The IETF’s recent differentiated services (DS) architecture, which specifies a scalable mechanism for treating packets differently, offers new opportunities for building end-to-end quality of service (QoS) systems. However, it also introduces new challenges. In particular, 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 DS mechanisms can be used with standard TCP implementations or whether it is necessary to wait for upcoming features introduced in future modified versions of TCP. The general-purpose architecture for reservation and allocation (GARA) supports flow-specific QoS specification, immediate and advance reservation, and online monitoring and control of both individual resources and heterogeneous resource ensembles. Using GARA, we evaluated actual DS mechanisms provided by Cisco routers. We present the results of this evaluation and discuss their impact on the performance of popular TCP implementations.