Emerging applications and operational scenarios raise strict requirements for long-distance data transmission, driving network operators to design wide area networks from a new perspective. Software-defined wide area network, i.e., SD-WAN, has been regarded as the promising architecture of next-generation wide area network. To demystify software-defined wide area network, we revisit the status and challenges of legacy wide area network. We briefly introduce the architecture of software-defined wide area network. In the order from bottom to top, we survey the representative advances in each layer of software-defined wide area network. As SD-WAN based multi-objective networking has been widely discussed to provide high-quality and complicated services, we explore the opportunities and challenges brought by new techniques and network protocols.