The transport layer is one of the key layers of the Internet protocol stack. It enrichs the network layer service to make it suitable for applications. Almost 40 years after its initial design, TCP remains the most widely used transport protocol. In the early 2000s, SCTP was proposed as an alternative to TCP. Despite a clean and extensible design and many useful features, it did not reach wide deployment. This failure is mainly caused by middleboxes. We'll describe their operation and explain why Multipath TCP, which is a backward compatible evolution to TCP, has better chances of being deployed. We'll explain the main principles behind Multipath TCP and the lessons that can be drawn from its design. We'll then analyse why Internet giants like Google and Microsoft now consider application-layer solutions like QUIC to replace standard protocols like TCP.