1. Related Links
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 RFC 5905.
The pool scheme expands a single DNS name into multiple peer entries. This is intended for, but not limited to, the NTP Pool Project, 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.
4. 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 tos command. The maxclock default is 10, but it should be changed in typical configuration to some lower number, usually two greater than the minclock option of the same command.
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 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 Configuration Options page. See that page for applicability and defaults.
5. Pool Scheme
The idea of targeting servers on a random basis to distribute and balance the load is not a new one; however, the 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, similar to the manycast discovery scheme, and prunes the excess. Cryptographic authentication is not required.
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. In principle, it is possible to use a configuration file containing a single line pool pool.ntp.org. The NTP Pool Project offers instructions on using the pool with the server command, which is suboptimal but works with older versions of ntpd predating the pool command. Use of the server command does a one-time DNS lookup, and uses the IP address returned thereafter. If the server becomes unavailable, the DNS will not be re-resolved. The pool command will use multiple servers that the DNS resolves to, refreshing as required.