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333 lines
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<TITLE>Mitigation Rules and the ``prefer'' Keyword
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<H3>
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Mitigation Rules and the <TT>prefer</TT> Keyword</H3>
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<HR>
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<H4>
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Introduction</H4>
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The mechanics of the NTP algorithms which select the best data sample from
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each available peer and the best subset of the peer population have been
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finely crafted to resist network jitter, faults in the network or peer
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operations, and to deliver the best possible accuracy. Most of the time
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these algorithms do a good job without requiring explicit manual tailoring
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of the configuration file. However, there are times when the accuracy can
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be improved by some careful tailoring. The following sections explain how
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to do this using explicit configuration items and special signals, when
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available, that are generated by some radio clocks.
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<P>In order to provide robust backup sources, primary (stratum-1) servers
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are usually operated in a diversity configuration, in which the server
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operates with a number of remote peers in addition to one or more radio
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or modem clocks operating as local peers. In these configurations the suite
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of algorithms used in NTP to refine the data from each peer separately
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and to select and weight the data from a number of peers are used with
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the entire ensemble of remote peers and local peers. As the result of these
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algorithms, a set of <I>survivors</I> are identified which can presumably
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provide the most reliable and accurate time. Ordinarily, the individual
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clock offsets of the survivors are combined on a weighted average basis
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to produce an offset used to control the system clock.
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<P>However, because of small but significant systematic time offsets between
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the survivors, it is in general not possible to achieve the lowest jitter
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and highest stability in these configurations. This happens because the
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selection algorithm tends to <I>clockhop</I> between survivors of substantially
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the same quality, but showing small systematic offsets between them. In
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addition, there are a number of configurations involving pulse-per-second
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(PPS) signals, modem backup services and other special cases, so that a
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set of mitigation rules becomes necessary to select a single peer from
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among the survivors. These rules are based on a set of special characteristics
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of the various peers and reference clock drivers specified in the configuration
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file.
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<H4>
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The <TT>prefer</TT> Peer</H4>
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The mitigation rules are designed to provide an intelligent selection between
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various peers of substantially the same statistical quality. They is designed
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to provide the best quality time without compromising the normal operation
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of the NTP algorithms. The mitigation scheme in its present form is not
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an integral component of the NTP Version 3 specification RFC- 1305. but
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is to be included in the version 4 specification when it is published.
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The scheme is based on the concept of <I>prefer peer</I>, which is specified
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by including the <TT>prefer</TT> keyword with the associated <TT>server</TT>
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or <TT>peer</TT> command in the configuration file. This keyword can be
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used with any peer or server, but is most commonly used with a radio clock.
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While the scheme does not forbid it, it does not seem useful to designate
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more than one peer as preferred, since the additional complexities to mitigate
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among them do not seem justified from on-air experience.
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<P>The prefer scheme works on the set of peers that have survived the sanity
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checks and intersection algorithms of the clock selection procedures. Ordinarily,
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the members of this set can be considered <I>truechimers</I> and any one
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of them could in principle provide correct time; however, due to various
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error contributions, not all can provide the most accurate and stable time.
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The job of the clustering algorithm, which is invoked at this point, is
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to select the best subset of the survivors providing the least variance
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in the combined ensemble average, compared to the variance in each member
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of the subset separately. The detailed operation of the clustering algorithm,
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which is given in the specification, is not important here, other than
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to point out it operates in rounds, where a survivor, presumably the worst
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of the lot, is discarded in each round until one of several termination
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conditions is met.
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<P>In the prefer scheme the clustering algorithm is modified so that the
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prefer peer is never discarded; on the contrary, its potential removal
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becomes a termination condition. If the original algorithm were about to
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toss out the prefer peer, the algorithm terminates right there. The prefer
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peer can still be discarded by the sanity checks and intersection algorithms,
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of course, but it will always survive the clustering algorithm. If it does
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not survive or for some reason it fails to provide updates, it will eventually
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become unreachable and the clock selection will remitigate to select the
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next best source.
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<P>Along with this behavior, the clock selection procedures are modified
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so that the combining algorithm is not used when a prefer peer is present.
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Instead, the offset of the prefer peer is used exclusively as the synchronization
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source. In the usual case involving a radio clock and a flock of remote
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stratum-1 peers, and with the radio clock designated a prefer peer, the
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result is that the high quality radio time disciplines the server clock
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as long as the radio itself remains operational and with valid time, as
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determined from the remote peers, sanity checks and intersection algorithm.
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<H4>
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Peer Classification</H4>
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In order to understand the effects of the various intricate schemes involved,
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it is necessary to understand some arcane details on how the algorithms
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decide on a synchronization source, when more than one source is available.
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This is done on the basis of a set of explicit mitigation rules, which
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define special classes of remote and local peers as a function of configuration
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declarations and reference clock driver type:
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<OL>
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<LI>
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The prefer peer is designated using the <TT>prefer</TT> keyword with the
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<TT>server</TT> or <TT>peer</TT> commands. All other things being equal,
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this peer will be selected for synchronization over all other survivors
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of the clock selection procedures.</LI>
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<BR>
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<LI>
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When a PPS signal is connected via the PPS Clock Discipline driver (type
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22), this is called the <I>PPS peer</I>. This driver provides precision
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clock corrections only within one second, so is always operated in conjunction
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with another peer or reference clock driver, which provides the seconds
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numbering. The PPS peer is active only under conditions explained below.</LI>
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<BR>
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<LI>
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When the Undisciplined Local Clock driver (type 1) is configured, this
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is called the <I>local clock peer</I>. This is used either as a backup
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reference source (stratum greater than zero), should all other synchronization
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sources fail, or as the primary reference source (stratum zero) in cases
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where the kernel time is disciplined by some other means of synchronization,
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such as the NIST <TT>lockclock</TT> scheme, or another synchronization
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protocol, such as the Digital Time Synchronization Service (DTSS).</LI>
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<BR>
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<LI>
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When a modem driver such as the Automated Computer Time Service driver
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(type 18) is configured, this is called the <I>modem peer</I>. This is
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used either as a backup reference source, should all other primary sources
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fail, or as the (only) primary reference source.</LI>
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<BR>
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<LI>
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Where support is available, the PPS signal may be processed directly by
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the kernel, as described in the <A HREF="kern.htm">A Kernel Model for Precision
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Timekeeping</A> page. This is called the <I>kernel discipline</I>. The
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PPS signal can discipline the kernel in both frequency and time. The frequency
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discipline is active as long as the PPS interface device and signal itself
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is operating correctly, as determined by the kernel algorithms. The time
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discipline is active only under conditions explained below.</LI>
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</OL>
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Reference clock drivers operate in the manner described in the <A HREF="refclock.htm">Reference
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Clock Drivers</A> page and its dependencies. The drivers are ordinarily
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operated at stratum zero, so that as the result of ordinary NTP operations,
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the server itself operates at stratum one, as required by the NTP specification.
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In some cases described below, the driver is intentionally operated at
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an elevated stratum, so that it will be selected only if no other survivor
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is present with a lower stratum. In the case of the PPS peer or kernel
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time discipline, these sources appear active only if the prefer peer has
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survived the intersection and clustering algorithms, as described below,
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and its clock offset relative to the current local clock is less than a
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specified value, currently 128 ms.
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<P>The modem clock drivers are a special case. Ordinarily, the update interval
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between modem calls to synchronize the system clock is many times longer
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than the interval between polls of either the remote or local peers. In
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order to provide the best stability, the operation of the clock discipline
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algorithm changes gradually from a phase-lock mode at the shorter update
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intervals to a frequency-lock mode at the longer update intervals. If both
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remote or local peers together with a modem peer are operated in the same
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configuration, what can happen is that first the clock selection algorithm
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can select one or more remote/local peers and the clock discipline algorithm
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will optimize for the shorter update intervals. Then, the selection algorithm
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can select the modem peer, which requires a much different optimization.
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The intent in the design is to allow the modem peer to control the system
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clock either when no other source is available or, if the modem peer happens
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to be marked as prefer, then it always controls the clock, as long as it
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passes the sanity checks and intersection algorithm. There still is room
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for suboptimal operation in this scheme, since a noise spike can still
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cause a clockhop either way. Nevertheless, the optimization function is
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slow to adapt, so that a clockhop or two does not cause much harm.
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<P>The local clock driver is another special case. Normally, this driver
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is eligible for selection only if no other source is available. When selected,
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vernier adjustments introduced via the configuration file or remotely using
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the <TT><A HREF="ntpdc.htm">ntpdc</A> </TT>program can be used to trim
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the local clock frequency and time. However, if the local clock driver
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is designated the prefer peer, this driver is always selected and all other
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sources are ignored. This behavior is intended for use when the kernel
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time is controlled by some means external to NTP, such as the NIST <TT>lockclock</TT>
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algorithm or another time synchronization protocol such as DTSS.
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In this case the only way to disable the local clock driver is to mark
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it unsynchronized using the leap indicator bits. In the case of modified
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kernels with the <TT>ntp_adjtime()</TT> system call, this can be done automatically
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if the external synchronization protocol uses it to discipline the kernel
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time.
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<H4>
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Mitigation Rules</H4>
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The mitigation rules apply in the intersection and clustering algorithms
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described in the NTP specification. The intersection algorithm first scans
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all peers with a persistent association and includes only those that satisfy
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specified sanity checks. In addition to the checks required by the specification,
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the mitigation rules require either the local-clock peer or modem peer
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to be included only if marked as the prefer peer. The intersection algorithm
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operates on the included population to select only those peers believed
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to represent the correct time. If one or more peers survive the operation,
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processing continues in the clustering algorithm. Otherwise, if there is
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a modem peer, it is declared the only survivor; otherwise, if there is
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a local-clock peer, it is declared the only survivor. Processing then continues
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in the clustering algorithm.
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<P>The clustering algorithm repeatedly discards outlyers in order to reduce
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the residual jitter in the survivor population. As required by the NTP
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specification, these operations continue until either a specified minimum
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number of survivors remain or the minimum select dispersion of the population
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is greater than the maximum peer dispersion of any member. The mitigation
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rules require an additional terminating condition which stops these operations
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at the point where the prefer peer is about to be discarded.
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<P>The mitigation rules establish the choice of <I>system peer</I>, which
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determine the stratum, reference identifier and several other system variables
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which are visible to clients of the local server. In addition, they establish
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which source or combination of sources control the local clock.
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<OL>
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<LI>
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If there is a prefer peer and it is the local-clock peer or the modem peer;
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or, if there is a prefer peer and the kernel time discipline is active,
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choose the prefer peer as the system peer and its offset as the system
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clock offset. If the prefer peer is the local-clock peer, an offset can
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be calculated by the driver to produce a frequency offset in order to correct
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for systematic frequency errors. In case a source other than NTP is controlling
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the system clock, corrections determined by NTP can be ignored by using
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the <TT>disable pll</TT> in the configuration file. If the prefer peer
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is the modem peer, it must be the primary source for the reasons noted
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above. If the kernel time discipline is active, the system clock offset
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is ignored and the corrections handled directly by the kernel.</LI>
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<LI>
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If the above is not the case and there is a PPS peer, then choose it as
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the system peer and its offset as the system clock offset.</LI>
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<LI>
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If the above is not the case and there is a prefer peer (not the local-clock
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or modem peer in this case), then choose it as the system peer and its
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offset as the system clock offset.</LI>
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<LI>
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If the above is not the case and the peer previously chosen as the system
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peer is in the surviving population, then choose it as the system peer
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and average its offset along with the other survivors to determine the
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system clock offset. This behavior is designed to avoid excess jitter due
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to clockhopping, when switching the system peer would not materially improve
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the time accuracy.</LI>
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<LI>
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If the above is not the case, then choose the first candidate in the list
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of survivors ranked in order of synchronization distance and average its
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offset along with the other survivors to determine the system clock offset.
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This is the default case and the only case considered in the current NTP
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specification.</LI>
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</OL>
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<H4>
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Using the Pulse-per-Second (PPS) Signal</H4>
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Most radio clocks are connected using a serial port operating at speeds
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of 9600 bps or higher. The accuracy using typical timecode formats, where
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the on-time epoch is indicated by a designated ASCII character, like carriage-return
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<TT><cr></TT>, is limited to a millisecond at best and a few milliseconds
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in typical cases. However, some radios produce a PPS signal which can be
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used to improve the accuracy with typical workstation servers to the order
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of a few tens of microseconds. The details of how this can be accomplished
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are discussed in the <A HREF="pps.htm">Pulse-per-second (PPS) Signal Interfacing</A>
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page. The following paragraphs discuss how the PPS signal is affected by
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the mitigation rules.
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<P>First, it should be pointed out that the PPS signal is inherently ambiguous,
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in that it provides a precise seconds epoch, but does not provide a way
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to number the seconds. In principle and most commonly, another source of
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synchronization, either the timecode from an associated radio clock, or
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even one or more remote NTP servers, is available to perform that function.
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In all cases, a specific, configured peer or server must be designated
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as associated with the PPS signal. This is done using the <TT>prefer</TT>
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keyword as described previously. The PPS signal can be associated in this
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way with any peer, but is most commonly used with the radio clock generating
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the PPS signal.
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<P>The PPS signal can be used in two ways to discipline the local clock,
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one using a special PPS driver described in the <A HREF="driver22.htm">PPS
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Clock Discipline</A> page, the other using PPS signal support in the kernel,
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as described in the <A HREF="kern.htm">A Kernel Model for Precision Timekeeping</A>
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page. In either case, the signal must be present and within nominal jitter
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and wander error tolerances. In addition, the associated prefer peer must
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have survived the sanity checks and intersection algorithms and the dispersion
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settled below 1 s. This insures that the radio clock hardware is operating
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correctly and that, presumably, the PPS signal is operating correctly as
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well. Second, the absolute offset of the local clock from that peer must
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be less than 128 ms, or well within the 0.5-s unambiguous range of the
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PPS signal itself. In the case of the PPS driver, the time offsets generated
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from the PPS signal are propagated via the clock filter to the clock selection
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procedures just like any other peer. Should these pass the sanity checks
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and intersection algorithms, they will show up along with the offsets of
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the prefer peer itself. Note that, unlike the prefer peer, the PPS peer
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samples are not protected from discard by the clustering algorithm. These
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complicated procedures insure that the PPS offsets developed in this way
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are the most accurate, reliable available for synchronization.
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<P>The PPS peer remains active as long as it survives the intersection
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algorithm and the prefer peer is reachable; however, like any other clock
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driver, it runs a reachability algorithm on the PPS signal itself. If for
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some reason the signal fails or displays gross errors, the PPS peer will
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either become unreachable or stray out of the survivor population. In this
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case the clock selection remitigates as described above.
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<P>When kernel support for the PPS signal is available, the PPS signal
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is interfaced to the kernel serial driver code via a modem control lead.
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As the PPS signal is derived from external equipment, cables, etc., which
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sometimes fail, a good deal of error checking is done in the kernel to
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detect signal failure and excessive noise. The way in which the mitigation
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rules affect the kernel discipline is as follows.
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<P>In order to operate, the kernel support must be enabled by the <TT>enable
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pll </TT>command in the configuration file and the signal must be present
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and within nominal jitter and wander error tolerances. In the NTP daemon,
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the PPS discipline is active only when the prefer peer is among the survivors
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of the clustering algorithm, and its absolute offset is within 128 ms,
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as in the PPS driver. Under these conditions the kernel disregards updates
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produced by the NTP daemon and uses its internal PPS source instead. The
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kernel maintains a watchdog timer for the PPS signal; if the signal has
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not been heard or is out of tolerance for more than some interval, currently
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two minutes, the kernel discipline is declared inoperable and operation
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continues as if it were not present.
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<HR>
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<ADDRESS>
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David L. Mills (mills@udel.edu)</ADDRESS>
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</BODY>
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</HTML>
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