In order to face the continuous increase of the data traffic on cellular networks, using WiFi to offload the LTE network can be an interesting solution for the operators. The convergence between fixed and mobile access allows the WiFi access points to be connected to the cellular eNodeBs. This is known as very tight coupling between LTE and WiFi. WiFi and LTE flows are merged just bellow the IP layer which allows faster attachment procedures to WiFi access points and dual WiFi/LTE transmission while providing a seamless experience to the end user. However, WiFi networks are very different from LTE in terms of delays and loss rates which may have a bad impact on higher layers such as TCP. In this paper, we study the performance of TCP with Very Tight Coupling. We perform experimentation on a real testbed based on the Open Air Interface framework.