NTP Overview 2053■ Receive Timestamp: the local time at which the request arrived at the servicehost.■ Transmit Timestamp: the local time at which the reply departed the service hostfor the client.■ Authenticator: authentication information.Operation Modes of NTP Devices running NTP can implement clock synchronization in one of the followingmodes:Server/client modeFigure 612 Server/client modeWhen working in the server/client mode, a client sends a clock synchronizationmessage to servers, with the Mode field in the message set to 3 (client mode).Upon receiving the message, the servers automatically work in the server modeand send a reply, with the Mode field in the messages set to 4 (server mode). Uponreceiving the replies from the servers, the client performs clock filtering andselection, and synchronizes its local clock to that of the optimal reference source.In this mode, a client can be synchronized to a server, but not vice versa.Symmetric peers modeFigure 613 Symmetric peers modeA device working in the symmetric active mode periodically sends clocksynchronization messages, with the Mode field in the message set to 1 (symmetricNetworkServerClientClocksynchronizationmessage(Mode 3)Automatically worksin server /clientmode and sends areplyReply (Mode 4)Performs clock filtering andselection, and synchronizes its localclock to that of the optimal referencesourceNetworkSymmetricpassive peerSymmetric activepeerClocksynchronizationmessage(Mode 1)Automaticallyworks insymmetricpeers modeand sends areplyReply (Mode 2)The symmetric peers mode isestablished and the two devicescan synchronize, or besynchronized by each other Synchronizeeach otherClock synchronization messageexchange (Mode 3 and Mode 4)
