A network node comprises a core processing part to perform a core processing function, an ingress processing part to receive incoming signalling messages, an egress processing part to output signalling messages for transmission from the network node, and a message manipulation function to manipulate signalling messages. In a first configuration, the message manipulation function is comprised in the ingress processing part, and in a second configuration the message manipulation function is comprised in the egress processing part. In the first configuration, an egress characteristic is determined, indicative of how an outgoing signalling message is to be outputted, and an incoming signalling message is manipulated based on the egress characteristic. In the second configuration, an ingress characteristic is determined, indicative of how an incoming signalling message has been received, and an outgoing signalling message is manipulated based on the ingress characteristic.