[Linuxptp-users] Doubts about PTP standard

[Namit A. <nam...@gm...>]

Hi All,
          My question is not related to linuxptp, but its rather a 
general question about PTP protocol. In the IEEE Std paper of 1588 
protocol, from page 201 to 219, there is an appendix in which they have 
given examples for use of TCs, BCs and OCs. One thing to notice is that 
the mean path delay calculated by means of residence time correction, 
asymmetry correction etc. comes out to be the same as the assumed mean 
path delay. Then why exactly do we take into consideration all this 
residence time, asymmetry etc. into our calculations??

Also, what exactly is the residence time because according to my 
understanding difference of the time from master to slave and vice versa 
will automatically give you the time (including residence time) it took 
to propagate from one end to other.

For eg. Suppose the path is like this :
Master -----------> Router1      -----------> Slave
           5s      1s             5s
                 (residence time)         

So suppose the packet started at t1 = 9:00 AM and reached slave at t2 = 
9:02 and 11 seconds. So master and slave clocks have an offset of 2 
minutes. But t2 - t1 ultimately takes into consideration the residence 
time also. 
Similarly on the other way:
Slave -----------> Router1 -------> Router2     -----------> Slave
           5s      1s             2s                 5s
                 (residence time)      

t4 - t3 will give us 13 seconds difference even if we do not calculate 
residence times separately.   

Why are we separately building TCs and BCs to calculate residence times 
?? What is the use of that??