vyos-documentation/docs/configuration/service/ntp.rst

.. _ntp:

###
NTP
###

: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.

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.

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.

NTP supplies a warning of any impending leap second adjustment, but no
information about local time zones or daylight saving time is transmitted.

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`.

.. note:: VyOS 1.4 uses chrony instead of ntpd (see :vytask:`T3008`) which will
   no longer accept anonymous NTP requests as in VyOS 1.3. All configurations
   will be migrated to keep the anonymous functionality. For new setups if you
   have clients using your VyOS installation as NTP server, you must specify
   the `allow-client` directive.

Configuration
=============

.. cfgcmd:: set service ntp server <address>

   Configure one or more servers for synchronisation. Server name can 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.

   * ``time1.vyos.net``
   * ``time2.vyos.net``
   * ``time3.vyos.net``

.. cfgcmd:: set service ntp server <address> <noselect | nts | pool | prefer | ptp | interleave>

   Configure one or more attributes to the given NTP server.

   * ``noselect`` marks the server as unused, except for display purposes. The
     server is discarded by the selection algorithm.

   * ``nts`` enables Network Time Security (NTS) for the server as specified 
     in :rfc:`8915`

   * ``pool`` mobilizes persistent client mode association with a number of
     remote servers.

   * ``prefer`` marks the server as preferred. All other things being equal,
     this host will be chosen for synchronization among a set of correctly
     operating hosts.

   * ``ptp`` enables the PTP transport for this server (see :ref:`ptp-transport`).

   * ``interleave`` enables NTP interleaved mode (see
     `draft-ntp-interleaved-modes`_), which can improve synchronization accuracy
     and stability when supported by both parties.

.. cfgcmd:: set service ntp listen-address <address>

   NTP process will only listen on the specified IP address. You must specify
   the `<address>` and optionally the permitted clients. Multiple listen
   addresses for same IP family is no longer supported. Only one IPv4 and one
   IPv6 address can be configured, using separate commands for each.

.. cfgcmd:: set service ntp allow-client address <address>

   List of networks or client addresses permitted to contact this NTP server.

   Multiple networks/client IP addresses can be configured.

.. cfgcmd:: set service ntp vrf <name>

   Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.

.. cfgcmd:: set service ntp leap-second [ignore|smear|system|timezone]

   Define how to handle leap-seconds.

   * `ignore`: No correction is applied to the clock for the leap second. The
     clock will be corrected later in normal operation when new measurements are
     made and the estimated offset includes the one second error.

   * `smear`: When smearing a leap second, the leap status is suppressed on the
     server and the served time is corrected slowly by slewing instead of
     stepping. The clients do not need any special configuration as they do not
     know there is any leap second and they follow the server time which
     eventually brings them back to UTC. Care must be taken to ensure they use
     only NTP servers which smear the leap second in exactly the same way for
     synchronisation.

   * `system`: When inserting a leap second, the kernel steps the system clock
     backwards by one second when the clock gets to 00:00:00 UTC. When deleting
     a leap second, it steps forward by one second when the clock gets to
     23:59:59 UTC.

   * `timezone`: This directive specifies a timezone in the system timezone
     database which chronyd can use to determine when will the next leap second
     occur and what is the current offset between TAI and UTC. It will
     periodically check if 23:59:59 and 23:59:60 are valid times in the
     timezone. This normally works with the right/UTC timezone which is the
     default

.. _draft-ntp-interleaved-modes: https://datatracker.ietf.org/doc/draft-ietf-ntp-interleaved-modes/07/

Hardware Timestamping of NTP Packets
======================================

The chrony daemon on VyOS can leverage NIC hardware capabilities to record the
exact time packets are received on the interface, as well as when packets were
actually transmitted. This provides improved accuracy and stability when the
system is under load, as queuing and OS context switching can introduce a
variable delay between when the packet is received on the network and when it
is actually processed by the NTP daemon.

Hardware timestamping depends on NIC support. Some NICs can be configured to
apply timestamps to any incoming packet, while others only support applying
timestamps to specific protocols (e.g. PTP).

When timestamping is enabled on an interface, chrony's default behavior is to
try to configure the interface to only timestamp NTP packets. If this mode is
not supported, chrony will attempt to set it to timestamp all packets. If
neither option is supported (e.g. the NIC can only timestamp received PTP
packets), chrony will leverage timestamping on transmitted packets only, which
still provides some benefit.

.. cfgcmd:: set service ntp timestamp interface <interface>

   Configures hardware timestamping on the interface <interface>. The special
   value `all` can also be specified to enable timestamping on all interfaces
   that support it.

   Configure the timestamping behavior with the following option:

   * ``receive-filter [all|ntp|ptp|none]`` selects the receive filter mode,
     which controls which inbound packets the NIC applies timestamps to. The
     selected mode must be supported by the NIC, or timestamping will be
     disabled for the interface.


The following `receive-filter` modes can be selected:

* `all`: All received packets will be timestamped.

* `ntp`: Only received  NTP protocol packets will be timestamped.

* `ptp`: Only received PTP protocol packets will be timestamped. Combined with
  the PTP transport for NTP packets, this can be leveraged to take advantage of
  hardware timestamping on NICs that only support the ptp filter mode.

* `none`: No received packets will be timestamped. Hardware timestamping of
  transmitted packets will still be leveraged, if supported by the NIC.

.. _ptp-transport:

PTP Transport of NTP Packets
=============================

The Precision Time Protocol (IEEE 1588) is a local network time synchronization
protocol that provides high precision time synchronization by leveraging
hardware clocks in NICs and other network elements. VyOS does not currently
support standards-based PTP, which can be deployed independently of
NTP.

For networks consisting of VyOS and other Linux systems running relatively
recent versions of the chrony daemon, NTP packets can be "tunneled" over
PTP. NTP over PTP provides the best of both worlds, leveraging hardware support
for timestamping PTP packets while retaining the configuration flexibility and
fault tolerance of NTP.

.. cfgcmd:: set service ntp ptp

   Enables the NTP daemon PTP transport. The NTP daemon will listen on the
   configured PTP port. Note that one or more servers must be individually
   enabled for PTP before the daemon will synchronize over the transport.

.. cfgcmd:: set service ntp ptp port <port>

   Configures the PTP port. By default, the standard port 319 is used.