A method of determining a passive Round Trip Time, RTT, delay in a telecommunications system for exchanging data packets (2a-2d) in accordance with a data transmission protocol between a first device and a second device operatively connected to said telecommunications system, said first device identified by a first device identification, and said second device, identified by a second device identification, said data packets (2a-2d) comprising an address part including a source address and a destination address, said method comprising the steps of: a. receiving, by a node in said telecommunications system, a data packet originating from said first device and directed to said second device, said source address of said address part of said data packet being said first device identification, said destination address of said address part of said data packet being said second device identification; b. modifying, by said node, said first device identification which is said source address of said received data packet; c. providing, from said received data packet, a modified data packet by said node, having an address part comprising said modified first device identification as source address and said second device identification as destination address; d. linking, by said node, in an address translation table, said address part of said received data packet with said address part of said modified data packet, by linking at least said source address of said address part of said data packet to said modified first device identification which is said source address of said address part of said modified data packet; e. transmitting, by said node, at a first point in time, said modified data packet to said second device and maintaining the first point in time as a first stamp in the node; f. receiving, by said node, at a second point in time, later than the second point in time, from said second device, a further data packet, the destination address of the address part of the further data packet being said modified first device identification, the source address of the address part of the further data packet being said second device identification, wherein said further data packet is received in response to said modified data packet; and maintaining the second point in time as a second time stamp in the node; g. modifying, by said node, the received further data packet by replacing the destination address of the address part of the further data packet with the source address of the address part of the data packet as maintained in the address translation table, thereby providing a modified further data packet; h. transmitting, by said node, said modified further data packet to said first device, and i. determining, by said node, said RTT delay from the first time stamp and the second time stamp, wherein said node is located in an access network of said telecommunications system. a. receiving, by a node in said telecommunications system, a data packet originating from said first device and directed to said second device, said source address of said address part of said data packet being said first device identification, said destination address of said address part of said data packet being said second device identification; b. modifying, by said node, said first device identification which is said source address of said received data packet; c. providing, from said received data packet, a modified data packet by said node, having an address part comprising said modified first device identification as source address and said second device identification as destination address; d. linking, by said node, in an address translation table, said address part of said received data packet with said address part of said modified data packet, by linking at least said source address of said address part of said data packet to said modified first device identification which is said source address of said address part of said modified data packet; e. transmitting, by said node, at a first point in time, said modified data packet to said second device and maintaining the first point in time as a first stamp in the node; f. receiving, by said node, at a second point in time, later than the second point in time, from said second device, a further data packet, the destination address of the address part of the further data packet being said modified first device identification, the source address of the address part of the further data packet being said second device identification, wherein said further data packet is received in response to said modified data packet; and maintaining the second point in time as a second time stamp in the node; g. modifying, by said node, the received further data packet by replacing the destination address of the address part of the further data packet with the source address of the address part of the data packet as maintained in the address translation table, thereby providing a modified further data packet; h. transmitting, by said node, said modified further data packet to said first device, and i. determining, by said node, said RTT delay from the first time stamp and the second time stamp,