Q:

To and from time estimation using round trip time

Assume that we got a lot of distributed computers, and their internal clocks unreliable. We can measure round trip times by a ping and a response, but how can one calculate the to and from times between all of the computers? 

A:

To get accurate one-way latency you need one of the following-

A single device that connects to both points in the network.  This is usually a traffic generator (a la iXia or similar) that will send a packet out one interface and receive on another.  This is how port-to-port latency on a network device is typically measured but would rarely be seen outside a specific test environment.
Hosts (and network devices) that do support a highly accurate time protocol - specifically IEEE 1588, the Precision Time Protocol, which allows for much more precise clock distribution than good old NTP.  One of the common uses of PTP is actually for network devices (and certain applications) to very precisely timestamp packets as they traverse the network.  Since the timing is both well-distributed and high resolution this provides an excellent ongoing view of one-way latency values on both a per-hop and an end-to-end basis.  PTP is a fairly common feature in a lot of DC-class switches nowadays (..although implementation quality varies) but its deployment tends to be sparse outside of certain industry verticals.  

