The current SCTP specification is not aware of QoS provided by a network. As a result, it is unable to support preferential treatment of the individual streams. We introduce the concept of dividing an association into subflows (SFs) so that the modified SCTP can support different levels of QoS. We define the necessary modifications to the current SCTP for this capability: namely, new data and SACK chunk formats to implement congestion control for each SF. To avoid the problem of false sharing, we modify SCTP such that each SF has its own congestion control mechanism. Using analytic models, we show that the modified SCTP is expected to perform better than the original SCTP. We verify these results through simulation experiments, where we observed that modified SCTP is able to take advantage of the diff-serv network and provides a better throughput.