A method of providing path characterisation information to a data sender relating to a network characteristic experienced by data units each traversing a path via one or more nodes across a data network from said data sender towards one of a plurality of data receivers, each of said data units having associated therewith information indicative of said network characteristic, the path being traversed by each data unit having at least a first path portion between said data sender and a proxy node along which data units may traverse having associated therewith at least a measure indicative of said network characteristic representable by any codepoint of a first set of codepoints, each of said data receivers being operable to provide data acknowledgement units in respect of data units they receive; the method comprising said proxy node performing steps of: receiving a first data unit sent from said data sender via a first path portion, said first data unit having associated therewith a receiver indication indicative of an intended receiver for said data unit; forwarding said first data unit along a second path portion towards said receiver; receiving a data acknowledgement unit from said receiver in respect of said first data unit; and inspecting said data acknowledgement unit and determining therefrom whether said data acknowledgement unit has associated therewith information indicative of said network characteristic in a format indicating that said information indicative of said network characteristic is representable by any of the codepoints of the first set of codepoints; the method further comprising said proxy node performing the following steps in respect of subsequent data units received from said data sender and having associated therewith receiver indications indicative of said receiver in the event that it is determined that said data acknowledgement unit has associated therewith information indicative of said network characteristic in a format indicating that said information indicative of said network characteristic is representable only by any codepoint of a second set of codepoints having less codepoints therein than said first set of codepoints: - storing information indicative of said network characteristic in respect of said subsequent data units; - forwarding said subsequent data units towards said receiver; - receiving subsequent data acknowledgement units from said receiver in respect of said subsequent data units: - inspecting said subsequent data acknowledgement units whereby to determine acknowledgement-based information indicative of said network characteristic; and - providing for said data sender path characterisation information in respect of said subsequent data units, said path characterisation information being dependent on said stored network characteristic information and on said acknowledgement-based information. receiving a first data unit sent from said data sender via a first path portion, said first data unit having associated therewith a receiver indication indicative of an intended receiver for said data unit; forwarding said first data unit along a second path portion towards said receiver; receiving a data acknowledgement unit from said receiver in respect of said first data unit; and inspecting said data acknowledgement unit and determining therefrom whether said data acknowledgement unit has associated therewith information indicative of said network characteristic in a format indicating that said information indicative of said network characteristic is representable by any of the codepoints of the first set of codepoints; the method further comprising said proxy node performing the following steps in respect of subsequent data units received from said data sender and having associated therewith receiver indications indicative of said receiver in the event that it is determined that said data acknowledgement unit has associated therewith information indicative of said network characteristic in a format indicating that said information indicative of said network characteristic is representable only by any codepoint of a second set of codepoints having less codepoints therein than said first set of codepoints: - storing information indicative of said network characteristic in respect of said subsequent data units; - forwarding said subsequent data units towards said receiver; - receiving subsequent data acknowledgement units from said receiver in respect of said subsequent data units: - inspecting said subsequent data acknowledgement units whereby to determine acknowledgement-based information indicative of said network characteristic; and - providing for said data sender path characterisation information in respect of said subsequent data units, said path characterisation information being dependent on said stored network characteristic information and on said acknowledgement-based information. - storing information indicative of said network characteristic in respect of said subsequent data units; - forwarding said subsequent data units towards said receiver; - receiving subsequent data acknowledgement units from said receiver in respect of said subsequent data units: - inspecting said subsequent data acknowledgement units whereby to determine acknowledgement-based information indicative of said network characteristic; and - providing for said data sender path characterisation information in respect of said subsequent data units, said path characterisation information being dependent on said stored network characteristic information and on said acknowledgement-based information.