The Software Defined Network (SDN) paradigm represents a major breakthrough in the networking field, due to its unprecedented capabilities in terms of flexibility and programmability. SDNs have been successfully deployed in data centers and small to medium enterprises. However, adopting the SDN paradigm in the context of wide area networks is more challenging, due to a number of factors including the higher probability that node and link failures occur and the unavailability of a dedicated control channel. In this paper, we present a DAG (Directed Acyclic Graph) based forwarding paradigm addressing the challenges that arise when the SDN concept is applied to large scale networks. Specifically, the proposed paradigm aims to limit the number of entries required on the SDN switches, to provide a fast local restoration of single node/link failures without the intervention of the SDN controller and to prevent the possibility of having inconsistent forwarding tables during updates. The proposed paradigm does not require any extension to the OpenFlow protocol and we show how it can be implemented by only using standard features. The DAG-based forwarding paradigm requires to compute a DAG between every pair of ingress-egress switches and to design an index-based hashing scheme to balance the load across the paths in the DAG while avoiding TCP reordering issues. In this paper, we present heuristic algorithms providing a solution to such problems and report the results of a simulation study conducted to assess the performance of the proposed forwarding paradigm.

中文翻译：

---

一种基于 DAG 的大规模软件定义网络转发范例

软件定义网络 (SDN) 范式代表了网络领域的重大突破，因为它在灵活性和可编程性方面具有前所未有的能力。SDN 已成功部署在数据中心和中小型企业中。然而，在广域网环境中采用 SDN 范式更具挑战性，原因有很多，包括节点和链路故障发生的可能性更高以及专用控制通道不可用。在本文中，我们提出了一种基于 DAG（有向无环图）的转发范式，以解决将 SDN 概念应用于大规模网络时出现的挑战。具体来说，提议的范式旨在限制 SDN 交换机所需的条目数量，在没有 SDN 控制器干预的情况下提供单个节点/链路故障的快速本地恢复，并防止在更新期间出现不一致的转发表。提议的范例不需要对 OpenFlow 协议进行任何扩展，我们展示了如何仅使用标准功能来实现它。基于 DAG 的转发范例需要计算每对入口-出口交换机之间的 DAG，并设计基于索引的散列方案以平衡 DAG 中路径之间的负载，同时避免 TCP 重新排序问题。在本文中，我们提出了为此类问题提供解决方案的启发式算法，并报告了为评估所提出的转发范式的性能而进行的模拟研究的结果。