Linux PTP

I am pretty new to Ubuntu but recently I had to sync the clocks of multiple machines in a LAN. I chose Linux PTP due to the fact that while all machines were different, they were all ruing either Ubuntu or Lubuntu as their OS and Linux PTP was free. When trying to implement IEEE 1588 protocol on all of these machines I ran into lots of problems and what documentation I did find was either over my head due to the fact I am a novice, or incomplete. What follows are the steps I found worked for me. You'll have to excuse me if any of the information or terminology is incorrect, like I said I'm pretty new to this. Hope it helps!

IEEE1588 Linux PTP

Linux PTP is a software implementation of PrecisionTime Protocol (PTP) that is in accordance with the IEEE 1588standard. Linux PTP makes use of the Linux keel, this allowed usto take advantage of the flexibility that is inherent to LinuxApplication Programming Interfaces (API). Another very practicalreason for our choice was cost. Linux PTP is free and readilyavailable for download.
Some useful features of Linux PTP are as follows.

• Supports hardware and software time stamping via the Linux SO_TIMESTAMPING socket option.
• Supports the Linux PTP Hardware Clock (PHC) subsystem by using the clock_gettime family of calls, including the new clock_adjtimex system call.
• Implements Boundary Clock (BC) and Ordinary Clock (OC).
• Transport over UDP/IPv4, UDP/IPv6, and raw Etheet (Layer 2).
• Supports IEEE 802.1AS-2011 in the role of end station.
• Modular design allowing painless addition of new transports and clock servos.

PTP Prerequisites

The machinerequirements for Linux PTP implementation

• Linux keel version 3.0 or newer
• Keel headers available at compile time
• A supported Etheet MAC device
• A supported PHY device paired with a MAC that allows time stamping in the PHY

www.linuxptp.sourceforge.net

Beforestarting the process of implementing Linux PTP it was imperative tosee if the machines at our disposal had the capabilities listeddirectly above by using the 'ethtool'.

$ethtool -T <network interface>

This command will list the time stamping capabilitiesof the chosen network interface. In our case only software timestamping is available. Another advantage of the ethtool ioctl isthat while ruing the ptp4l program (ptp4l does the heavy liftingfor clock syncronization), it will use ethtool in order to discoverthe proper PHC device.

Some other tools that will be needed in order tomanipulate, compile, and build the appropriate keel are

• fakeroot
• build-essential
• crash
• kexec-tools (when prompted do not let kexec handle reboots)
• makedumpfile
• keel-wedge

All of these tools can be obtained with the apt-getinstall command.

LinuxKeel Manipulation

The most difficult part of Linux PTP implementation(for novices to Linux like me) is the reconfiguration of the LinuxKeel. In order to sync the clocks of two or more machines threekeel configuration options are needed.

• CONFIG_PPS
• CONFIG_NETWORK_PHY_TIMESTAMPING
• PTP_1588_CLOCK

The environment I worked to synchronize was anon-homogeneous one in that the machines were both physicallydifferent and the operating systems (along with the keels) varied. By looking at the .config file for each keel it was evident that inall cases the CONFIG_NETWORK_PHY_TIMESTAMPING option was set to 'n'(not available). In most cases the other two options were set to 'm'(modular). It has been my experience that either 'y' (on) or 'm' areacceptable states for these options.
What follows will be generalized script of commands andactions I have taken in order to get the Linux keel in anacceptable state for the implementation of IEEE 1588. These steps(with slight variations) are able to build a working keel in bothUbuntu and Lubuntu.

KeelConfiguration Set Up

• $sudo apt-get update
• $sudo apt-get -f upgrade These two steps make sure all of the software installed on the machine is the latest. It should also fix any potential dependency issues.
• $cd /usr/src Changes to the directory where the keels reside
• $sudo apt-get source linux-image-$(uname -r)
• $sudo apt-get build-dep linux-image-$(uname -r) These last two steps place a copy of the latest version of the currently ruing keel in the /usr/src directory. It also places the correct debian files needed for the keel build in the /usr/src directory
• $cd <to newest version of the keel>
• $cd debian
• $sudo chmod a+x scripts/
• $cd scripts a+x misc/ These steps alter the permissions to the debian scripts which will do the heavy lifting in building the keel
• $cd<to newest version of the keel>
• $sudo fakeroot debian/rules clean
• $sudo fakeroot debian/rules editconfigs These steps bring up a GUI for the configuration file of the keel

KeelReconfiguration (While in Menu-Config)

• Navigate to CONFIG_PPS, CONFIG_NETWORK_PHY_TIMESTAMPING, and PTP_1588_CLOCK and switch their status to 'y'. After this save settings and exit.

KeelBuild

• $sudo fakeroot debian/rules clean
• $sudo fakeroot debian/rules binary-headers binary-generic These steps will begin the compilation process. Compilation will take some time. Due to the varying processing capabilities of the machines used I found this can take anywhere between 2-16 hours.
• $cd /usr/src
• $ls *.deb Shows only the new .deb files
• $sudo dpkg -i linux <desired verion>.deb
• $sudo reboot

LinuxPTP Installation

• Download latest release from Source Forge www.linuxptp.sourceforge.net
• $cd <path to LinuxPTP>
• $sudo make clean
• $sudo make
• $sudo make install Linux PTP should now be ready

Useof Linux PTP

After Linux PTP stack gets made and installed on thedesired machines it is possible to synchronize their clocks. This isaccomplished by proper networking and the ptp4l command. There are two ways to utilize ptp4l.
The first way is to specify the desired options in thecommand line. The least specific command that will start 1588protocol is $ptp4l -i <desired networking interface>. However for the environment I was working in I needed a few extracommand options. For our purposes $ptp4l -i <net int> -m -Swas the command used.

• -m was used to display the current state of the ptp4l program to the standard out
• -S was necessary to specify the use of Software Timestamping

• [global]
• verbose 1
• time_stamping software
• tx_timestamp_timeout 100
• [eth0]
• [tap0]

This file basically does the same thing as laying outthe options in the command line. The one important difference isthat in a configuration file it is possible to identify multiplenetwork interfaces. This is necessary if the machine is to be set upas a boundary clock. Many more options are laid out in the manualfor ptp4l.

InformationGathering

It is easy to tell if ptp4l is doing it's job. Eitherthe -m in the command line or the verbose 1 option inthe configuration file will update the status of synchronization tothe standard out every second by default. This information can bebroken down as follows

• master offset- Measured offset from the master in nanoseconds
• s0, s1, s2- Indicates clock servo states (0 unlocked, 1 clock-step, 2 locked)
• freq value- Frequency adjustment of the clock in parts per billion
• path delay- estimated delay of the synchronization message sent from the master in nanoseconds
• Port 0- UNIX domain socket used for local PTP management
• Port 1- Network interface being used for synchronization