In what may be the most common client/server model, a client sends an NTP message to one or moreservers and processes the replies as received. The server interchanges addresses and ports, overwritescertain fields in the message, recalculates the checksum and returns the message immediately.Information included in the NTP message allows the client to determine the server time regarding localtime and adjust the local clock accordingly. In addition, the message includes information to calculatethe expected timekeeping accuracy and reliability, as well as select the best from possibly several servers.Following conventions established by the telephone industry [BEL86], the accuracy of each server isdefined by a number called the stratum, with the topmost level (primary servers) assigned as one andeach level downwards (secondary servers) in the hierarchy assigned as one greater than the precedinglevel.Dell Networking OS synchronizes with a time-serving host to get the correct time. You can set DellNetworking OS to poll specific NTP time-serving hosts for the current time. From those time-servinghosts, the system chooses one NTP host with which to synchronize and serve as a client to the NTP host.As soon as a host-client relationship is established, the networking device propagates the timeinformation throughout its local network.Protocol OverviewThe NTP messages to one or more servers and processes the replies as received. The server interchangesaddresses and ports, fills in or overwrites certain fields in the message, recalculates the checksum, andreturns it immediately.Information included in the NTP message allows each client/server peer to determine the timekeepingcharacteristics of its other peers, including the expected accuracies of their clocks. Using thisinformation, each peer is able to select the best time from possibly several other clocks, update the localclock, and estimate its accuracy.Figure 122. NTP FieldsSystem Time and Date 893
