The rate of traffic generated by MTC is growing at a very fast pace, putting strong pressure on existing network infrastructures. One of the most challenging tasks today is being able to support M2M/IoT data exchanges by providing connectivity between any pair of M2M devices all over the world. For this reason, we analyze the role of SINs in supporting MTC in this work. Horizontal solutions are analyzed herein in order to allow interworking by acting as relay entities among different protocol stacks and services, vertically implemented over different network segments. We analyze the still pending challenges hampering interworking, and propose a possible protocol stack for M2M/IoT communications based on the oneM2M standard. Eventually, this article compares the performance achievable by using two of the most diffused application protocols, CoAP and MQTT, shedding light on their efficiency and differences.