= Automatic Server Discovery Schemes
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|image:pic/alice51.gif[]|
{millshome}pictures.html[from 'Alice's Adventures in Wonderland', Lewis Carroll]

Make sure who your friends are.

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== Related Links

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== Table of Contents

* link:#assoc[Association Management]
* link:#pool[Server Pool Scheme]

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[[modes]]
== Introduction

The NTPv4 *reference specification* supports three automatic server discovery
schemes: broadcast, manycast, and server pool. However, NTPsec only supports
the pool mechanism. For details on the reference specification,
please see https://tools.ietf.org/html/rfc5905[RFC 5905].

The pool scheme expands a single DNS name into multiple peer entries.
This is intended for, but not limited to, the
https://www.ntppool.org[NTP Pool], a worldwide set of servers
volunteered for public use.

The mechanism might be described as _grab-n'-prune._ Through one means or
another, a number of associations are "grabbed" either directly or indirectly
from the configuration file, and they are ordered from best to worst according
to the NTP mitigation algorithms, and surplus associations are pruned.

[[assoc]]
== Association Management

Pool discovery uses an iterated process to discover new preemptable client
associations as long as the total number of client associations is less
than the +maxclock+ option of the link:miscopt.html#tos[+tos+] command,
which should typically be changed from the default.

Pool discovery uses a stratum filter to select just those servers with
strata considered useful. This can avoid large numbers of clients
ganging up on a small number of low-stratum servers and avoids servers
below or above specified stratum levels. By default, servers of all
strata are acceptable; however, the +tos+ command can be used to
restrict the acceptable range from the +floor+ option, inclusive, to the
+ceiling+ option, exclusive. Potential servers operating at the same
stratum as the client will be avoided.  Additional filters can be
supplied using the methods described on the
link:authentic.html[Authentication Support] page.

The pruning process uses a set of unreach counters, one for each
association created by the configuration or discovery process. At each
poll interval, the counter is increased by one. If an acceptable packet
arrives for an association, the counter is set to zero. If the
the counter reaches an arbitrary threshold of 10, the association
becomes a candidate for pruning.

The pruning algorithm is very simple. If an ephemeral or preemptable
association becomes a candidate for pruning, it is immediately
demobilized. If a persistent association becomes a candidate for
pruning, it is not demobilized, but its poll interval is set at the
maximum. The pruning algorithm design avoids needless discovery/prune
cycles for associations that wander in and out of the survivor list, but
otherwise have similar characteristics.

Following is a description of the pool scheme. Note that
reference to options applies to the commands described on the
link:confopt.html[Configuration Options] page. See that page
for applicability and defaults.

[[pool]]
== Pool Scheme

The idea of targeting servers on a random basis to distribute and
balance the load is not a new one; however, the
https://www.ntppool.org/[NTP Pool Project] puts
this on steroids. At present, several thousand operators around the
globe have volunteered their servers for public access. In general,
NTP is a lightweight service and servers used for other purposes don't
mind an additional small load. The trick is to randomize over the
population and minimize the load on any one server while retaining the
advantages of multiple servers using the NTP mitigation algorithms.

To support this service, custom DNS software is used by pool.ntp.org
and its subdomains to discover a random selection of participating
(in-country) servers in response to a DNS query. The client receiving
this list mobilizes some or all of them, as described above.  Cryptographic
authentication is not supported.

The pool scheme is configured using one or more +pool+ commands with DNS
names indicating the pool from which to draw. The +pool+ command can be
used more than once; duplicate servers are detected and discarded.  For
configuration examples, see link:quick.html#pool[Using the NTP Pool].

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