原文章：

A. Masters and Slaves
In PTP, master clocks provide the reference time for one 
or more slave clocks through the exchange of messages over a 
network. The protocol determines a unique master among a 
group of clocks using the Best Master Clock algorithm 
(BMC). The BMC selects the most stable and accurate clock.
B. Sync Messages PTP masters send Sync messages. The master records the 
send time of Sync messages (t1), and slaves record the receipt 
time (t2). The difference between the send and receipt times of 
Sync messages is the master-to-slave delay (dm2s):dm2s = t1 – t2. (2.1) 
Sync messages are sent once per Sync interval (Tsync) 
(typically 2 s). This makes the master-to-slave delay sampling 
period (Tm2s): Tm2s = Tsync = 2 s. (2.2) 
C. Delay Request Messages PTP slaves send Delay Request messages. Slaves record 
the send time of Delay Request messages (t3), and the master 
records the receipt time (t4). The difference between the send 
and receipt times of Delay Request messages is the slave-tomaster delay (ds2m): ds2m = t3 – t4. (2.3) 
Delay Request messages are sent on intervals uniformly 
distributed between 2 and 30 Sync intervals. This makes the 
slave-to-master delay sampling period (Ts2m): Ts2m = Tsync * U[2,30]. (2.4) 
D. One-Way Delay PTP calculates an estimate of the message propagation 
delay. This calculation assumes symmetric propagation 
delays, so that an average of the master-to-slave and slave-tomaster delays cancels the time offset between master and 
slave. This yields the message propagation delay, which the 
specification refers to as the one-way delay (dprop): dprop = (dm2s + ds2m)/2. (2.5) Assuming symmetric propagation delays is often, but not 
always, valid. Asymmetric propagation delays cannot be 
observed by the protocol. They will cause a constant bias in 
the one-way delay and, in turn, the overall time coordination. 
The bias will equal half of the magnitude of the delay 
asymmetry. Assuming a constant delay asymmetry, an asymmetric 
delay bias can be eliminated by adding a latency correction to 
the master-to-slave or slave-to-master delay that cancels the 
asymmetry; however, assuming constant delay asymmetry 
also may be invalid. 
E. Offset From Master PTP estimates the time difference between master and 
slave clocks. This is the master-to-slave delay corrected for 
message propagation delay, and it is referred to as the offset 
from master (∆t): 
∆t = dm2s – dprop. (2.6) 
III. PTPd IN BRIEF