Apparatuses, methods, and systems are disclosed for measuring RTT. One apparatus 400 includes a processor 405 and a transceiver 425 that communicates 705 with a mobile core network via an interworking function. The processor transmits 710 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 715 a first message from the interworking function and transmits 720 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 comprising an echo response indication.