NTP服务器和客户端的设置及使用(EN版)

[size=2]1. About NTP[/size]

NTP is used to synchronize the time of a computer client or server

to another server or a referenced time source such as a radio, a

satellite receiver or a modem. It provides client accuracy typically

within a ms on a LAN or within 10ms over a WAN relative to a primary

server synchronized to Coordinated Universal Time (UTC). Bringing up

a NTP primary server requires a radio or satellite receiver or

modem.

Time is distributed through a hierarchy of NTP servers, with each

server adopting Primary (stratum 1) servers which are at the top of

the pile and have access to some external time source, usually a

radio clock. A stratum-2 server is one which is currently obtaining

time from a stratum-1 server, a stratum-3 server would get its time

from a stratum 2 server and so on. To avoid long lived

synchronization loops the number of strata is limited to 15. If your

network is isolated and you cannot access other people's servers

across the Internet, a radio clock may make a good investment.

Otherwise you would configure your server to one of the known

primary or secondary NTP servers.

Each client in the synchronization subnet (which also may be a

server itself) chooses exactly one of the available servers to

synchronize to, usually from among the lowest stratum servers it has

access to. Since this is not always an optimal configuration, NTP

operates under another premise as well; i.e., each server's time

should be viewed with a certain amount of distrust. NTP really

prefers to have access to several sources of lower stratum time (at

least three) since it can then apply an agreement algorithm to

detect insanity on the part of any one of these. Normally, when all

servers are in agreement, NTP will choose the best of these in terms

of lowest stratum, closest (in terms of network delay) and claimed

precisions, along with several other considerations.

Finally, there is the issue of association modes. There are a number

of modes to choose from depending on the association between each of

the servers. Configuring an association in symmetric-active mode

(usually indicated by a peer declaration in the configuration file)

indicates to the remote server that one wishes to obtain time from

the remote server and that one is also willing to supply time to the

remote server if need be. This mode is appropriate in configurations

involving a number of redundant time servers interconnected via

diverse network paths, which is presently the case for most stratum

- 1 and stratum - 2 servers on the internet today. Configuring an

association in client mode (usually indicated by a server

declaration in the configuration file) indicates that one wishes to

obtain time from the remote server, but that one is not willing to

provide the time to the remote server. This mode is appropriate for

file server and workstation clients that do not provide

synchronization to other local clients. Where the requirements in