communication path asymmetry

IEEE-1588 chapter 6.2(d)中对asymmetry的介绍：
Like all message-based time transfer protocols, PTP time accuracy is degraded by asymmetry in the paths taken by event messages; see 7.4.2. Specifically the time offset error is 1/2 of the asymmetry. Asymmetry is not detectable by PTP; however, if known, PTP corrects for asymmetry. Asymmetry can be introduced in the physical layer, e.g., via transmission media asymmetry, by bridges and routers, and in large systems by the forward and reverse paths traversed by event messages taking different routes through the network. Systems should be configured and components selected to minimize these effects guided by the required timing accuracy. In single subnet systems with distances of a few meters, asymmetry is not usually a concern for time accuracies above a few 10s of ns.

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From <IEEE 1588-2008> 7.4.2

在two-step clock的系统中，message从Master/Responder发送到Slave/Requestor，以及从Slave/Requestor发送到Master/Responder，都应该沿着同一条path进行传输，以将非对称带来的误差最小化。

在PTP协议中，message从Master发送到Slave，以及从Slave发送到Master花费的时间，需要被测量出来。
在peer delay mechanism中，message从requestor发送到responder，以及从responder发送到requestor的时间，需要被测量出来。
一般来说，2个相反方向测出来的2个时间，是不一样的。

Propagation asymmetry

<meanPathDelay>

delayAsymmetry定义为：
<meanPathDelay> + delayAsymmetry
<meanPathDelay> - delayAsymmetry
也就是说，当master-to-slave/responder-to-requestor的propagation time比反方向要长时，delayAsymmetry是正的。

本协议不包含测量delayAsymmetry的方法。但如果delayAsmmetry算出来了，应该按照这里介绍的model，对timing computation进行校正，见11.6。

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From <IEEE 1588-2008> 11.6

Asymmetry correction for PTP version 2 event messages

对于end-to-end模式下的orinary clock、boundary clock来说，对于asymmetry需要做这些调整：

1. Sync message
   发送方不需要调整，接受方需要将delayAsymmetry加到Sync的correction field。
2. Delay_req message
   发送方需要将Delay_req的correction field减去delayAsymmetry，接受方不需要调整。

也就是说，只有slave clock需要考虑delayAsymmetry，master clock不需要。