As the Internet evolves toward the global multi-service network of the future, a key consideration is support for services with guaranteed quality of service (QoS). The current Internet has a number of barriers to QoS support for real-time data delivery such as the lack of control architectures. The proposed differentiated services (Diffserv) framework is seen as the key technology to achieve this. This paper examines standard transport layer protocols, i.e. UDP and TCP, suited for real-time data delivery over Diffserv-capable networks where bandwidth guarantees are provided. We designed a streaming video application that buffers a portion of data on the receiver before starting the playback and operates in a streaming mode. Streaming media applications currently used in the Internet use UDP because of the general belief that TCP's reliable delivery mechanisms such as congestion control and retransmission are unsuitable for time-sensitive delivery. However, TCP showed the better simulation results than UDP when packets are lost over Diffserv networks. In our simulation, TCP can adapt its sending rate to the guaranteed bandwidth and the buffer on the receiver allows TCP to retransmit lost packets before the playback. As for TCP, the employment of SACK option that can recover lost packets efficiently contributes to the better performance.