HP OpenVMS Systems Documentation
Compaq TCP/IP Services for OpenVMS
The Network Time Protocol (NTP) synchronizes time and coordinates time distribution throughout a TCP/IP network. NTP provides accurate and dependable timekeeping for hosts on TCP/IP networks. TCP/IP Services NTP software is an implementation of the NTP Version 3 specification and maintains compatibility with NTP versions 1 and 2.
NTP provides synchronization traceable to clocks of high absolute accuracy and avoids synchronization to clocks keeping incorrect time.
Time synchronization is important in client/server computing. For example, systems that share common databases require coordinated transaction processing and timestamping of instrumental data.
This chapter reviews key concepts and describes:
Synchronized timekeeping means that hosts with accurate system timestamps send time quotes to each other. Hosts running NTP may be either time servers or clients although they are often both servers and clients.
NTP does not attempt to synchronize clocks to each other. Rather, each server attempts to synchronize to Universal Coordinated Time (UTC) using the best available source and best available transmission paths to that source. NTP expects that the time being distributed from the root of the synchronization subnet will be derived from some external source of UTC (for example, a radio clock).
If your network is isolated and you cannot access other NTP servers on
the internet, you can designate one of your nodes as the reference
clock to which all other hosts will synchronize.
12.1.1 Time Distributed Through a Hierarchy of Servers
In the NTP environment, time is distributed through a hierarchy of NTP time servers. Each server adopts a stratum that indicates how far away it is operating from an external source of UTC. NTP times are an offset of UTC. Stratum 1 servers have access to an external time source, usually a radio clock. A stratum 2 server is one that is currently obtaining time from a stratum 1 server; a stratum 3 server gets its time from a stratum 2 server, and so on. To avoid long-lived synchronization loops, the number of strata is limited to 15.
Stratum 2 (and higher) hosts might be company or campus servers that obtain time from some number of primary servers and provide time to many local clients. In general:
Internet time servers are usually stratum 1 servers. Other hosts
connected to an internet time server have stratum numbers of 2 or
higher and may act as time servers for other hosts on the network.
Clients usually choose one of the lowest accessible stratum servers
from which to synchronize.
12.1.2 How Hosts Negotiate Synchronization
Each host has its identifying stratum number encoded within UDP datagrams. Peers communicate by exchanging these timestamped UDP datagrams. NTP uses these exchanges to construct a list of possible synchronization sources then sorts them according to stratum and synchronization distance. Peers are accepted or rejected, leaving only the most accurate and precise sources.
NTP evaluates any new peer to determine whether it qualifies as a new (more suitable) synchronization source.
NTP accepts the peer under the following conditions:
The OpenVMS system clock is maintained as a software timer with a resolution of 100 nanoseconds, updated at 10-millisecond intervals. A clock update is triggered when a register, loaded with a predefined value, has decremented to zero. Upon reaching zero, an interrupt is triggered that reloads the register, thus repeating the process.
The smaller the value loaded into this register, the more quickly it
reaches zero and triggers an update. The clock runs more quickly in
such an instance. A larger value means more time between updates;
therefore, the clock runs more slowly. A clock tick is
the amount of time between clock updates.
12.1.4 How NTP Makes Adjustments to System Time
Once NTP has selected a suitable synchronization source, NTP compares the source's time with that of the local clock. If NTP determines that the local clock is running ahead of or behind the synchronization source, NTP uses a general drift mechanism to slow down or speed up the clock as needed. NTP accomplishes this by issuing a series of new clock ticks. For example, if NTP detects that the local clock is drifting ahead by +0.1884338 second, it issues a series of new ticks in an effort to reduce the difference between the synchronization source and the local clock.
If the local system time is not reasonably correct, NTP will not set the local clock. For example, if the new time is more than 1000 seconds off in either direction, NTP does not set the clock. In this case, NTP logs the error and shuts down.
NTP maintains a record of the resets it makes along with informational
messages in the NTP log file, TCPIP$NTP_RUN.LOG. See Section 12.6 for
more details about event logging and help in interpreting an NTP log
12.1.5 Configuring the Local Host
As the system manager of the local host, you determine which network hosts to use for synchronization and populate an NTP configuration file with a list of the participating hosts.
NTP hosts may be configured in one or more of the following modes:
The NTP service can be shut down and started independently of TCP/IP Services. The following files are provided:
To preserve site-specific parameter settings and commands, create the following files. These files are not overwritten when you reinstall TCP/IP Services:
The NTP configuration file TCPIP$NTP.CONF contains a list of hosts your system will use for time synchronization. Before configuring your host, you must:
To simplify configuration file maintenance, avoid configuring peer
associations with higher stratum servers.
12.3.1 Creating the Configuration File
To create a configuration file for your local host, edit a copy of the file TCPIP$NTP.TEMPLATE (located in SYS$SPECIFIC:[TCPIP$NTP]) to add the names of participating hosts, then save the file as SYS$SPECIFIC:[TCPIP$NTP]TCPIP$NTP.CONF. This file is not overwritten when you install subsequent versions of TCP/IP Services.
If you had a previous version of NTP configured on your system, your TCPIP$NTP.CONF file is created automatically and is populated with entries from the file UCX$NTP.CONF when you run the TCPIP$CONFIG procedure.
NTP configuration statements are formatted as follows:
|key ID||For all packets sent to the address, includes authentication fields encrypted using the specified key identifier, an unsigned 32-bit integer. The default is no encryption.|
|version number||Specifies the version number to be used for outgoing NTP packets. Versions 1, 2, and 3 are the choices. The default is 3.|
|prefer||Marks the server as preferred. This host will be chosen for synchronization among a set of correctly operating hosts.|
|minpoll interval||Specifies the minimum polling interval for NTP messages, in seconds to the power of two. The allowable range is 4 (16 seconds) to 14 (16384 seconds), inclusive. This option is not applicable to reference clocks. The default is 6 (64 seconds).|
|maxpoll interval||Specifies the maximum polling interval (in seconds), for NTP messages. The allowable range is 4 (16 seconds) to 14 (16384 seconds) inclusive. The default is 10 (1024 seconds). (This option does not apply to reference clocks.)|
|ttl nn||Specifies the time-to-live for multicast packets. Used only with broadcast mode.|
|auth||Controls synchronization with unconfigured peers only if the peer has been correctly authenticated using a trusted key and key identifier. By default, auth is enabled.|
|bclient||Controls the server to listen for messages from broadcast or multicast servers. By default, bclient is disabled.|
|monitor||Controls the monitoring facility. By default, monitor is enabled.|
|pll||Controls whether the server adjusts its local clock by means of NTP. If disabled, the local clock free-runs at its intrinsic time and frequency offset. This flag is useful if the local clock is controlled by some other device or protocol and NTP is used only to provide synchronization to other clients. In this case, the local clock driver is used. By default, pll is enabled.|
|stats||Enables the statistics facility. By default, stats is enabled.|
TCP/IP Services NTP includes a comprehensive monitoring facility suitable for continuous, long term recording of server and client timekeeping performance. See the statistics command below for a listing and example of each type of statistics currently supported. Statistic files are managed using file generation sets and scripts.
You can specify the following monitoring commands in your configuration file:
48773 10847.650 0.0001307 17.3478 2
48773 10847.650 127.127.4.1 9714 -0.001605 0.00000 0.00142
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
51554 79509.68 220.127.116.11 18.104.22.168 3156617109.664603 3156617109.673268 3156617109.673268 31 56617109.673268 3156617109.666556
A sample of the NTP configuration template follows:
# Copyright 2000 Compaq Computer Corporation # # Example NTP Configuration File # # Rename this template to TCPIP$NTP.CONF. # # See the Compaq TCP/IP Services for OpenVMS Management manual for # additional commands and detailed instructions on using this # configuration file. # # The Network Time Protocol (NTP) provides synchronized timekeeping among # a set of distributed time servers and clients. The local OpenVMS host # maintains an NTP configuration file, TCPIP$NTP.CONF, of participating peers. # TCPIP$NTP.CONF is maintained in the SYS$SPECIFIC:[TCPIP$NTP] directory. # # As the system manager populating this file, you must determine the # peer hosts with which the local hosts should negotiate and synchronize. # Include at least one (but preferably three) hosts that you are # certain have the following characteristics: # # * provide accurate time # * synchronize to Internet Time Servers (if they are not themselves # Internet Time Servers) # # The NTP configuration file is not dynamic, and therefore requires # restarting NTP after being edited to make the changes take effect. # However, you can make run-time configuration requests interactively # using the TCPIP$NTPDC utility. # Your NTP configuration file should always include the following # driftfile entry. The driftfile is the name of the file that stores # the clock drift (also known as frequency error) of the system clock. driftfile SYS$SPECIFIC:[TCPIP$NTP]TCPIP$NTP.DRIFT # Sample peer entries follow. Replace them with your own list of hosts # and identify the appropriate association mode. If you specify # multiple hosts, NTP can choose the best source with which to # synchronize. This also provides reliability in case one of the hosts # becomes unavailable. # Identify each peer with a fully qualified DNS host name or with # an IP address in dotted-quad notation. peer 22.214.171.124 peer 126.96.36.199 peer 188.8.131.52 peer parrot # The following commands allow interoperation of NTP with another time service # such as DTSS. If enabled (by removing #), NTP will not set the system clock. # server 127.127.1.0 prefer # fudge 127.127.1.0 stratum 0 # The following commands allow this node to act as a backup NTP server (or as # the sole NTP server on an isolated network), using its own system clock as # the reference source. If enabled (by removing #), this NTP server will # become active only when all other normal synchronization sources are # unavailable. # server 127.127.1.0 # fudge 127.127.1.0 stratum 8