[ntp:questions] determining time difference between two not-directly-connected computers

matthew.garman at gmail.com matthew.garman at gmail.com
Thu Jan 4 23:21:51 UTC 2007

I've got two (x86_64 Linux) machines between which I need to
determine the relative time difference.  The problem is, the
machines are not directly connected and only one is connected to the

The topology looks like this:

box_1 <--VPN--> box_common <--LAN--> box_2

It's not so important that box_1 and box_2 are synced, but that
their time difference is accurately measured.

So, box_1 and box_common sync via NTP to (different) public stratum
2 NTP servers.  box_2 syncs to box_common.

I wrote a simple program to try an determine the time difference
between box_1 and box_2.  The program works like this:

box_1 and box_2 run the program in "server" mode; when queried, the
server program calls gettimeofday() and returns the timeval result
via UDP.

box_common runs the program in "client" mode.  client mode does the

    - record the current time via gettimeofday() as t1
    - send a UDP packet to the remote server
    - wait for the UDP response (i.e. the remote machine's actual
    - record the time of the UDP response as t2
    - define "remote calculated time" as the midpoint between t1 and
    - calculate the difference between "remote calculated time" and
      the actual remote time as provided by the UDP response; call
      this t_d

Once I've done the above for box_1 and box_2, I subtract t_d1 and

I'm just looking for some feedback on my approach: is it valid (i.e.
am I overlooking something?), are there better approaches, etc?


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