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How to Set Up a PTP Network Using Linux PTP and SUSE® Linux Enterprise
======================================================================
                                                         March 2014

Scope
-----
This how-to guide provides a conceptual overview of Linux PTP (Precision Time
Protocol)  and explains the basic setup of a PTP network consisting of a single
Ethernet segment using Linux PTP and SUSE Linux Enterprise.

Why Precision Time Protocol?
----------------------------
Precision Time Protocol (PTP) provides  accurate clock synchronization across a
local group of hosts.  As opposed to Network Time Protocol (NTP), it does not
aim at minimizing a host's difference from official world time but tries to
minimize the difference from other hosts in the group. Traditionally, the task
of precise synchronization in a group has been achieved with GPS time
synchronization at each host or a central clock synchronizing the hosts via
specialized hardware.  PTP offers comparable accuracy using Ethernet based
hardware, assuming stable network latencies and symmetric transport delays.

About the Linux Implementation of PTP
-------------------------------------
Linux PTP takes advantage of the hardware support implemented in some network
interface cards (NICs). In addition, this implementation can work with some
NICs that do not provide hardware PTP facilities. However, the necessary
functionality is implemented in their hardware drivers.

There are three components of the PTP software:
  * The in-kernel PTP support, consisting of the kernel PTP subsystem,
    pulse-per-second subsystem and NIC driver code changes necessary for time
    stamping passing packets
  * Daemon for synchronizing the NIC's clock and system clock needed in
    hardware time stamping setup
  * Daemon for implementing the protocol itself, which does the time
    calculations and responds to the inquiries from other hosts

The Beginning
-------------
First, we need to check what type of time stamping the NIC in question does.
That can be determined using ethtool utility with option -T.  Example output is
as follows:
# ethtool -T enp4s0f1
Time stamping parameters for enp4s0f1:
Capabilities:
	hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
	software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
	hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
	software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
	software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
	hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 3
Hardware Transmit Timestamp Modes:
	off                   (HWTSTAMP_TX_OFF)
	on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
	none                  (HWTSTAMP_FILTER_NONE)
	ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
	ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
	ptpv2-l4-sync         (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
	ptpv2-l4-delay-req    (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
	ptpv2-l2-sync         (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
	ptpv2-l2-delay-req    (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
	ptpv2-event           (HWTSTAMP_FILTER_PTP_V2_EVENT)

For us the most important section is Capabilities.  That shows the card in the
example can do both hardware time stamping (“hardware-transmit” and
“hardware-receive”) using hardware clock (“hardware-raw-clock“) and software
time stamping using kernel system clock (“software-transmit“,
“software-receive“ and “software-system-clock“).

Cards that support only software time stamping display only the “software-“
capabilities and none of the “hardware-“ ones.  The time stamping mode will
later be needed for configuring the user space daemon.

The User Space Daemons
----------------------
The daemons are primarily configured via options in /etc/sysconfig/ptp4l and
/etc/sysconfig/phc2sys. These files control what options will be provided to
the two daemons: the protocol daemon, called ptp4l and the synchronization
daemon, called phc2sys. Apart from /etc/sysconfig/ there is the /etc/ptp4l.conf
configuration file controlling further behavior of the protocol daemon.

Configuring Linux PTP with Hardware Time Stamping
-------------------------------------------------
The hardware time stamping setup  includes an independent clock running on the
network card. This independent clock is used for timing calculations in PTP.

The NIC PTP clock is synchronized to the system clock by the phc2sys daemon.
Therefore, in this setup you always need to enable the phc2sys service.  The
NIC clock synchronization daemon phc2sys always synchronizes clocks in one
direction, for example, slave clock to master clock.  This is important when
configuring a group master.

For a group master, you usually need to synchronize the group to its system
clock, which itself is synchronized to a precise time using NTP, for example.
In this scenario the options for phc2sys will be “-s CLOCK_REALTIME -c ethX -w”
where CLOCK_REALTIME is a special string recognized by phc2sys corresponding to
the system clock, and ethX corresponds to the network interface that is
connected with the PTP clock. For a PTP slave host the configuration is simpler
and involves only telling phc2sys which network interface is associated with
the slave PTP clock. Having the system clock as master is the default here, so
the options are like “-s ethX -w”

You might have noticed “-w” in the example options above. That is connected
with the fact that the PTP clock is running in International Atomic Time (TAI),
which is not the Coordinated Universal Time (UTC) used by system clock. There
is a certain offset between these time scales. The “-w” option ensures the
offset between TAI and UTC is read from the ptp4l daemon. See the phc2sys(8)
manual page for more details about time scale usage.

No extra options need to be passed to ptp4l as hardware time stamping is the
default.  The option string in /etc/sysconfig/ptp4l will typically look like
“-i ethX -f /etc/ptp4l.conf” where ethX is a placeholder for the actual
interface name.

Configuring Linux PTP with Software Time Stamping
-------------------------------------------------
With software time stamping the situation is simpler. There is no independent
PTP clock to be synchronized, so phc2sys is not in use and the phc2sys service
can be disabled.

The only change that must be made is telling ptp4l that the NIC in question
uses software time stamping mode.  That can be accomplished by using “-S” in
OPTIONS in /etc/sysconfig/ptp4l. Therefore, the typical option string
will look like “-i ethX -f /etc/ptp4l.conf -S” where ethX is again a
placeholder for the actual interface name.

When there is a PTP group master using software time stamping, the time
announced to the network is not using native PTP time scale. That may cause
interoperability issues with third-party PTP solutions and should be taken into
account when designing the system.

Configuring the Group Master
----------------------------
In every group there is one master clock, and the other hosts run slave clocks.
The election of group master is done automatically based on the host's clock
parameters. Most important is the host priority value in the PTP domain.

As there is a difference in the system setup for slave clocks and master
clocks, the group master should be selected by the system administrator, and
its priority value should prioritize it over other hosts in the group.  That
should be done by setting the lower priority1 value in /etc/ptp4l.conf.

Domains and Transport Mechanisms
--------------------------------
It is possible for multiple PTP host groups to share a single Ethernet segment.
These groups can be separated by setting different values for the domainNumber
in /etc/ptp4l.conf. Hosts belonging to one group share the same value of
domainNumber.

There are also different transport options for PTP packets over Ethernet. PTP
messages can be encapsulated either in UDP4 (which is the default), UDP6 or
directly into Ethernet frames. The transport mechanism can be selected either
via command line option to ptp4l or in the /etc/ptp4l.conf configuration file.
See the ptp4l(8) manual page for more details.

Examples of Configurations
--------------------------
Configuration examples in this section are not full configuration files but
rather minimal list of changes that should be done to the indicated files. The
string ethX always stands for the actual network interface name in your setup.

Slave clock using software time stamping:
/etc/sysconfig/ptp4l:
  OPTIONS=”-f /etc/ptp4l.conf -i ethX”
No changes made to the distribution /etc/ptp4l.conf.

Slave clock using hardware time stamping:
/etc/sysconfig/ptp4l:
  OPTIONS=”-f /etc/ptp4l.conf -i ethX”
/etc/sysconfig/phc2sys:
  OPTIONS=”-s ethX -w”
No changes made to the distribution /etc/ptp4l.conf.

Master clock using hardware time stamping:
/etc/sysconfig/ptp4l:
  OPTIONS=”-f /etc/ptp4l.conf -i ethX”
/etc/sysconfig/phc2sys:
  OPTIONS=”-s CLOCK_REALTIME -c ethX -w”
/etc/ptp4l.conf:
  priority1 127

Master clock using software time stamping, not generally recommended:
/etc/sysconfig/ptp4l:
  OPTIONS=”-f /etc/ptp4l.conf -i ethX”
/etc/ptp4l.conf:
  priority1 127

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