Methods and systems for determining origin-specific network metrics in respect of a data network having at least one ingress node, the method comprising: receiving data items from outside the network; forwarding the data items with headers comprising fields for carrying origin information relating to the origin of the data item, and path metric information indicative of a characteristic being monitored; causing the data items to traverse the network; updating the fields carrying path metric information as the data items traverse the network; determining the origin information and path metric information after the data items have traversed the network and deriving in dependence thereon an origin-specific path metric relating to the characteristic in respect of the relevant portion of a path across the network; and combining origin-specific path metrics derived in respect of different data items if they relate to characteristics in respect of data items having a common origin. Also, methods and systems for determining target-specific network metrics in respect of a data network having at least one egress node.