In this paper, we propose an extension to current TCP protocol named concurrent TCP (cTCP), which can improve throughput of connections through balancing traffic load on multiple end-to-end paths. We keep the single-sliding-window architecture of TCP untouched for compatibility reasons. The two core components of the design are a packet scheduling switch that balances load among paths, and a duplicated acknowledgement classifier that handles packet reordering. We implemented our protocol in FreeBSD kernel and compared the performance of cTCP with theoretical results. Experiment outcomes show that throughputs of our protocol are close to the ideal values.