Patent ID: 8762796

Claim:
A communication device that is connected to a plurality of time servers via a network configured with one or more transfer devices, the communication device comprising: a network controller configured to receive a message containing time information which indicates a timing counted by one of the time servers and containing a network identifier, and configured to obtain a receiving timing of receiving the message; a network processing unit configured to, when the network identifier specified in the message does not match with any network identifier assigned to any one of the plurality of time servers, destroy the message; a protocol processing unit configured to, when the network identifier has a match, calculate a time error by referring to the time information specified in the message and by referring to the receiving timing, configured to detect whether a first time server is malfunctioning, and configured to, when it is detected that the first time server is malfunctioning, output the time error calculated by referring to a message containing a network identifier assigned to a second time server; a servo configured to calculate an operation amount by referring to the time error; and a clock configured to vary a clock rate according to the operation amount.