The transmission of high quality video streams over IP networks becomes more and more attractive for providing IP based TV or video on demand services. Still a reasonable high bandwidth is required for achieving high quality streaming. Instead of using hard reservations to guaranty the quality of the video transmissions we present an adaptive streaming algorithm based on adaptive packet marking which provides soft guarantees. A prototype streaming server and client have been developed supporting RTP-encapsulated MPEG-2 transmission, into which the adaptive streaming algorithm has been integrated. The software has been successfully used in a testbed, demonstrating that the algorithm is effective and has very promising properties.