Commit c800de4b authored by Thomas Markwalder's avatar Thomas Markwalder
Browse files

[3979] Did some clean up and rewording

parent 94847b16
......@@ -133,8 +133,8 @@ configuration file.</para>
the lease database (if true), or they should be left in the
<emphasis>expired-reclaimed</emphasis> state (if false). The latter
facilitates lease affinity, i.e. ability to re-assign expired lease to
the same client which used this lease before. See the
<xref linkend="lease-affinity"/> for the details. Also, see the
the same client which used this lease before. See
<xref linkend="lease-affinity"/> for the details. Also, see
<xref linkend="lease-reclamation"/> for the general information
about the processing of expired leases (leases reclamation).</para>
</section>
......
......@@ -6,38 +6,41 @@
<chapter id="lease-expiration">
<title>Lease Expiration in DHCPv4 and DHCPv6</title>
<para>The major role of the DHCP server is to assign addresses or/and
delegate prefixes to the DHCP clients. These addresses and delegated
prefixes are often referred to as 'leases'. The leases are typically
assigned to the clients for a finite amount of time, known as
'valid lifetime'. The DHCP client willing to continue using the assigned
leases, will periodically renew them by sending appropriate message
to the DHCP server. The DHCP server records the time when the lease
is renewed and calculates a new expiration time for it.
<para>The primary role of the DHCP server is to assign addresses and/or
delegate prefixes to DHCP clients. These addresses and prefixes are
often referred to as 'leases'. Leases are typically assigned to clients
for a finite amount of time, known as 'valid lifetime'. DHCP clients who
wish to continue using their assigned leases, will periodically renew them
by sending the appropriate message to the DHCP server. The DHCP server records
the time when these leases are renewed and calculates new expiration times
for them.
</para>
<para>If the client does not renew a lease and its valid lifetime
<para>If the client does not renew a lease before its valid lifetime
elapses, the lease is considered expired. There are many situations
when the client may cease lease renewals.
The most obvious one is the shutdown of the machine running the
client.</para>
when the client may cease lease renewals. A common scenario is when
the machine running the client shuts down for an extended period of
time.
.</para>
<para> The process through which the DHCP server makes expired leases
available for reassignment is referred to as "lease reclamation" and expired
leases returned to availability through this process are referred to as
"reclaimed".
<para>The DHCP server makes expired leases available for assignment.
This process is referred to as 'lease reclamation', and consequently
each expired lease made available for assignment is called 'reclaimed'.
The DHCP server should reclaim an expired lease as soon as it detects
that it has expired. One possible way in which the server may detect
expiration is when it is trying to allocate a lease to a client and
it finds this lease is already present in the database. If this lease
is expired, it may be allocated to the same or another client, but it
must be first reclaimed. Another way the server detects
expired leases is by periodically quering the lease database. The
further sections of this chapter explain how to configure the server
to periodically query for the expired leases and how to minimize the
impact of the periodic leases reclamation process on the server's
responsiveness. Finally, the 'lease affinity' is explained, which
provides means to assign the same lease to the returning client
after its lease has expired.
that it has expired. One way in which the server may detect expiration
is when it is trying to allocate a lease to a client and finds this
lease already present in the database but expired. Another way the
server detects expired leases is by periodically querying the lease
database for them. Regardless of how an expired lease is detected, before
it my assigned to a client, it must be reclaimed.
This chapter explains how to configure the server to periodically query
for the expired leases and how to minimize the impact of the periodic leases
reclamation process on the server's responsiveness. Finally, 'lease affinity',
which provides the means to assign the same lease to the returning client
after its lease has expired, is explained.
</para>
<para>Although, all configuration examples in this section are provided
......@@ -47,7 +50,7 @@
<section id="lease-reclamation">
<title>Lease Reclamation</title>
<para>The lease reclamation is a process in which an expired lease
<para>lease reclamation is the process through which an expired lease
becomes available for assignment to the same or a different client.
This process involves the following steps for each reclaimed lease:
</para>
......@@ -73,30 +76,29 @@
</listitem>
</itemizedlist>
<para>Please refer to the <xref linkend="dhcp-ddns-server"/> to see
how to configure the DNS updates in Kea, and to the
<para>Please refer to <xref linkend="dhcp-ddns-server"/> to see
how to configure DNS updates in Kea, and to
<xref linkend="hooks-libraries"/> for information about using
hooks libraries.</para>
</section>
<section id="lease-reclaim-config">
<title>Configuring Leases Reclamation</title>
<para>Kea can be configured to periodically detect and process expired
<para>Kea can be configured to periodically detect and reclaim expired
leases. During this process the lease entries in the database are
modified or removed, therefore the server will not process incoming DHCP
modified or removed. Therefore the server will not process incoming DHCP
messages to avoid issues with concurrent access to database information.
As a result, the server will be unresponsive when the leases reclamation
is performed, the DHCP queries will accumulate and responses will be
As a result, the server will be unresponsive while lease reclamation
is performed. DHCP queries will accumulate and responses will be
sent once the leases reclamation cycle is complete.</para>
<para>In the deployments where the response time is critical, the
administrators want to minimize the interruptions in the service
caused by the processing of expired leases. Kea provides a set of
configuration parameters to control the frequency of leases reclamation,
the maximum number of leases processed in a single cycle and the
timeout after which the reclamation should be interrupted. The
following configuration examples demonstrate how these parameters
can be used:
<para>In deployments where response time is critical, administrators may
wish to minimize the interruptions in service caused by lease reclamation.
Toward this end, Kea provides configuration parameters to control: the
frequency of lease reclamation cycles, the maximum number of leases
processed in a single reclamation cycle, and the maximum amount of time a
single reclamation cycle is allowed to run before being interrupted. The
following examples demonstrate how these parameters can be used:
<screen>
"Dhcp4": {
......@@ -120,7 +122,7 @@
<screen>
| c1 | | c2 | |c3| | c4 |
| c1 | | c2 | |c3| | c4 |
|&#x3c;----&#x3e;|&#x3c;----------&#x3e;|&#x3c;--&#x3e;|&#x3c;----------&#x3e;|&#x3c;&#x3e;|&#x3c;----------&#x3e;|&#x3c;--&#x3e;|
---------------------------------------------------------------->
| | 5s | | 5s | | 5s | | time
......@@ -142,18 +144,17 @@
queries and does not perform leases reclamation. The
<command>max-reclaim-leases</command> and
<command>max-reclaim-time</command> are set to 0, which implies that
there is no restriction on the maximum number of reclaimed leases
in the particular cycle, and the maximum duration of each cycle.
there is no restriction on the maximum number of leases reclaimed
in the particular cycle, or the maximum duration of each cycle.
</para>
<para>In the deployments with high lease pool utilization, relatively
short valid lifetimes and clients often disconnecting allowing the
leases to expire, the number of expired leases requiring reclamation
at the given time may rise significantly. In this case it is often
desired to apply restrictions on the maximum duration of the leases
reclamation cycle or the number of leases that can be reclaimed in
this cycle. The following configuration demonstrates how this
can be done:
<para>In deployments with high lease pool utilization, relatively
short valid lifetimes, and frequently disconnecting clients which
allow leases to expire; the number of expired leases requiring reclamation
at any given time may rise significantly. In this case it is often
desirable to apply restrictions on the maximum duration of a reclamation
cycle or the maximum number of leases reclaimed in a cycle. The following
configuration demonstrates how this can be done:
<screen>
"Dhcp4": {
......@@ -175,10 +176,10 @@
</para>
<para>The <command>max-reclaim-leases</command> parameter limits the number
of leases reclaimed in the single cycle to 100. The
of leases reclaimed in a single cycle to 100. The
<command>max-reclaim-time</command> limits the maximum duration of each
cycle to 50ms. The lease reclamation cycle will be interrupted when
first of these limitations is hit. The reclamation of all unreclaimed
cycle to 50ms. The lease reclamation cycle will be interrupted if either
of these limitations is reached. The reclamation of all unreclaimed
leases will be attempted in subsequent cycles.</para>
<para>The following diagram illustrates the behavior of the system in the
......@@ -200,50 +201,47 @@
more than 50ms, and thus is interrupted according to the value of the
<command>max-reclaim-time</command>. This results in equal durations of
all reclamation cycles over time. Note that in this example the limitation
of maximum 100 leases is not hit. This may be the case when the database
transactions are slow or the callouts in the hook libraries attached to
the server are slow. In any case, the choice between the selecting the
specific number of leases or the maximum time for the lease reclamation
strongly depends on the particular deployment, used lease database
backend, hooks libraries etc.</para>
<para>If the limits are applied on the maximum number of reclaimed leases
or the maximum time for a single reclamation cycle, there is a risk
that the server will not be able to catch up the number of expired
leases to reclaim. This should not be the problem if the server is
dealing with a temporary burst of expirations, because it should be
able to eventually deal with them over time. However, if leases
expire at the high rate for a longer period of time, the unreclaimed
leases will pile up in the database. In order to notify the administrator
that the current configuration does not satisfy the needs for
reclamation of expired leases, the server issues a warning message
in the log, if it was unable to reclaim all leases within the last
couple of reclamation cycles. The number of cycles after which such
warning is issued is specified with the
of maximum 100 leases is not reached. This may be the case when database
transactions are slow or callouts in the hook libraries attached to
the server are slow. Regardless, the choosing values for either the
maximum number of leases or a maximum cycle time strongly depends on the
particular deployment, lease database backend being used, and any hooks
libraries etc. Administrators may need to experiment to tune the system
to suit the dynamics of their deployment.</para>
<para>It is important to realize that with the use of these limits, there
is a risk that expired leases will accumulate faster than the server can
reclaim them. This should not be the problem if the server is dealing
with a temporary burst of expirations, because it should be able to
eventually deal with them over time. However, if leases expire at the high
rate for a longer period of time, the unreclaimed leases will pile up in
the database. In order to notify the administrator that the current
configuration does not satisfy the needs for reclamation of expired
leases, the server issues a warning message in the log, if it was unable
to reclaim all leases within the last couple of reclamation cycles. The
number of cycles after which such warning is issued is specified with the
<command>unwarned-reclaim-cycles</command> configuration parameter.
</para>
<para>Setting the <command>reclaim-timer-wait-time</command> to 0 disables
periodic reclamation of the expired leases.</para>
</section>
<section id="lease-affinity">
<title>Configuring Lease Affinity</title>
<para>Suppose that the laptop goes to a sleep mode after a period of user's
inactivity. While the laptop is in the sleep mode, the DHCP client
running on it will not renew leases obtained from the server and the
leases will eventually expire. When the laptop wakes up from the
sleep mode, it is often desired that it can continue using previous
IP addresses. In order to facilitate it, the server needs to correlate
returning clients with the expired leases they were using in the past.
When the client returns, the server will first check for those
leases and re-assign them if they are still available (not assigned
to another client). The ability of the server to re-assign the same
lease to the returning client is referred to as 'lease affinity'.
<para>Suppose that a laptop goes to a sleep mode after a period of user
inactivity. While the laptop is in sleep mode, its DHCP client will not
renew leases obtained from the server and these leases will eventually
expire. When the laptop wakes up, it is often desired that it continue
using its previous assigned IP addresses. In order to facilitate this,
the server needs to correlate returning clients with their expired leases
When the client returns, the server will first check for those leases and
re-assign them if they have not assigned to another client. The ability
of the server to re-assign the same lease to a returning client is
referred to as 'lease affinity'.
</para>
<para>When the lease affinity is enabled, the server would still
<para>When lease affinity is enabled, the server will still
reclaim leases according to the parameters described in
<xref linkend="lease-reclaim-config"/>, but the reclaimed leases
will be held in the database (rather than removed) for the specified
......@@ -251,12 +249,12 @@
if there are any reclaimed leases associated with this client and
re-assign them if possible. However, it is important to note that
any reclaimed lease may be assigned to another client if that client
asks for it. Therefore, the lease affinity provides no guarantee that
the reclaimed lease will be available for the client who used it
before. It merely increases the chances for the client to be assigned
the same lease. If the lease pool is small (mostly applies to DHCPv4
for which address space is small), there is an increased likelihood
that the expired lease will be hijacked by another client.
specifically asks for it. Therefore, the lease affinity does not
guarantee that the reclaimed lease will be available for the client
who used it before. It merely increases the chances for the client to
be assigned the same lease. If the lease pool is small (mostly applies
to DHCPv4 for which address space is small), there is an increased
likelihood that the expired lease will be hijacked by another client.
</para>
<para>Consider the following configuration:
......@@ -279,27 +277,24 @@
<para>The <command>hold-reclaim-time</command> specifies how many seconds
after an expiration a reclaimed lease should be held in the database
for re-assignment to the same client. In the example given above, the
for re-assignment to the same client. In the example given above,
reclaimed leases will be held for 30 minutes (1800s) after their
expiration. During this time, the server will likely be able to
re-assign the same lease to the returning client, unless another client
requests this lease and the server assigns it.</para>
<para>The server must occasionally remove reclaimed leases for which the
time indicated by <command>hold-reclaim-time</command> elapsed. The
<para>The server must periodically remove reclaimed leases for which the
time indicated by <command>hold-reclaim-time</command> has elapsed. The
<command>flush-reclaimed-timer-wait-time</command> controls how
often the server removes such leases. In the example provided
above, the server will initiate removal of leases 5 seconds after
above, the server will initiate removal of such leases 5 seconds after
the previous removal attempt was completed. Setting this value to 0
disables lease affinity, in which case leases will be removed from the
lease database when they are reclaimed. If the lease affinity is
enabled, it is recommended that this parameter is set to significantly
higher value than the <command>reclaim-timer-wait-time</command>
because timely removal of expired-reclaimed leases is not critical,
while this removal impacts the server's responsiveness, because the
server doesn't process DHCP messages while it removes leases from
the database.</para>
lease database when they are reclaimed. If lease affinity is enabled, it
is recommended that hold-reclaim-time be set to a value significantly
higher than the <command>reclaim-timer-wait-time</command>, as timely
removal of expired-reclaimed leases is less critical while the removal
process may impact server responsiveness.
</section>
<section id="lease-reclamation-defaults">
......
......@@ -285,24 +285,24 @@ The @c TimerMgr allows for registering timers and associating them with
user callback functions, which are executed without waiting for the
call to the @c select() function to return as a result of the timeout.
When the particular timer elapses, the blocking call to select is
interrupted by sending data over the dedicated (for a timer)
@c isc::util::WatchSocket. Each timer has an instance of the
interrupted by sending data over a dedicated (for a timer)
@c isc::util::WatchSocket. Each timer has an instance of
@c isc::util::WatchSocket associated with it, and each such socket
is registered in the @c IfaceMgr using the @c IfaceMgr::addExternalSocket.
is registered with the @c IfaceMgr using the @c IfaceMgr::addExternalSocket.
When the transmission of the data over the watch socket interrupts the
@c select() call, the user callback is executed by the
@c select() call, the user callback is executed by
@c isc::dhcp::IfaceMgr and the watch socket is cleared to accept
subsequent events for the particular timer.
subsequent events for that particular timer.
The timers are implemented using the @c isc::asiolink::IntervalTimer class.
They are run in a dedicated thread which is owned (created and destroyed)
in the @c isc::dhcp::TimerMgr. This worker thread runs an instance
of the @c isc::asiolink::IOService object, associated with all
by @c isc::dhcp::TimerMgr. This worker thread runs an instance
of @c isc::asiolink::IOService object which is associated with all
registered timers. The thread uses a common callback function which
is executed when the timer elapses. This callback function receives
is executed when a timer elapses. This callback function receives
a name of the elapsed timer as an argument and, based on that, selects the
appropriate @c isc::util::WatchSocket to be marked as ready. In order to
overcome the race conditions with a main thread, the worker thread blocks
overcome the race conditions with the main thread, the worker thread blocks
right after it marks the watch socket as ready, and waits for this
socket to be cleared by the main thread. This is the indication
that the timer specific callback function has been invoked and the
......@@ -311,7 +311,7 @@ their readiness when they elapse.
@section leaseReclamationRoutine Leases Reclamation Routine
The lease reclamation is the process in which the expired lease becomes
Lease reclamation is the process in which the expired lease becomes
available for re-assignment to the same or another client. When the
server reclaims the lease it executes the callouts registered for the
"lease4_expire" and "lease6_expire" hook points, performs the DNS update
......@@ -320,7 +320,7 @@ marks a lease as reclaimed in the lease database. The lease may be
marked as reclaimed by setting its state to @c Lease::STATE_EXPIRED_RECLAIMED
or by being removed from the database.
The leases reclamation is performed periodically for a bulk of expired
Reclamation is performed periodically for a bulk of expired
leases in the lease reclamation routine. The lease reclamation routines
for both DHCP servers are implemented in the @c isc::dhcp::AllocEngine:
- @c isc::dhcp::AllocEngine::reclaimExpiredLeases4 (DHCPv4)
......@@ -330,23 +330,23 @@ Note that besides the reclamation of the leases, these methods also
update the relevant statistics, i.e. decrease the number of assigned
leases and increase the number of reclaimed leases.
The leases reclamation routines are executed periodically according to
The reclamation routines are executed periodically according to
the server configuration (see the documentation for the
"expired-leases-processing" configuration map). Internally, they are
registered as callback functions in the @c isc::dhcp::TimerMgr
(see @ref timerManager for the details), during the servers' startup
or reconfiguration.
Execution of the leases reclamation routine may take relatively
Execution of the reclamation routine may take a relatively
long period of time. It depends on the complexity of the callouts,
whether the DNS update is required for leases, and the type of the
lease database used. While the leases reclamation routine is
executed, the server is not processing any DHCP messages to avoid
lease database used. While the reclamation routine is
executed, the server will not process any DHCP messages to avoid
race conditions being a result of concurrent access to the lease
database to allocate and reclaim leases. To make sure that the
server remains responsive, it is possible to limit the number of
leases being processed by the leases reclamation routine and/or
limit the time for the lease reclamation routine to process
limit the time for the reclamation routine to process
leases. Both limits are specified in the respective arguments
passed to the lease reclamation routines.
......@@ -357,7 +357,7 @@ passed to the reclamation routine. The first approach is desired
when the server should provide "lease affinity", i.e. ability to
re-assign the same lease to the returning client. By only
updating the lease state, the server preserves association of the
lease with the particular client. When the client returns the
lease with a particular client. When that client returns the
server may assign the same lease to the client, assuming that this
lease is still available. The lease is removed during the
reclamation when the lease affinity is not required and it is
......
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