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Once I read somewhere that Time has been invented in the universe so that everything would not happen at once.

NTP (Network Time Protocol) provides accurate and syncronised time across the Internet. This introductory article will try to show you how to use NTP to control and synchronize your system clock.

First approach

NTP is organised in a hierarchical client-server model. In the top of this hierarchy there are a small number of machines known as reference clocks. A reference clock is known as stratum 0 and is typically a cesium clock or a Global Positioning System (GPS) that receives time from satellites. Attached to these machines there are the so-called stratum 1 servers (that is, stratum 0 clients), which are the top level time servers available to the Internet, that is, they are the best NTP servers available.

Note: in the NTP lingo measure for synchronization distance is termed as stratum: the number of steps that a system lies from a primary time source.

Following this hierarchy, the next level in the structure are the stratum 2 servers which in turn are the clients for stratum 1 servers. The lowest level of the hierarchy is made up by stratum 16 servers. Generally speaking, every server syncronized with a stratum n server is termed as being at stratum n+1 level. So, there are a few stratum 1 servers which are referenced by stratum 2 servers, wich in turn are refenced by stratum 3 servers, which are referenced by stratum 4 and so on.

NTP servers operating in the same stratum may be associated with others in a peer to peer basis, so they may decide who has the higher quality of time and then can synchronise to the most accurate.

In addition to the client-server model and the peer to peer model, a server may broadcast time to a broadcast or multicast IP addresses and clients may be configured to synchronise to these broadcast time signals.

So, at this point we know that NTP clients can operate with NTP servers in three ways:

• in a client-server basis
• in a peer to peer mode
• sending the time using broadcast/multicast

How does it work

Whenever ntpd starts it checks its configuration file (/etc/ntp.conf) to determine syncronization sources, authentication options, monitoring options, access control and other operating options. It also checks the frequency file (/etc/ntp/drift) that contains the latest estimate of clock frequency error. If specified, it will also look for a file containing the authentication keys (/etc/ntp/keys).

Note that the path and/or name of these configuration files may vary in your system. Check the -c command line option.

Once the NTP daemon is up and running, it will operate by exchanging packets (time and sanity check exchanges) with its configured servers at poll intervals and its behaviour will depend on the delay between the local time and its reference servers. Basically, the process starts when the NTP client sends a packet containing its timestamp to a server. When the server receives such a packet, it will in turn store its own timestamp and a transmit timestamp into the packet and send it back to the client. When the client receives the packet it will log its receipt time in order to estimate the travelling time of the packet.

The packet exchange takes place until a NTP server is accepted as a synchronization source, which take about five minutes. The NTP daemon tries to adjust the clock in small steps and will continue until the client gets the accurate time. If the delay between both the server and client is big enough the daemon will terminate and you will need to adjust the time manually and start the daemon again.
Sample ntp.conf configuration file

server 134.214.100.6
server pool.ntp.org

peer 192.168.100.125
peer 192.168.100.126
peer 192.168.100.127

driftfile /etc/ntp/drift
#multicastclient # listen on default 224.0.1.1
#broadcastdelay 0.008

authenticate no

#keys /etc/ntp/keys
#trustedkey 65535
#requestkey 65535
#controlkey 65535

# by default ignore all ntp packets
restrict 0.0.0.0 mask 0.0.0.0 ignore

# allow localhost
restrict 127.0.0.1 mask 255.255.255.255

# accept packets from…
restrict 192.168.100.125 mask 255.255.255.255
restrict 192.168.100.126 mask 255.255.255.255
restrict 192.168.100.127 mask 255.255.255.255

Take a look at references below to understand the configuration options.

References:

• NTP homepage
• ntpd
• Network time protocol (version 3) specification
• Public NTP Time Servers

This entry was posted on Saturday, January 17th, 2004 at 2:41 am and is filed under Articles. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.