精度

• NTP: ms, s
• PTP: μs, ns

误差的最大源头就是在各种switch/router中的queue. NTP没有针对这个的解决方案, 但PTP有. 就是利用特殊的switch或者router, 这些设备叫做transparent clock or boundary clock.

Furthermore the biggest source of error in network timing is often due to the variations in queuing time in switches and routers. NTP does not have a solution for this, PTP does. The solution is to use special switches and routers called transparent clocks or boundary clocks.

但是PTP也有缺点

• ease of implementation: 几乎所有设备都支持NTP, 也有很多NTP的app可以使用
• cost:
• robustness. see below:

PTP fault tolerance
PTP网络内只能出现一个Master, 其他passive master需要等positive master失效了以后才能接任.

image.png

NTP fault tolerance
NTP网络中可以同时出现多个Server, 并且都给client发送时间消息, 而client会从中选一个最好的.

image.png

Redundant servers in an NTP network

原文

https://blog.meinbergglobal.com/2013/11/22/ntp-vs-ptp-network-timing-smackdown/