3. Network Virtualization

单词学习

翻译

In this section, we discuss network virtualization, a prominent early “use case” for SDN. Network virtualization presents the abstraction of a network that is decoupled from the underlying physical equipment. Network virtualization allows multiple virtual networks to run over a shared infrastructure, and each virtual network can have a much simpler (more abstract) topology than the underlying physical network. Forexample, a Virtual Local Area Network (VLAN) provides the illusion of a single LAN spanning multiple physical subnets, and multiple VLANs can run over the same collection of switches and routers. Although network virtualization is conceptually independent of SDN, the relationship between these two technologies has become much closer in recent years

在本节中，我们将讨论网络虚拟化，这是SDN早期的一个突出的“用例”。网络虚拟化暗示从底层物理设备疏散的网络的抽象，网络虚拟化允许多个虚拟网络在共享根本设施上运行，并且每个虚拟网络可以具有比底层物理网络更简单（更抽象）的拓扑。例如，虚拟局域网（VLAN）供给了一个跨越多个物理子网的单一LAN的假象，多个VLAN可以在不异的交换机和路由器调集上运行。尽管网络虚拟化在观点上独立于SDN，但近年来这两种技术之间的关系变得越发密切。

We preface our discussion of network virtualization with three caveats. First, a complete history of network virtualization would require a separate survey; we focus on developments in network virtualization that relate directly to innovations in programmable networking. Second, although network virtualization has gained prominence as a use case for SDN, the concept predates modern-day SDN and has in fact evolved in parallel with programmable networking. The two technologies are in fact tightly coupled: Programmable networks often presumed mechanisms for sharing the infrastructure (across multiple tenants in a data center, administrative groups in a campus, or experiments in an experimental facility) and supporting logical network topologies that differ from the physical network, both of which are central tenets of network virtualization. Finally, we caution that a precise definition of “network virtualization” is elusive, and experts naturally disagree as to whether some of the mechanisms we discuss (e.g., slicing) represent forms of network virtualization. In this article, we define the scope of network virtualization to include any technology that facilitates hosting a virtual network on an underlying physical network infrastructure.

在讨论网络虚拟化之前，我们有三个注意事项：首先，一个完整的网络虚拟化历史将需要一个单独的查询拜访，我们专注于网络虚拟化的成长，这些成长与可编程网络的创新直接相关。第二，虽然网络虚拟化作为sdn的一个用例已经获得了显著的职位地方，但这个观点早于现代SDN，事实上已经与可编程网络并行成长。这两种技术实际上是紧密耦合的——可编程网络凡是被认为是共享根本设施的机制（在数据中心的多个租户之间，在校园的打点组之间，或者在尝试设施中进行尝试），并撑持差别于物理网络的逻辑网络拓扑，这两者都是网络虚拟化的核心原则。最后，“网络虚拟化”的精确界说是难以捉摸的，对付我们讨论的一些机制（例如切片）是否代表网络虚拟化的形式，专家们自然差别意。在本文中，，我们界说了网络虚拟化的范畴，以包孕任何有助于在底层物理网络根本设施上托管虚拟网络的技术。

Network Virtualization before SDN. For many years, network equipment has supported the creation of virtual networks, in the form of VLANs and virtual private networks. However, only network administrators could create these virtual networks, and these virtual networks were limited to running the existing network protocols. As such, incrementally deploying new technologies proved difficult. Instead, researchers and practitioners resorted to running overlay networks, where a small set of upgraded nodes use tunnels to form their own topology on top of a legacy network. In an overlay network, the upgraded nodes run their own controlplane protocol, and direct data traffic (and control-plane messages) to each other by encapsulating packets, sending them through the legacy network, and decapsulating them at the other end. The Mbone (for multicast) [50], the 6bone (for IPv6) [43], and the X-Bone [76] were prominent early examples