tells to its partner in the potential-
expiration-time option of a BNDUPD message.
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o acknowledged potential lease interval
The acknowledged potential lease interval is the potential lease
interval the partner server has most recently acknowledged in
the potential-expiration-time option of a BNDACK message.
The key restriction (and guarantee) that any server makes with
respect to lease intervals is that the actual client lease interval
never exceeds the acknowledged potential lease interval (if any) by
more than a fixed amount. This fixed amount is called the "Maximum
Client Lead Time" (MCLT).
The MCLT MAY be configurable on the primary server, but for correct
server operation it MUST be the same and known to both the primary
and secondary servers. The secondary server determines the MCLT from
the MCLT option sent from the primary server to the secondary server
in the CONNECT message.
A server MUST record in its stable storage both the actual lease
interval and the most recently acknowledged potential lease interval
for each IP address binding. It is assumed that the desired client
lease interval can be determined through techniques outside of the
scope of this protocol. See section 7.1.5 for more details concern-
ing the times that the server MUST record in its stable storage and
the way that they interact with the lease time that may be offered to
a DHCP client.
Again, the fundamental relationship among these times which MUST be
maintained is:
actual lease interval <
( acknowledged potential lease interval + MCLT )
Figure 5.2.1-1 illustrates an initial lease to a client using the
rules discussed in the example which follows it. Note that this is
only one example -- as long as the fundamental relationship is
preserved, the actual times used could be quite different.
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DHCP Primary Secondary
time Client Server Server
| (time in intervals) | (absolute time) |
| | |
| >-DHCPDISCOVER-> | |
| <---DHCPOFFER-< | |
| | |
| >-DHCPREQUEST-> | |
| (selecting) | |
| | |
t | <--------DHCPACK-< | |
| lease-time=MCLT | |
| | >-BNDUPD--> |
| | lease-expiration=t+MCLT
| | potential-expiration=t+(MCLT/2)+X
| | |
| | <-BNDACK-< |
| | potential-expiration=t+(MCLT/2)+X
... ... ...
| | |
t+MCLT/2 | >-DHCPREQUEST-> | |
| (renew) | |
| | |
t1 | <--------DHCPACK-< | |
| lease-time=X | |
| | >-BNDUPD--> |
| | lease-expiration=t1+X
| | potential-expiration=t1+(X/2)+X
| | |
| | <-BNDACK-< |
| | potential-expiration=t1+(X/2)+X
... ... ...
Figure 5.2.1-1: Lazy Update Message Traffic
X = Desired Lease Interval
Assumes renewal interval = lease interval / 2
DISCUSSION:
This protocol mandates only that the above fundamental relation-
ship concerning lease intervals is preserved.
In the interests of clarity, however, let's examine a specific
example. The MCLT in this case is 1 hour. The desired lease
interval is 3 days, and its renewal time is half the lease
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interval.
The rules for this example are:
o What to tell the client:
Take the remainder of the acknowledged potential lease interval.
If this is a new lease, then this value will be zero. If this
remainder plus the MCLT is greater than the desired lease inter-
val, give the client the desired lease interval else give the
client the remainder plus the MCLT.
o What to tell the failover partner server:
Take the renewal interval (typically half of the actual client
lease interval), add to it the desired lease interval, and add
it to the current time to yield the value that goes into the
potential-expiration-time option.
Also tell the failover partner the actual lease interval by
adding it to the current time to yield the value that goes into
the lease-expiration option.
In operation this might work as follows:
When a server makes an offer for a new lease on an IP address to a
DHCP client, it determines the desired lease interval (in this
case, 3 days). It then examines the acknowledged potential lease
interval (which in this case is zero) and determines the remainder
of the time left to run, which is also zero. To this it adds the
MCLT. Since the actual lease interval cannot be allowed to exceed
the remainder of the current acknowledged potential lease interval
plus the MCLT, the offer made to the client is for the remainder
of the current acknowledged potential lease interval (i.e., zero)
plus the MCLT. Thus, the actual lease interval is 1 hour.
Once the server has performed the BNDACK to the DHCP client, it
will update the secondary server with the lease information. How-
ever, the desired potential lease interval will be composed of the
one half of the current actual lease interval added to the desired
lease interval. Thus, the secondary server is updated with a
BNDUPD with a lease interval of 3 days + 1/2 hour specified in the
potential-expiration-time option.
When the primary server receives an ACK to its update of the
secondary server's (partner's) potential lease interval, it
records that as the acknowledged potential lease interval. A
server MUST NOT send a BNDACK in response to a BNDUPD message
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until it is sure that the information in the BNDUPD message
resides in its stable storage. Thus, the primary server in this
case can be sure that the secondary server has recorded the poten-
tial lease interval in its stable storage when the primary server
receives a BNDACK message from the secondary server.
When the DHCP client attempts to renew at T1 (approximately one
half an hour from the start of the lease), the primary server
again determines the desired lease interval, which is still 3
days. It then compares this with the remaining acknowledged
potential lease interval (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 lease interval is
3 days. Adding the MCLT to this yields 3 days plus 1 hour, which
is more than the desired lease interval of 3 days. So the client
is renewed for the desired lease interval -- 3 days.
When the primary DHCP server updates the secondary DHCP server
after the DHCP client's renewal ACK is complete, it will calculate
the desired potential lease interval as the T1 fraction of the
actual client lease interval (1/2 of 3 days this time = 1.5 days).
To this it will add the desired client lease interval of 3 days,
yielding a total desired partner server lease interval 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
lease interval so as to be able to always offer the client the
desired client lease interval.
Once the initial actual client lease interval of the MCLT is past,
the protocol operates effectively like the DHCP protocol does
today in its behavior concerning lease intervals. However, the
guarantee that the actual client lease interval will never exceed
the remaining acknowledged partner server lease interval by more
than the MCLT allows full recovery from a variety of failures.
5.2.2. Controlled re-allocation of IP addresses
When in PARTNER-DOWN state there is a waiting period after which an
IP address can be re-allocated to another client. For leases which
are available when the server enters PARTNER-DOWN state, the period
is the MCLT from entry into PARTNER-DOWN state. For IP addresses
which are not available when the server enters PARTNER-DOWN state,
the period is the MCLT after the lease becomes available. See sec-
tion 9.4.2 for more details.
In any other state, a server cannot reallocate an address from one
client to another without first notifying its partner (through a
BNDUPD message) and receiving acknowledgement (through a BNDACK
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message) that its partner is aware that that first client is not
using the address.
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" IP address on a server may be allocated to any client.
An IP address 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 IP address was EXPIRED or RELEASED through a BNDUPD. When
the sending server received the BNDACK for that IP address showing it
was FREE, it would move the IP address from EXPIRED or RELEASED to
FREE, and it would be available for allocation by the primary server
to any clients.
A server MAY reallocate an IP address 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.
5.3. Load balancing
In order to implement load balancing between a primary and secondary
server pair, each server must respond to DHCPDISCOVER requests from
some clients and not from other clients. In order to do this suc-
cessfully, each server must be able to determine immediately upon
receipt of a DHCP client request whether it is to service this
request or to ignore it in order to allow the other server to service
the request.
In addition, it should be possible to configure the percentage of
clients which will be serviced by either the primary or secondary
server. This configuration should be more or less continuous, from
all clients serviced by the primary through an even split with half
serviced by each, to all clients serviced by the secondary.
The technique chosen to support these goals is described in [LOADB].
A bitmap-style Hash Bucket Assignment (as described in [LOADB]) is
used to determine which DHCP clients can be processed. There are two
potential HBA's in a failover server -- a server HBA and a failover
HBA. The way that a server acquires a server HBA is outside of the
scope of the failover protocol, but both servers in a failover pair
MUST have the same server HBA. The failover HBA is sent by the
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primary server to the secondary server whenever a connection is esta-
blished, using the hash-bucket-assignment option defined in section
12.11.
When using the server HBA (if any) and the failover HBA (if any), to
decide whether to process a DHCP request, the server HBA always
applies in every failover state, and the failover HBA (which MUST be
a subset of the server HBA) is used by the secondary server to decide
which packets to process when in NORMAL state.
5.4. Operating in NORMAL state
When in NORMAL state, each server services DHCPDISCOVER's and all
other DHCP requests other than DHCPREQUEST/RENEWAL or
DHCPREQUEST/REBINDING from the client set defined by the load balanc-
ing algorithm [LOADB]. Each server services DHCPREQUEST/RENEWAL or
DHCPDISCOVER/REBINDING requests from any client.
In general, whenever the binding database is changed in stable
storage (other than a change resulting from receiving a BNDUPD from
the failover partner), then a BNDUPD message is sent with the con-
tents of that change to the partner server. The partner server then
writes the information about that binding in its bindings database in
stable storage and replies with a BNDACK message.
The binding database in a DHCP server would normally be changed as a
result of DHCP protocol activity with a DHCP client (e.g., granting
a lease to a DHCP client through the familiar
DISCOVER/OFFER/REQUEST/ACK cycle or extending a lease due to a
renewal from a DHCP client) or possibly (on some servers) because a
lease has expired or undergone another state change that must be
recorded in the DHCP binding database. These are the state changes
that would be communicated to the partner server using a BNDUPD mes-
sage. Of course, receipt of a BNDUPD message itself will normally
cause an update of the binding database for all of the IP addresses
contained in the BNDUPD, and a binding database change such as this
MUST NOT trigger a corresponding BNDUPD message to the partner.
5.5. Operating in COMMUNICATIONS-INTERRUPTED state
When operating in COMMUNICATIONS-INTERRUPTED state, each server is
operating independently, but does not assume that its partner is not
operating. The partner server might be operating and simply unable
to communicate with this server, or might not be operating.
Each server responds to the full range of DHCP client messages that
it receives, but in such a way that graceful reintegration is always
possible when its partner comes back into contact with it.
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5.6. Operating in PARTNER-DOWN state
When operating in PARTNER-DOWN state, a server assumes that its
partner is not currently operating, but does make allowances for the
possibility that that server was operating in the past, though possi-
bly out of communications with this server. It responds to all DHCP
client requests in PARTNER-DOWN state.
5.7. Operating in RECOVER state
A server operating in RECOVER state assumes that it is reintegrating
with a server that has been operating in PARTNER-DOWN state, and that
it needs to update its bindings database before it services DHCP
client requests.
A server may also operate in RECOVER state in order to fully recover
its bindings database from its partner server.
5.8. Operating in STARTUP state
A server operating in STARTUP state assumes that failover is opera-
tional, and it spends a short time whenever it comes up attempting to
contact the partner. During this time (generally a few seconds), the
server is unresponsive to DHCP client requests. This period exists
in order to give a server a chance to determine that its partner has
changed state since it was last in communications, and to react to
that changed state (if any) prior to responding to DHCP client
requests.
The period of time a server remains in STARTUP state SHOULD be long
enough to ensure that it will connect to the other server if that
server is available for connections.
5.9. Time synchronization between servers
The failover protocol is designed to operate between two servers
which have time values which differ by an arbitrarily large amount.
A particular implementation MAY choose to only support servers whose
time values differ by an arbitrarily small amount.
In any event, whether large or only small differences in time values
are supported, every message that is received MUST be tagged with a
time value as soon as possible after receipt. This time value is
used along with the time value that is sent in every message between
the failover partners to develop a delta time between the servers.
This delta time is used during the connection process to establish a
baseline delta time between the servers, and upon receipt of each
message, the delta time for that message is used to refine the delta
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time for the server pair.
While the algorithm for this refinement of delta time is not speci-
fied as part of this protocol, a server SHOULD allow the delta time
value for a pair of failover servers to be periodically updated to
account for time drift. In addition, the delta time value between
servers SHOULD be smoothed in some fashion, so that transient network
delays will not cause it to vary wildly.
A server SHOULD recognize a drastic change in the delta time value as
an event to be signaled to a network administrator, as well as reset-
ting the time delta between the failover partners.
The specific definitions of a minor or drastic change in delta time
as well as the algorithm used to smooth minor changes into the run-
ning delta time are implementation issues and are not further
addresses in this document.
5.10. IP address binding-status
In most DHCP servers an IP address can take on several different
binding-status values, sometimes also called states. While no two
DHCP servers probably have exactly the same possible binding-status
values, the DHCP RFC 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
binding-status-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. It is a goal of
this protocol that any DHCP server can map the various IP address
binding-status values that it uses internally into these failover IP
address binding-status values on transmission of binding database
updates to its partner, and likewise that it can map any failover IP
address binding-status values it received in a binding update into
its internal IP address binding-status values.
The IP address binding-status values defined for the failover proto-
col are listed below. Unless otherwise noted below, there MAY be
client information associated with each of these binding-status
values.
o
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o ACTIVE -- Lease is assigned to a client. Client identification
MUST appear.
o EXPIRED -- indicates that a client's binding on an IP address
has expired. When the partner server ACK's the BNDUPD of an
EXPIRED IP address, the server sets its internal state to FREE.
It is then available for allocation to any client of the primary
server. It may be allocated to the same client on the server
where the lease expired if a BNDUPD containing the EXPIRED state
has not yet been sent to the partner (e.g., in the event that
the servers are not in communication). Client identification
SHOULD appear.
o RELEASED -- indicates that a DHCP client sent in a DHCPRELEASE
message. When the partner server ACK's the BNDUPD of an
RELEASED IP address, the server sets its internal state to FREE,
and it is available for allocation by the primary server to any
DHCP client. It may be allocated to the same client if a BNDUPD
has not yet been sent to the partner. Client identification
SHOULD appear.
o FREE -- is used when a DHCP server needs to communicate that an
IP address is unused by any DHCP client, but it was not just
released, expired, or reset by a network administrator. When
the partner server ACK's the BNDUPD of a FREE IP address, the
server sets its internal state such that it is available for
allocation by the primary DHCP server to any DHCP client. (Note
that in PARTNER-DOWN state, after waiting the MCLT, the IP
address MAY be allocated to a DHCP client by the secondary
server.)
Note that when an IP address that was allocated by the secondary
reverts to the FREE state, it must (like any other IP address)
be assigned to the secondary through the POOLREQ/BNDUPD process
before the secondary can reallocate it.
Client identification MAY appear.
o ABANDONED -- indicates that an IP address is considered unusable
by the DHCP subsystem. An IP address for which a valid PING
response was received SHOULD be set to ABANDONED. An IP address
for which a DHCPDECLINE was received should be set to ABANDONED.
Client identification MUST NOT appear.
o RESET -- indicates that this IP address was made available by
operator command. This is a distinct state so that the reason
that the IP address became FREE can be determined. Client iden-
tification MAY appear.
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o BACKUP -- indicates that this IP address can be allocated by the
secondary server to a DHCP client at any time. When the MCLT has
passed after its time of entry into PARTNER-DOWN state, the IP
address may be allocated by the primary to any DHCP client.
Client identification MAY appear.
These binding-status values are communicated from one failover
partner to another using the binding-status option, see section 12.3
for details of this option. Unless otherwise noted above there MAY
be client information associated with each of these binding-status
values.
An IP address will move between these binding-status values using the
following state transition diagram:
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DHCP client DECLINE or
server detected problem
from any state
+----------+ V +---------+
External >---->| RESET | | |ABANDONED|
command | | +-->| |
+----------+ +---------+
|
Comm w/Parter(1)
V
+---------+ Comm(1) +----------+ Comm(1) +---------+
| EXPIRED |--------->| FREE |<----------| RELEASED|
| | w/Parter | | w/Partner | |
+---------+ +----------+ +---------+
^ ^ | | +-----------+ ^
| | | | | |
| Exp. grace IP | IP addr alloc. IP addr |
| period ends address to sec.(2) reserved |
| | leasedy V V |
| | by | +----------+ +---------+ |
| | primary| BACKUP | | BACKUP- | |
| wait for | | | | RESERVED| |
| grace period | +----------+ +---------+ |
| | | | |
| | | IP addr leased by |
| Expired grace | secondary |
| period exists V V |
| | +----------+ |
| | Lease on | ACTIVE | DHCPRELEASE |
+-----+-IP addr---| |------------------+
expires +----------+
Figure 5.10-1: Transitions between binding-status values.
(1) This transition MAY also occur if the server is in
PARTNER-DOWN state and the MCLT has passed since the entry
in the RELEASED, EXPIRED, or RESET states.
(2) This transition MAY occur if the server is the secondary
and the MCLT has passed since its entry into PARTNER-DOWN state.
Again, note that a DHCP server implementing the failover protocol
does not have to implement either this state machine or use these
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particular binding-status values in its normal operation of allocat-
ing IP addresses to DHCP clients. It only needs to map its internal
binding-status-values onto these "standard" binding-status values,
and map these "standard" binding-status values back into its internal
binding-status values. For example, a server which implements a
grace period for a IP address binding SHOULD simply wait to update
its partner server until the grace period on that binding has run
out.
The process of setting an IP address to FREE deserves some detailed
discussion. When an IP address is moved to the EXPIRED,RELEASED, or
RESET binding-status on a server, it will send a BNDUPD with the
binding-status of EXPIRED, RELEASED, or RESET to its partner. If its
partner agrees that is acceptable (see sections 7.1.2 and 7.1.3 con-
cerning why a server might not accept a BNDUPD) it will return a
BNDACK with no reject-reason, signifying that it accepted the update.
As part of the BNDUPD processing, the server returning the BNDACK
will set the binding-status of the IP address to FREE, and upon
receipt of the BNDACK the server which sent the BNDUPD will set the
binding-status of the IP address to FREE. Thus, the EXPIRED,
RELEASED, or RESET binding-status is something of a transitory state.
This process is encoded in the transition diagram above by "Comm
w/Partner".
5.11. DNS dynamic update considerations
DHCP servers (and clients) can use DNS Dynamic Updates as described
in [RFC 2136] to maintain DNS name-mappings as they maintain DHCP
leases. Many different administrative models for DHCP-DNS integra-
tion 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 [DDNS]. The nature of the DHCP failover
protocol introduces some issues concerning dynamic DNS updates that
are not part of non-failover DHCP environments. This section
describes these issues, and defines the information which failover
partners should exchange and the protocol which they should follow in
order to ensure consistent behavior. The presence of this section
should not be interpreted as requiring that implementations of the
DHCP failover protocol must also support DDNS updates. The purpose
of this discussion is to clarify the areas where the DHCP failover
and DHCP-DDNS protocols intersect for the benefit of implementations
which support both protocols, not to introduce a new requirement into
the DHCP failover protocol. Thus, a DHCP 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.
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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.
5.11.1. Relationship between failover and dynamic DNS update
The failover protocol describes the conditions under which each fail-
over 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 fail-
over 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 assignments; allowing clients to continue using leases which
they obtained from one failover partner even if they can only commun-
icate with the other partner; allowing the backup 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:
1. Allow timely DDNS updates from the server which grants a
client a lease. 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 lease is sub-
sequently 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 fail-
over servers are attempting to perform DDNS updates for the
same lease-client binding. 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.
5.11.2. Use of the DDNS option
In order for either server to be able to complete a DDNS update, or
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to remove DNS records which were added by its partner, both servers
need to know the FQDN associated with the lease-client binding. The
FQDN associated with the client's A RR and PTR RR SHOULD be communi-
cated from the server which adds records into the DNS to its partner.
The initiating server SHOULD use the DDNS option in the BNDUPD mes-
sages to inform the partner server of the status of any DDNS updates
associated with a lease binding. Failover servers MAY choose not to
include the DDNS option in BNDUPD messages if there has been no
change in the status of any DDNS update related to the lease binding.
The partner server receiving BNDUPD messages containing the DDNS
option SHOULD compare the status flags and the FQDN contained in the
option data with the current DDNS information it has associated with
the lease binding, and update its notion of the DDNS status accord-
ingly.
The initiating server MAY send a BNDUPD to its partner before the
DDNS update has been successfully completed. If it does so, it SHOULD
leave the 'C' bit in the Flags field clear, to indicate to the
partner that the DDNS update may not be complete. When the DDNS
update has been successfully acknowledged by the DNS server, the ini-
tiating DHCP server SHOULD include the DDNS option in its next BNDUPD
message about the binding, so that the partner server will be able to
record the final status of the DDNS update. The initiating server
SHOULD set the 'C' bit in the DDNS option if the DDNS update was suc-
cessfully accepted by the DNS server.
Some implementations will 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 Domain Name field in the DDNS option contains the FQDN that will
be associated with the A RR (if the server is performing an A RR
update for the client) and the PTR RR. This FQDN may be composed in
any of several ways, depending on server configuration and the infor-
mation 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 config-
ured to attempt to use the information the client supplies, it may be
configured with an FQDN to use for the client, or it may be config-
ured to synthesize an FQDN. The responsive server SHOULD include the
FQDN that it will be using in DDNS updates it initiates when it sends
the DDNS option.
Since the responsive server may not have completed the DDNS update at
the time it sends the first BNDUPD about the lease binding, there may
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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"). Alter-
natively, at sites which use some or all of the information which
clients supply to form the FQDN, it's possible that a client's confi-
guration may be changed so that it begins to supply new data. The
responsive server may react by removing the DNS records which it ori-
ginally added for the client, and replacing them with records that
refer to the client's new FQDN. In such cases, the responsive server
SHOULD include the actual FQDN that was used in subsequent DDNS
options. The responsive server SHOULD include relevant client-option
data in the client-request-options option 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.
5.11.3. Adding RRs to the DNS
A failover server which is going to perform DDNS updates SHOULD ini-
tiate the DDNS update when it grants a new lease to a client. The
non-responsive partner 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 [DDNS].
The server may be configured to perform an A 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. Since both
servers are responsive to renewals in NORMAL state, it is not
enough to simply require the non-responsive server to avoid a
DNS update in this case. The server which would be responsive
to a DHCPDISCOVER from this client (even though the current
request is a DHCPREQUEST/RENEW) is the server which should
initiate the DDNS update.
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
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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.
5.11.4. Deleting RRs from the DNS
The failover server which makes an IP address 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 an IP address FREE" when it
initiates a BNDUPD with a binding-status of FREE, EXPIRED, or
RELEASED. Its partner confirms this status by acking that BNDUPD,
and upon receipt of the ACK the server has "made the IP address
FREE". Conversely, a server in PARTNER-DOWN state "makes an IP
address FREE" when it sets the binding-status to FREE, since in
PARTNER-DOWN state not 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 send-
ing 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 NAKed these BNDUPD messages.
The failover protocol ensures that only one of the two partner
servers will be able to make a lease FREE. The server making the
lease 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 IP addresses are moved into the FREE state during
that period.
5.12. Reservations and failover
Some DHCP servers support a capability to offer specific pre-
configured IP addresses 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 IP address -- and they
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offer that IP address to no other clients. Such a capability has
several names, but it is sometimes called a "reservation", in that
the IP address is reserved for a particular DHCP client.
In a situation where there are two DHCP servers serving the same sub-
net without using failover, the two DHCP server's need to have dis-
joint IP address 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 an IP address 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 pro-
tocol.
The general solution with regards to reservations is as follows.
Whenever a reserved IP address becomes FREE (i.e., when first config-
ured or whenever a client frees it or it expires or is reset), the
primary server MUST show that IP address as FREE (and thus available
for its own allocation) and it MUST send it to the secondary server
as BACKUP-RESERVED, in order that the secondary server be able to
allocate it as well.
Note that this implies that a reserved IP address goes through the
normal state changes from FREE to ACTIVE (and possibly back to FREE).
The failover protcol supports this approach to reservations, i.e.,
where the IP address undergoes the normal state changes of any IP
address, but it can only be offered to the client for which it is
reserved. Other approaches to the support of reservations exist in
some DHCP server implementations (e.g., where the IP address is
apparently leased to a particular client forever, without any expira-
tion). The goal is for the failover protocol to support any of the
usual approaches to reservations, both those that allow an IP address
to go through different states when reserved, and those that don't.
From the above, it follows that a reservation soley on the secondary
will not necessarily allow the secondary to offer that address to
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 IP
address to the client to which is is reserved.
When the reservation on an IP address is cancelled, if the IP address
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.
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5.13. Dynamic BOOTP and failover
Some DHCP servers support a capability to offer IP addresses to BOOTP
clients without having a particular address previously allocated for
those clients. This capability is often called something like
"dynamic BOOTP". It is discussed briefly in RFC 1534 [RFC 1534].
This capability has a negative interaction with the fundamental ele-
ments of the failover protocol, in that an address handed out to a
BOOTP device has no term (or effectively no term, in that usually
they are considered leases for "forever"). There is no opportunity
to hand out a lease which is only the MCLT long when first hearing
from a BOOTP device, because they may only interact once with the
DHCP server and they have no notion of a lease expiration time. Thus
the entire concept of the MCLT and waiting the MCLT after entering
PARTNER-DOWN state is defeated when dealing with BOOTP devices.
With some restrictions, however, dynamic BOOTP devices can be sup-
ported in a server on a subnet where failover is supported. The only
restriction (and it is not small) is that on any portion of the sub-
net (in any address pool) where dynamic BOOTP devices can be allo-
cated IP addresses, a DHCP server MUST NOT ever use any of the IP
addresses which were previously available for allocation by its fail-
over partner. Thus, the addresses allocated by the primary to the
secondary for allocation that might have been allocated to BOOTP dev-
ices MUST NOT ever be used by the primary server even if it is in
PARTNER-DOWN state and has waited the MCLT after entering that state.
Conversely, addresses available for allocation by the primary MUST
NOT be used by the secondary even it is in PARTNER-DOWN state. The
reason for this is because one of those IP address could have been
allocated by the secondary server to a BOOTP device, and the primary
server would have no way of ever knowing that happened.
5.14. Guidelines for selecting MCLT
There is no one correct value for the MCLT. There is an explicit
tradeoff between various factors in selecting an MCLT value.
5.14.1. Short MCLT
A short MCLT value will mean that after entering PARTNER-DOWN state,
a server will only have to wait a short time before it can start
allocating its partner's IP addresses to DHCP clients. Furthermore,
it will only have to wait a short time after the expiration of a
lease on an IP address before it can reallocate that IP address to
another DHCP client.
However the downside of a short MCLT value is that the initial lease
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interval that will be offered to every new DHCP client will be short,
which will cause increased traffic as those clients will need to send
in their first renew in a half of a short MCLT time. In addition,
the lease extensions that a server in COMMUNICATIONS-INTERRUPTED
state can give will be only the MCLT after the server has been in
COMMUNICATIONS-INTERRUPTED for around the desired client lease
period. If a server stays in COMMUNICATIONS-INTERRUPTED for that
long, then the leases it hands out will be short and that will
increase the load on that server, possibly causing difficulty.
5.14.2. Long MCLT
A long MCLT value will mean that the initial lease period will be
longer and the time that a server in COMMUNICATIONS-INTERRUPTED state
will be able to extend leases (after it has been in COMMUNICATIONS-
INTERRUPTED state for around the desired client lease period) will be
longer.
However, a server entering PARTNER-DOWN state will have to wait the
longer MCLT before being able to allocate its partner's IP addresses
to new DHCP clients. This may mean that additional IP addresses are
required in order to cover this time period. Further, the server in
PARTNER-DOWN will have to wait the longer MCLT from every lease
expiration before it can reallocate an IP address to a different DHCP
client.
6. Common Message Format
This section discusses the common message format that all failover
messages have in common, including the message header format as well
as the common option format. See section 12 for the the definitions
of the specific options used in the failover protocol.
6.1. Message header format
The options contained in the payload data section of the failover
message all use a two byte option number and two byte length format.
All failover protocol messages are sent over the TCP connection
between failover endpoints and encoded using a message format
specific to the failover protocol.
There exists a common message format for all failover messages, which
utilizes the options in a way similar to the DHCP protocol. For each
message type, some options are required and some are optional. In
addition, when a message is received any options that are not under-
stood by the receiving server MUST be ignored.
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All of the fields in the fixed portion of the message MUST be filled
with correct data in every message sent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length (2) | msg type (1) |payload off (1)|
+---------------+---------------+---------------+---------------+
| time (4) |
+---------------------------------------------------------------+
| xid (4) |
+---------------------------------------------------------------+
| 0 or more additional header bytes (variable) |
+---------------------------------------------------------------+
| payload data (variable) |
| |
| formatted as DHCP-style options |
| using a two byte option code and two byte length |
| See section 6.2 for details. |
+---------------------------------------------------------------+
message length - 2 bytes, network byte order
This is the length of the message. It includes the two byte message
length itself. The maximum length is 2048 bytes. The minimum length
is 12.
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msg type - 1 byte
The message type field is used to distinguish between messages.
The following message types are defined:
Value Message Type
----- ------------
0 reserved not used
1 POOLREQ request allocation of addresses
2 POOLRESP respond with allocation count
3 BNDUPD update partner with binding info
4 BNDACK acknowledge receipt of binding update
5 CONNECT establish connection with the secondary
6 CONNECTACK respond to attempt to establish connection with partner
7 UPDREQALL request full transfer of binding info
8 UPDDONE ack send and ack of req'd binding info
9 UPDREQ req transfer of un-acked binding info
10 STATE inform partner of current state or state change
11 CONTACT probe communications integrity with partner
12 DISCONNECT close a connection
New message types should be defined in one of two ranges, 0-127 or
129-255. The range of 0-127 is used for messages that MUST be sup-
ported by every server, and if a server receives a message in the
range of 0-127 that it doesn't understand, it MUST close the TCP con-
nection. The range of 128-255 is used for messages which MAY be sup-
ported but are not required, and if a server receives a message in
this range that it does not understand it SHOULD ignore the message.
payload offset - 1 byte
The byte offset of the Payload Data, from the beginning of the
failover message header. The value for the current protocol version
(version 1) is 8.
time - 4 bytes, network byte order
The absolute time in GMT when the message was transmitted,
represented as seconds elapsed since Jan 1, 1970 (i.e., similar to
the ANSI C time_t time value representation). While the ANSI C
time_t value is signed, the value used in this specification is
unsigned.
A server SHOULD set this time as close to the actual transmission of
the message as possible.
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xid - 4 bytes, network byte order
This is the transaction id of the failover message. The sender of a
failover protocol message is responsible for setting this number, and
the receiver of the message copies the number over into any response
message, treating it as opaque data. The sender MUST ensure that
every message sent from a particular failover endpoint over the
associated TCP connection has a unique transaction id.
For failover messages that have no corresponding response message,
the XID value is meaningless, but MUST be supplied. The XID value is
used solely by the receiver of a response message to determine the
corresponding request message.
Requests messages where the XID is used in the corresponding response
messages are: POOLREQ, BNDUPD, CONNECT, UPDREQALL, and UPDREQ. The
corresponding response messages are POOLRESP, BNDACK, CONNECTACK,
UPDDONE, and UPDDONE, respectively.
As requests/responses don't survive connection reestablishment, XIDs
only need to be unique during a specific connection.
payload data - variable length
The options are placed after the header, after skipping payload
offset bytes from beginning of the message. The payload data options
are not preceded by a "cookie" value.
The payload data is formatted as DHCP style options using two byte
option codes and two byte option lengths. The option codes are in a
namespace which is unique to the failover protocol.
The maximum length of the payload data in octets is 2048 less the
size of the header, i.e., the maximum message length is 2048 octets.
6.2. Common option format
The options contained in the payload data section of the failover
message all use a two byte option number and two byte length format.
The option numbers are drawn from an option number space unique to
the failover protocol. All of the message types share a common
option number space and common options definitions, though not all
options are required or meaningful for every message.
In contrast to the options which appear in DHCP client and server
messages, the options in failover message are ordered. That is, for
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some messages the order in which the options appear in the payload
data area is significant. The messages for which option ordering is
significant explicitly describe the ordering requirements. If no
ordering requirements are mentioned, then the order is not signifi-
cant for that message.
For all options which refer to time, they all use an absolute time in
GMT. Time synchronization has already been achieved between the
source and the target server using the CONNECT message and is updated
and refined using the time in every packet.
The time value is an unsigned 32 bit integer in network byte order
giving the number of seconds since 00:00 UTC, 1st January 1970. This
can be converted to an NTP timestamp by adding decimal 2208988800.
This time format will not wrap until the year 2106. Until sometime
in 2038, it is equal to the ANSI C time_t value (which is a signed 32
bit value and will overflow into a negative number in 2038).
Options should appear once only in each message (except for BNDUPD
and BNDACK messages where bulking is used, see section 6.3 for
details.) An option that appears twice is not concatenated, but
treated as an error.
Specific option values are described in section 12.
See section 13 for how to define additional options.
6.3. Batching multiple binding update transactions in one BNDUPD mes-
sage
Implementations of this protocol MAY send multiple binding update
transactions in one BNDUPD message, where a binding update transac-
tion is defined as the set of options which are associated with the
update of a single IP address. All implementations of this protocol
MUST be prepared to receive BNDUPD messages which contain multiple
binding update transactions and respond correctly to them, including
replying with a BNDACK message which contains status for the multiple
binding update transactions contained in the BNDUPD message.
In the discussion of sending and receiving BNDUPD messages in section
7.1 and BNDACK messages in section 7.2, each BNDUPD message and
BNDACK message is assumed to contain a single binding update transac-
tion in order to reduce the complexity of the discussions in section
7.
Multiple binding update transactions MAY be batched together in one
BNDUPD protocol message with the data sets for the individual tran-
sactions delimited by the assigned-IP-address option, which MUST
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appear first in the option set for each transaction. Ordering of
options between the assigned-IP-address options is not significant.
This is illustrated in the following schematic representation:
Non-IP Address/Non-client specific options first
assigned-IP-address option for the first IP address
Options pertaining to first address, including
at least the binding-status option and others as
required.
assigned-IP-address option for the second IP address
Options pertaining to second address, including
at least the binding-status option and others as
required.
...
Trailing options (message digest).
There MUST be a one-to-one correspondence between BNDUPD and BNDACK
messages, and every BNDACK message MUST contain status for all of the
binding update transactions in the corresponding BNDUPD message.
The BNDACK message corresponding to a BNDUPD message MUST contain
assigned-IP-address options for all of the binding update transac-
tions in the BNDUPD message. Thus, every BNDACK message contains
exactly the same assigned-IP-address options as does its correspond-
ing BNDUPD message. The order of the assigned-IP-address options
MAY, however, be different. Here is a schematic representation of a
BNDACK:
Non-IP Address/Non-client specific options first
assigned-IP-address option for the first IP address
If rejected, reject-reason option and message option.
assigned-IP-address option for the second IP address
If rejected, reject-reason option and message option.
...
Trailing options (message digest).
In case the server chooses to reject some or all of the IP address
binding information in a BNDUPD message in a BNDACK reply, the BNDACK
message MUST contain a reject-reason option following every
assigned-IP-address option in order to indicate that the binding
update transaction for that IP address was not accepted and why. As
with a BNDACK message containing a single binding update transaction,
an assigned-IP-address option without any associated reject-reason
option indicates a successful binding update transaction.
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7. Protocol Messages
This section contains the detailed definition of the protocol mes-
sages, including the information to include when sending the message,
as well as the actions to take upon receiving the message. The mes-
sage type for each message appears as [n] in the heading for the mes-
sage (see section 6.1).
7.1. BNDUPD message [3]
The binding update (BNDUPD) message is used to send the binding data-
base changes (known as binding update transactions) to the partner
server, and the partner server responds with a binding acknowledge-
ment (BNDACK) message when it has successfully committed those
changes to its own stable storage.
The rest of the failover protocol exists to determine whether the
partner server is able to communicate or not, and to enable the
partners to exchange BNDUPD/BNDACK messages in order to keep their
binding databases in stable storage synchronized.
The rest of this section is written as though every BNDUPD message
contains only a single binding update transaction in order to reduce
the complexity of the discussion. See section 6.3 for information on
how to create and process BNDUPD and BNDACK messages which contain
multiple binding update transactions. 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.
The following table summarizes the various options for the BNDUPD
message.
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binding-status BACKUP
RESET
ABANDONED
Option ACTIVE EXPIRED RELEASED FREE
------ ------ ------- -------- ----
assigned-IP-address (3) MUST MUST MUST MUST
binding-status MUST MUST MUST MUST
client-identifier MAY MAY MAY MAY(2)
client-hardware-address MUST MUST MUST MAY(2)
lease-expiration-time MUST MUST NOT MUST NOT MUST NOT
potential-expiration-time MUST MUST NOT MUST NOT MUST NOT
start-time-of-state SHOULD SHOULD SHOULD SHOULD
client-last-trans.-time MUST SHOULD MUST MAY
DDNS(1) SHOULD SHOULD SHOULD SHOULD
client-request-options SHOULD SHOULD NOT SHOULD SHOULD NOT
client-reply-options SHOULD SHOULD NOT SHOULD NOT SHOULD NOT
(1) MUST if server is performing dynamic DNS for this IP address, else
MUST NOT.
(2) MUST NOT if binding-status is ABANDONED.
(3) assigned-IP-address MUST be the first option for an IP address
Table 7.1-1: Options used in a BNDUPD message
7.1.1. Sending the BNDUPD message
A BNDUPD message SHOULD be generated whenever any binding changes. A
change might be in the binding-status, the lease-expiration-time, or
even just the last-transaction-time. In general, any time a DHCP
server writes its stable storage, a BNDUPD message SHOULD be gen-
erated. This will often be the result of the processing of a DHCP
client request, but it might also be the result of a successful
dynamic DNS update operation.
BNDUPD (and BNDACK) messages refer to the binding-status of the IP
address, and this protocol defines a series of binding-statuses, dis-
cussed in more detail below. Some servers may not support all of
these binding-statuses, and so in those cases they will not be sent.
Upon receipt of a BNDUPD message which contains an unsupported
binding-status, a reasonable interpretation should be made (see sec-
tion 5.10).
All BNDUPD messages MUST contain the IP address of the binding update
transaction in the assigned-IP-address option.
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All binding update transactions contain a binding-status option, and
it will have one of the values found in section 5.10. Client infor-
mation consists of client-hardware-address and possibly a client-
identifier, and is explained in more detail later in this section.
The following table indicates whether client information should or
should not appear with each binding-status in a binding update tran-
saction:
binding-status includes client information
------------------------------------------------
ACTIVE MUST
EXPIRED SHOULD
RELEASED SHOULD
FREE MAY
ABANDONED MUST NOT
RESET MAY
BACKUP MAY
Table 7.1.1-1: Client information required by various
binding-status values.
The ACTIVE binding-status requires some options to indicate the
length of the binding:
o lease-expiration-time
The lease-expiration-time option MUST appear, and be set to the
expiration time most recently ACKed to the DHCP client. Note
that the time ACKed to a DHCP client is a lease duration in
seconds, while the lease-expiration-time option in a BNDUPD mes-
sage is an absolute time value.
o potential-expiration-time
The potential-expiration-time option MUST appear, and be set to
a value beyond that of the lease-expiration time. This is the
value that is ACKed by the BNDACK message. A server sending a
BNDUPD message MUST be able to recover the potential-
expiration-time sent in every BNDUPD, not just those that
receive a corresponding BNDACK, in order to be able to protect
against possible duplicate allocation of IP addresses after
transitioning to PARTNER-DOWN state. See section 5.2.1 for
details as to why the potential-expiration-time exists and
guidelines for how to decide on the value.
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The following option information applies to all BNDUPD messages,
regardless of the value of the binding-status, unless otherwise
noted.
o Identifying the client
For many of the binding-status values a client MUST appear while
for others a client MAY appear, and for some a client MUST NOT
appear.
A client is identified in a BNDUPD message by at least one and pos-
sibly two options. The client-hardware-address option MUST appear
any time that a client appears in a BNDUPD message, and contains
the hardware type and chaddr information from the DHCP request
packet. A failover client-identifier option MUST appear any time
that a client appears in a BNDUPD message if and only if that
client used a DHCP client-identifier option when communicating with
the DHCP server. See section 12.5 and 12.4 for details of how to
construct these two options from a DHCP request packet.
o start-time-of-state
The start-time-of-state SHOULD appear. It is set to the time at
which this IP address first took on the state that corresponds to
the current value of binding-status.
o last-transaction-time
The last-transaction-time value SHOULD appear. This is the time at
which this DHCP server last received a packet from the DHCP client
referenced by the client-identifier or client-hardware-address that
was associated with the IP address referenced by the assigned-IP-
address.
o DDNS
If the DHCP server is performing dynamic DNS operations on behalf
of the DHCP client represented by the client-identifier or client-
hardware-address, then it should include a DDNS option containing
the domain name and status of any dynamic DNS operations enabled.
o client-request-options
If the BNDUPD was triggered by a request from a DHCP client (typi-
cally those with binding-status of ACTIVE and RELEASED), then the
server SHOULD include options of interest to a failover partner
from the client's request packet in the client-request-options for
transmission to its partner (see section 12.8).
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A server sending a BNDUPD SHOULD remember the "interesting" options
or the information that would appear in an "interesting" option for
transmission at a time when the BNDUPD is not closely associated
with a DHCP client request.
A server SHOULD send the following "interesting" options. It MAY
send any DHCP client options. As new options are defined, the RFC
defining these options SHOULD include information that they are
"interesting to failover servers" if they should be sent as part of
a BNDUPD.
option option
number name
-----------------------------------------
12 host-name
81 client-FQDN [DDNS]
82 relay-agent-information [AGENTINFO]
TBD user-class [USERCLASS]
60 vendor-class-identifier
Table 7.1.1-2: Options which SHOULD be sent in
the client-request-options option in a BNDUPD message.
o client-reply-options
If the BNDUPD was triggered by a request from a DHCP client (typi-
cally those with binding-status of ACTIVE and RELEASED), then the
server SHOULD include options of interest to a failover partner
from the server's DHCP reply packet in the client-reply-options for
transmission to its partner (see section 12.7).
A server sending a BNDUPD SHOULD remember the "interesting" options
or the information that would appear in an "interesting" option for
transmission at a time when the BNDUPD is not closely associated
with a DHCP client request.
A server SHOULD send the following "interesting" options. It MAY
send any DHCP client options. As new options are defined, the RFC
defining these options SHOULD include information that they are
"interesting to failover servers" if they should be sent as part of
a BNDUPD.
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option option
number name
-----------------------------------------
58 renewal-time
59 rebinding-time
Table 7.1.1-3: Options which SHOULD be sent in
the client-reply-options option in a BNDUPD message.
The BNDUPD message SHOULD be sent as soon as possible from the time
that the DHCP client received a response and the lease bindings data-
base is written on stable storage.
7.1.2. Receiving the BNDUPD message
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 resolu-
tion 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:
o Two clients, one IP address conflict
This is the duplicate IP address allocation conflict. There are
two different clients each allocated the same address. See sec-
tion 7.1.3 for how to resolve this conflict.
o Two IP addresses, one client conflict
This conflict exists when a client on one server is associated
with a one IP address, and on the other server with a different
IP address in the same or a related subnet. This does not refer
to the case where a single client has addresses in multiple dif-
ferent subnets or administrative domains, but rather the case
where on the same subnet the client has as 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. In the event that a DHCP server requires
that a DHCP client have only one outstanding lease for an IP
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address on one subnet, this conflict should be resolved by
accepting the update which has the latest client-last-
transaction-time.
o binding-status conflict
This is normal conflict, where one server is updating the other
with newer information. See section 7.1.3 for details of how to
resolve these conflicts.
7.1.3. Deciding whether to accept the binding update transaction in a
BNDUPD message
IP addresses undergo binding status changes for several reasons,
including receipt and processing of DHCP client requests, administra-
tive inputs and receipt of BNDUPD messages. Every DHCP server needs
to respond to DHCP client requests and administrative inputs with
changes to its internal record of the binding-status of an IP
address, and this response is not in the scope of the failover proto-
col. However, the receipt of BNDUPD messages implies at least a pos-
sible change of the binding-status for an IP address, and must be
discussed here. See section 7.1.2 for general actions to take upon
receipt of a BNDUPD message.
When receiving a BNDUPD message, it is important to note that it may
not be current, in that the server receiving the BNDUPD message may
have had a more recent interaction with the DHCP client than its
partner who sent the BNDUPD message. In this case, the receiving
server MUST reject the BNDUPD message. In addition, it is worth not-
ing that two (and possibly three) binding-status values are the
direct result of interaction with a DHCP client, ACTIVE and RELEASED
(and possibly ABANDONED). All other binding-status values are either
the result of the expiration of a time period or interaction with an
external agency (e.g., a network administrator).
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 IP address), then there will be
no client-last-transaction-time option in the BNDUPD message.
The list in Figure 7.1.3-1 is indexed by the binding-status that a
server receives in a BNDUPD message. In many cases, the binding-
status of an IP address within the receiving server's data storage
will have an affect upon the checks performed prior to accepting the
new binding-status in a BNDUPD message.
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In Figure 7.1.3-1, to "accept" a BNDUPD means to update the server's
bindings database with the information contained in the BNDUPD and
once that update is complete, send a BNDACK message corresponding to
the BNDUPD message. To "reject" a BNDUPD means to respond to the
BNDUPD with a BNDACK with a reject-reason option included.
When interpreting the rules in the following list, 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.
The second rule concerns clients and IP addresses. If the clients in
a BNDUPD message and in a receiving server's binding differ, then if
the receiving server's binding-status is ACTIVE and the binding-
status in the BNDUPD is ACTIVE, then if the receiving server is a
secondary server accept it, else reject it.
binding-status in received BNDUPD
binding-status
in receiving FREE RESET
server ACTIVE EXPIRED RELEASED BACKUP ABANDONED
ACTIVE accept time(2) time(1) time(2) accept
EXPIRED time(1) accept accept accept accept
RELEASED time(1) time(1) accept accept accept
FREE/BACKUP accept accept accept accept accept
RESET time(3) accept accept accept accept
ABANDONED reject reject reject reject accept
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
servers' lease-expiration-time, accept it, else reject it.
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.
Figure 7.1.3-1: Accepting BNDUPD messages
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7.1.4. Accepting the BNDUPD message
When accepting a BNDUPD message, the information contained in the
client-request-options and client-reply-options SHOULD be examined
for any information of interest to this server. For instance, a
server which wished to detect changes in client specified host names
might want to examine and save information from the host-name or
client-FQDN options. Servers which expect to utilize information
from the relay-agent-information option would want to store this
information.
7.1.5. Time values related to the BNDUPD message
There are four time values that MAY be sent in a BNDUPD message.
o lease-expiration-time
The time that the server gave to the client, i.e., the time that
the server believes that the client's lease will expire.
o potential-expiration-time
The time that the server wants to be sure its partner waits
(added to the MCLT) before assuming that this lease has expired.
Typically some time beyond the desired client lease time.
o client-last-transaction-time
The time that the client last interacted with this server.
o start-time-of-state
The time at which the binding first went into the current state.
As discussed in section 5.2, each server knows what its partner has
ACKed with regard to potential-expiration time. In addition, each
server needs to remember what it has told its partner as the
potential-expiration-time. Moreover, each server must remember what
it has acked to the *other* server as the most recent potential-
expiration-time from that server.
Remember that each server sends a potential-expiration-time and
receives an ACK for that as well as receiving a potential-
expiration-time and needing to remember what it has acked for that.
While they don't have to be named in any particular way, the times
that a server needs to remember for every IP address in order to
implement the failover protocol are:
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o lease-expiration-time
The time that a server gave to the DHCP client. A DHCP server
needs to remember this time already, just to be a DHCP server.
A server SHOULD update this time with the lease-expiration time
received from a partner in a BNDUPD if the received lease-
expiration time is later than the lease-expiration time recorded
for this binding.
o sent-potential-expiration-time
The latest time sent to the partner for a potential-expiration-
time.
o acked-potential-expiration-time
The latest time that the partner has acked for a potential
expiration time. Typically the same as sent-potential-
expiration-time if there is not a BNDUPD outstanding.
o received-potential-expiration-time
The latest time that this server has ever received as a
potential-expiration-time from its partner in a BNDUPD that this
server ACKed.
So, a server has to remember two additional times concerning BNDUPD
messages that it has initiated, and one additional time concerning
BNDUPD message that it has received. How are these times used?
First, let's look at the time that a DHCP server can offer to a DHCP
client. A server can offer to a DHCP client a time that is no longer
than the MCLT beyond the max( received-potential-expiration-time,
acked-potential-expiration-time). One might think that the server
should be able to offer only the MCLT beyond the acked-potential-
expiration-time, and while that is certainly simple and easy to
understand, it has negative consequences in actual operation.
To illustrate this, in the simple case where the primary updates the
secondary for a while and then fails, if the secondary can then renew
the client for only the MCLT beyond the acked-potential-expiration-
time, then the secondary will only be able to renew the client for
the MCLT, because the secondary has never sent a BNDUPD packet to the
primary concerning this IP address and client, and so its acked-
potential-expiration-time is zero.
However, since the secondary is allowed to renew the client with the
MCLT beyond the max( received-potential-expiration-time, acked-
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potential-expiration-time), then the secondary can usually renew the
client for the full lease period, at least for the first renew it
sees from the client, since the received-potential-expiration-time is
generally longer than the client's desired lease interval. The
difference in renew times could make a big difference in server load
on the secondary in this case.
What are the consequences of allowing a server to offer a DHCP client
a lease term of the MCLT beyond the max( received-potential-
expiration-time, acked-potential-expiration-time)? The consequences
appear whenever a server enters PARTNER-DOWN state, and affect how
long that server has to wait before reallocating expired leases.
With this approach, when a server goes into PARTNER-DOWN state, it
must wait the MCLT beyond the max( lease-expiration-time, sent-
potential-expiration-time, acked-potential-expiration-time,
received-potential-expiration-time ) for each IP address before it
can reallocate that IP address to another DHCP client. One might
normally think that it needed to wait only the MCLT beyond the max(
lease-expiration-time, received-potential-expiration-time ), i.e.,
beyond what it has told the client and what it has explicitly acked
to the other server. But with the optimization discussed above --
where either server can offer the DHCP client a lease term of the
MCLT beyond the max( received-potential-expiration-time, acked-
potential-expiration-time), then the additional times sent-
potential-expiration-time and acked-potential-expiration-time must be
added into the expression, since the partner could have used those
times as part of its own lease time calculation.
Thus this optimization may require a longer waiting time when enter-
ing PARTNER-DOWN state, but will generally allow servers to operate
considerably more effectively when running in COMMUNICATIONS-
INTERRUPTED state.
7.2. BNDACK message [4]
A server sends a binding acknowledgement (BNDACK) message when it has
processed a BNDUPD message and after it has successfully committed to
stable storage any binding database changes made as a result of pro-
cessing the BNDUPD message. A BNDACK message is used to both accept
or reject a BNDUPD message. A BNDACK message which contains a
reject-reason option is a rejection of the corresponding BNDUPD mes-
sage.
In order to reduce the complexity of the discussion, the rest of this
section is written as though every BNDUPD message contains only a
single binding update transaction and thus every corresponding BNDACK
message would also contain reply information about only a single
binding update transaction. See section 6.3 for information on how
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to create and process BNDUPD and BNDACK messages which contain multi-
ple binding update transactions.
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 multi-
ple binding update transactions. If a server does not ever create
BNDUPD messages which contain multiple binding update transactions,
then it does not need to be able to process a received BNDACK message
with multiple binding update transactions. However, all servers MUST
be able to create BNDACK messages which deal with multiple binding
update transactions received in a BNDUPD message.
Every BNDUPD message that is received by a server MUST be responded
to with a corresponding BNDACK message. The receiving server SHOULD
respond quickly to every BNDUPD message but it MAY choose to respond
preferentially to DHCP client requests instead of BNDUPD messages,
since there is no absolute time period within which a BNDACK must be
sent in response to a BNDUPD message, while DHCP clients frequently
have strict time constraints.
A BNDACK message can only be sent in response to a BNDUPD message
using the same TCP connection from which the BNDUPD message was
received, since the XID's in BNDUPD messages are guaranteed unique
only during the life of a single TCP connection. When a connection
to a partner server goes down, a server with unprocessed BNDUPD mes-
sages MAY simply drop all of those messages, since it can be sure
that the partner will resend them when they are next in communica-
tions (albeit with a different XID), or it MAY instead choose to pro-
cess those BNDUPD messages, but it MUST NOT send any BNDACK messages
in response.
The following table summarizes the options for the BNDACK message.
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binding-status BACKUP
RESET
ABANDONED
Option ACTIVE EXPIRED RELEASED FREE
------ ------ ------- -------- ----
assigned-IP-address (3) MUST MUST MUST MUST
binding-status MUST MUST MUST MUST
client-identifier MAY MAY MAY MAY(2)
client-hardware-address MUST MUST MUST MAY(2)
reject-reason MAY MAY MAY MAY
message MAY MAY MAY MAY
lease-expiration-time MUST MUST NOT MUST NOT MUST NOT
potential-expiration-time MUST MUST NOT MUST NOT MUST NOT
start-time-of-state SHOULD SHOULD SHOULD SHOULD
client-last-trans.-time SHOULD SHOULD SHOULD MAY
DDNS(1) SHOULD SHOULD SHOULD SHOULD
(1) MUST if server is performing dynamic DNS for this IP address, else
MUST NOT.
(2) MUST NOT if binding-status is ABANDONED.
(3) assigned-IP-address MUST be the first option for an IP address
Table 7.2-1: Options used in a BNDACK message
7.2.1. Sending the BNDACK message
The BNDACK message MUST contain the same xid as the corresponding
BNDUPD message.
The assigned-IP-address option from the BNDUPD message MUST be
included in the BNDACK message. Any additional options from the
BNDUPD message SHOULD NOT appear in the BNDACK message. Note that
any information sent in options (e.g, a later lease-expiration time)
in the BNDACK message MUST NOT be assumed to necessarily be recorded
in the stable storage of the server who receives the BNDACK message
because there is no corresponding ACK of the BNDACK message. Any
information that SHOULD be recorded in the partner server's stable
storage MUST be transmitted in a subsequent BNDUPD.
If the server is accepting the BNDUPD, the BNDACK message includes
only the assigned-IP-address option. If the server is rejecting the
BNDUPD, the additional option reject-reason MUST appear in the BNDACK
message, and the message option SHOULD appear in this case containing
a human-readable error message describing in some detail the reason
for the rejection of the BNDUPD message.
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If the server rejects the BNDUPD message with a BNDACK and a reject-
reason option, it may be because the server believes that it has
binding information that the other server should know. A server
which is rejecting a BNDUPD may initiate a BNDUPD of its own in order
to update its partner with what it believes is better binding infor-
mation, but it MUST ensure through some means that it will not end up
in a situation where each server is sending BNDUPD messages as fast
as possible because they can't agree on which server has better bind-
ing data. Placing a considerable delay on the initiation of a BNDUPD
message after sending a BNDACK with a reject-reason would be one way
to ensure this situation doesn't occur.
7.2.2. Receiving the BNDACK message
When a server receives a BNDACK message, if it doesn't contain a
reject-reason option that means that the BNDUPD message was accepted,
and the server which sent the BNDUPD SHOULD update its stable storage
with the potential-expiration-time value sent in the BNDUPD message
and returned in the BNDACK message. Other values sent in the BNDUPD
message MAY be used as desired.
If the BNDACK message contains a reject-reason option, that means
that the BNDUPD was rejected. There SHOULD be a message option in
the BNDACK giving a text reason for the rejection, and the server
SHOULD log the message in some way. The server MUST NOT immediately
try to resend the BNDUPD message as there is no reason to believe the
partner won't reject it a second time. However a server MAY choose
to send another BNDUPD at some future time, for instance when the
server next processes an update request from its partner.
7.3. UPDREQ message [9]
The update request (UPDREQ) message is used by one server to request
that its partner send it all of the binding database information that
it has not already seen. Since each server is required to keep
track at all times of the binding information the other server has
received and ACKed, one server can request transmission of all un-
ACKed binding database information held by the other server by using
the UPDREQ message.
The UPDREQ message is used whenever the sending server cannot proceed
before it has processed all previously un-ACKed binding update infor-
mation, since the UPDREQ message should yield a corresponding UPDDONE
message. The UPDDONE message is not sent until the server that sent
the UPDREQ message has responded to all of the BNDUPD messages gen-
erated by the UPDREQ message with BNDACK messages (they may either be
accepted or rejected by the BNDACK messages, but they MUST have been
responded to). Thus, the sender of the UPDREQ message can be sure
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upon receipt of an UPDDONE message that it has received and committed
to stable storage all outstanding binding database updates.
See section 9, Failover Endpoint States, for the details of when the
UPDREQ message is sent.
7.3.1. Sending the UPDREQ message
The UPDREQ message has no message specific options.
7.3.2. Receiving the UPDREQ message
A server receiving an UPDREQ message MUST send all binding database
changes that have not yet been ACKed by the sending server. These
changes are sent as undistinguished BNDUPD messages.
However, the server which received and is processing the UPDREQ mes-
sage MUST track the BNDACK messages that correspond to the BNDUPD
messages triggered by the UPDREQ message and, when they are all
received, the server MUST send an UPDDONE message.
The server processing the UPDREQ message and sending BNDUPD messages
to its partner SHOULD only track the BNDUPD and BNDACK message pairs
for unACKed binding database changes that were present upon the
receipt of the UPDREQ message. A server which has received an UPDREQ
message SHOULD send BNDUPD messages for binding database changes that
occur after receipt of the UPDREQ message, but it SHOULD NOT include
those additional BNDUPD messages and their corresponding BNDACK mes-
sages in the accounting necessary to consider the UPDREQ complete and
subsequently send the UPDDONE message. If some additional binding
database changes end up becoming part of the set of BNDUPD messages
considered as part of the UPDREQ (due to whatever algorithm the
server uses to scan its bindings database for unacked changes) it
will probably not cause any difficulty, but a server MUST NOT attempt
to include all such later BNDUPD messages in the accounting for the
UPDREQ in order to be able to transmit an UPDDONE message.
When queuing up the BNDUPD messages for transmission to the sender of
the UPDREQ message, the server processing the UPDREQ message MUST
honor the value returned in the max-unacked-bndupd option in the CON-
NECT or CONNECTACK message that set up the connection with the send-
ing server. It MUST NOT send more BNDUPD messages without receiving
corresponding BNDACKs than the value returned in max-unacked-bndupd.
7.4. UPDREQALL message [7]
The update request all (UPDREQALL) message is used by one server to
request that its partner send it all of the binding database
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information. This message is used to allow one server to recover
from a failure of stable storage and to restore its binding database
in its entirety from the other server.
A server which sends an UPDREQALL message cannot proceed until all of
its binding update information is restored, and it knows that all of
that information is restored when an UPDDONE message is received.
See section 9, Protocol state transitions, for the details of when
the UPDREQALL message is sent.
The UPDREQALL message has no message specific options.
7.4.1. Sending the UPDREQALL message
The UPDREQALL is sent.
7.4.2. Receiving the UPDREQALL message
A server receiving an UPDREQALL message MUST send all binding data-
base information to the sending server. These changes are sent as
undistinguished BNDUPD messages. Otherwise the processing is the same
as for the UPDREQ message. See section 7.3.2 for details.
7.5. UPDDONE message [8]
The update done (UPDDONE) message is used by a server receiving an
UPDREQ or UPDREQALL message to signify that it has sent all of the
BNDUPD messages requested by the UPDREQ or UPDREQALL request and that
it has received a BNDACK for each of those messages.
While a BNDACK message MUST have been received for each BNDUPD mes-
sage prior to the transmission of the UPDDONE message, this doesn't
necessarily mean that all of the BNDUPD messages were accepted, only
that all of them were responded to with a BNDACK message. Thus, a
NAK (comprised of a BNDACK message containing a reject-reason option)
could be used to reject a BNDUPD, but for the purposes of the UPDDONE
message, such NAK would count as a response to the associated BNDUPD
message, and would not block the eventual transmission of the UPDDONE
message.
The xid in an UPDDONE message MUST be identical to the xid in the
UPDREQ or UPDREQALL message that initiated the update process.
The UPDDONE message has no message specific options.
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7.5.1. Sending the UPDDONE message
The UPDDONE message SHOULD be sent as soon as the last BNDACK message
corresponding to a BNDUPD message requested by the UPDREQ or
UPDREQALL is received from the server which sent the UPDREQ or
UPDREQALL. The XID of the UPDDONE message MUST be the same as the
XID of the corresponding UPDREQ or UPDREQALL message.
7.5.2. Receiving the UPDDONE message
A server receiving the UPDDONE message knows that all of the informa-
tion that it requested by sending an UPDREQ or UPDREQALL message has
now been sent and that it has recorded this information in its stable
storage. It typically uses the receipt of an UPDDONE message to move
to a different failover state. See sections 9.5.2 and 9.8.3 for
details.
7.6. POOLREQ message [1]
The pool request (POOLREQ) message is used by the secondary server to
request an allocation of IP addresses from the primary server. It
MUST be sent by a secondary server to a primary server to request IP
address allocation by the primary. The IP addresses allocated are
transmitted using normal BNDUPD messages from the primary to the
secondary.
The POOLREQ message SHOULD be sent from the secondary to the primary
whenever the secondary transitions into NORMAL state. It SHOULD
periodically be resent in order that any change in the number of
available IP addresses on the primary be reflected in the pool on the
secondary. The period may be influenced by the secondary server's
leasing activity.
The POOLREQ message has no message specific options.
7.6.1. Sending the POOLREQ message
The POOLREQ message is sent.
7.6.2. Receiving the POOLREQ message
When a primary server receives a POOLREQ message it SHOULD examine
the binding database and determine how many IP addresses the secon-
dary server should have, and set these IP addresses to BACKUP state.
It SHOULD then send BNDUPD messages concerning all of these IP
addresses to the secondary server.
Servers frequently have several kinds of IP addresses available on a
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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 IP addresses on every network segment
participating in the failover protocol. The primary server is
responsible for allocating the secondary server the correct propor-
tion of available IP addresses of each kind, and the secondary server
is responsible for being configured in such a way that it can tell
the kind of every IP address based solely on the IP address itself.
A primary server MUST keep track of how many IP addresses were allo-
cated as a result of processing the POOLREQ message, and send that
number in the POOLRESP message.
A primary server MAY choose to defer processing a POOLREQ message
until a more convenient time to process it, but it should not depend
on the secondary server to resend the POOLREQ message in that case.
If a secondary server receives a POOLREQ message it SHOULD report an
error.
7.7. POOLRESP message [2]
A primary server sends a POOLRESP message to a secondary server after
the allocation process for available addresses to the secondary
server is complete. Typically this message will precede some of the
BNDUPD messages that the primary uses to send the actual allocated IP
addresses to the secondary.
The xid in the POOLRESP message MUST be identical to the xid in the
POOLREQ message for which this POOLRESP is a response.
7.7.1. Sending the POOLRESP message
The POOLRESP message MUST contain the same xid as the corresponding
POOLREQ message.
Only one option MUST appear in a POOLREQ message:
o addresses-transferred
The number of addresses allocated to the secondary server by the
primary server as a result of a POOLREQ is contained in the
addresses-transferred option in a POOLRESP message. Note this
is the number of addresses that are transferred to the secondary
in the primary's binding database as a result of the correspond-
ing POOLREQ message, and that it may be some time before they
can all be transmitted to the secondary server through the use
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of BNDUPD messages.
7.7.2. Receiving the POOLRESP message
When a secondary server receives a POOLRESP message, it SHOULD send
another POOLREQ message if the value of the addresses-transferred
option is non-zero.
Typically, no other action is taken on the reception of a POOLRESP
message.
7.8. CONNECT message [5]
The connect message is used to establish an applications level con-
nection over a newly created TCP connection. It gives the source
information for the connection, and critical configuration informa-
tion. It MUST be sent only by the primary server. Either server can
initiate a TCP connection, but the CONNECT message is only sent by
the primary server.
The CONNECT message MUST be the first message sent down a newly esta-
blished connection, and it MUST be sent only by the primary server.
The following table summarizes the options that are associated with
the CONNECT message:
Option
------
sending-server-IP-address MUST
max-unacked-bndupd MUST
receive-timer MUST
vendor-class-identifier MUST
protocol-version MUST
TLS-request MUST (1)
MCLT MUST
hash-bucket-assignment MUST
(1) MUST NOT if CONNECT is being sent over a TLS connection
Table 7.8-1: Options used in a CONNECT message
7.8.1. Sending the CONNECT message
The CONNECT message MUST be the first message sent by the primary
server after the establishment of a new TCP connection with a secon-
dary server participating in the failover protocol.
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The xid of the CONNECT message must be unique.
The IP address of the primary server MUST be placed in the sending-
server-IP-address option. This information is placed in an option
inside of the message in order to allow the identity of the sender to
be covered by a shared secret.
The number of BNDUPD messages the primary server can accept without
blocking the TCP connection MUST be placed in the max-unacked-bndupd
option. This MUST be a number equal to or greater than 1, SHOULD be
a number greater than 10, and SHOULD be a number less than 100.
The length of the receive timer (tReceive, see section 8.3) MUST be
placed in the receive-timer option.
The MCLT MUST be placed in the MCLT option.
The hash-bucket-assignment option MUST be included in the CONNECT
message. In the event that load balancing is not configured for this
server, the hash-bucket-assignment option will indicate that. The
value of the hash-bucket-assignment option is determined from the
specific buckets that the primary server has determined that the
secondary server MUST service as part of the load-balancing algo-
rithm. The way in which the primary server determines this informa-
tion is outside the scope of this protocol definition. The primary
server SHOULD be configured with a percentage of clients that the
secondary server will be instructed to service, and the primary
server SHOULD use the algorithm in [LOADB] to generate a Hash Bucket
Assignment which it sends to the secondary server.
The vendor class identifier MUST be placed in the vendor-class-
identifier option.
The protocol-version option MUST be included in every CONNECT mes-
sage. The current value of the protocol version is 1.
The TLS-request option MUST be sent and contains the desired TLS con-
nection request as well as information concerning whether TLS is sup-
ported. If this CONNECT message is being sent over a already
created TLS connection, the TLS-request MUST NOT appear.
7.8.2. Receiving the CONNECT message
When a server receives a TCP connection on the failover port, if it
is a PRIMARY server it should send a CONNECT message, and if it is a
secondary server it should wait for a CONNECT message before sending
any messages. To avoid denial of service attacks, a secondary should
only wait for a CONNECT message on a new connection for a limited
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amount of time and close the connection if none is received during
that time.
When a secondary server receives a CONNECT message it should:
1. Record the time at which the message was received.
2. Examine the protocol-version option, and decide if this server
is capable of interoperating with another server running that
protocol version. If not, send the CONNECTACK message with
the appropriate reject-reason. The server MUST include its
protocol-version in the CONNECTACK message.
3. Examine the TLS-request option. Figure out the TLS-reply
value based on the capabilities and configuration of this
server. If the result for the TLS-reply value is a 1 and the
connection is accepted, indicating use of TLS, then immedi-
ately send the CONNECTACK message and go into TLS negotiation.
If the TLS-reply value implies rejection of the connection,
then immediately send the CONNECTACK message with the TLS-
reply value and the appropriate reject-reason option value.
In all other cases, save the TLS-reply option information for
the eventual CONNECTACK message.
The possibilities for TLS-request and TLS-reply are:
CONNECT CONNECTACK
TLS TLS
request reply
Reject
t1 t1 Reason Comments
-- -- ------ --------
0 0 no TLS used
0 1 11 primary won't use TLS, secondary requires TLS
1 0 primary desires TLS, secondary doesn't
1 1 primary desires TLS, secondary will use TLS
2 0 9, 10 primary requires TLS and secondary won't
2 1 primary requires TLS and secondary will use TLS
4. Check to see if there is a message-digest option in the CON-
NECT message. If there was, and the server does not support
message-digests, then reject the connection with the appropri-
ate reject-reason in the CONNECTACK. If the server does sup-
port message-digests, then check this message for validity
based on the message-digest, and reject it if the digest indi-
cates the message was altered.
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5. Determine if the sender (from the sending-server-IP-address
option) and the implicit role of the sender (i.e., primary)
represents a server with which the receiver was configured to
engage in failover activity. This is performed after any TLS
or message digest processing so that it occurs after a secure
connection is created, to ensure that there is no tampering
with the IP address of the partner.
If not, then the receiving server should reject the CONNECT
request by sending a CONNECTACK message with a reject-reason
value of: 8, invalid failover partner.
If it is, then the receiving failover endpoint should be
determined.
6. Decide if the time delta between the sending of the message,
in the time field, and the receipt of the message, recorded in
step 1 above, is acceptable. A server MAY require an arbi-
trarily small delta in time values in order to set up a fail-
over connection with another server. See section 5.9 for
information on time synchronization.
If the delta between the time values is too great, the server
should reject the CONNECT request by sending a CONNECTACK mes-
sage with a reject-reason of 4, time mismatch too great.
If the time mismatch is not considered too great then the
receiving server MUST record the delta between the servers.
The receiving server MUST use this delta to correct all of the
absolute times received from the other server in all time-
valued options. Note that servers can participate in failover
with arbitrarily great time mismatches, as long as it is more
or less constant.
7. Examine the MCLT option in the CONNECT request and use the
value of the MCLT as the MCLT for this failover endpoint.
The secondary server SHOULD be able to operate with any MCLT
sent by the primary, but if it cannot, then it should send a
CONNECTACK with a reject-reason of 5, MCLT mismatch.
8. The server MUST store hash-bucket-assignment option for use
during processing during NORMAL state. If this hash bucket
assignment conflicts with the secondary server's configured
hash bucket assignment for use in other than NORMAL state, the
secondary server should send a CONNECTACK with a reject reason
of 19, Hash bucket assignment conflict.
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9. The receiving server MAY use the vendor-class-identifier to do
vendor specific processing.
7.9. CONNECTACK message [6]
The CONNECTACK message is sent to accept or reject a CONNECT message.
It is sent by the secondary server which received a CONNECT message.
Attempting immediately to reconnect after either receiving a CONNEC-
TACK with a reject-reason or after sending a CONNECTACK with a
reject-reason could yield unwanted looping behavior, since the reason
that the connection was rejected may well not have changed since the
last attempt. A simple suggested solution is to wait a minute or two
after sending or receiving a CONNECTACK message with a reject-reason
before attempting to reestablish communication.
The following table summarizes the options associated with the CON-
NECTACK message:
Option
------
sending-server-IP-address MUST
max-unacked-bndupd MUST
receive-timer MUST
vendor-class-identifier MUST
protocol-version MUST
TLS-request MUST(1)
reject-reason MAY(2)
message MAY
MCLT MUST NOT
hash-bucket-assignment MUST NOT
(1) MUST NOT if sending CONNECTACK after TLS negotiation
(2) Indicates a rejection of the CONNECT message.
Table 7.9-1: Options used in a CONNECTACK message
7.9.1. Sending the CONNECTACK message
The xid of the CONNECTACK message MUST be that of the corresponding
CONNECT message.
The IP address of the sending server MUST be placed in the sending-
server-IP-address option. This information is placed in an option
inside of the message in order to allow the identity of the sender to
be covered by a shared secret.
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The protocol-version option MUST be included in every CONNECTACK mes-
sage. The current value of the protocol version is 1.
If the connection has been rejected, the reject-reason option MUST be
placed in the CONNECTACK message with an appropriate reason, and a
message option SHOULD be included with a human-readable error message
describing the reason for the rejection in some detail. If the
reject-reason option appears, then the remaining options listed below
do not appear. The sending server should close the connection after
sending the CONNECTACK if the connection was rejected.
The results of the TLS negotiation MUST be placed in the TLS-reply
option. If this CONNECTACK message is being sent over an already TLS
secured connection, then there MUST NOT be a TLS-reply option.
If there was a message-digest option in the CONNECT message, then
there MUST be a message-digest in the CONNECTACK message and any sub-
sequent messages if the CONNECTACK does not contain a reject-reason.
The number of BNDUPD messages the server can accept without blocking
the TCP connection MUST be placed in the max-unacked-bndupd option.
This SHOULD be a number greater than 10, and SHOULD be a number less
than 100.
The length of the receive timer (tReceive, see section 8.3) MUST be
placed in the receive-timer option.
The vendor class identifier MUST be placed in the vendor-class-
identifier option.
After a connection is created (either by sending a CONNECTACK message
to the first CONNECT message, or sending a CONNECTACK message to a
CONNECT message received over a TLS connection), the server MUST send
a STATE message.
After a connection is created, the server MUST start two timers for
the connection: tSend and tReceive. The tSend timer SHOULD be
approximately 33 percent of the time in the receiver-timer option in
the corresponding CONNECT message. The tReceive timer SHOULD be the
time sent in the receiver-timer option in the CONNECTACK message.
The tReceive timer is reset whenever a message is received from this
TCP connection. If it ever expires, the TCP connection is dropped
and communications with this partner is considered not ok.
The tSend timer is reset whenever a message is sent over this connec-
tion. When it expires, a CONTACT message MUST be sent.
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7.9.2. Receiving the CONNECTACK message
If a CONNECTACK message is received with a different XID from the one
in the CONNECT that was sent, it SHOULD be ignored.
When a CONNECTACK message is received, the following actions should
be taken:
1. Record the time the message was received.
2. Check to see if the xid on the CONNECTACK matches an outstand-
ing CONNECT message on this TCP connection.
3. Check to see if there is a reject-reason option in the CONNEC-
TACK message. If not, continue with step 3. If there is a
reject-reason option, the server SHOULD report the error code.
If a message option appears a server SHOULD display the string
from the message option in a user visible way. The server
MUST close the connection if a reject-reason option appears.
4. Check the value of the TLS-reply option (if any, which there
won't be if this CONNECT is taking place utilizing TLS), and
if it was 1, then skip processing of the rest of the CONNEC-
TACK message, and immediately enter into TLS connection setup.
This step occurs prior to steps 5 and 6 in order to allow
creation of a secure connection (if required) prior to pro-
cessing the protocol version and IP address information.
5. Examine the value of the protocol-version option. If this
server is able to establish connections with another server
running this protocol version, then continue, else close the
connection.
6. Decide if the time delta between the sending of the message,
in the time field, and the receipt of the message, recorded in
step 1 above, is acceptable. A server MAY require an arbi-
trarily small delta in time values in order to set up a fail-
over connection with another server.
If the delta between the time values is too great, the server
should drop the TCP connection.
If the time mismatch is not considered too great then the
receiving server MUST record the delta between the servers.
The receiving server MUST use this delta to correct all of the
absolute times received from the other server in all time-
valued options. Note that the failover protocol is
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constructed so that two servers can be failover partners with
arbitrarily great time mismatches.
7. The receiving server MAY use the vendor-class-identifier to do
vendor specific processing.
8. After accepting a CONNECTACK message, the server MUST send a
STATE message.
After receiving a CONNECTACK message, the server MUST start
two timers for the connection: tSend and tReceive. The tSend
timer SHOULD be approximately 20 percent of the time in the
receiver-timer option in the corresponding CONNECTACK message.
The tReceive timer SHOULD be set to the time sent in the
receiver-timer option in the CONNECT message.
The tReceive timer is reset whenever a message is received
from this TCP connection. If it ever expires, the TCP connec-
tion is dropped and communications with this partner is con-
sidered not ok.
The tSend timer is reset whenever a message is sent over this
connection. When it expires, a CONTACT message MUST be sent.
7.10. STATE message [10]
The state (STATE) message is used to communicate the current failover
state to the partner server.
The STATE message MUST be sent after sending a CONNECTACK message
that didn't contain a reject-reason option, and MUST be sent after
receiving a CONNECTACK message without a reject-reason option.
A STATE message MUST be sent whenever the failover endpoint changes
its failover state and a connection exists to the partner.
The STATE message requires no response from the failover partner.
The following table shows the options that MUST appear in a STATE
message:
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Option
------
sending-state MUST
server-flags MUST
start-time-of-state MUST
Table 7.10-1: Options used in a STATE message
7.10.1. Sending the STATE message
The current failover state is placed in the server-state option and
the current state of the STARTUP flag is placed in the server-flags
option.
The message is sent with a unique xid.
A server SHOULD only send the STATE message either when the connec-
tion is created (i.e, after sending or receiving a CONNECTACK message
with no reject-reason option), or when there is a change from the
values sent in a previous STATE message.
7.10.2. Receiving the STATE message
Every STATE message SHOULD indicate a change in state or a change in
the flags.
When a STATE message is received, any state transitions specified in
section 9 are taken.
No response to a STATE message is required.
7.11. CONTACT message [11]
The contact (CONTACT) message is sent to verify communications
integrity with a failover partner. The CONTACT message is sent when
no messages have been sent to the failover partner for a specified
period of time. This is determined by the tSend timer expiring (see
section 8.3).
The CONTACT message has no message specific options.
7.11.1. Sending the CONTACT message
The CONTACT message is sent.
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7.11.2. Receiving the CONTACT message
When a CONTACT message is received, the tReceive timer is reset (as
it is with any message that is received).
A server SHOULD use the time in the time field and the time the mes-
sage was received to refine the delta time calculations between the
servers.
7.12. DISCONNECT message [12]
The DISCONNECT is the last message sent over a connection before
dropping an established connection (note that an established connec-
tion is one where a CONNECTACK has been sent without a reject rea-
son).
After sending or receiving a DISCONNECT message, a server needs to
have some mechanism to prevent an error loop. Simply reconnecting to
the partner immediately is not the best option, especially after
several consecutive attempts.
A simple suggested solution is to wait a minute or two after sending
or receiving a DISCONNECT before attempting to reestablish communica-
tion.
The DISCONNECT message MUST be the last message sent down a connec-
tion before it is closed.
The following table summarizes the options that are associated with
the DISCONNECT message:
Option
------
reject-reason MUST
message SHOULD
Table 7.12-1: Options used in a DISCONNECT message
7.12.1. Sending the DISCONNECT message
The DISCONNECT message MUST be the last message sent by the a server
which is dropping a TCP connection.
The xid of the DISCONNECT message must be unique.
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The reject-reason option MUST appear giving a reason why the connec-
tion was dropped. A message option SHOULD appear giving a human
readable error message with possibly more details.
7.12.2. Receiving the DISCONNECT message
When a server receives a DISCONNECT message it should log the message
if there was one and possibly raise an alarm of some sort if the
reject reason was one that was sufficiently serious.
8. Connection Management
Servers participating in the failover protocol communicate over TCP
connections. These TCP connections are used both to transmit bind-
ing information from one server to another as well as to allow each
server to determine whether communications is possible with the other
server.
Central to the operation of the failover protocol is a notion of
"communications okay" or "communications failed". Failover state
transitions are taken in many cases when the status of communications
with the partner changes, and the existence or non-existence of a TCP
connections between failover endpoints is used to determine if com-
munications is "okay" or "failed".
A single TCP connection exists which connects two failover endpoints.
8.1. Connection granularity
There exists one TCP connection between each set of failover end-
points. See section 5.1.1 for an explanation of failover endpoints.
There are a maximum of two TCP connections between any two servers
implementing the failover protocol, one for each of the possible
failover endpoints between these two servers. There is a minimum of
one TCP connection between one server and every other failover server
with which it implements the failover protocol.
8.2. Creating the TCP connection
There are two ports used for initiating TCP connections, correspond-
ing to the two roles that a server can fill with respect to another
server. Every server implementing the failover protcol MUST listen
on at least one of these ports. Port 647 is the port to which pri-
mary servers will attempt a connection, and port TBD is the port to
which secondary servers will attempt a connection. When a connection
attempt is received on port 647 it is therefore from a primary
server, and it is attempting to connect to this server to become a
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secondary server for it. Likewise, when an attempt to connect is
received on port TBD the connection attempt is from a secondary
server, and it is attempting to connect to this server to be a pri-
mary server. The source port of any TCP connection is unimportant.
See the schematic representation below:
Primary Server
--------------
Listens on port TBD for secondary server to connect to it
Periodically connects on port 647 to contact secondary
Secondary Server
--------------
Listens on port 647 for primary server to connect to it
Periodically connects on port TDB to contact primary
Every 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, a
server SHOULD reduce the frequency with which it attempts to connect
to that server but it SHOULD continue to attempt to connect periodi-
cally.
If a connection attempt has been received from another server in a
particular role (i.e., from a specific failover endpoint) then the
receiving server MUST NOT initiate a connection attempt to the
partner server in that same role.
If both servers happen to attempt to connect simultaneously, the
secondary server MUST drop its attempt in favor of the primary's
attempt. Thus, in the event that a secondary server receives a con-
nection attempt to port 647 from a primary server when it has already
initiated a connection attempt to port TBD on the same primary
server, it MUST accept the connection to port 647 and it MUST drop
drop the connection attempt to port TBD. In the event that a primary
server receives a connection attempt to port TBD from a secondary
server when it has already initiated a connection attempt to port 647
on that same server, it MUST reject the connection attempt to port
TBD and continue to pursue the connection attempt on port 647.
Once a connection is established, the primary server MUST send a CON-
NECT message across the connection. A secondary server MUST wait for
the CONNECT message from a primary server.
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Every CONNECT message includes a TLS-request option, and if the CON-
NECTACK 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 associ-
ated CONNECTACK message as they had in the first messages.
The second message sent over a new connection (either a bare TCP con-
nection or a connection utilizing TLS) is a STATE message. Upon the
receipt of this message, the receiver can consider communications up.
It is entirely possible that two servers will attempt to make connec-
tions to each other essentially simultaneously, and in this case the
secondary server will be waiting for a CONNECT message on each con-
nection. The primary server MUST send a CONNECT message over one
connection and it MUST close the other connection.
A secondary server MUST NOT respond to the closing of a TCP connec-
tion with a blind attempt to reconnect -- there may be another TCP
connection to the same failover partner already in use.
8.3. Using the TCP connection for determining communications status
The TCP connection is used to determine the communications status of
the other server, i.e., communications-ok, or communications-
interrupted.
Three things must happen for a server to consider that communications
are ok with respect to another server:
1. A TCP connection must be established to the other server.
2. A CONNECT message must be received and a CONNECTACK message
sent in response. The CONNECT message is used to determine
the identify of the failover endpoint of the other end of the
TCP connection -- without it, the failover endpoint cannot be
uniquely determined. Without knowledge of the failover end-
point, then the entity with which communications is ok is
undetermined.
3. A STATE message must be received from the other server over
the connection. This STATE message initializes important
information necessary to the operation of the state machine
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the governs the behavior of this failover endpoint.
There are two ways that a server can determine that communications
has failed:
1. The TCP connection can go down, yielding an error when
attempting to send or receive a message. This will happen at
least as often as the period of the tSend timer.
2. The tReceive timer can expire.
In either of these cases, communications is considered interrupted.
Several difficulties arise when trying to use one TCP connection for
both bulk data transfer as well as to sense the communications status
of the other server. One aspect of the problem stems from the dif-
ferent requirements of both uses. The bulk data transfer is of
course critically important to the protocol, but the speed with which
it is processed is not terribly significant. It might well be
minutes before a BNDUPD message is processed, and while not optimal,
such an occasional delay doesn't compromise the correctness of the
protocol. However, the speed with which one server detects the other
server is up (or, more importantly, down) is more highly constrained.
Generally one server should be able to detect that the other server
is not communicating within a minute or less.
These differing time constraints makes it difficult to use the same
TCP connection for data transfer as well as to sense communications
integrity. See section 3.5 for additional details on TCP.
The solution to this problem is to require that some message be
received by each end of the connection within a limited time or that
the connection will be considered down. If no messages have been
sent recently, then a CONTACT message is sent.
In the case where there is no data queued to be sent, this is not a
problem, but in the case where there is data queued to be sent to the
partner, then the CONTACT message will not actually be transmitted
until the queued data is sent. Section 3.5 explains why waiting for
TCP to determine that the connection is down is not acceptable, and
leads a requirement that the receiving server never block the sending
server from sending CONTACT messages.
In order to meet this requirement, each server tells the other server
the number of outstanding BNDUPD messages that it will accept. The
receiving server is required to always be able to accept that many
BNDUPD messages off of the connection's input queue even if it cannot
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process them immediately, and to accept all other messages immedi-
ately.
Thus, the sending server's TCP is never blocked from sending a mes-
sage except for very short periods, less than a few seconds unless
the network connection itself has problems. In this case, if the
CONTACT messages don't make it to the partner then the partner will
close the connection.
DISCUSSION:
When implementing this capability, one needs to be careful when
sending any message on the TCP connection as TCP can easily block
the server if the local TCP send buffers are full. This can't be
prevented because if the receiver is not reachable (via the net-
work), the sending TCP can't send and thus it will be unable to
empty the local TCP send buffers. So, all send operations either
need to assume they may block for some time or non-blocking sends
must be used.
8.4. Using the TCP connection for binding data
Binding data, in the form of BNDUPD messages and BNDACK messages to
respond to them, are sent across the TCP connection.
In order to support timely detection of any failure in the partner
server, the TCP connection MUST NOT block for more than a very short
time, on the order of a few seconds. Therefore, a server that is
sending BNDUPD messages MUST send only a restricted number before
receiving BNDACK messages about previous messages sent.
The number of outstanding BNDUPD messages that each server will
accept without causing TCP to block transmission of additional data
(i.e, CONTACT messages) is sent by each server in the CONNECT and
CONNECTACK messages in the max-unacked-bndupd option.
8.5. Using the TCP connection for control messages
The TCP connection is used for control messages: POOLREQ, UPDREQ,
STATE, CONTACT, UPDREQALL and the corresponding reply messages: POOL-
RESP, UPDDONE. A server MUST immediately accept all of these mes-
sages from the TCP connection. A server MUST immediately accept any
BNDACK which is received as well.
8.6. Losing the TCP connection
When the TCP connection is lost, then communications is not ok with
the other server. A server which has lost communications SHOULD
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immediately attempt to reconnect to the other server, and should
retry these connection attempts periodically.
An acknowledgement message (BNDACK, POOLRESP, UPDDONE) message can
only be sent in response to a request message (BNDUPD, POOLREQ,
UPDREQ, UPDREQALL) on the same TCP connection from which the request
was received, in part since the XID's in the request messages are
guaranteed unique only during the life of a single TCP connection.
When a connection to a partner server goes down, a server with unpro-
cessed request messages MAY simply drop all of those messages, since
it can be sure that the partner will resend them when they are next
in communications. A server with unprocessed BNDUPD messages when a
TCP connection goes down MAY instead choose to process those BNDUPD
messages, but it MUST NOT send any BNDACK messages in response (again
because of the issues surrounding XID uniqueness).
When the TCP connection is closed explicitly, the DISCONNECT message
with a reject-reason option (and, ideally, a message option) MUST be
sent over the TCP connection.
9. Failover Endpoint States
This section discusses the various states that a failover endpoint
may take, and the server actions required when entering the state,
operating in the state, and leaving the state, as well as the events
that cause transitions out of the state into another state.
The state transition diagram in Figure 9.2-1 is relevant for this
section. This is the common state transition diagram for both servers
in a failover pair. In the event that the textual description of a
state differs from the state transition diagram, the textual descrip-
tion is to be considered authoritative.
9.1. Server Initialization
When a server starts it starts out in STARTUP state. See section 9.3
below for details.
9.2. Server State Transitions
Whenever a server transitions into a new state, it MUST record the
state and the time at which it entered that state in stable storage.
If communications is "ok", it MUST also send a STATE message to its
failover partner.
Figure 9.2-1 is the diagram of the server state transitions. The
remainder of this section contains information important to the
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understanding of that diagram.
The server stays in the current state until all of the actions speci-
fied on the state transition are complete. If communications fails
during one of the actions, the server simply stays in the current
state and attempts a transition whenever the conditions for a transi-
tion are later fulfilled.
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.
The legend "responsive", "balanced", or "unresponsive" in each state
indicates whether the server is responsive to all DHCP client
requests, running in load balanced mode, or totally unresponsive in
the respective state. The terms "responsive" and "unresponsive" have
the obvious meanings, while "balanced" means that a DHCP server may
respond to all DHCPREQUEST messages that are RENEWAL or REBINDING,
and to all other messages from clients for which the load balancing
algorithm indicates that it MUST respond to. See sections 5.3 and
9.6.2 for details on load balancing.
In the state transition diagram below, when communication is reesta-
blished between the two servers, each must record the state of the
partner when communication was restored. State transitions on one
server in some cases imply state transitions on the partner server,
so a record of the current state of the partner server must be kept
by each server.
If the state of the partner changes while communicating a server
moves through the communications-failed transition and into whatever
state results. It then immediately moves through whatever state
transition is appropriate given the current state of the partner
server. A server performing this operation SHOULD NOT close the TCP
connection to its partner.
DISCUSSION:
The point of this technique is simplicity, both in explanation of
the protocol and in its implementation. The alternative to this
technique of memory of partner state and automatic state transi-
tion on change of partner state is to have every state in the fol-
lowing diagram have a state transition for every possible state of
the partner. With the approach adopted, only the states in which
communications are reestablished require a state transition for
each possible partner state.
The current state of a server MUST be recorded in stable storage and
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thus be available to the server after a server restart.
+---------------+ V +--------------+
| RECOVER - | | | STARTUP - |
|(unresponsive) | +->|(unresponsive)|
+---------------+ +--------------+
Comm. OK +-----------------+
Other State:-RECOVER | PARTNER DOWN - |<-----------------+
| | | (responsive) | |
All POTENTIAL- +-----------------+ +--------------+ |
Others CONFLICT------------ | --------+ | RESOLUTION -| |
| Comm. OK | | INTERRUPTED | |
UPDREQ(ALL) Other State: | +-| (responsive) | |
Wait UPDDONE | | | | +--------------+ |
Wait MCLT from fail RECOVER All Others| Comm. OK ^ | |
+--------------+ | V V V | Ext. |
|RECOVER-DONE +| +--+ +--------------+ Comm. Cmd. |
|(unresponsive)| | | POTENTIAL + | Failed | |
+--------------+ Wait for +>| CONFLICT |------+ +-->|
Comm. OK Other | |(unresponsive)|<--------+ |
+--Other State:-+ State: | +--------------+ | |
| | | RECOVER | | | |
| All POTENT. DONE | Resolve Conflict | |
| Others: CONFLICT-- | ----+ (see 9.8) | |
| Wait for V V | |
| Other State: NORMAL +-----------------+ | |
| V | NORMAL + | External | |
| +--+----------+-->| (balanced) |-Command---+-- | -----+
| ^ ^ +-----------------+ | |
| | | | | |
| Wait for Comm. OK Comm. External |
| Other Other Failed Command |
| State: State: | or | |
|RECOVER-DONE NORMAL Start Safe Safe | |
| | COMM. INT. Period Timer Period | |
| Comm. OK. | V expiration |
| Other State: | +------------------+ | |
| RECOVER +--| COMMUNICATIONS - |-----------+ |
V +-------------| INTERRUPTED | Comm. OK |
RECOVER | (responsive) |--Other State:-+
RECOVER-DONE--------->+------------------+ All Others
Figure 9.2-1: Server state diagram.
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9.3. STARTUP state
The STARTUP state affords an opportunity for a server to probe its
partner server, before starting to service DHCP clients.
DISCUSSION:
Without the STARTUP state, a server would likely start in a state
derived from its previously stored state (held in stable storage),
if any. However, this may be inconsistent with the current state
of the partner. The STARTUP state affords the opportunity for a
server to potentially learn the partner's state and determine if
that state is consistent with its derived starting state or
whether some significant state change has occurred at the partner
that forces the server to start in another state. This is
especially critical if significant time has elapsed while the
server was down.
9.3.1. Operation while in STARTUP state
Whenever a server is in STARTUP state, it MUST be unresponsive to
DHCP client requests, and so the time spent in the STARTUP state is
necessarily short, typically on the order of a few seconds to a few
tens of seconds. The exact time spent in the STARTUP state is imple-
mentation dependent, and the primary and secondary server are not
required to spend the same amount of time in the STARTUP state.
Whenever a STATE message is sent to the partner while in STARTUP
state the STARTUP bit MUST be set in the server-flags option and the
previously recorded failover state MUST be placed in the server-state
option.
9.3.2. Transition out of STARTUP state
Each server starts out in startup state every time it initializes
itself, and performs the following algorithm as part of its initiali-
zation:
1. Is there any record in stable storage of a previous failover
state? If yes, set previous-state to the last recorded state
in stable storage, and continue with step 2.
Is there any configuration information that indicates that
this server was previously running but lost its stable
storage? Such information must typically come from some
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administrative intervention, since it is difficult for a
server to distinguish first startup from a startup after it
has lost its stable storage. If yes, then set the previous-
state to RECOVER, and set the time-of-failure to whatever time
was configured, and go on to step 2. This time-of-failure
will be used in the transition out of the RECOVER state into
the RECOVER-DONE state, below.
If there is no record of any previous failover state in stable
storage nor of any previous operational activity for this
server, then set the previous-state to PARTNER-DOWN if this
server is a primary and RECOVER if this server is a secondary,
and set the time-of-failure to a time before the maximum-
client-lead-time before now. If using standard Posix times, 0
would typically do quite well.
2. Is the previous-state NORMAL? If yes, set the previous-state
to COMMUNICATIONS-INTERRUPTED.
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 and SHOULD be configur-
able. It SHOULD be long enough for a TCP connection to be
created to a heavily loaded partner across a slow network.
4. Attempt to create a TCP connection to the failover partner.
See section 8.2.
5. Wait for "communications okay", i.e., the process discussed in
section 8.2 "Creating the TCP Connection", to complete,
including the receipt of a STATE message from the partner.
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
the 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 the current state to
POTENTIAL-CONFLICT.
Then, transition to the current state and take the "communica-
tions okay" state transition based on the current state of
this server and the partner.
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7. If the startup time expires, take an implementation dependent
action: The server MAY go to the previous-state, or the
server MAY wait.
Reasons to go to previous-state and begin processing:
If the current server is the only operational server, then if
it waits, there will be no operational DHCP servers. This
situation could occur very easily where one server fails and
then the other crashes and reboots. If the rebooting server
doesn't start processing DHCP client requests without first
being in communication with the other server, then the level
of DHCP redundancy is not particularly high. This is an
appropriate approach if the possibility of partition is low,
or if the safe period expiration time is well beyond the time
at which an operator would notice and react to a partition
situation. It is also quite appropriate if the safe period
will never expire.
Reasons to wait:
If the current server has been down for longer than the
maximum-client-lead-time, and it is partitioned from the other
server, then when it returns it will attempt to use its own
available addresses to allocate to new DHCP clients, and the
other server may well be in PARTNER-DOWN state and may have
already allocated some of those available addresses to DHCP
clients. In cases where the possibility of partition is high,
and the safe period expiration time is less than the likely
operator reaction time, this is a good approach to use.
9.4. PARTNER-DOWN state
PARTNER-DOWN state is a state either server can enter. When in this
state, the server does not assume that the other server could still
be operating and servicing a different set of clients, but instead
assumes that it is the only server operating. If one server is in
PARTNER-DOWN state, the other server MUST NOT be operating.
9.4.1. Upon entry to PARTNER-DOWN state
No special actions are required when entering PARTNER-DOWN state.
The server should continue to attempt to connect to the partner
periodically.
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9.4.2. Operation while in PARTNER-DOWN state
A server in PARTNER-DOWN state MUST respond to DHCP client requests.
It will allow renewal of all outstanding leases on IP addresses, and
will allocate IP addresses from its own pool, and after a fixed
period of time (the MCLT interval) has elapsed from entry into
PARTNER-DOWN state, it will allocate IP addresses from the set of all
available IP addresses.
Once a server has entered NORMAL state, the PARTNER-DOWN state is
entered only on command of an external agency (typically an adminis-
trator of some sort) or after the expiration of an externally config-
ured minimum safe-time after the beginning of COMMUNICATIONS-
INTERRUPTED state.
Any available IP address 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 maximum-client-lead-time beyond the entry
into PARTNER-DOWN state has elapsed.
A server in PARTNER-DOWN state MUST NOT allocate an IP address to a
DHCP client different from that to which it was allocated at the
entrance to PARTNER-DOWN state until the maximum-client-lead-time
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. See section
7.1.5 for details. 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.
Two options exist for lease times given out while in PARTNER-DOWN
state, with different ramifications flowing from each.
If the server wishes the Failover protocol to protect it from loss of
stable storage in PARTNER-DOWN state, then it should ensure that the
MCLT based lease time restrictions in Section 5.1 are maintained,
even in PARTNER-DOWN state.
If the server wishes to forego the protection of the Failover proto-
col in the event of loss of stable storage, then it need recognize no
restrictions on actual client lease times while in PARTNER-DOWN
state.
A server in PARTNER-DOWN state MUST continue to attempt to establish
communications and synchronization with its partner.
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9.4.3. Transitions 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 value of the server-
state option in the received STATE message:
o partner in NORMAL, COMMUNICATIONS-INTERRUPTED, PARTNER-DOWN or
POTENTIAL-CONFLICT state
transition to POTENTIAL-CONFLICT state
o partner in RECOVER state
stay in PARTNER-DOWN state
o partner in RECOVER-DONE state
transition into NORMAL state
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
A server in RECOVER MUST NOT respond to DHCP client requests.
A server in RECOVER state will attempt to reestablish communications
with the other server.
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9.5.2. Transitions out of RECOVER state
If the other server is in POTENTIAL-CONFLICT state when communica-
tions are reestablished, then the server in RECOVER state will move
to POTENTIAL-CONFLICT state itself.
If the other server is in RECOVER state, then this server SHOULD sig-
nal an error and halt processing.
If the other server is in any other state, then the server in RECOVER
state will request an update of missing binding information by send-
ing an UPDREQ message. If the server has been instructed (through
configuration or other external agency) that it has lost its stable
storage, or if it has deduced that from the fact that 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 start a timer whose expiration is set to a time equal to the
time the server went down (if known) or the current time (if the
down-time is unknown) plus the maximum-client-lead-time. When this
timer goes off, 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.
See Figure 9.5.2-1.
DISCUSSION:
The actual requirement on this wait period in RECOVER is that it
start not before the recovering server went down, not necessarily
when it came back up. If the time when the recovering server
failed is known, it could be communicated to the recovering server
(perhaps through actions of the network administrator), and the
wait period could be reduced to the maximum-client-lead-time less
the difference between the current time and the time the server
failed. In this way, the waiting period could be minimized.
Various heuristics could be used to estimate this time, for exam-
ple if the recovering server periodically updates stable storage
with a time stamp, the wait period could be calculated to start at
the time of the last update of stable storage plus the time
required for the next update (which never occurred). This esti-
mate is later than the server went down, but probably not too much
later.
If an UPDDONE message isn't received within an implementation
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dependent amount of time, and no BNDUPD messages are being received,
the connection SHOULD be dropped.
A B
Server Server
| |
RECOVER PARTNER-DOWN
| |
| >--UPDREQ--------------------> |
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
... ...
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
| |
| <--------------------UPDDONE--< |
| |
Wait MCLT from last known |
time of operation |
| |
RECOVER-DONE |
| |
| >--STATE-(RECOVER-DONE)------> |
| NORMAL
| <-------------(NORMAL)-STATE--< |
NORMAL |
| >---- State-(NORMAL)--------------->
| |
| |
Figure 9.5.2-1: Transition out of RECOVER state
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9.6. 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.
9.6.1. Upon entry to NORMAL state
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 IP addresses for
allocation using the POOLREQ message.
9.6.2. Processing DHCP client requests and load balancing
In NORMAL state, a server MUST process every DHCPREQUEST/RENEWAL or
DHCPREQUEST/REBINDING request it receives. And, it processes other
requests only for those clients as dictated by the load balancing
algorithm specified in [LOADB].
As discussed in section 5.3, each server will take the client-
identifier from each DHCP client request (or the client-hardware-
address, i.e., the htype concatenated to the front of the chaddr if
no client-identifier is present in the request) and use it as the
'Request ID' specified in [LOADB]. After applying the algorithm
specified in [LOADB] and comparing the result with the hash bucket
assignment (performed during connect processing between failover
servers), each failover server will be able to unambiguously deter-
mine if it should process the DHCP client request.
9.6.3. Operation in NORMAL state
When in NORMAL state, for every DHCP client request that it
processes, as determined by the algorithm described in section 9.6.2,
above, a server will operate in the following manner:
o Lease time calculations
As discussed in section 5.2.1, "Control of lease time", the
lease interval given to a DHCP client can never be more than the
MCLT greater than the most recently received potential-
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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. One possible approach is dis-
cussed in section 5.2.1, but that particular approach is in no
way required by this protocol.
See section 7.1.5 for details concerning the storage of time
associated IP addresses and how to use these times when calcu-
lating lease times for DHCP clients.
o Lazy update of partner server
After an ACK of a IP address binding, the server servicing a
DHCP client request attempts to update its partner with the new
binding information. The lease time used in the update of the
secondary MUST be at least that given to the DHCP client in the
DHCPACK, and the potential-expiration-time MUST be at least the
lease time, and SHOULD be considerably longer.
o Reallocation of IP addresses between clients
Whenever a client binding is released or expires, a BNDUPD mes-
sage must be sent to partner, setting the binding state to
RELEASED or EXPIRED. However, until a BNDACK is received for
this message, the IP address cannot be allocated to another
client. It can be allocated to the same client again.
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 the primary server. It MUST ensure that the infor-
mation is recorded in stable storage prior to sending the BNDACK mes-
sage back to its partner.
9.6.4. Transitions 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.
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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 every tSend seconds, 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), 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.
9.7. COMMUNICATIONS-INTERRUPTED State
A server goes into COMMUNICATIONS-INTERRUPTED state whenever it is
unable to communicate with the other server. Primary and secondary
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 IP
address allocation can occur while the servers cycle between these
states.
9.7.1. Upon entry to COMMUNICATIONS-INTERRUPTED state
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 10), 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 adminis-
trative staff to a potential problem in the DHCP subsystem.
9.7.2. Operation in COMMUNICATIONS-INTERRUPTED State
In this state a server MUST respond to all DHCP client requests, and
the algorithm for load balancing described in section 5.3 MUST NOT be
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used. When allocating new IP addresses, each server allocates from
its own IP address pool, where the primary MUST allocate only FREE IP
addresses, and the secondary MUST allocate only BACKUP IP addresses.
When responding to renewal requests, each server will allow continued
renewal of a DHCP client's current lease on an IP address irrespec-
tive 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-expiration-
time already acknowledged by the other server, or 2) the lease-
expiration-time, or 3) the potential-expiration-time received from
the partner server.
However, since the server cannot communicate with its partner in this
state, the acknowledged-potential-expiration time will not be updated
in any new bindings. This is likely to eventually cause the actual-
client-lease-times to be the current time plus the maximum-client-
lead-time (unless this is greater than the desired-client-lease-
time).
9.7.3. 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.
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 partner in NORMAL or COMMUNICATIONS-INTERRUPTED
The partner SHOULD NOT be in NORMAL state here, since upon res-
toration of communications it MUST have created a new TCP con-
nection which would have forced it into COMMUNICATIONS-
INTERRUPTED state. Still, we should account for every state
just in case.
Transition into the NORMAL state.
o partner in RECOVER
Stay in COMMUNICATIONS-INTERRUPTED state.
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o partner in RECOVER-DONE
Transition into NORMAL state.
o partner in PARTNER-DOWN or POTENTIAL-CONFLICT
Transition into POTENTIAL-CONFLICT state.
o partner in PAUSED
Stay in COMMUNICATIONS-INTERRUPTED state.
o partner in SHUTDOWN
Transition into PARTNER-DOWN state.
The following figure illustrates the transition from NORMAL to
COMMUNICATIONS-INTERRUPTED state and then back to NORMAL state again.
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Primary Secondary
Server Server
NORMAL NORMAL
| >--CONTACT-------------------> |
| <--------------------CONTACT--< |
| [TCP connection broken] |
COMMUNICATIONS : COMMUNICATIONS
INTERRUPTED : INTERRUPTED
| [attempt new TCP connection] |
| [connection succeeds] |
| |
| >--CONNECT-------------------> |
| <-----------------CONNECTACK--< |
| <-------------------STATE-----< |
| NORMAL
| >--STATE---------------------> |
NORMAL |
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
| |
| <---------------------BNDUPD--< |
| >------BNDACK----------------> |
... ...
| |
| <--------------------POOLREQ--< |
| >--POOLRESP-(2)--------------> |
| |
| >--BNDUPD-(#1)---------------> |
| <---------------------BNDACK--< |
| |
| <--------------------POOLREQ--< |
| >--POOLRESP-(0)--------------> |
| |
| >--BNDUPD-(#2)---------------> |
| <---------------------BNDACK--< |
| |
Figure 9.7.3-1: Transition from NORMAL to COMMUNICATIONS-
INTERRUPTED and back (example with 2
addresses allocated to secondary)
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9.8. POTENTIAL-CONFLICT state
This state indicates that the two servers are attempting to re-
integrate 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 IP address has been offered and accepted by two different
DHCP clients.
It is a goal of this protocol to minimize the possibility that
POTENTIAL-CONFLICT state is ever entered.
9.8.1. Upon entry to POTENTIAL-CONFLICT state
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.
9.8.2.
Any server in POTENTIAL-CONFLICT state MUST NOT process any incoming
DHCP requests.
9.8.3. Transitions 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 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 transitioned from POTENTIAL-CONFLICT to NORMAL state, it
SHOULD send an UPDREQ message to the primary server.
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Primary Secondary
Server Server
| |
POTENTIAL-CONFLICT POTENTIAL-CONFLICT
| |
| >--UPDREQ--------------------> |
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
... ...
| |
| <---------------------BNDUPD--< |
| >--BNDACK--------------------> |
| |
| <--------------------UPDDONE--< |
NORMAL |
| >--STATE--(NORMAL)-----------> |
| <---------------------UPDREQ--< |
| |
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
... ...
| >--BNDUPD--------------------> |
| <---------------------BNDACK--< |
| |
| >--UPDDONE-------------------> |
| NORMAL
| |
| <--------------------POOLREQ--< |
| >------POOLRESP-(n)----------> |
| addresses |
Figure 9.8.3-1: Transition out of POTENTIAL-CONFLICT
9.9. RESOLUTION-INTERRUPTED state
This state indicates that the two servers were attempting to re-
integrate 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.
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9.9.1. Upon entry to RESOLUTION-INTERRUPTED state
When a server enters RESOLUTION-INTERRUPTED state it SHOULD raise an
alarm condition to alert administrative staff of a problem in the
DHCP subsystem.
9.9.2. Operation in RESOLUTION-INTERRUPTED state
In this state a server MUST respond to all DHCP client requests, and
any load balancing (described in section 5.3) MUST NOT be used. When
allocating new IP addresses, each server SHOULD allocate from its own
IP address pool (if that can be determined), where the primary SHOULD
allocate only FREE IP addresses, and the secondary SHOULD allocate
only BACKUP IP addresses. When responding to renewal requests, each
server will allow continued renewal of a DHCP client's current lease
on an IP address 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-expiration-time already acknowledged by the other
server or 2) the lease-expiration-time or 3) `potential-expiration-
time received from the partner server.
However, since the server cannot communicate with its partner in this
state, the acknowledged-potential-expiration time will not be updated
in any new bindings.
9.9.3. Transitions 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.
9.10. 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.
9.10.1. Operation in RECOVER-DONE state
A server in RECOVER-DONE state MUST respond only to
DHCPREQUEST/RENEWAL and DHCPREQUEST/REBINDING DHCP messages.
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9.10.2. Transitions out of RECOVER-DONE state
When a server in RECOVER-DONE state determines that its partner
server has entered NORMAL state, then it will transition into NORMAL
state as well.
If communications fails while in RECOVER-DONE state, a server will
stay in RECOVER-DONE state.
9.11. PAUSED state
This state exists to allow one server to inform another that it will
be out of service for what is predicted to be a relatively short
time, and to allow the other server to transition to COMMUNICATIONS-
INTERRUPTED state immediately and to begin servicing all DHCP clients
with no interruption in service to new DHCP clients.
A server which is aware that it is shutting down temporarily SHOULD
send a STATE message with the server-state option containing PAUSED
state and close the TCP connection.
While a server may or may not transition internally into PAUSED
state, the 'previous' state determined when it is restarted MUST be
the state the server was in prior to receiving the command to shut-
down and restart and which precedes its entry into the PAUSED state.
See section 9.3.2 concerning the use of the previous state upon
server restart.
9.11.1. Upon entry to PAUSED state
When entering PAUSED state, the server MUST store the previous state
in stable storage, and use that state as the previous state when it
is restarted.
9.11.2. Transitions out of PAUSED state
A server transitions out of PAUSED state by being restarted. At that
time, the previous state MUST be the state the server was in prior to
entering the PAUSED state.
9.12. SHUTDOWN state
This state exists to allow one server to inform another that it will
be out of service for what is predicted to be a relatively long time,
and to allow the other server to transition immediately to PARTNER-
DOWN state, and take over completely for the server going down.
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A server which is aware that it is shutting down SHOULD send a STATE
message with the server-state field containing SHUTDOWN.
While a server may or may not transition internally into SHUTDOWN
state, the 'previous' state determined when it is restarted MUST be
the state active prior to the command to shutdown. See section 9.3.2
concerning the use of the previous state upon server restart.
9.12.1. Upon entry to SHUTDOWN state
When entering SHUTDOWN state, the server MUST record the previous
state in stable storage for use when the server is restarted. It
also MUST record the current time as the last time operational.
A server which is aware that it is shutting down SHOULD send a STATE
message with the server-state field containing SHUTDOWN.
9.12.2. Operation in SHUTDOWN state
A server in SHUTDOWN state MUST NOT respond to any DHCP client input.
If a server receives any message indicating that the partner has
moved to PARTNER-DOWN state while it is in SHUTDOWN state then it
MUST record RECOVER state as the previous state to be used when it is
restarted.
A server SHOULD wait for a few seconds after informing the partner of
entry into SHUTDOWN state (if communications are okay) to determine
if the partner entered PARTNER-DOWN state.
9.12.3. Transitions out of SHUTDOWN state
A server transitions out of SHUTDOWN state by being restarted.
10. Safe Period
Due to the restrictions imposed on each server while in
COMMUNICATIONS-INTERRUPTED state, long-term operation in this state
is not feasible for either server. One reason that these states
exist at all, is to allow the servers to easily survive transient
network communications failures of a few minutes to a few days
(although the actual time periods will depend a great deal on the
DHCP activity of the network in terms of arrival and departure of
DHCP clients on the network).
Eventually, when the servers are unable to communicate, they will
have to move into a state where they no longer can re-integrate
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without some possibility of a duplicate IP address allocation. There
are two ways that they can move into this state (known as PARTNER-
DOWN).
They can either be informed by external command that, indeed, the
partner server is down. In this case, there is no difficulty in mov-
ing into the PARTNER-DOWN state since it is an accurate reflection of
reality and the protocol has been designed to operate correctly (even
during reintegration) as long as, when in PARTNER-DOWN state the
partner is, indeed, down.
The more difficult scenario is when the servers are running unat-
tended for extended periods, and in this case an option is provided
to configure something called a "safe-period" into each server. This
OPTIONAL safe-period is the period after which either the primary or
secondary server will automatically transition to PARTNER-DOWN from
COMMUNICATIONS-INTERRUPTED state. If this transition is completed
and the partner is not down, then the possibility of duplicate IP
address allocations will exist.
The goal of the "safe-period" is to allow network operations staff
some time to react to a server moving into COMMUNICATIONS-INTERRUPTED
state. During the safe-period the only requirement is that the net-
work operations staff determine if both servers are still running --
and if they are, to either fix the network communications failure
between them, or to take one of the servers down before the expira-
tion of the safe-period.
The length of the safe-period is installation dependent, and depends
in large part on the number of unallocated IP addresses within the
subnet address pool and the expected frequency of arrival of previ-
ously unknown DHCP clients requiring IP addresses. Many environments
should be able to support safe-periods of several days.
During this safe period, either server will allow renewals from any
existing client. The only limitation concerns the need for IP
addresses for the DHCP server to hand out to new DHCP clients and the
need to re-allocate IP addresses to different DHCP clients.
The number of "extra" IP addresses required is equal to the expected
total number of new DHCP clients encountered during the safe period.
This is dependent only on the arrival rate of new DHCP clients, not
the total number of outstanding leases on IP addresses.
In the unlikely event that a relatively short safe period of an hour
is all that can be used (given a dearth of IP addresses or a very
high arrival rate of new DHCP clients), even that can provide sub-
stantial benefits in allowing the DHCP subsystem to ride through
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minor problems that could occur and be fixed within that hour. In
these cases, no possibility of duplicate IP address allocation
exists, and re-integration after the failure is solved will be
automatic and require no operator intervention.
11. Security
The Failover protocol communicates DHCP lease activity and this data
is generally easily discovered via other means, such as by pinging
addresses and doing DNS lookups. Therefore, the need to encrypt the
data over the wire is likely not great (though some sites may feel
differently).
However, it is very desirable to assure the integrity of failover
partners and to thus ensure proper operation of the servers. For
example, denial of service attacks are possible by the communication
of invalid state information to one or both servers.
Therefore, the Failover protocol MUST be capable of being secured by
using a simple shared secret message digest which covers each mes-
sage. This provides authentication of the servers, but does not pro-
vide encryption of the data exchange.
The Failover protocol MAY also be secured by using TLS [RFC 2246]
(Transport Layer Security) if encryption of the data exchange is
desired. The use of the shared secret or TLS will not protect
against TCP or IP layer attacks (such as someone sending fake TCP RST
segments). IPsec SHOULD be used to protect against most (if not all)
of these kinds of attacks.
11.1. Simple shared secret
Messages between the failover partners are authenticated through the
use of a shared secret, which is never sent over the network and must
be known by each server. How each server is told about this shared
secret and secures its storage of the shared secret is outside the
scope of this document. If a server is configured with a shared
secret for a partner, it MUST send the message-digest option in ALL
messages to that partner and it MUST treat any messages received from
that partner without a message-digest option as failing authentica-
tion.
If a server is not configured with a shared secret for a partner, it
MUST NOT send the message-digest option in any message to that
partner and it MUST treat any messages received from that partner
with a message-digest option as failing authentication.
The shared secret is used to calculate a 16 octet message-digest
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which is sent in every failover message as the message-digest option.
See section 12.16. The message-digest contains a one-way 16 octet MD5
[RFC 1321] hash calculated over a stream of octets consisting of the
entire message concatenated with the shared secret.
For calculation, the message includes the message-digest option with
the message-digest data zeroed (16-octets of zero). Once the calcula-
tion is complete, these 16 octets of zero are replaced by the 16-
octet MD5 hash and the message is sent.
For verification, the 16-octet message-digest is saved and replaced
with 16-octets of zero and calculated per above. The resulting MD5
hash is compared to the received hash and if they match, the message
is assumed authenticated.
A failover partner that fails to authenticate a received message or
receives a message without a message-digest option when configured
with a shared secret MUST close the connection immediately and take
steps to notify operators.
This use of the shared secret is very similar to that used for RADIUS
Accounting [RFC 2139].
11.2. TLS
TLS, Transport Layer Security, as specified in [RFC 2246] MAY be
used. The use of TLS would be similar to the way it is used with
SMTP [RFC 2487] and IMAP/POP3/ACAP [RFC 2595].
To request the use of TLS, the server that successfully opened a con-
nection to its peer MUST send the TLS option as part of the CONNECT
message. The server receiving the TLS option MUST respond with a
TLS-reply option indicating its acceptance or rejection of the TLS-
request in the CONNECT message.
If the CONNECTACK message contained a TLS-reply of 1 , then both
servers begin TLS negotiation.
Upon completion of this negotiation, the server which originally sent
the CONNECT message MUST resend its CONNECT message without any TLS-
request, and must wait for a corresponding CONNECTACK.
Implementation of the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [RFC 2246]
cipher suite is REQUIRED in Failover servers supporting TLS. This is
important as it assures that any two compliant implementations can be
configured to interoperate.
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12. Failover Options
This section lists all of the options that are currently defined to
be used with the failover protocol. See section 6.2 for details con-
cerning time values.
12.1. addresses-transferred
A 32 bit unsigned long in network byte order. Reports the number of
addresses transferred by the primary to the secondary server
(addresses to be used for the secondary server's private address
pool).
Code Len Number of Addresses
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 1 | 0 | 4 | n1 | n2 | n3 | n4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.2. assigned-IP-address
The DHCP managed IP address to which this message refers.
Code Len Address
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 2 | 0 | 4 | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+----+-----+-----+-----+
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12.3. binding-status
This option is used to convey the current state of a binding.
Code Len Type
+-----+-----+-----+-----+-----+
| 0 | 3 | 0 | 1 | 1-7 |
+-----+-----+-----+-----+-----+
Legal values for this option are:
Value Binding Status
----- ------------------------------------------------
1 FREE Lease is currently available
2 ACTIVE Lease is assigned to a client
3 EXPIRED Lease has expired
4 RELEASED Lease has been released by client
5 ABANDONED A server, or client flagged address as unusable
6 RESET Lease was freed by some external agent
7 BACKUP Lease belongs to secondary's private address pool
8 BACKUP-RESERVED Lease belongs to secondary's private address pool
as well as primary's since it is reserved on primary.
12.4. client-identifier
This is the client-identifier for the client associated with a
binding. The client-identifier data is subject to the same
conventions as DHCP option 81 [RFC 2132].
Code Len Client Identifier
+-----+-----+-----+-----+----+-----+---
| 0 | 4 | 0 | n | i1 | i2 | ...
+-----+-----+-----+-----+----+-----+--
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12.5. client-hardware-address
This is the hardware address for the client associated with a
binding. Byte t1 (type) MUST be set to the proper ARP hardware
address code, as defined in the ARP section of RFC 1700 (it MUST NOT
be zero!)
Code Len htype chaddr
+-----+-----+-----+-----+----+-----+-----+---
| 0 | 5 | 0 | n | t1 | c1 | c2 | ...
+-----+-----+-----+-----+----+-----+-----+---
12.6. client-last-transaction-time
The time at which this server last received a DHCP request from a
particular client expressed as an absolute time (see section 6.2).
Code Len client last transaction time
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 6 | 0 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.7. client-reply-options
This option contains options from a DHCP server's reply to a DHCP
client request. It is sent in a BNDUPD message. The first 4 bytes
of the option contain the "magic number" of the option area from
which the DHCP reply options were taken and serves to define the
format of the rest of the sub-options contained in this option.
After the magic number, the options included are in the normal
options format appropriate for that magic number.
A server SHOULD NOT include all of the options in a DHCP server's
reply to a client's request in this option, but rather a server
SHOULD include only those options which are of likely interest to its
partner server. See section 7.1 for details.
Code Len Magic Number Embedded options
+-----+-----+-----+-----+----+----+----+----+----+----+--
| 0 | 7 | 0 | n | m1 | m2 | m3 | m4 | b1 | b2 | ...
+-----+-----+-----+-----+----+----+----+----+----+----+--
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12.8. client-request-options
This option contains options from a DHCP client's request. It is
sent in a BNDUPD message. The first 4 bytes of the option contain
the "magic number" of the option area from which the DHCP client's
request options were taken and serves to define the format of the
rest of the sub-options contained in this option. After the magic
number, the options included are in the normal options format
appropriate for that magic number.
A server SHOULD NOT include all of the options in a DHCP client
request in this option, but rather a server SHOULD include only those
options which are of likely interest to its partner server. See
section 7.1 for details.
Code Len Magic Number Embedded options
+-----+-----+-----+-----+----+----+----+----+----+----+--
| 0 | 8 | 0 | n | m1 | m2 | m3 | m4 | b1 | b2 | ...
+-----+-----+-----+-----+----+----+----+----+----+----+--
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12.9. DDNS
If an implementation supports Dynamic DNS updates, this option is
used to communicate the status of the DDNS update associated with a
particular lease binding. The Flags field conveys the types of DNS
RRs that are to be updated by the DHCP server, and the status of the
DDNS update. The Domain Name field conveys the DNS FQDN that the
DHCP server is using to refer to the client, in DNS encoding as
specified in [RFC 1035].
Code Len Flags Domain Name
+-----+-----+-----+-----+-----+------+------+-----+------
| 0 | 9 | 0 | n | flags | d1 | d2 | ...
+-----+-----+-----+-----+-----+------+------+-----+------
The Flags field is a 16-bit field; several bit positions are
specified here.
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|A|D|P| MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The bits (numbered from the least-significant bit in network
byte-order) are used as follows:
0 (C): A RR update successfully completed
1 (A): Server is controlling A RR on behalf of the client
2 (D): PTR RR update successfully completed (Done)
3 (P): Server is controlling PTR RR on behalf of the client
4-15 : Must be zero
All of the unspecified bit positions SHOULD be set to 0 by servers
sending the Failover-DDNS option, and they MUST be ignored by servers
receiving the option.
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12.10. delayed-service-parameter
The delayed-service-parameter is an optional load balancing tuning
parameter, defined in [LOADB]. If it is used, it MUST be sent in the
same message as the hash-bucket-assignment option (see section
12.11). Format :
Code Len Seconds
+-----+-----+-----+-----+----+
| 0 | 10 | 0 | 1 | S |
+-----+-----+-----+-----+----+
S is a one byte value, 1..255.
12.11. hash-bucket-assignment
A set of load balancing hash values for the secondary server. See
section 5.3 for more information on how this option is used.
The format and usage of the data in this option is defined in
[LOADB].
Code Len Hash Buckets
+-----+-----+-----+-----+-----+-----+-----+-----+
| 0 | 11 | 0 | 32 | b1 | b2 | ... | b32 |
+-----+-----+-----+-----+-----+-----+-----+-----+
12.12. lease-expiration-time
The lease expiration time is the lease interval that a DHCP server
has ACKed to a DHCP client added to the time at which that ACK was
transmitted -- expressed as an absolute time (see section 6.2).
Code Len Time
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 12 | 0 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+----+-----+-----+-----+
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12.13. max-unacked-bndupd
The maximum number of BNDUPD message that this server is prepared to
accept over the TCP connection without causing the TCP connection to
block. A 32 bit unsigned integer value, in network byte order.
Code Len Maximum Unacked BNDUPD
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 13 | 0 | 4 | n1 | n2 | n3 | n4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.14. MCLT
Maximum Client Lead Time, an interval, in seconds. A 32 bit unsigned
integer value, in network byte order.
Code Len Time
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 14 | 0 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.15. message
This option is used to supply a human readable message text. It may
be used in association with the Reject Reason Code to provide a human
readable error message for the reject.
Code Len Text
+-----+-----+-----+-----+------+-----+--
| 0 | 15 | 0 | n | c1 | c2 | ...
+-----+-----+-----+-----+------+-----+--
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12.16. message-digest
The message digest for this message.
This option consists of a variable number of bytes which contain the
message digest of the message prior to the inclusion of this option.
When this option appears in a message, it MUST appear as the last
option in the message. It MUST appear in every message if message
digests are required.
Code Len Message Digest
+-----+-----+-----+-----+----+-----+-----
| 0 | 16 | 0 | n | d1 | d2 | ...
+-----+-----+-----+-----+----+-----+-----
12.17. potential-expiration-time
The potential expiration time is the time that one server tells
another server that it may wish to grant in a lease to a DHCP client.
It is an absolute time. See section 6.2.
Code Len Time
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 17 | 0 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.18. receive-timer
The number of seconds (an interval) within which the server must
receive a message from its partner, or it will assume that
communications with the partner is not ok. An unsigned 32 bit
integer in network byte order.
Code Len Receive Timer
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 18 | 0 | 4 | s1 | s2 | s3 | s4 |
+-----+-----+-----+-----+----+-----+-----+-----+
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12.19. protocol-version
The protocol version being used by the server. It is only sent in the
CONNECT and CONNECTACK messages. The current value for the version
is 1.
Code Len Version
+-----+-----+-----+-----+-----+
| 0 | 19 | 0 | 1 | 1 |
+-----+-----+-----+-----+-----+
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12.20. reject-reason
This option is used to selectively reject binding updates. It MAY be
used in a BNDACK message or a CONNECTACK message, always associated
with an assigned-IP-address option, which contains the IP address of
the update being rejected.
Code Len Reason Code
+-----+-----+-----+-----+-----+
| 0 | 20 | 0 | 1 | R1 |
+-----+-----+-----+-----+-----+
Reason codes :
0 Reserved
1 Illegal IP address (not part of any address pool).
2 Fatal conflict exists: address in use by other client.
3 Missing binding information.
4 Connection rejected, time mismatch too great.
5 Connection rejected, invalid MCLT.
6 Connection rejected, unknown reason.
7 Connection rejected, duplicate connection.
8 Connection rejected, invalid failover partner.
9 TLS not supported.
10 TLS supported but not configured.
11 TLS required but not supported by partner.
12 Message digest not supported.
13 Message digest not configured.
14 Protocol version mismatch.
15 Outdated binding information.
16 Less critical binding information.
17 No traffic within sufficient time.
18 Hash bucket assignment conflict.
19-253, reserved.
254 Unknown: Error occurred but does not match any reason code.
255 Reserved for code expansion.
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12.21. sending-server-IP-address
The IP address of the server sending this message. This option is
required for all messages if the message digest option used.
Code Len Address
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 21 | 0 | 4 | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+----+-----+-----+-----+
12.22. server-flags
This option is used to convey the current flags of the failover
endpoint in the sending server.
Code Len Server Flags
+-----+-----+-----+-----+-------+
| 0 | 22 | 0 | 1 | flags |
+-----+-----+-----+-----+-------+
The flags field is an 8-bit field; one bit position is
specified here.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S| MBZ |
+-+-+-+-+-+-+-+-+
The bits (numbered from the least-significant bit in network
byte-order) are used as follows:
0 (S): STARTUP,
Bit 0 MUST be set to 1 whenever the server is in STARTUP state,
and set to 0 otherwise. (Note that when in STARTUP state, the
state transmitted in the server-state option is usually the last
recorded state from stable storage, but see section 9.3 for
details.)
1-7 : Must be zero
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12.23. server-state
This option is used to convey the current state of the failover
endpoint in the sending server.
Code Len Server State
+-----+-----+-----+-----+-----+
| 0 | 23 | 0 | 1 | 1-9 |
+-----+-----+-----+-----+-----+
Legal values for this option are:
Value Server State
----- -------------------------------------------------------------
0 reserved
1 STARTUP Startup state (1)
2 NORMAL Normal state
3 COMMUNICATIONS-INTERRUPTED Communication interrupted (safe)
4 PARTNER-DOWN Partner down (unsafe mode)
5 POTENTIAL-CONFLICT Synchronizing
6 RECOVER Recovering bindings from partner
7 PAUSED Shutting down for a short period.
8 SHUTDOWN Shutting down for an extended
period.
9 RECOVER-DONE Interlock state prior to NORMAL
10 RESOLUTION-INTERRUPTED Comm. failed during resolution
(1) The STARTUP state is never sent to the partner server, it is
indicated by the STARTUP bit in the server-flags options (see section
12.22).
12.24. start-time-of-state
This option is used for different states in different messages. In a
BNDUPD message it represents the start time of the state of the lease
in the BNDUPD message. In a STATE message, it represents the start
time of the partner server's failover state. In all cases it is an
absolute time.
Code Len Start Time of State
+-----+-----+-----+-----+----+-----+-----+-----+
| 0 | 24 | 0 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+----+-----+-----+-----+
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12.25. TLS-reply
This option contains information relating to TLS security
negotiation. It is sent in a CONNECTACK message
A t1 value of 0 indicates no TLS operation, a value of 1 indicates
that TLS operation is required.
Code Len TLS
+-----+-----+-----+-----+-----+
| 0 | 25 | 0 | 1 | t1 |
+-----+-----+-----+-----+-----+
12.26. TLS-request
This option contains information relating to TLS security
negotiation. It is sent in a CONNECT message.
The t1 byte is the TLS request from this server. A value of 0
indicates no TLS operation (to communicate the other server MUST NOT
require TLS), a value of 1 indicates that TLS operation is desired
but not required (to communicate, the other server MAY utilize TLS),
and a value of 2 indicates that TLS operation is required (to
communicate the other server MUST utilize TLS) to establish
communications with this server.
Code Len TLS
+-----+-----+-----+-----+-----+
| 0 | 26 | 0 | 1 | t1 |
+-----+-----+-----+-----+-----+
12.27. vendor-class-identifier
A string which identifies the vendor of the failover protocol
implementation.
Code Len vendor class string
+-----+-----+-----+-----+----+-----+---
| 0 | 27 | 0 | n | c1 | c2 | ...
+-----+-----+-----+-----+----+-----+---
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12.28. vendor-specific-options
This option is used to convey options specific to a particular
vendor's implementation. The vendor class identifier is used to
specify which option space the embedded options are drawn from.
It functions similarly to the vendor class identifier and vendor
specific options in the DHCP protocol.
This option contains other options in the same two byte code, two
byte length format. If this option appears in a message without a
corresponding vendor class identifier, it MUST be ignored.
Code Len Embedded options
+-----+-----+-----+-----+----+-----+---
| 0 | 28 | 0 | n | c1 | c2 | ...
+-----+-----+-----+-----+----+-----+---
13. IANA Considerations
This document defines several number spaces (failover options, fail-
over message types, and failover reject reason codes). For all of
these number spaces, certain values are defined in this specifica-
tion. New values may only be defined by IETF Consensus, as described
in [RFC 2434]. Basically, this means that they are defined by RFCs
approved by the IESG.
14. Acknowledgments
Ralph Droms started it all, by sketching out an initial interserver
draft that embodied ideas from several past IETF meetings. In that
draft, he acknowledged contributions by Jeff Mogul, Greg Minshall,
Rob Stevens, Walt Wimer, Ted Lemon, and the DHC working group.
Kim Kinnear and Bob Cole each extended that draft, separately and
then together, until they created an interserver draft that supported
any number of servers. The complexity of that approach was just too
great, and that draft wasn't greeted with enthusiasm by many, includ-
ing its authors.
It did however lead to a much simpler approach embodied in the first
Failover draft by Greg Rabil, Mike Dooley, Arun Kapur and Ralph
Droms. This draft posited only two servers -- a primary and a
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secondary.
Kim Kinnear then wrote the Safe Failover draft to layer on top of the
Failover Draft and increase its robustness in the face of certain
rare network failures.
At the spring 1998 IETF meeting in LA, the DHC working group said
that they wanted a merged Failover and Safe Failover draft. Steve
Gonczi and Bernie Volz stepped up and produced the raw material for
such a merged draft, along with a new message format designed around
DHCP options and other extensions and clarifications. Kim Kinnear
edited their work into draft format and made other changes in time
for the Summer Chicago IETF meeting.
During the summer and fall of 1998, two groups worked on separate
implementations of the UDP failover draft. Bernie Volz and Steve
Gonczi constituted one group, and Kim Kinnear, Mark Stapp and Paul
Fox made up the other. These two groups worked together to produce
considerable changes and simplifications of the protocol during that
period, and Steve Gonczi and Kim Kinnear edited those changes into
-03 draft in time for submission to the December 1998 Orlando IETF
meeting.
In February of 1999 Kim Kinnear and Mark Stapp hosted a meeting on
people interested in the failover draft. During that meeting a gen-
eral agreement was reached to recast the failover protocol to use TCP
instead of UDP. In addition, the group together brainstormed a work-
able load-balancing technique. Kim Kinnear rewrote the entire draft
to include the changes made at that meeting as well as to restructure
the draft along guidelines suggested by Thomas Narten. The result
was the -04 draft, submitted prior to the Oslo IETF meeting.
The initial idea for a hash-based load balancing approach was offered
by Ted Lemon, and the determination of an algorithm and its integra-
tion into the draft was done by Steve Gonczi. The security section
was spearheaded by Bernie Volz. Both contributed considerably to the
ideas and text in the rest of the draft with several reviews.
In early October of 1999, three conference calls were held to discuss
the -04 draft. The -05 includes changes as a result of those calls,
perhaps the largest of which was to remove the load balancing
approach into a separate draft. Thanks to all of the many people
who participated in the conference calls. Changes were made because
of contributions by: Ted Lemon, David Erdmann, Richard Jones, Rob
Stevens, Thomas Narten, Diana Lane, and Andre Kostur.
Another conference call was held in mid-January of 2000, and the -06
draft was produced to tighten up the the -05 draft both technically
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as well as editorially.
This, the -07 draft was edited by Kim Kinnear and was based in part
on reviews by Richard Jones, Bernie Volz, and Steve Gonczi. It embo-
dies several technical updates as well as numerous editorial revi-
sions that enhance both correctness as well as clarity.
These most recent changes have not been widely circulated among the
other authors prior to submission to the IETF.
Many people have reviewed the various earlier drafts that went into
this result. At American Internet, ideas were contributed by Brad
Parker. At Cisco Systems Paul Fox and Ellen Garvey contributed to
the design of the protocol.
Glenn Waters of Nortel Networks contributed ideas and enthusiasm to
make a Failover protocol that was both "safe" and "lazy".
15. References
[AGENTINFO] Patrick, M., "draft-ietf-dhc-agent-options-11.txt", July,
2000.
[DDNS] Rekhter, Y., Stapp, M., "draft-ietf-dhc-dhcp-dns-12.txt",
March, 2000.
[LOADB] Volz, B., Gonczi, S., Lemon, T., Stevens, R., "draft-ietf-
dhc-loadb-02.txt", July, 1999.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specification", November, 1987.
[RFC 1321] Rivest, R., and Dusse, S., "The MD5 Message-Digest Algo-
rithm", RFC 1321, MIT Laboratory for Computer Science, RSA Data
Security Inc., April 1992.
[RFC 1534] Droms, R., "Interoperation between DHCP and BOOTP", RFC
1534, October 1993.
[RFC 2119] Bradner, S. "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Droms, et. al. Expires January 2001 [Page 118]
Internet Draft DHCP Failover Protocol July 2000
Extensions", Internet RFC 2132, March 1997.
[RFC 2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April
1997
[RFC 2139] Rigney, C., "Radius Accounting", RFC 2139, Livingston
Enterprises, April 1997.
[RFC 2246] Dierks, T., "The TLS Protocol, Version 1.0", RFC 2246,
January 1999.
[RFC 2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC 2487] Hoffman, P., "SMTP Service Extension for Secure SMTP over
TLS", RFC 2487, January 1999.
[RFC 2595] Newman, C., "Using TLS with IMAP, POP3, and ACAP", RFC
2595, June 1999.
[USERCLASS] Droms, R., Demirtjis A., Stump, G., Gu, Y., Vyaghrapuri,
R., Beser, B., Privat, J. "draft-ietf-dhc-userclass-08.txt", July,
2000.
16. Author's information
Ralph Droms
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
EMail: droms@bucknell.edu
Kim Kinnear
Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8000
EMail: kkinnear@cisco.com
mjs@cisco.com
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Bernie Volz
IPWorks, Inc.
959 Concord St.
Framingham, MA 01701
Phone: (508) 879-1809
EMail: volz@ipworks.com
Steve Gonczi
Network Engines, Inc.
25 Dan Road
Canton, MA 02021-2817
Phone: (781) 332-1165
Email: steve.gonczi@networkengines.com
Greg Rabil, Mike Dooley, Arun Kapur
Lucent Technologies
400 Lapp Road
Malvern, PA 19355
Phone: (800) 208-2747
EMail: grabil@lucent.com
mdooley@lucent.com
akapur@lucent.com
17. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and dis-
tributed, in whole or in part, without restriction of any kind, provided
that the above copyright notice and this paragraph are included on all
such copies and derivative works. However, this document itself may not
be modified in any way, such as by removing the copyright notice or
references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet Stan-
dards process must be followed, or as required to translate it into
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languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT-
NESS FOR A PARTICULAR PURPOSE.
Open Issues
These issues need to be resolved:
1. Get another port number for connections.
2. Resolve how to handle secondary IP address allocation.
3. Figure out a better way to identify vendors. How about an
SNMP Enterprise MIB value?
4. Need to tie reject-reasons to text of draft, remove obsolete
reject-reasons.
Droms, et. al. Expires January 2001 [Page 121]
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