Abstract-We consider the issue of designing a generic transport layer protocol for energy-constrained sensor networks. We present the requirements for such a transport protocol and propose Sensor Transmission Control Protocol (STCP). STCP is a generic, scalable and reliable transport layer protocol where a majority of the functionalities are implemented at the base station. STCP offers controlled variable reliability, congestion detection and avoidance, and supports multiple applications in the same network. We present the design and implementation of STCP and evaluate the protocol with different scenarios and network characteristics.I. REQUIREMENTS OF A GENERIC TRANSPORT LAYER Sensor networks are deployed for a wide range of applications in the military, health, environment, agriculture and office domain. Every application has different characteristics and requirements such as sensed data type, rate of data transmission and reliability. Existing transport layer protocols for sensor networks are either tailored for certain applications or assume that the nodes employ a particular network layer or MAC layer protocol. As a result, their approaches may not be applicable across many sensor network deployments.Heterogenity: Sensor nodes may have multiple sensors (light, temperature, seismic, etc.) with different transmission characteristics. Packets from a sensor for an application constitute its data flow. Each flow can be either continuous or eventdriven. In continuous flow applications, nodes transmit packets periodically to a base station. In event-driven applications, nodes transmit data only when a pertinent event occurs. Both types of flows may exist in the same network. The transport layer protocol should support multiple heterogeneous applications in the same network.Reliability: Every application may require different reliability. For example, in military surveillance, data transmitted by the sensor nodes must always reach the base station. While in temperature monitoring, a few packets may be lost. The transport protocol should exploit this variable reliability model and save network resources.Congestion Control: Packets from all the nodes in the network converge at nodes located near the base station. These nodes forward more packets and hence, there is a possibility of congestion close to the base station. High data rates, sudden burst of data and collisions are other reasons of congestion in sensor networks [13]. Nodes might drop packets due to memory limitations and energy spent in forwarding these dropped packets is effectively wasted. Congestion also increases latency. Hence, the transport layer should support congestion detection and avoidance.It is desirable to design a transport layer protocol that can support multiple applications in the same network, provide controlled variable reliability, address congestion issues, reduce latency and maximize throughput. We propose Sensor Transmission Control Protocol (STCP) and show, through simulations, that it meets these requirements.We summarize the requi...