Apparatuses, methods, and systems are disclosed for measuring RTT. One apparatus includes a processor and a transceiver that communicates with a mobile core network via an interworking function. The processor transmits a first request to establish a data connection between the apparatus and the mobile core network, wherein the first request includes a first indicator that the apparatus supports round trip time (“RTT”) measurements, the RTT measurements calculated using messages transmitted via the data connection and using an encapsulation header. The processor receives a first message from the interworking function and transmits a second message to the interworking function, wherein the first message includes a first encapsulation header comprising an echo request indication wherein the second message includes a second encapsulation header including an echo response indication.