Association modes
NTP supports the following association modes:
Client/server mode
Symmetric active/passive mode
Broadcast mode
Multicast mode
Table 2: NTP association mode
Mode | Working process | Principle | Application scenario |
|---|---|---|---|
Client/server | On the client, specify the IP address of the NTP server. A client sends a clock synchronization message to the NTP servers. Upon receiving the message, the servers automatically operate in server mode and send a reply. If the client can be synchronized to multiple time servers, it selects an optimal clock and synchronizes its local clock to the optimal reference source after receiving the replies from the servers. | A client can synchronize to a server, but a server cannot synchronize to a client. | As Figure 33 shows, this mode is intended for configurations where devices of a higher stratum synchronize to devices with a lower stratum. |
Symmetric active/passive | On the symmetric active peer, specify the IP address of the symmetric passive peer. A symmetric active peer periodically sends clock synchronization messages to a symmetric passive peer. The symmetric passive peer automatically operates in symmetric passive mode and sends a reply. If the symmetric active peer can be synchronized to multiple time servers, it selects an optimal clock and synchronizes its local clock to the optimal reference source after receiving the replies from the servers. | A symmetric active peer and a symmetric passive peer can be synchronized to each other. If both of them are synchronized, the peer with a higher stratum is synchronized to the peer with a lower stratum. | As Figure 33 shows, this mode is most often used between servers with the same stratum to operate as a backup for one another. If a server fails to communicate with all the servers of a lower stratum, the server can still synchronize to the servers of the same stratum. |
Broadcast | A server periodically sends clock synchronization messages to the broadcast address 255.255.255.255. Clients listen to the broadcast messages from the servers to synchronize to the server according to the broadcast messages. When a client receives the first broadcast message, the client and the server start to exchange messages to calculate the network delay between them. Then, only the broadcast server sends clock synchronization messages. | A broadcast client can synchronize to a broadcast server, but a broadcast server cannot synchronize to a broadcast client. | A broadcast server sends clock synchronization messages to synchronize clients in the same subnet. As Figure 33 shows, broadcast mode is intended for configurations involving one or a few servers and a potentially large client population. The broadcast mode has a lower time accuracy than the client/server and symmetric active/passive modes because only the broadcast servers send clock synchronization messages. |
Multicast | A multicast server periodically sends clock synchronization messages to the user-configured multicast address. Clients listen to the multicast messages from servers and synchronize to the server according to the received messages. | A multicast client can synchronize to a multicast server, but a multicast server cannot synchronize to a multicast client. | A multicast server can provide time synchronization for clients in the same subnet or in different subnets. The multicast mode has a lower time accuracy than the client/server and symmetric active/passive modes. |
In this document, an "NTP server" or a "server" refers to a device that operates as an NTP server in client/server mode. Time servers refer to all the devices that can provide time synchronization, including NTP servers, NTP symmetric peers, broadcast servers, and multicast servers.