Use of PTPd on RedHat/CentOS
<p>I need to create a reliable and accurate synchronization between two CentOS 6 machines connected through a direct Ethernet connection.</p>

<p>I've seen that on Linux several implementation of <a href="https://en.wikipedia.org/wiki/Precision_Time_Protocol" rel="nofollow noreferrer">the IEEE 1588 Precision Time Protocol (PTP)</a> exist:</p>

<ul>
<li><a href="https://sourceforge.net/projects/ptpd/" rel="nofollow noreferrer">PTPd</a>:

<ul>
<li>Apparently, this is the original implentation</li>
<li>Source code available on <a href="http://www.github.com/ptpd/ptpd" rel="nofollow noreferrer">GitHub</a> (appparently, <s>still maintained</s> almost unmaintained)</li>
</ul></li>
<li><a href="http://ptpd2.sourceforge.net" rel="nofollow noreferrer">PTPd2</a>: 

<ul>
<li>A new version meant to supersede the previous implementation</li>
<li>Apparently unmaintained</li>
<li>For CentOS 6, available only in the EPEL repositories</li>
</ul></li>
<li><a href="http://code.google.com/p/ptpv2d" rel="nofollow noreferrer">PTPv2d</a>: 

<ul>
<li>A further implementation</li>
<li>Unmaintained as well</li>
</ul></li>
<li><a href="http://linuxptp.sourceforge.net" rel="nofollow noreferrer">linuxptp</a>: 

<ul>
<li>A specific implementation for Linux</li>
<li>Maintained </li>
<li>Available on the CentOS repositories</li>
<li>Suggested by the RedHat documentation for both <a href="https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/ch-Configuring_PTP_Using_ptp4l.html" rel="nofollow noreferrer">RedHat 6</a> and <a href="https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/sec-Using_PTP.html" rel="nofollow noreferrer">RedHat 7</a></li>
</ul></li>
</ul>

<p>My questions follow:</p>

<ul>
<li>Why does the RedHat documentation <a href="https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s1-Installing_PTP.html" rel="nofollow noreferrer">suggest the use of linuxptp</a> for RedHat 6 (based on Linux kernel 2.6) despite <a href="http://linuxptp.sourceforge.net/" rel="nofollow noreferrer">the linuxptp documentation says that a Linux kernel version 3.0 or newer is needed</a> ?</li>
<li>Which are differences between PTPd2 and Linuxptp in terms of reliability and timing accuracy ? </li>
<li>Which one should I prefer on CentOS 6 and on CentOS 7, respectively ?</li>
<li>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 <code>date</code> to make the machine synchronize ?</li>
</ul>