NQPTP – Not Quite PTP

nqptp is a daemon that monitors timing data from PTP clocks it sees on ports 319 and 320. It maintains records for one clock, identified by its Clock ID.

It is a companion application to Shairport Sync and provides timing information for AirPlay 2 operation.

Installation

This guide is for recent Linux and FreeBSD systems.

As usual, you should first ensure everything is up to date.

Please use git!

As you probably know, you can download the repository in two ways: (1) using git to clone it -- recommended -- or (2) downloading the repository as a ZIP archive. Please use the git method. The reason it that when you use git, the build process can incorporate the git build information in the version string you get when you execute the command $ nqptp -V. This will be very useful for identifying the exact build if you are making comments or bug reports. Here is an example:

Version with git build information:
Version: 1.1-dev-24-g0c00a79. Shared Memory Interface Version: 5.

Version without git build information:
Version: 1.1-dev. Shared Memory Interface Version: 5.

Remove Old Service Files

Linux

If you are updating from version 1.2.4 or earlier in Linux, remove the service file nqptp.service from the directories /lib/systemd/system and /usr/local/lib/systemd/system (you'll need superuser privileges):

# rm /lib/systemd/system/nqptp.service
# rm /usr/local/lib/systemd/system/nqptp.service
# systemctl daemon-reload

Don't worry if you get a message stating that the files doesn't exist -- no harm done.

FreeBSD

At present, there is no need to remove the old startup script as (in FreeBSD only) it is always replaced during the # make install step.

The startup script is at /usr/local/etc/rc.d/nqptp.

Build and Install

Note that you will need superuser privileges to install, enable and start the daemon.

Linux

$ git clone https://github.com/mikebrady/nqptp.git
$ cd nqptp
$ autoreconf -fi
$ ./configure --with-systemd-startup
$ make
# make install

FreeBSD

$ git clone https://github.com/mikebrady/nqptp.git
$ cd nqptp
$ autoreconf -fi
$ ./configure --with-freebsd-startup
$ make
# make install

The make install installs a startup script as requested. You should enable it and start it in the normal way:

First Install or Update?

Linux

First Install

If you are installing nqptp for the first time, enable it and start it:

# systemctl enable nqptp
# systemctl start nqptp

If Shairport Sync is already running, you should restart it after starting nqptp:

# systemctl restart shairport-sync

Update

If you are updating an existing installation of nqptp, after installing it you should restart it. You should then also restart Shairport Sync:

# systemctl restart nqptp
# systemctl restart shairport-sync

FreeBSD

First Install

If you are installing nqptp for the first time, add an automatic startup entry for it in /etc/rc.local and start it:

1. Edit /etc/rc.conf and add the following line:

   nqptp_enable="YES"
   

2. When you have finished editing /etc/rc.conf, you can start nqptp from the command line:

   # service nqptp start
   

If Shairport Sync is already running, you should you restart it after starting nqptp:

# service shairport_sync restart

Update

If you are updating an existing installation of nqptp, after installing it you should restart it. You should then also restart Shairport Sync:

# service nqptp restart
# service shairport_sync restart

Firewall

If your system runs a firewall, ensure that ports 319 and 320 are open for UDP traffic in both directions. These ports are associated with PTP service and may be referred to as "PTP" in firewall rules. For example, the following would open ports 319 and 320 for Fedora, which uses firewalld:

# firewall-cmd --add-service=ptp
# firewall-cmd --permanent --add-service=ptp # make it permanent across reboots

Notes

The nqptp application requires exclusive access to ports 319 and 320. This means that it can not coexist with any other user of those ports, such as full PTP service daemons. In Linux, nqptp runs as a low-priviliged user but is given special access to ports 319 and 320 using systemd AmbientCapabilities. In FreeBSD, nqptp runs as root user.

Programming Notes

Commands and status information are sent to nqptp over port 9000.

Information about the PTP clock is provided via a POSIX shared memory interface.

Here are details of the interface:

typedef struct {
  uint64_t master_clock_id;             // the current master clock
  uint64_t local_time;                  // the time when the offset was calculated
  uint64_t local_to_master_time_offset; // add this to the local time to get master clock time
  uint64_t master_clock_start_time;     // this is when the master clock became master
} shm_structure_set;

// The actual interface comprises a shared memory region of type struct shm_structure.
// This comprises two records of type shm_structure_set. 
// The secondary record is written strictly after all writes to the main record are
// complete. This is ensured using the __sync_synchronize() construct.
// The reader should ensure that both copies match for a read to be valid.
// For safety, the secondary record should be read strictly after the first.

struct shm_structure {
  uint16_t version; // check this is equal to NQPTP_SHM_STRUCTURES_VERSION
  shm_structure_set main;
  shm_structure_set secondary;
};

Clock records that are not updated for a period are deleted.

Known Issues

• nqptp has not been thoroughly checked or audited for security issues. Note that it runs in root mode on FreeBSD.
• It's probably buggy!
• nqptp does not take advantage of hardware timestamping.

Things Can Change

The nqptp daemon is under active development and, consequently, everything here can change, possibly very radically.

NQPTP is not PTP!

nqptp uses just a part of the IEEE 1588-2008 protocol. It is not a PTP clock.