Latency is a measure of time delay experienced in a system. Network latency in a packet-switched network is measured either one-way (the time from the source sending a packet to the destination receiving it), or two-way (the one-way latency from source to destination plus the one-way latency from the destination back to the source). Where precision is important, one-way latency for a link can be more strictly defined as the time from the start of packet transmission to the start of packet reception.
There are many possible techniques to synchronize time between a source and a destination. It is possible to achieve time synchronization by using synchronized clocks that use Global Positioning System (GPS) technology. GPS considers a satellite environment as reference time for the synchronization of the source and the destination. The synchronization accuracy depends on the precision with which the source and destination hosts are able to synchronize their internal clock to the GPS signal. Using GPS for synchronizing has several drawbacks:                the synchronization of several devices, each of which is equipped with a GPS receiver, can be expensive        the GPS antenna has to be located within a specific distance from the receiver, limiting the positioning of monitoring devices        
Another synchronization system that can be used is Network Time Protocol (NTP) servers. The synchronization is obtained through the time reference offered by public NTP servers located across the Internet. This is the cheapest synchronization technique, but it does not provide as accurate results as GPS does; the accuracy depends on the characteristics of the paths followed by the NTP synchronization messages and on the distance between the NTP server and the source and destination that must be synchronized.
U.S. Pat. No. 7,283,568, with the title “Methods, systems and computer program products for synchronizing clocks of nodes on a computer network”, discloses an algorithm for clock synchronization between two nodes using virtual clocks, a generalization of the clock synchronization for many nodes, and using many round-trip-delays to compute an average one-trip delay. A key feature of the embodiment described is that each node manages a virtual clock relying on average measurements for every other node it synchronizes with.