1. A method of determining an origin-specific network metric in respect of a data network having at least one ingress node, said method comprising: receiving data items from one or more sources outside said data network via one or more ingress nodes of said data network; forwarding each of a plurality of said data items from said one or more ingress nodes with one or more headers comprising fields for carrying the following information: origin information indicative of either or both of the source from which the data item has been received and the ingress node via which the data item has been received; and path metric information indicative of a characteristic required to be monitored as the data item traverses a portion of a path across the data network; causing each of said data items to traverse a portion of a path across the data network; updating the fields carrying path metric information in respect of said data items in dependence on said characteristic as the data items traverse the network; determining in respect of each of a plurality of data items the origin information and the path metric information after the data item has traversed a portion of a path across the data network, and deriving in dependence on said determined path metric information and on said origin information an origin-specific path metric relating to the characteristic in respect of the portion of the path across the data network from the ingress node via which the data item has been received; and in respect of a plurality of data items at least some of which are traversing different end-to-end paths, combining origin-specific path metrics derived in respect of said data items in the event that said origin-specific path metrics relate to characteristics in respect of data items having origin information indicative of a common origin, whereby to determine an origin-specific network metric.