GE Mark VIe, Системное руководство

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4.6.3 NTP Timestamps
The 64-bit timestamps used by NTP consist of a 32-bit seconds part and a 32-bit fractional second part, giving NTP a time
scale of 2
32 seconds (136 years) and a theoretical resolution of 2 -32 seconds (233 picoseconds). The timescale wraps around
every 2 32 seconds (136 years). NTP uses an epoch of January 1, 1900. The first rollover will occur in 2036.
Note The current NTPv4 format supports Era Number and Era Offset, which corrects date rollover issues.
Implementations should disambiguate NTP time using a knowledge of the approximate time from other sources. Since NTP
only works with the differences between timestamps and never their absolute values, the wraparound is invisible as long as
the timestamps are within 68 years of each other. This means that the rollover will be invisible for most running systems,
since they will have the correct time to within a very small tolerance. However, systems that are starting up need to know the
date within no more than 68 years. The Mark VIe control uses a battery powered hardware clock to avoid this problem.
4.6.4 Clock Synchronization Algorithm
To synchronize its clock with a remote server, the NTP client must compute the round-trip delay time. The round-trip delay =
(t
3
- t
0
) - (t
2
- t
1
).
The time of the request packet transmission is t
0
, the time of the packet reception is t
1
, the time of the response packet
transmission is t
2
, and the time of the response packet reception is t
3
. The elapsed time on the client side between the emission
of the request packet and the reception of the response packet is t
3
- t
0
. The time the server waited before sending the answer
is t
2
- t
1
.
The offset = ((t
1
- t
0
) + (t
2
- t
3
)) / 2.
The NTP synchronization is correct when both the incoming and outgoing routes between the client and the server have a
symmetrical nominal delay. If the routes do not have a common nominal delay, the synchronization has a systematic bias of
half the difference between the forward and backward travel time.
4.6.5 Clients and Servers
The relationship between NTP servers and clients can be configured to operate in several ways. Computers using NTP can
operate in different modes with respect to different machines. For example, a single machine could be a client of a machine
with a lower stratum number, while being a peer to a machine on the same stratum, and a broadcast server to a number of
clients at higher stratum numbers.
Servers
provide time to clients. Clients send requests to the server and the server sends back a time stamped response,
along with information such as accuracy and stratum.
Clients
get time responses from a NTP server or servers, and uses the information to calibrate its clock. This consists of the
client determining how far its clock is off and adjusting its time to match that of the server. The maximum error is determined
based on the round-trip delay time for the packet to be received.
Peers
are members of a group of NTP servers that are tightly coupled. In a group of two peers, at any given time, the most
accurate peer is acting as a server and the other peers are acting as clients. The result is that peer groups will have closely
synchronized times without requiring a single server to be specified.
Communication Protocols GEH-6721_Vol_I_BP System Guide 113
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