Methods, apparatus and other means for providing path characterisation information to a data sender relating to a network characteristic such as congestion experienced by data units traversing a path across a data network from the data sender towards one of a plurality of data receivers, the data units having associated therewith information indicative of the network characteristic, the path being traversed by the data units having at least a first path portion between said data sender and a proxy node along which data units may traverse conveying a measure indicative of the network characteristic that is representable by any of a first set of codepoints, the data receivers being operable to provide data acknowledgement units in respect of data units they receive via a second path portion in which information indicative of the network characteristic may only be representable using codepoints of a second set of codepoints having less codepoints than the first set of codepoints.