This is a brief overview of PTPd's source code intended to aid developers looking through PTPd.
This documentation is incomplete, and it will be worked on as time permits. More information, particularly on PTPd's clock servo, is available in this paper written for the 2005 Conference on IEEE 1588.
The PTP daemon (PTPd) implements the Precision Time protocol (PTP) as defined by the IEEE 1588 standard. PTP was developed to provide very precise time coordination of LAN connected computers. The IEEE 1558 specification dictates most of PTPd's operation, so one should obtain a copy of the spec before going through PTPd's source code. PTPd keeps all of the names used in the spec, so it should be easy to match up PTPd's source code with the spec. Descriptions herein will assume knowledge of the spec.
The best way to become familiar with a program is to use it. One should be able to run PTPd on standard Linux systems without much trouble. To better observe the operation of PTPd, compile it with the PTPD_DBG flag to have the program produce debugging output, and run PTPd with the '-c' argument to prevent it from backgrounding itself.
For PTPd to operate, it must be run on at least two computers connected through an Ethernet hub or switch. It is possible to have PTPd coordinate two (and only two) computers through a routed connection by running PTPd on each computer with the '-u' argument followed by the IP address of the other computer.
Right out of the box, PTPd should easily be able to coordinate the clocks of your computers with microsecond-level precision. The default distribution achieved coordination within ten microseconds running on fairly busy embedded systems with 66MHz m68k processors. While it should be easy to get PTPd to provide a precisely coordinated time-base on Linux systems, it is more difficult to use the coordinated time-base. This is because the Linux API currently (as of early 2.6) provides no mechanism to generate high-precision timed events. Jiffy timers are currently Linux's standard timing mechanism. Jiffy timers have a maximum precision dictated by the kernel's HZ value, which is typically 100 on 2.4 kernels (ten millisecond resolution) or 1000 on 2.6 kernels (one millisecond resolution).
There are ways to generate high-precision timed events with Linux, but timing in Linux, or any software system, is a tricky issue. It is also typically platform and application specific. PTPd's maintainers would be a good source of knowledge on precise timing in software systems; however, they can only do so much for free.
PTPd's source is grouped into a few components. The component delineations are based on the functionality defined by the spec, but the delineations are not specifically defined by the spec.
The following is a block diagram of PTPd's major components, in which occlusion indicates interfaces between components.
In general, PTPd's source is divided into platform-independent and platform-dependent code. Platform-independent code is located in the top level of the PTPd source tree, and platform-dependent code is located the dep/ sub-directory.
PTPd's major source code components are:
The IEEE 1588 spec does not define how the offset from master produced by a PTP slave is used to bring the slave clock into coordination with master clock time. This procedure, called clock discipline, is not trivial, and there are many possible design approaches and trade-offs. For these reasons, this documentation devotes a section exclusively to the clock servo component.
The following shows the message send and receive paths in a typical system running PTPd, along with the associated time stamps that form the basis of the master-to-slave and slave-to-master delay measurements.
The following is a system diagram of PTPd's clock servo. The FIR filtering of the offset from master input is a simple, two-sample average. The IIR filtering of the input one-way delay is described below. The PI controller that mediates the tick rate adjustment output has the difference equation: y[n] = e[n]/Ap + a[n], a[n] = e[n]/Ai + a[n-1].
The following are plots of the frequency response of the one-way delay filter. It is a variable cutoff/delay low-pass, infinite impulse response (IIR) filter. The one-way delay filter has the difference equation: s*y[n] - (s-1)*y[n-1] = x[n]/2 + x[n-1]/2, where increasing the stiffness (s) lowers the cutoff and increases the delay.