3Com Switch 8800 Advanced Software V5 Configuration Guide

NTP Overview 1067
■ Receive Timestamp: the local time at which the request arrived at the service
host.
■ Transmit Timestamp: the local time at which the reply departed the service host
for the client.
■ Authenticator: authentication information.
Operation Modes of NTP Devices running NTP can implement clock synchronization in one of the following
modes:
Server/client mode
In server/client mode, a client can be synchronized to a server, but not vice versa.
When working in the server/client mode, a client sends a clock synchronization
message 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 mode
and send a reply, with the Mode field in the messages set to 4 (server mode). Upon
receiving the replies from the servers, the client performs clock filtering and
selection, and synchronizes its local clock to that of the optimal reference source.
Symmetric peers mode
After the symmetric peers mode is configured, the symmetric active peer sends
clock synchronization messages with the Mode field set to 3 (client mode) to the
symmetric passive peer. The device that receives the message automatically enters
the symmetric passive mode and sends a reply, with the Mode field in the message
set to 4 (server mode). By exchanging messages, the symmetric peers mode is
established between the two devices. Then, the two devices can synchronize, or
be synchronized by, each other. In this case, the Mode field is set to 1 (symmetric
active peer) in the clock synchronization messages sent by the symmetric active
peer, and that is set to 2 (symmetric passive peer) in the response messages sent
by the symmetric passive peer. If both devices have reference clocks, the device
whose local clock has a lower stratum level will synchronize the clock of the other
device.
Broadcast mode
In the broadcast mode, a server periodically sends clock synchronization messages
to the broadcast address 255.255.255.255. Clients listen to the broadcast
messages from servers. After a client receives the first broadcast message, the
client initiates the server/client mode request to acquire the network delay
between the client and the server. Then, the client enters the broadcast client
mode and continues listening to broadcast messages, and synchronizes its local
clock based on the received broadcast messages.
Multicast mode
In the multicast mode, a server periodically sends clock synchronization messages
to the user-configured multicast address, or, if no multicast address is configured,
to the default NTP multicast address 224.0.1.1. Clients listen to the multicast
messages from servers. After a client receives the first multicast message, the
client initiates the server/client mode request to acquire the network delay
between the client and the server. Then, the client enters the multicast client mode
and continues listening to multicast messages, and synchronizes its local clock
based on the received multicast messages.