Traditional fair bandwidth sharing by leveraging AIMD-based congestion control mechanisms faces great challenges in data center networks. Much work has been done to solve one of the various challenges. However, no single transport layer protocol can solve all of them. In this article, we focus on the transport layer in data centers, and present a comprehensive survey of existing problems and their current solutions. We hope that this article can help readers quickly understand the causes of each problem and learn about current research progress, so as to help them make new contributions in this field.