New business infrastructures over the Internet pose a new set of traffic constraints. In particular, multimedia and interactive contents require guarantees of bandwidth and delivery time. The broad deployment and real-time nature of this class of applications require the provisioning of specific resources in the network to guarantee a certain level of Quality of Service (QoS). QoS techniques need ways of obtaining feedback about the status of the QoS enabled paths, for example, for checking the fulfilment of Service Level Agreements (SLA). A possible technique for obtaining such feedback is by passively monitoring the network traffic. The issue with traffic monitoring is the additional bandwidth needed by the control traffic generated by the different collection points in order to synchronise its acquired QoS metrics. Moreover, passive monitoring at line speed is an expensive process both in terms of resource consumption and price. However, some of these requirements can be significantly reduced by using traffic sampling. This paper presents a novel methodology for intra-domain on-line distributed QoS monitoring, which makes an efficient use of the network resources by employing distributed sampling mechanisms. The proposal is validated by performing real tests on an European-wide testbed. Our results show that the sampling technique can significantly reduce the traffic overhead, while obtaining very accurate estimations of the One Way Delay performance metric