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3. Association Modes
This page describes the various modes of operation provided in NTPv4. There are three types of associations in NTP: persistent, preemptable and ephemeral. Persistent associations are mobilized by a configuration command and never demobilized. Preemptable associations are mobilized by a configuration command which includes the preempt option or upon arrival of an automatic server discovery packet. They are demobilized by timeout or when preempted by a "better" server, as described on the Automatic Server Discovery Schemes page.
There are two principal modes of operation in NTP: client/server and broadcast. There are three automatic server discovery schemes in NTP: broadcast and pool described on the Automatic Server Discovery Schemes page. In addition, the burst options and orphan mode can be used in appropriate cases.
Following is a summary of the operations in each mode. Note that reference to option applies to the commands described on the Server Commands and Options page. See that page for applicability and defaults.
4. Client/Server Mode
Client/server mode is the most common configuration in the Internet today. It operates in the classic remote-procedure-call (RPC) paradigm with stateless servers and stateful clients. In this mode a host sends a client (mode 3) request to the specified server and expects a server (mode 4) reply at some future time. In some contexts this would be described as a "pull" operation, in that the host pulls the time and related values from the server.
A host is configured in client mode using the server or pool command and specifying the server DNS name or IPv4 or IPv6 address; the server requires no prior configuration (but see Access Control). The iburst option described later on this page is recommended for clients, as this speeds up initial synchronization from several minutes to several seconds. The burst option described later on this page can be useful to reduce jitter on very noisy dial-up or ISDN network links.
Ordinarily, the program automatically manages the poll interval between the default minimum and maximum values. The minpoll and maxpoll options can be used to bracket the range. Unless noted otherwise, these options should not be used with reference clock drivers.
5. Symmetric Active/Passive Mode
Symmetric active/passive mode is intended for configurations where a clique of low-stratum peers operate as mutual backups for each other. Each peer operates with one or more primary reference sources, such as a reference clock, or a set of secondary (stratum 2) servers known to be reliable and authentic. Should one of the peers lose all reference sources or simply cease operation, the other peers will automatically reconfigure so that time and related values can flow from the surviving peers to all hosts in the subnet. In some contexts this would be described as a "push-pull" operation, in that the peer either pulls or pushes the time and related values depending on the particular configuration.
A symmetric active peer sends a symmetric active (mode 1) message to a designated peer. If a matching configured symmetric active association is found, the designated peer returns a symmetric active message. If no matching association is found, the designated peer mobilizes an ephemeral symmetric passive association and returns a symmetric passive (mode 2) message. Since an intruder can impersonate a symmetric active peer and cause a spurious symmetric passive association to be mobilized, symmetric passive mode should always be cryptographically validated.
Due to unresolvable security issues with symmetric mode, NTPsec includes only partial support for it. The deprecated peer directive which formerly set up a symmetric active association is now a synonym for server. Servers which receive symmetric active messages will immediately reply with symmetric passive responses without setting up any new association; essentially they treat such messages exactly like client-mode messages, aside from putting a different mode number into the response.
6. Broadcast/Multicast Modes
These modes cannot be effectively secured and are deprecated in NTPsec. Client-mode support has been removed; server-side support is retained for backward compatibility but may be removed in a future release.
NTP broadcast modes are intended for configurations involving one or a few servers and a possibly very large client population. Broadcast mode can be used with Ethernet, FDDI and WiFi spans interconnected by hubs or switches. Ordinarily, broadcast packets do not extend beyond a router.
A server is configured to send broadcast messages using the broadcast command and specifying the subnet address for broadcast.
7. Manycast and Pool Modes
Manycast is no longer supported by NTPsec.
For more information on pool mode, see the Automatic Server Discovery Schemes page.
8. Poll Interval Management
NTP uses an intricate heuristic algorithm to automatically control the poll interval for maximum accuracy consistent with minimum network overhead. The algorithm measures the incidental offset and jitter to determine the best poll interval. When ntpd starts, the interval is the default minimum 64 sec. Under normal conditions when the clock discipline has stabilized, the interval increases in steps to the default maximum 1024 sec. In addition, should a server become unreachable after some time, the interval increases in steps to the maximum in order to reduce network overhead. Additional information about the algorithm is on the Poll Program page.
The default poll interval range is suitable for most conditions, but can be changed using options on the Server Commands and Options and Miscellaneous Options pages. However, when using maximum intervals much larger than the default, the residual clock frequency error must be small enough for the discipline loop to capture and correct. The capture range is 500 PPM with a 64-sec interval decreasing by a factor of two for each interval doubling. At a 36-hr interval, for example, the capture range is only 0.24 PPM.
In the NTPv4 specification and reference implementation, the poll interval is expressed in log2 units, properly called the poll exponent. It is constrained by the lower limit minpoll and upper limit maxpoll options of the server command. The limits default to 6 (64 sec) and 10 (1024 sec), respectively, which are appropriate for the vast majority of cases.
As a rule of thumb, the expected errors increase by a factor of two as the poll interval increases by a factor of four. The poll interval algorithm slowly increases the poll interval when jitter dominates the error budget, but quickly reduces the interval when wander dominates it. More information about this algorithm is on the How NTP Works page.
There is normally no need to change the poll limits, as the poll interval is managed automatically as a function of prevailing jitter and wander. The most common exceptions are the following.
With fast, lightly loaded LANs and modern processors, the nominal Allan intercept is about 500 sec. In these cases the expected errors can be further reduced using a poll exponent of 4 (16 sec). In the case of the pulse-per-second (PPS) driver, this is the recommended value.
With symmetric modes the most stable behavior results when both peers are configured in symmetric active mode with matching poll intervals of 6 (64 sec).
The poll interval should not be modified for reference clocks, with the single exception the ACTS telephone modem driver. In this case the recommended minimum and maximum intervals are 12 (1.1 hr) and 17 (36 hr), respectively.
9. Burst Options
Occasionally it is necessary to send packets temporarily at intervals less than the poll interval. For instance, with the burst and iburst options of the server command, the poll program sends a burst of several packets at 2 second intervals. In either case the poll program avoids sending needless packets if the server is not responding. The client begins a burst with a single packet. When the first packet is received from the server, the client continues with the remaining packets in the burst. If the first packet is not received within 64 s, it will be sent again for two additional retries before beginning backoff. The result is to minimize network load if the server is not responding. Additional details are on the Poll Program page.
There are two burst options where a single poll event triggers a burst. They should be used only with the server and pool commands, but not with reference clock drivers nor symmetric mode peers. In both modes, received server packets update the clock filter, which selects the best (most accurate) time values. When the last packet in the burst is sent, the next received packet updates the system variables and adjusts the system clock as if only a single packet exchange had occurred.
The iburst option is useful where the system clock must be set quickly or when the network attachment requires an initial calling or training sequence, as in PPP or ISDN services. In general, this option is recommended for server and pool commands. A burst is sent only when the server is unreachable; in particular, when first starting up. Ordinarily, the clock is set within a few seconds after the first received packet. See the Clock State Machine page for further details about the startup behavior.
The burst option is useful in cases of severe network jitter or when the network attachment requires an initial calling or training sequence. This option is recommended when the minimum poll exponent is larger than 10 (1024 sec). A burst is sent only when the server is reachable. The number of packets in the burst is determined by the poll interval so that the average interval between packets (headway) is no less than the minimum poll interval for the association.