The IETF's differentiated services (diffserv) architecture, which specifies a scalable mechanism for treating packets differently, offers new opportunities for building end-to-end Quality of Services (QoS) systems. However, it also introduces new challenges. It is not clear whether TCP's flow and congestion control mechanisms work well with the mechanisms used for end-to-end QoS. For that reason it is essential to analyze whether the existing differentiated services mechanisms can be used with standard TCP implementations, or if it is necessary to wait for upcoming features introduced in future modified versions of TCP. We performed a careful evaluation of high-bandwidth TCP performance while using Ciscos's implementation of diffserv mechanisms. We drew conclusions about how to best configure diffserv to provide a premium service and what a bandwidth broker implementation needs to provide to support this.