Data networks, nodes making up parts of data networks, and information are related to the characterization of paths taken by data travelling between nodes in the networks. Path characterization information is arranged to be conveyed to nodes such that it may be used by nodes subsequently forwarding data. In particular, the invention relates to nodes receiving such path characterization information from upstream nodes, deriving therefrom information indicative of characteristics of a path downstream of said nodes, and using such information to make informed decisions such as routing decisions when forwarding data onward in data networks.