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Internet performance measurement is commonly perceived as a high-cost control-plane activity and until now it has tended to be implemented on top of the network's forwarding operation. Consequently, measurement mechanisms have often had to trade relevance and accuracy over non-intrusiveness and cost effectiveness. In this paper, we present the software implementation of an in-line measurement mechanism that uses native structures of the Internet Protocol version 6 (IPv6) stack to piggyback measurement information on data-carrying traffic as this is routed between two points in the network. We carefully examine the overhead associated with both the measurement process and the measurement data, and we demonstrate that direct two-point measurement has minimal impact on throughput and on system processing load. The results of this paper show that adequately engineered measurement mechanisms that exploit selective processing do not compromise the network's forwarding efficiency, and can be deployed in an always-on manner to reveal the true performance of network traffic over small timescales.