Recently, there is an increasing interests in providing differentiated services in Internet. However, research efforts have almost exclusively focused on routers by improving their policies of packet scheduling and queue management. There has been much less work on transport level approaches to support differentiated services. The mechanism presented by Chang-Biao Xu, DSAS-TCP and MulTCP are the only pieces of the works in this direction known to the authors. Up to now, there is no paper to discuss the interrelation between these mechanisms. Regarding throughput as TCP criteria to support proportional-differentiated-services (PDS), this paper deeply explores the variants of AIMD(a,b)-based TCP congestion control and their effect on differentiated services, and presents a transport level approach for TCP to support PDS, namely PDS_TCP which can be obtained by introducing weighted factor to a or b of AIMD(a,b)-based TCP congestion control. PDS_TCP also takes into account the influence of slow start for timeout. From the analysis, this paper draws the conclusion that the existing mechanisms are only variants of PDS_TCP. For the example of PDS_TCP, the principles, implementation and simulation results of PDS_a_TCP are discussed in detail. The theory analysis and simulation results show that the proposed mechanism PDS_TCP can be implemented with lower additional overheads and support controlled PDS very well without the loss of flexibility.