Transmission Control Protocol (TCP) is the main transport protocol that provides reliable transmission in the current Internet. However, TCP still has problems that may affect its performance on various network environments. Packet reordering, which is one of the problems on TCP, refers to the out-of-order event of packets. There are many reasons that can cause packet reordering. In this chapter, we propose RA-TCP, a novel TCP variant which modifies the retransmission occasion and the congestion response of the traditional TCP mechanism, to improve the performance on packet reordering. In particular, we replace the three duplicate ACKs with a retransmission detection timer (RDT) for the trigger of fast retransmit. Our simulation studies reveal that RA-TCP performs consistently better than existing mechanisms that try to make TCP more robust to packet reordering.