A method of determining a passive Round Trip Time, RTT, delay in a telecommunications system for exchanging data packets in accordance with a data transmission protocol between a first device and a second device. The first and second devices are identified by first and second device identifications, respectively. The data packets include an address part including a source address and a destination address. The method is performed in a node by modifying the address part of a received data packet from the first device, and creating a first timestamp before transmitting the modified data packet to the second device. Upon receiving, at the node, from the second device in response to the modified data packet, a data packet having the modified address part of the modified data packet, the node creates a second timestamp that provides for passively measuring the RTT delay.