We consider the problem of streaming packetized media over the Internet through an intermediate proxy server to a client, in a rate-distortion optimized way. The proxy employs a hybrid receiver/sender driven transmission scheme to communicate with both the media server and the client. Computing the optimal transmission policy for the proxy involves estimation of the probability that a single packet will be communicated to the client in error as a function of the expected redundancy (or cost) used to communicate the packet. In this paper, we show how to compute this error-cost function, and thereby optimize the proxy’s transmission policy.