5G will support transparent multi-path connectivity for mobile users through the Access Traffic Steering, Switching and Splitting function. For real-time traffic, this can be implemented by a multi-path proxy that implements transport layer tunnels between users and a proxy node in the packet core network. However, the choice of transport layer protocol for such a multi-path tunnel could greatly impact the user Quality-of-Experience. For example, re-transmitting lost packets could reduce video playback artefacts, but the additional delay caused by re-transmission could lead to the video freezing. In this paper, we develop mathematical models that estimate the lower bound on the average delay imposed by a reliable or semi-reliable multi-path tunnel. We validate our model using extensive network emulations and evaluate model accuracy for different scenarios. Finally, we use the model to derive design principles for a semi-reliable multipath tunnel protocol.