Huawei & USTC Huawei (USA) Huawei (USA) The introduction of SDN fundamentally changes the way that the application layer traffic optimization (ALTO) works. This draft describes two architectures, the Vertical Architecture and the Horizontal Architecture, allowing coherent coexistence of ALTO and software defined network (SDN). Unique requirements for design and operations are identified and summarized, suggesting that the Vertical Architecture allows better division, management, flexibility, privacy control and long-term evolution of the network. We also define the main interactions and information flows, and present a set of use cases to illustrate how we extend ALTO to support SDN, in the Vertical Architecture.