For improved IP fragmentation and transmission, user payload is transmitted through a transmission path enabled by at least a first established tunnel (17,18) and a second (17,18) established tunnel. The tunnels connect a first node (11, 13) and a second (11,13) node in a Packet Core Network (PCN). A method comprises the first node fragmenting (19) at least one received payload packet (15) into fragments (16) on the basis of a minimum Maximum Transmission Unit (MTU) for an upper IP layer of the transmission path; encapsulating (20) the fragments at the entry of the first tunnel, and, determining (21) the MTU for the upper IP layer of the transmission path on the basis of an MTU of a lower IP layer of the transmission path and on the basis of the size of at least one additional tunnel header for the encapsulated fragments (16).