Overlay network
An overlay network is a computer network that is layered on top of another (logical as opposed to physical) network. The concept of overlay networking is distinct from the traditional model of OSI layered networks, and almost always assumes that the underlay network is an IP network of some kind. [1]
Uses[edit]
Telecommunications Operators[edit]
Telecom Operators use overlay networks to provide services over their physical infrastructure. In the networks that connect physically diverse sites (Wide Area Networks, WANs), one common overlay network technology is BGP VPNs. These VPNs are provided in the form of a service to enterprises to connect their own sites and applications. The advantage of these kinds of overlay networks are that the telecom operator does not need to manage addressing or other enterprise specific network attributes.
Within data centers, it was more common to use VXLAN, however due to its complexity and the need to stitch Layer 2 VXLAN-based overlay networks to Layer 3 IP/BGP networks, it has become more common to use BGP within data centers to provide Layer 2 connectivity between Virtual Machines or Kubernetes Clusters.
Enterprise networks[edit]
Enterprise private networks were first overlaid on telecommunication networks such as Frame Relay and Asynchronous Transfer Mode packet switching infrastructures but migration from these (now legacy) infrastructures to IP-based MPLS networks and virtual private networks started (2001~2002) and is now completed, with very few remaining Frame Relay or ATM networks.
From an enterprise point of view, while an overlay VPN service configured by the operator might fulfill their basic connectivity requirements, they lack flexibility. For example, connecting services from competitive operators, or an enterprise service over an internet service and securing that service is impossible with standard VPN technologies, hence the proliferation of SD-WAN overlay networks that allow enterprises to connect sites and users regardless of the network access type they have.
Over the Internet[edit]
The Internet is the basis for more overlaid networks that can be constructed in order to permit routing of messages to destinations not specified by an IP address. For example, distributed hash tables can be used to route messages to a node having a specific logical address, whose IP address is not known in advance.
Advantages[edit]
Resilience[edit]
The objective of resilience in telecommunications networks is to enable automated recovery during failure events in order to maintain a wanted service level or availability. As telecommunications networks are built in a layered fashion, resilience can be used in the physical, optical, IP or session/application layers. Each layer relies on the resilience features of the layer below it. Overlay IP networks in the form of SD-WAN services therefore rely on the physical, optical and underlying IP services they are transported over. Application layer overlays depend on the all the layers below them. The advantage of overlays are that they are more flexible/programmable than traditional network infrastructure, which outweighs the disadvantages of additional latency, complexity and bandwidth overheads.
Overlay network protocols based on TCP/IP include:
Overlay network protocols based on UDP/IP include: