ntp: document additional options

* listen-address
* allow-clients

1 files changed, 47 insertions, 10 deletions

diff --git a/docs/system/ntp.rst b/docs/system/ntp.rst
index 95ff0692..0836f2fa 100644
--- a/docs/system/ntp.rst
+++ b/docs/system/ntp.rst
@@ -1,19 +1,56 @@
 .. _ntp:
 
+###
 NTP
-===
+###
 
-there are 3 default NTP server set. You are able to change them.
+:abbr:`NTP (Network Time Protocol`) is a networking protocol for clock
+synchronization between computer systems over packet-switched, variable-latency
+data networks. In operation since before 1985, NTP is one of the oldest Internet
+protocols in current use.
 
-.. code-block:: none
+NTP is intended to synchronize all participating computers to within a few
+milliseconds of :abbr:`UTC (Coordinated Universal Time)`. It uses the
+intersection algorithm, a modified version of Marzullo's algorithm, to select
+accurate time servers and is designed to mitigate the effects of variable
+network latency. NTP can usually maintain time to within tens of milliseconds
+over the public Internet, and can achieve better than one millisecond accuracy
+in local area networks under ideal conditions. Asymmetric routes and network
+congestion can cause errors of 100 ms or more.
 
-  set system ntp server 0.pool.ntp.org
-  set system ntp server 1.pool.ntp.org
-  set system ntp server 2.pool.ntp.org
+The protocol is usually described in terms of a client-server model, but can as
+easily be used in peer-to-peer relationships where both peers consider the other
+to be a potential time source. Implementations send and receive timestamps using
+:abbr:`UDP (User Datagram Protocol)` on port number 123.
 
-To set up VyOS as an NTP responder, you must specify the listen address and optionally the permitted clients.
+NTP supplies a warning of any impending leap second adjustment, but no
+information about local time zones or daylight saving time is transmitted.
 
-.. code-block:: none
+The current protocol is version 4 (NTPv4), which is a proposed standard as
+documented in :rfc:`5905`. It is backward compatible with version 3, specified
+in :rfc:`1305`.
 
-  set system ntp listen-address 192.168.199.1
-  set system ntp allow-clients address 192.168.199.0/24
+Configuration
+=============
+
+.. cfgcmd:: set system ntp server '<address | fqdn>'
+
+   Configure one or more servers for synchronisation. Server name cen be either
+   an IP address or :abbr:`FQDN (Fully Qualified Domain Name)`.
+
+   There are 3 default NTP server set. You are able to change them.
+
+   * 0.pool.ntp.org
+   * 1.pool.ntp.org
+   * 2.pool.ntp.org
+
+.. cfgcmd:: set system ntp listen-address '<address>'
+
+   Setup VyOS as an NTP responder, you must specify the `<address>` and
+   optionally the permitted clients. Multiple listen addresses can be
+   configured.
+
+.. cfgcmd:: set system ntp allow-clients address '<address>'
+
+   List of networks or client addresses permitted to contact this NTP server.
+   Multiple networks can be configured.