Differences

This shows you the differences between two versions of the page.

--- doc:appunti:linux:sa:ntp [2005/12/15 15:35] – external edit 127.0.0.1
+++ doc:appunti:linux:sa:ntp [2011/11/17 10:36] (current) – [ntpq] niccolo
@@ Line 57: / Line 57: @@
 </code>
+===== ntpq =====
+Riportiamo dalla pagina [[http://www.meinberg.de/english/info/ntp.htm|NTP Overview]] una spiegazione dell'output del comando.
+<code>
+   remote      refid      st t when poll reach   delay   offset  jitter
+=======================================================================
+ LOCAL(0)      LOCAL(0)   12 l   30   64  377    0.000    0.000   0.000
+*GENERIC(0)    .DCFa.      0 -   24   64  377    0.000    0.050   0.003
++172.16.3.103  .PPS.       1 u   36   64  377    1.306   -0.019   0.043
+</code>
+If the first character of a line is not blank then it contains a qualifier for the corresponding reference time source. Immediately after the daemon has been started all qualifiers are blank. The NTP daemon needs several polling cycles to check the available time sources and declare one of them as the reference it synchronizes to.
+An asterisk * in the first column marks the reference time source which is currently preferred by the NTP daemon, the + character marks high quality candidates for the reference time which could be used if the currently selected reference time source should become unavailable.
+The column **remote** displays the IP address or the host name of the reference time source, where **LOCAL** refers to the local clock. The **refid** shows the type of the reference clock, where e.g. **LOCAL** or **LCL** refers to the local clockagain, **.DCFa.** refers to a standard DCF77 time source, and **.PPS.** indicates that the reference clock is disciplined by a hardware pulse-per-second signal. Other identifiers are possible, depending on the type of the reference clock.
+The column **st** reflects the stratum number of the reference time source. In the example above, the local clock has stratum 12, the remote time server at 172.16.3.103 has stratum 1 which is the best you can see across the network, and the local radio clock has stratum 0, so the radio clock is currently being preferred.
+Every time a **when** count reaches the **poll** number in the same line, the NTP daemon queries the time from the corresponding time source and resets the **when** count to 0. The query results of each polling cycle are filtered and used as a measure for the clock's quality and reachability.
+The column **reach** shows if a reference time source could be reached at the last polling intervals, i.e. data could be read from the reference time source, and the **reference time source was synchronized**. The value must be interpreted as an 8 bit shift register whose contents is for historical reasons displayed as octal values. If the NTP daemon has just been started, the value is 0. Each time a query was successful a '1' is shifted in from the right, so after the daemon has been started the sequence of reach numbers is 0, 1, 3, 7, 17, 37, 77, 177, 377. The maximum value 377 means that the eight last queries were completed successfully.
+Queries are considered successful if data can be received from the time source, and the time source in turn claims to be synchronized to some other timesource. In case of a hardware reference clock this means the query considered unsuccessful if the hardware reference clock is not synchronized to its incoming time signal, e.g. because the clock's antenna has been disconnected, or if no data can be received e.g. because the serial cable to an external device has been disconnected.
+The NTP daemon must have reached a reference time source several times (reach not 0) before it selects a preferred time source and puts an asterisk in the first column.
+The columns **delay**, **offset** and **jitter** show some timing values which are derived from the query results. In some versions of **ntpq** the last column is labeled **disp** (for dispersion) instead of **jitter**. All values are in in milliseconds. The **delay** value is derived from the roundtrip time of the queries. The **offset** value shows the difference between the reference time and the system clock. The **jitter** value indicates the magnitude of jitter between several time queries.
 ===== Client =====