Time synchronization

A method of ensuring that all servers in a Directory tree report the same time.

Clocks in computers can deviate slightly, resulting in different times on different servers. Time synchronization corrects these deviations so that all servers in a Directory tree report the same time and provide a time stamp to order NDS events.

Whenever an event occurs in the Directory, such as when a password is changed, or an object renamed, NDS requests a time stamp.

A time stamp is a unique code that includes the time and identifies this event. The NDS event is assigned a time stamp so that the order in which Directory replicas are updated is correct.

NDS uses time stamps to

• Establish the order of events (such as object creation and Directory partition replication)
• Record real world time values
• Set expiration dates

Time stamps are especially important when Directory partitions are replicated and need to be concurrent with one another.

Replication allows Directory partition updates to originate from many locations. As various users update the Directory replicas of the Directory partition, some updates inevitably pertain to the same data.

For example, a user might delete an object and then recreate it. But without a method of recording the order of these events, the Directory could try to create the object and then delete it.

Time stamps allow the Directory to reproduce the order of events accurately.

Time Servers

When you install NetWare 4 on a server, you are prompted to designate it as a Single Reference, Primary, Reference, or Secondary time server.

Each designation performs a particular time synchronization function:

• Single Reference time server Provides time to Secondary time servers and to workstations.

  This server determines the time for the entire network and is the only source of time on the network. The network supervisor sets the time on the Single Reference time server.

  Because the Single Reference time server is the source of time on the network, all other servers must be able to contact it.

  The following figure illustrates a Single Reference time server providing time to Secondary time servers and to its own workstations. The Secondary time servers, in turn, provide time to their workstations.

  Figure 71
  Single Reference Time Server
  

  The Single Reference time server works on networks of any size, but this configuration is used primarily for small networks that aren't WANs.

  IMPORTANT:  If you use a Single Reference time server, don't use any Primary or Reference time servers on the network.

• Primary time server Synchronizes the time with at least one other Primary or Reference time server and provides the time to Secondary time servers and to workstations.

  Primary time servers also vote with other Primary or Reference time servers to determine what the common network time should be.

  The following figure shows Primary time servers in various locations providing time to their respective Secondary time servers. Secondary time servers, in turn, provide time to their workstations.

  Figure 72
  Primary Time Servers
  

  Use the Primary time server on larger networks to increase fault tolerance by providing redundant paths for Secondary time servers.

  If a Primary time server goes down, the Secondary time server can get the time from an alternate Primary time server.

  Place a Primary time server in each geographically distinct area so that secondary servers and workstations can access them without using WAN links.

  You must have at least one other Primary or Reference time server that the Primary time server can contact. Whenever Primary and Reference time servers are on a network, they must be able to contact each other for polling.

  However, Primary servers do adjust their internal clocks to synchronize with that common network time. Because all Primary servers adjust their clocks, network time may drift slightly.

• Reference time server Provides a time to which all other time servers and workstations synchronize.

  Reference time servers may be synchronized with an external time source, such as a radio clock.

  Reference time servers vote with other Primary or Reference time servers to determine what the common network time should be.

  However, Reference time servers don't adjust their internal clocks; instead, the Primary servers' internal clocks are adjusted to synchronize with the Reference time server.

  Therefore, a Reference time server acts as a central point to set network time. Eventually, all Primary time servers adjust their clocks to agree with a Reference time server.

  The following figure shows a Reference time server synchronized to an external clock. The Reference time server, in turn, provides time to its own secondary servers and workstations, as well as to the Primary time server at another location.

  Figure 73
  Reference Time Server
  

  Use a Reference time server when it is important to have a central point to control time on the network. Usually, only one Reference time server is installed on a network.

  You can use more than one Reference time server on a network, but you must synchronize each Reference time server with an external time source, such as a radio clock.

  You must have at least one other Primary time server that the Reference time server can contact.

  Whenever Primary and Reference time servers are on a network, they must be able to contact each other for polling.

• Secondary time server Secondary time servers obtain the time from a Single Reference, Primary, or Reference time server. They adjust their internal clocks to synchronize with the network time, and they provide the time to workstations.

  A Secondary time server doesn't participate in determining the correct network time.

  If you have designated a server on the network as a Single Reference time server, then designate all other servers on the network as Secondary time servers.

  If you have designated several servers on the network as Primary or Reference time servers, then designate all other servers on the network as Secondary time servers.

  To keep network traffic to a minimum, connect Secondary time servers to Primary or Reference time servers that are physically nearby.

  For optimal time synchronization, minimize the number of intervening routers and slow LAN segments between Secondary time servers and their Single Reference, Primary, or Reference time server.

SAP and Custom Configuration

Time source servers use one of two methods to find each other: SAP and custom configuration.

• SAP By default, Primary, Reference, and Single Reference servers use SAP to announce their presence on the network.

  Primary and Reference time servers use SAP information to determine the other servers to poll to determine the network time.

  Secondary time servers use SAP information to pick a time server to follow.

  An advantage of SAP is that it allows quick installation without regard to the network layout. It also allows automatic reconfiguration if operating modes are changed or if new servers are added to the network.

  A disadvantage of the SAP method is that it generates a small amount of additional network traffic.

  Another disadvantage is that the SAP method can be disruptive in large network environments where test servers come and go, especially if the test server is polled and its time is substantially different than network time.

• Custom configuration You can list the specific time servers that a particular server should contact.

  You can also specify that a server shouldn't listen for SAP information from other time sources and that it shouldn't advertise its presence using SAP.

  An advantage of custom configuration is that the network supervisor maintains complete control of the time synchronization environment.

  Also, custom configuration helps eliminate nonessential network SAP traffic, as well as errors associated with accidental reconfiguration.

  A disadvantage of custom configuration is the increased time required for planning and installation.

  Also, it is more difficult to install or remove Primary, Reference, or Single Reference time servers. You must manually change the approved server list maintained on each server.

Time Synchronization Methods

On small networks where it is unlikely that servers will be added or reconfigured after initial installation, we recommend that you use a Single Reference time server using SAP (the installation defaults).

On larger networks, or on networks subject to frequent accidental reconfiguration when servers are added or removed, custom configuration is recommended.

Related utility: TIMESYNC in Utilities Reference .