What is overlay networking?
The concept of overlay networking has been around for a while, but has made a return to the spotlight with the rise of scalable network virtualization. It is largely leveraged by the telecom industry, which has increasingly embraced network virtualization over the past few years. But what exactly is an overlay network and how does it work in general?
Definition and examples
Simply put, an overlay network is a virtual network of nodes and logical links, which are built on top of an existing network. Using overlay networks, multiple layers of network abstraction can be created to run virtualized network layers, which are supported by a physical infrastructure. The aim of an overlay network is to enable a new service or function without having to reconfigure the entire network design.
The internet, for example, is the most widely used overlay network. Before it was commercialized in the 1980s, the web was a government-based research network, which ran on top of the physical infrastructure of the Public Switched Telecommunications Network (PSTN). Today, the majority of overlay networks run on top of the public internet. Other examples of overlay networks include virtual private networks, peer-to-peer networks and voice over IP services like Skype.
Encapsulation, protocols and SDN
The bulk of overlay networks use some kind of encapsulation in which one packet resides inside of another packet while being forwarded to a particular endpoint. The encapsulated packet is then de-encapsulated upon arriving at the endpoint. In some cases, encryption is used to secure the data between the endpoints, which requires the end-users to be authenticated to use the connection.
Various network protocols and standards have been developed for overlay networks. Among these protocols include IP, virtual extensible LAN (VXLAN), generic routing encapsulation, network virtualization overlays 3 (NVO3), IP multicast, among others. The introduction of software-defined network (SDN), which involves separating the network control plane from the forwarding plane, has also produced novel overlay technologies and solutions, such as VMWare’s NSX.
Advantages
Overlay networks have several advantages, such as enabling developers to create and implemented protocols on the web easily, from data routing to file sharing management. Additionally, data routing in overlay networks provides a great deal of flexibility with support for multi-path forwarding. Moreover, with overlay networks, IT managers are able to move virtual machines to different locations without being restricted by the physical network.
Drawbacks
This doesn’t mean virtual networks pose no challenges. The multiple layers of software and processing provided by overlay networks can increase performance overhead and make the network more complicated. Additionally, the process of encapsulating and de-encapsulating packets can demand a significant amount of computing power. Furthermore, the physical network isn’t able to adjust automatically to changes in virtual networks.
