作者:  来源:  发布日期:2007-8-30 11:24:36  点击次数:

1. About NTP

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 accuracy and reliability are modest, clients can be configured to use broadcast or multicast modes. Where maximum accuracy and reliability are required, clients and servers operate in either client, server, or symmetric modes.