End-to-end real-time transport protocol (RTP) packet transmission delay measurement apparatus and method

In a wireless call between two mobile phones, metrics of conversational latency between the two parties determines a measure of Quality of Service (QoS) for the one or more carriers of real-time transport protocol RTP packets. Packages of metrics are collected at the two terminal devices, each of which may have an independent local clock. A Quality of Service estimation apparatus transforms metric packages into a pair of probability distribution functions for a plurality of real-time transport protocol packets between two endpoints. The offset between the two independent local clocks is determined or eliminated. A key performance indicator is determined for End-to-End RTP stream delay by transforming the timestamps associated with transmission and reception at user equipment hosting a QoS agent.