70 NTP CONFIGURATIONIntroduction to NTP Network time protocol (NTP) is a time synchronization protocol defined byRFC1305. It is used for time synchronization among a set of distributed timeservers and clients. NTP transmits packets through UDP port 123.NTP is intended for time synchronization of all devices that have clocks in anetwork, so that the clocks of all devices can keep consistent. This enables theapplications that require unified time.A system running NTP not only can be synchronized by other clock sources, butalso can serve as a clock source to synchronize other clocks. Besides, it cansynchronize, or be synchronized by other systems by exchanging NTP packets.Applications of NTP NTP is mainly applied to synchronizing the clocks of all the network devices in anetwork. For example:■ In network management, the analysis of the log information and debugginginformation collected from different devices is meaningful and valid only whennetwork devices that generate the information adopts the same time.■ The accounting system requires that the clocks of all the network devices beconsistent.■ Some functions, such as restarting all the network devices in a networksimultaneously require that they adopt the same time.■ When multiple systems cooperate to handle a rather complex event, to ensurea correct execution order, they must adopt the same time.■ To perform incremental backup operations between a backup server and ahost, you must make sure they adopt the same time.As setting the system time manually in a network with many devices leads to a lotof workload and cannot ensure the accuracy, it is unfeasible for an administratorto perform the operation. However, an administrator can synchronize the devicesin a network with required accuracy by performing NTP configuration.NTP benefits from the following advantages:■ Defining the accuracy of clocks by strata to synchronize the time of all thedevices in a network quickly■ Supporting access control and MD5 authentication■ Sending protocol packets in unicast, multicast or broadcast moden The accuracy of a clock is determined by its stratum, which ranges from 1 to 16.The stratum of the reference clock ranges from 1 to 15. The accuracy descends
