Video transmission over unreliable packet network is in general hampered by the packet losses and constraint by stringent playback deadline. With these two key factors in consideration, Multiple Description Coding (MDC), comprising balanced and Unbalanced MDC has been proposed as an errorrobust source coding technique. Recently, transmitting multiple descriptions over a single path is interesting due to the unavailability of multiple independent paths. Therefore, in this paper, we investigate the problem of rate allocation for the High-Resolution (HR) and Low-Resolution (LR) descriptions in UMDC transmission over single path. We first propose an approximate but efficient rate allocation model with the aid of two-state Markov link model and a simple distortion model at the sender side. Then we conduct extensive experiments to verity the proposed model and more excitedly the simulation results clearly demonstrate the effectiveness of proposed model.