A method and system to allow congestion charging signals to be generated and congestion charges to be applied on an inter-domain basis, on an aggregated basis and not on a per flow basis. Each network domain charges its neighbouring domain for any ECN marks that it forwards to them. Since a flow will collect marks as it progresses across the network the last network domain (typically an Internet service provider (ISP) located at the edge of the network domains) will levy a charge based on a higher number of marks than any of the charges levied by other domains involved in the transmission of the flow, and this higher charge can be passed on to the end user, preferably as a single aggregated charge per subscriber per accounting period. Each network domain (whether an edge ISP or other domain such as an intermediate ISP) will forward traffic to other network domains and consequently can produce a single aggregated charge per connected domain.