This paper describes the design, implementation, and experimental evaluation of OverQoS, an overlay-based architecture for enhancing the best-effort service of today's Internet. Using a Controlled loss virtual link (CLVL) abstraction to bound the loss rate observed by a traffic aggregate, OverQoS can provide a variety of services including: (a) smoothing packet losses; (b) prioritizing packets within an aggregate; (c) statistical loss and bandwidth guarantees. We demonstrate the usefulness of OverQoS using two sample applications. First, RealServer can use OverQoS to improve the signal quality of multimedia streams by protecting more important packets at the expense of less important ones. Second, Counterstrike, a popular multi-player game, can use OverQoS to avoid frame drops and prevent end-hosts from getting disconnected in the presence of loss rates as high as 10%. Using a wide-area overlay testbed of 19 hosts, we show that: (a) OverQoS can simultaneously provide statistical ...