[CentOS-devel] Use of PTPd on CentOS 6

I need to create a reliable and accurate synchronization between two CentOS
6 machines connected through a direct Ethernet connection.

I've seen that on Linux several implementation of the IEEE 1588 Precision
Time Protocol (PTP) <https://en.wikipedia.org/wiki/Precision_Time_Protocol>
exist:

   - PTPd <http://ptpd.sourceforge.net>:
      - Apparently, this is the original implentation
      - Apparently, it is still maintained <http://www.github.com/ptpd/ptpd>
   - PTPd2 <http://ptpd2.sourceforge.net>:
      - A new version meant to supersede the previous implementation
      - Apparently unmaintained
      - For CentOS 6, available only in the EPEL repositories
   - PTPv2d <http://code.google.com/p/ptpv2d>:
      - A further implementation
      - Unmaintained as well
   - linuxptp <http://linuxptp.sourceforge.net>:
      - A specific implementation for Linux
      - Maintained
      - Available on the CentOS repositories
      - Suggested by the RedHat documentation for both RedHat 6
      <https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/ch-Configuring_PTP_Using_ptp4l.html>
      and RedHat 7
      <https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/sec-Using_PTP.html>

My questions follow:

   - Why does the RedHat documentation suggest the use of linuxptp
   <https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s1-Installing_PTP.html>
   for RedHat 6 (based on Linux kernel 2.6) despite the linuxptp
   documentation says that a Linux kernel version 3.0 or newer is needed
   <http://linuxptp.sourceforge.net/> ?
   - Which are differences between PTPd2 and Linuxptp in terms of
   reliability and timing accuracy ?
   - Which one should I prefer on CentOS 6 and on CentOS 7, respectively ?
   - Why either PTPd2 and Linuxptp do not synchronize immediately and often
   need me to start/stop the service several times or manually change system
   time through date to make the machine synchronize ?

Many thanks for any hint.
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