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T6630: docs for ntp: support hardware timestamp and interleave

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Lucas Christian 2024-09-23 00:34:30 -07:00
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docs/configuration/service/ntp.rst
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@ -50,7 +50,7 @@ Configuration
* ``time2.vyos.net``
* ``time3.vyos.net``
.. cfgcmd:: set service ntp server <address> <noselect | nts | pool | prefer>
.. cfgcmd:: set service ntp server <address> <noselect | nts | pool | prefer | ptp | interleave>
Configure one or more attributes to the given NTP server.
@ -67,6 +67,12 @@ Configuration
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
@ -112,3 +118,80 @@ Configuration
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.