An apparatus for a base station for a radio access network, the apparatus configured to: detect a change in a radio network congestion status associated with downlink traffic of a cell or cell sector of the base station that is being used to serve a mobile device, the change being a change to a given one of a plurality of possible radio congestion levels; start a packet modification process to modify one or more packets in each of one or more uplink data flows flowing from the mobile device to a core network connected to the radio access network, wherein each uplink data flow is a flow of data from a client socket at the mobile device to a server socket at an origin server (11 0a, 100b) over a transport layer connection, the client socket being defined by a client IP address and a client port number and the server socket being defined by a server IP address and a server port number and wherein for each uplink data flow there is a respective downlink data flow from the server socket at the origin server to the client socket at the mobile device over the transport layer connection, the one or more packets in each of the one or more uplink data flows being modified by the packet modification process to signal, to a traffic management component in the core network, the change in congestion status to the given one of the plurality of possible radio congestion levels, wherein the packet modification process is performed by the apparatus, and wherein the apparatus is configured to modify the one or more packets in each of the one or more uplink data flows by updating a field in an IP packet header of each of the one or more packets to signal in each packet the change in congestion status; wherein the apparatus is further configured to: detect, in each of the one or more downlink data flows flowing from the core network to the mobile device, at least one packet which has been modified by the traffic management component to indicate an acknowledgement of the signalled change in congestion status; and stop the packet modification process in response to detecting the acknowledgement of the change in congestion status. detect a change in a radio network congestion status associated with downlink traffic of a cell or cell sector of the base station that is being used to serve a mobile device, the change being a change to a given one of a plurality of possible radio congestion levels; start a packet modification process to modify one or more packets in each of one or more uplink data flows flowing from the mobile device to a core network connected to the radio access network, wherein each uplink data flow is a flow of data from a client socket at the mobile device to a server socket at an origin server (11 0a, 100b) over a transport layer connection, the client socket being defined by a client IP address and a client port number and the server socket being defined by a server IP address and a server port number and wherein for each uplink data flow there is a respective downlink data flow from the server socket at the origin server to the client socket at the mobile device over the transport layer connection, the one or more packets in each of the one or more uplink data flows being modified by the packet modification process to signal, to a traffic management component in the core network, the change in congestion status to the given one of the plurality of possible radio congestion levels, wherein the packet modification process is performed by the apparatus, and wherein the apparatus is configured to modify the one or more packets in each of the one or more uplink data flows by updating a field in an IP packet header of each of the one or more packets to signal in each packet the change in congestion status; wherein the apparatus is further configured to: detect, in each of the one or more downlink data flows flowing from the core network to the mobile device, at least one packet which has been modified by the traffic management component to indicate an acknowledgement of the signalled change in congestion status; and stop the packet modification process in response to detecting the acknowledgement of the change in congestion status. detect, in each of the one or more downlink data flows flowing from the core network to the mobile device, at least one packet which has been modified by the traffic management component to indicate an acknowledgement of the signalled change in congestion status; and stop the packet modification process in response to detecting the acknowledgement of the change in congestion status.