48System Time and DateSystem time and date settings and the network time protocol (NTP) are supported on the Z9000platform.You can set system times and dates and maintained through the NTP. They are also set through the DellNetworking Operating System (OS) command line interfaces (CLIs) and hardware settings.Network Time ProtocolThe network time protocol (NTP) synchronizes timekeeping among a set of distributed time servers andclients.The protocol also coordinates time distribution in a large, diverse network with various interfaces. In NTP,servers maintain the time and NTP clients synchronize with a time-serving host. NTP clients choose fromamong several NTP servers to determine which offers the best available source of time and the mostreliable transmission of information.NTP is a fault-tolerant protocol that automatically selects the best of several available time sources tosynchronize to. You can combine multiple candidates to minimize the accumulated error. Temporarily orpermanently insane time sources are detected and avoided.Dell Networking recommends configuring NTP for the most accurate time. In Dell Networking OS, youcan configure other time sources (the hardware clock and the software clock).NTP is designed to produce three products: clock offset, roundtrip delay, and dispersion, all of which arerelative to a selected reference clock.• Clock offset — represents the amount to adjust the local clock to bring it into correspondence withthe reference clock.• Roundtrip delay — provides the capability to launch a message to arrive at the reference clock at aspecified time.• Dispersion — represents the maximum error of the local clock relative to the reference clock.Because most host time servers synchronize via another peer time server, there are two components ineach of these three products, those determined by the peer relative to the primary reference source ofstandard time and those measured by the host relative to the peer.In order to facilitate error control and management of the subnet itself, each of these components ismaintained separately in the protocol. They provide not only precision measurements of offset and delay,but also definitive maximum error bounds, so that the user interface can determine not only the time, butthe quality of the time as well.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 localSystem Time and Date 795
