A network interface device having an FPGA for providing an FPGA application. A first interface between a host computing device and the FPGA application is provided, allowing the FPGA application to make use of data-path operations provided by a transport engine on the network interface device, as well as communicate with the host. The FPGA application sends and receives data with the host via a memory that is memory mapped to a shared memory location in the host computing device, whilst the transport engine sends and receives data packets with the host via a second memory. A second interface is provided to interface the FPGA application and transport engine with the network, wherein the second interface is configured to back-pressure the transport engine.