如何提供弹性的控制是将软件定义网络（SDN）部署到广域网中的关键问题之一。软件定义的广域网通常被划分为多个域，每个域由一些SDN交换机和控制这些交换机的一个SDN控制器所组成。当控制器失效时，其控制的交换机便会失去控制成为离线交换机。离线交换机上的流会丧失路径可编程性，无法切换路径成为离线流。当前解决方案采用基于交换机的粗粒度控制流方法，只能恢复有限的离线流，而且忽略了路径可编程性。本项目拟构建离线流和控制器之间新型控制关系，从而有效恢复离线流的路径可编程性。本项目拟设计一种细粒度的新型离线流恢复方法，来提高被恢复流的数量。基于此方法，拟提出一种算法来准确地计算出被恢复流的可编程路径数量。在此基础上，拟开发两种算法来决定离线流与控制器之间的控制关系：当流量历史信息未知时，保证恢复流具有均衡的路径可编程性；当流量历史信息已知时，通过分析历史信息获取重要离线流，并提高重要离线流的路径可编程性。

Providing resilient control is a critical concern for deploying Software-Defined Networking (SDN) into wide-area networks. A software-defined wide-area network is usually divided into multiple domains, each of which consists of switches and is controlled by one controller. Under controller failures, switches, which were controlled by failed controllers, become offline. Flows from offline switches lose the path programmability to change their paths and become offline. Existing solutions are based on the switch-level flow control. They can only recover a limited number of offline flows and do not consider the path programmability. This project will study how to efficiently establish the control mapping between offline flows and controllers to recover the path programmability of offline flows. It will design a fine-grained novel offline flow recovery method to improve the number of recovered offline flows and then propose an algorithm for accurately calculating the path programmability of recovered offline flows. Based on the proposed method and algorithm, it will further develop two algorithms to decide the mapping between offline flows and controllers. One algorithm is to guarantee the balanced path programmability of recovered offline flows when the traffic history is unknown; the other one is to improve the path programmability of important offline flows, which are obtained by analyzing traffic history.