The Identifier and Sequence Number can be used by the client to match the Timestamp Reply with the timestamp request.
The data transmitted (originate timestamp) in the Timestamp is 32 bits in length and is the number of milliseconds since midnight UT.
If the time is not available in miliseconds or cannot be provided with respect to midnight UT then any time can be inserted in a timestamp provided the high order bit of the timestamp is also set to indicate this non-standard value.
