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Compaq TCP/IP Services for OpenVMS
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Appendix B
Configuring and Managing NTP

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 4 specification and maintains compatibility with NTP Versions 1, 2, and 3.

Time synchronization is important in client/server computing. For example, systems that share common databases require coordinated transaction processing and timestamping of instrumental data.

NTP provides synchronization that is traceable to clocks of high absolute accuracy and avoids synchronization to clocks that keep incorrect time.

This chapter reviews key concepts and describes:

B.1 Key Concepts

Synchronized timekeeping means that hosts with accurate system timestamps send time quotes to each other. Hosts that run NTP can 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 Coordinated Universal Time (UTC) using the best available source and the 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.

B.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:

  • Lower-strata hosts act as time servers.
  • Higher-strata hosts are clients that adjust their time clocks according to the servers.

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.

B.1.2 How Hosts Negotiate Synchronization

The identifying stratum number of each host is 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 rejects the peer under the following conditions:

  • The peer is not synchronized.
  • The stratum is higher than the current source's stratum.
  • The peer is synchronized to the local node.

NTP accepts the peer under the following conditions:

  • There is no current time source.
  • The current source is unreachable.
  • The current source is not synchronized
  • The new source's stratum is lower than the current source.
  • The new source's stratum is the same as the current source, but its distance is closer to the synchronization source by more than 50 percent.

B.1.3 How the OpenVMS System Maintains the System Clock

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 the register reaches zero and triggers an update. Consequently, the clock runs more quickly. 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.

B.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 to reduce the difference between the synchronization source and the local clock.

If the local system time is not reasonably correct, NTP does 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. For details about event logging and for help interpreting an NTP log file, see Section B.5.

B.1.5 Configuring the Local Host

The system manager of the local host, determines which network hosts to use for synchronization and populates an NTP configuration file with a list of the participating hosts.

NTP hosts can be configured in any of the following modes:

  • Client/server mode
    This mode indicates that the local host wants to obtain time from the remote server and is willing to supply time to the remote server. This mode is appropriate in configurations involving a number of redundant time servers interconnected through diverse network paths. Internet time servers generally use this mode.
    Indicate this mode with a peer statement in the configuration file, as shown in the following example:

  • Client mode
    This mode indicates that the local host wants to obtain time from the remote server but it is not willing to provide time to the remote server. Client mode is appropriate for file server and workstation clients that do not provide synchronization to other local clients. A host with higher stratum generally uses this mode.
    Indicate client mode with the server statement in the configuration file, as shown in the following example:

  • Broadcast mode
    This mode indicates that the local server will send periodic broadcast messages to a client population at the broadcast/multicast address specified. This specification normally applies to the local server operating as a sender.
    Indicate this mode with a broadcast statement in the configuration file, as shown in the following example:

  • Multicast mode
    A multicast client is configured using the broadcast statement, but with a multicast group (class D) address instead of a local subnet broadcast address. However, there is a subtle difference between broadcasting and multicasting. Broadcasting is specific to each interface and local subnet address. If more than one interface is attached to a machine, a separate broadcast statement applies to each one.
    IP multicasting is a different paradigm. A multicast message has the same format as a broadcast message and is configured with the same broadcast statement, but with a multicast group address instead of a local subnet address. By design, multicast messages travel from the sender via a shortest-path or shared tree to the receivers, which might require these messages to emit from one or all interfaces but to carry a common source address. However, it is possible to configure multiple multicast group addresses using multiple broadcast statements. Other than these differences, multicast messages are processed just like broadcast messages. Note that the calibration feature in broadcast mode is extremely important, since IP multicast messages can travel far different paths through the IP routing fabric than can ordinary IP unicast messages.
    The Internet Assigned Number Association (IANA) has assigned multicast group address to NTP, but you should use this address only where the multicast span can be reliably constrained to protect neighbor networks. In general, you should use group addresses that have been given out by your administrator, as described in RFC 2365, or GLOP group addresses, as described in RFC 2770.
  • Manycast mode
    Manycasting is an automatic discovery and configuration paradigm new to NTP Version 4. It is intended as a means for a multicast client to survey the nearby network neighborhood for cooperating manycast servers, to validate them using cryptographic means, and to evaluate their time values with respect to other servers that might be in the vicinity. The intended result is that each manycast client mobilizes client associations with the best three of the available manycast servers and automatically reconfigures to sustain this number of servers if one or more fail >B.2 NTP Service Startup and Shutdown

    The NTP service can be shut down and started independently of TCP/IP Services. The following files are provided:

    • SYS$STARTUP:TCPIP$NTP_STARTUP.COM allows you to start the NTP service.
    • SYS$STARTUP:TCPIP$NTP_SHUTDOWN.COM allows you to shut down the NTP service.

    To preserve site-specific parameter settings and commands, create the following files. These files are not overwritten when you reinstall TCP/IP Services:

    • SYS$STARTUP:TCPIP$NTP_SYSTARTUP.COM can be used as a repository for site-specific definitions and parameters to be invoked when the NTP service is started.
    • SYS$STARTUP:TCPIP$NTP_SYSHUTDOWN.COM can be used as a repository for site-specific definitions and parameters to be invoked when the NTP service is shut down.

    B.3 Configuring Your NTP Host

    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 do the following:

    1. Select time sources.
    2. Obtain the IP addresses or host names of the time sources.
    3. Obtain the version number of NTP that the hosts are running.

    To ensure reliable synchronization, select multiple time sources that you are certain provide accurate time and that are synchronized to an Internet time server.

    To minimize common points of failure, avoid synchronizing the following:

    • The local host to another peer at the same stratum, unless the latter is receiving time from a lower stratum source to which the local host cannot connect.
    • More than one host in a particular administrative domain to the same time server outside that domain.

    To simplify configuration file maintenance, avoid configuring peer associations with higher stratum servers.

    B.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 a UCX version of NTP is 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.

    B.3.2 Configuration Statements and Options

    In the following configuration statements, the various modes are determined by the statement keyword and the type of the required IP address. Addresses are classsed by type as ( s ) a remote server or peer (IP class A, B, and C), ( b ) the broadcast address of a local interface, ( m ) a multicast address (IP class D), or ( r ) a reference clock address (127.127.x.x).

    NTP configuration statements are formatted as follows:

    • peer address [key ID] [version number] [prefer] [minpoll interval] [maxpoll interval]
      server address [key ID] [version number] [prefer ][burst] [iburst] [minpoll interval] [maxpoll interval]
      broadcast address [key ID] [version number][minpoll interval][ttl nn]
      manycastclient address [key ID] [version number][[minpoll interval] [maxpoll interval][ttl nn]
      These four statements specify the time server name or address to be used and the mode in which to operate. The address can be either a DNS name or an IP address in dotted-quad notation.
      • peer --- For type s addresses only, this statement mobilizes a persistent symmetric-active mode association with the specified remote peer. This statement should not be used for type b , type m , or type r addresses.
      • server --- For type s and type r addresses only, this statement mobilizes a persistent client mode association with the specified remote server or local reference clock. This statement should not be used for type b or type m addresses.
      • broadcast --- For type b and type m addresses only, this statement mobilizes a persisent broadcast mode association. Multiple statements can be used to specify multiple local broadcast interfaces (subnets) and/or multiple multicast groups. Note that local broadcast messages go only to the interface associated with the subnet specified, but multicast messages go to all interfaces.
      • manycastclient --- For type m addresses only, this statement mobilizes a manycast client mode association for the multicast address specified. In this case, a specific address must be supplied that matches the address used on the manycastserver statement for the designated manycast servers.
        The manycastclient statement specifies that the local server is to operate in client mode with the remote servers that are discovered as the result of broadcast/multicast messages. The client broadcasts a request message to the group address associated with the specified address and specifically enabled servers respond to these messages. The client selects the servers providing the best time and continues as with the server statement. The remaining servers are discarded as if never heard.

      The following table describes the options to the previous statements:
      Option Description
      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, 3, and 4 are the choices. The default is 4.
      prefer Marks the server as preferred. This host will be chosen for synchronization among a set of correctly operating hosts.
      burst When the server is reachable and at each poll interval, send a burst of eight packets instead of the usual one packet. The spacing between the first and the second packets is about 16 seconds to allow a modem call to complete, while the spacing between the remaining packets is about 2 seconds. This is designed to improve timekeeping quality with the server command and s addresses.
      iburst When the server is unreachable and at each poll interval, send a burst of eight packets instead of the usual one. As long as the server is unreachable, the spacing between packets is about 16 seconds to allow a modem call to complete. Once the server is reachable, the spacing between packets is about 2 seconds. This is designed to speed the initial synchronization acquisition with the server command and s addresses.
      minpoll interval Specifies the minimum polling interval for NTP messages, in seconds to the power of 2. 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 and manycast modes.

    • broadcastclient
      This statement enables reception of broadcast server messages to any local interface (type b) address. Upon receiving a message for the first time, the broadcast client measures the nominal server propagation delay using a brief client/server exchange with the server, then enters broadcastclient mode, in which it listens for and synchronizes to succeeding broadcast messages. Note that to avoid accidental or malicious disruption in this mode, both the server and client should use authentication and the same trusted key and key identifier.
    • broadcastdelay