When the filter and selection algorithms suggested in Section 4 are used, the following state variables are defined in addition to the variables described previously.
Filter Register (peer.filter): This is a shift register of NTP.SHIFT stages, where each stage stores a 3-tuple consisting of the measured delay, measured offset and calculated dispersion associated with a single observation. These 3-tuples enter from the most significant (leftmost) right and are shifted towards the least significant (rightmost) end and eventually discarded as new observations arrive.
Valid Data Counter (peer.valid): This is an integer counter indicating the valid samples remaining in the filter register. It is used to determine the reachability state and when the poll interval should be increased or decreased.
Offset (peer.offset): This is a signed, fixed-point number indicating the offset of the peer clock relative to the local clock, in seconds.
Delay (peer.delay): This is a signed fixed-point number indicating the roundtrip delay of the peer clock relative to the local clock over the network path between them, in seconds. Note that this variable can take on both positive and negative values, depending on clock precision and skew-error accumulation.
Dispersion (peer.dispersion): This is a signed fixed-point number indicating the maximum error of the peer clock relative to the local clock over the network path between them, in seconds. Only positive values greater than zero are possible.