ntp / ptp

NTP - Network Time Protocol
PTP - Precision Time Protocol

NTP
As of June 2010, the current protocol is version 4 (NTPv4)
NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC).^[1]:3 It uses a modified version of Marzullo's algorithm to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.

PTP
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems.
IEEE 1588-2002

IEEE 1588-2008

[링크 : http://en.wikipedia.org/wiki/Network_Time_Protocol]
[링크 : http://en.wikipedia.org/wiki/Precision_Time_Protocol]


For example when ntpq -c rl prints precision=-16, the precision is about 15 microseconds (2^-16 s).
[링크 : http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm]
[링크 : http://linux.die.net/man/8/ntpq]
[링크 : http://linux.die.net/man/8/ntpdc]


Why is IEEE 1588 so accurate? Two words: Hardware timestamping.
In some cases the delay can be quite long, many microseconds, or even milliseconds. 
[링크 : http://blog.meinbergglobal.com/2013/09/14/ieee-1588-accurate/]