A new modular transport layer protocol for wireless sensor networks (WSNs) is proposed: WMTP – Wireless Modular Transport Protocol. This protocol not only allows the simultaneous use of all the main features commonly found in WSN transport protocols, namely congestion-control, fairness, and reliability, but also does so in a modular fashion. This way, the application layer can choose to use exactly the features that it requires. Additionally, WMTP also provides a set of uncommon features such as throttling, flow-control, transport layer quality-of-service, and optional integration with service-discovery. Focusing specifically on the fairness feature, experimental results show that the network resources are appropriately divided whether a simple fairness semantic is used, or in the presence of weighted differentiation. Additionally, this feature is shown to operate with minimal energy cost, as the protocol overhead is less than 10%.