Cisco SystemsAbstract: In this paper we propose a new way of implementing transport protocols. Instead of building the whole sender/ receiver functionality as one piece, the sender functionality is split into three modules: data transmission, congestion control and error detection/recovery. This enables us to better examine the interactions between error detection and congestion control, and to introduce updates/changes to these modules independently. We present a re-implementation of TCP using this modular design technique and subsequently show the interactions between different congestion control and error detection strategies over a lossy link. The investigation enabled by our modular design aims at improvement of TCP performance over wireless links.