The single default IP path determined by routing protocols can hardly meet the transmission requirements of growing QoS-sensitive applications. Multipath transport has been shown to be an effective way to improve the efficiency of data delivery. However, current multipath transport methods either only apply to a specific kind of applications, or require specific network setting of terminal devices or behaviours of network devices. In this paper, we presents a general multipath transport framework based on application-level relay (MPTS-AR). MPTS-AR defines three kinds of logical entities including user agent, relay and controller. MPTS-AR also defines a relay service control protocol named OpenPath in control plane to manage relay service and a suite of multipath transport protocols (MPTPs) in data plane to facilitate multipath data transport and meet the transmission requirements of various upper applications. In addition, we present a relay path selection algorithm facilitated by means of Application Layer Traffic Optimization (ALTO). The goal of the path selection algorithm is to find superior relay paths, while trying to balance the overlay traffic among the provider-defined domains and relays. Simulation results have shown that the proposed algorithm performs well in selecting superior paths.