In the symmetric modes the client/server distinction (almost)
disappears. Symmetric passive mode is intended for use by time servers
operating near the root nodes (lowest stratum) of the synchronization
subnet and with a relatively large number of peers on an intermittent
basis. In this mode the identity of the peer need not be known in
advance, since the association with its state variables is created only
when an NTP message arrives. Furthermore, the state storage can be
reused when the peer becomes unreachable or is operating at a higher
stratum level and thus ineligible as a synchronization source.

Symmetric active mode is intended for use by time servers operating near
the end nodes (highest stratum) of the synchronization subnet. Reliable
time service can usually be maintained with two peers at the next lower
stratum level and one peer at the same stratum level, so the rate of
ongoing polls is usually not significant, even when connectivity is lost
and error messages are being returned for every poll.

Normally, one peer operates in an active mode (symmetric active, client
or broadcast modes) as configured by a startup file, while the other
operates in a passive mode (symmetric passive or server modes), often
without prior configuration. However, both peers can be configured to
operate in the symmetric active mode. An error condition results when
both peers operate in the same mode, but not symmetric active mode. In
such cases each peer will ignore messages from the other, so that prior
associations, if any, will be demobilized due to reachability failure.

Broadcast mode is intended for operation on high-speed LANs with
numerous workstations and where the highest accuracies are not required.
In the typical scenario one or more time servers on the LAN send
periodic broadcasts to the workstations, which then determine the time
on the basis of a preconfigured latency in the order of a few
milliseconds. As in the client/server modes the protocol machine can be
considerably simplified in this mode; however, a modified form of the
clock selection algorithm may prove useful in cases where multiple time
servers are used for enhanced reliability.

Event Processing

The significant events of interest in NTP occur upon expiration of a
peer timer (peer.timer), one of which is dedicated to each peer with an
active association, and upon arrival of an NTP message from the various
peers. An event can also occur as the result of an operator command or
detected system fault, such as a primary reference source failure. This
section describes the procedures invoked when these events occur.

Notation Conventions

The NTP filtering and selection algorithms act upon a set of variables
for clock offset (<$Etheta ,~THETA>), roundtrip delay (<$Edelta
,~DELTA>) and dispersion (<$Eepsilon ,~EPSILON>). When necessary to
distinguish between them, lower-case Greek letters are used for
variables relative to a peer, while upper-case Greek letters are used
for variables relative to the primary reference source(s), i.e., via the
peer to the root of the synchronization subnet. Subscripts will be used
to identify the particular peer when this is not clear from context. The
algorithms are based on a quantity called the synchronization distance
(<$Elambda ,~LAMBDA>), which is computed from the roundtrip delay and
dispersion as described below.

As described in Appendix H, the peer dispersion <$Eepsilon> includes
contributions due to measurement error <$Erho~=~1~<< <<~roman
sys.precision>, skew-error accumulation <$Ephi tau>, where
<$Ephi~=~roman {NTP.MAXSKEW over NTP.MAXAGE}> is the maximum skew rate
and <$Etau~=~roman {sys.clock~-~peer.update}> is the interval since the
last update, and filter (sample) dispersion <$Eepsilon sub sigma>
computed by the clock-filter algorithm. The root dispersion <$EEPSILON>
includes contributions due to the selected peer dispersion <$Eepsilon>
and skew-error accumulation <$Ephi tau>, together with the root
dispersion for the peer itself. The system dispersion includes the
select (sample) dispersion <$Eepsilon sub xi> computed by the clock-
select algorithm and the absolute initial clock offset <$E| THETA |>
provided to the local-clock algorithm. Both <$Eepsilon> and <$EEPSILON>
are dynamic quantities, since they depend on the elapsed time <$Etau>
since the last update, as well as the sample dispersions calculated by
the algorithms.

Each time the relevant peer variables are updated, all dispersions
associated with that peer are updated to reflect the skew-error
accumulation. The computations can be summarized as follows:

<$Etheta~==~roman peer.offset> ,
<$Edelta~==~roman peer.delay> ,
<$Eepsilon~==~roman peer.dispersion~=~rho~+~phi tau~+~epsilon sub sigma>
,
<$Elambda~==~epsilon~+~{| delta |} over 2> ,

where <$Etau> is the interval since the original timestamp (from which
<$Etheta> and <$Edelta> were determined) was transmitted to the present
time and <$Eepsilon sub sigma> is the filter dispersion (see clock-
filter procedure below). The variables relative to the root of the
synchronization subnet via peer i are determined as follows:

<$ETHETA sub i~==~theta sub i> ,
<$EDELTA sub i~==~roman peer.rootdelay~+~delta sub i> ,
<$EEPSILON sub i~==~roman peer.rootdispersion~+~epsilon sub i~+~phi tau
sub i> ,
<$ELAMBDA sub i~==~EPSILON sub i~+~{| DELTA sub i |} over 2> ,

where all variables are understood to pertain to the ith peer. Finally,