Measures, including methods, systems and computer-readable storage mediums, for use in operating a network node in a network. Signalling information for an in-progress communication session dialog is processed according to a stateless operating mode. The processing according to the stateless operating mode includes storing communication session dialog state data associated with the communication session dialog. An operating mode change trigger is detected. In response to the detection, the stored communication session dialog state data associated with the communication session dialog is retrieved. Signalling information for the in-progress communication session dialog is processed according to a state-full operating mode. The processing according to the state-full operating mode is carried out at least in part on the basis of the retrieved communication session dialog state data associated with the communication session dialog.