[Linuxptp-users] Master Clock Choice

[Vanderpool, C. <cly...@ax...>]

PTP users,

I have a question about how the choice of a parent clock is decided.  I
have included a block diagram of the LAN that I have synchronized with
ptp4l (software time stamping) in the attachment of this email.  Names of
various 'links' are inside the green squares.

The left side of the diagram is synchronized first, with the Ground Gateway
assuming the Grand Master role.  Then the right side of the diagram is
brought up.  On the right side link 1-A (which has no direct connection to
the left side) and the Air Gateway will always pick link 3-A as it's parent
boundary clock (despite the order in which they are synced to the LAN).  I
find this strange due to the fact that the path to the Grand Master needs
to go over a radio (3-A-->3-G-->Ground Gateway) instead of going through a
direct Ethernet connection (2-A-->2-G-->Ground Gateway).

I then tried to take link 3-G offline.  This caused links 1-A, 3-A, and the
Air Gateway to pick 2-A as their parent boundary clock.  This makes sense,
as 2-A offers the only connection to the Grand Master.  When I bring back
link 3-G, link 3-A picks it as it's parent boundary clock.  This makes
sense to me as it requires less steps to get to the Grand Master.  What
does not make sense to me is that both link 1-A and the Air Gateway drop
link 2-A as their parent and once again sync to link 3-A despite the fact
that the steps removed from the Grand Master remains the same.

I performed a ping test to try see how long it takes to send/receive
information on both of the previous pathways to the Grand Master.  The
results are as follows.

*3-A-->3-G-->Ground Gateway *

Average ping time - 1.203 micro sec
Stand Dev - .342 micro sec

*2-A-->2-G-->Ground Gateway*

Average ping time - 67.806 micro sec
Stand Dev - 6.333 micro sec


I was just wondering why the two machines with no direct connection to the
Grand Master would consistently choose the longer pathway (time wise) to
the Grand Master.  Is there something about the Best Master Clock Algorithm
that makes this so?

I apologize if this is a bit wordy and hard to follow, but I cannot reason
out how the choice of parent clocks is made.  Any information is greatly
appreciated and if anyone needs any further clarification please feel free
to ask.  Thank you in advance!

Clyde