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) exist:

• PTPd:
  • Apparently, this is the original implentation
  • Apparently, it is still maintained
• PTPd2:
  • A new version meant to supersede the previous implementation
  • Apparently unmaintained
  • For CentOS 6, available only in the EPEL repositories
• PTPv2d:
  • A further implementation
  • Unmaintained as well
• linuxptp:
  • A specific implementation for Linux
  • Maintained
  • Available on the CentOS repositories
  • Suggested by the RedHat documentation for both RedHat 6 and RedHat 7

My questions follow:

• Why does the RedHat documentation suggest the use of linuxptp 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 ?
• 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.