'''[wiki:GraspExtrasSoftware STARGRASP NTP client]'''

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As of SVN revision 4427 of the stage 2 software, the STARGRASP controller has included experimental support for synchronising with an NTP server to provide timing for various events. The client can be started via the '''[wiki:GraspSwControllerCmdNtp ntp]''' command.

== Startup Performance ==

When first started, the NTP client first sets the system time from the first NTP transaction in a single step. Since it cannot make any assumptions about the system's clock drift, it initially starts with a drift correction of zero and attempts to figure out the drift from first principles over successive updates. The result is an initial overshoot that's typically on the order of a couple of milliseconds (the vertical scale below is in usec):

[[Image(stargrasp_ntp_startup.png)]]

After a couple of minutes, the NTP client typically settles to an NTP offset in the region of 100-200 usec, barring perturbations from the NTP server.

== Stargrasp Client: Observed Stability ==

Show time offset when connected to NTP server via 100Mbit and Gigabit, long-term stability, etc.


== Linux NTP Daemon: Observed Stability ==

The Linux NTP client can be quite unstable at times. The following is the reported NTP time offset (in milliseconds) from a cluster of identical Linux systems synced with an NTP server on the local network:

[[Image(linux_ntp_stability.png)]]

There are both sudden large offsets observed (tens of milliseconds) and smaller, longer-term offsets compared to the server's clock that are believed to be real (audited by means of a simple program that returns the server's time directly and compares it against each system's clock without heed of the round-trip time on the LAN, which is typically sub-1ms). These departures are as yet unexplained.