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Anoverlay network is a logicalcomputer network that islayered on top of a physical network. The concept of overlay networking is distinct from the traditional model ofOSI layered networks, and almost always assumes that the underlay network is anIP network of some kind.[1]
Some examples of overlay networking technologies are,VXLAN,BGP VPNs, and IP-over-IP technologies, such asGRE,IPSEC tunnels, orSD-WAN.
Nodes in an overlay network can be thought of as being connected by logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example,distributed systems such aspeer-to-peer networks are overlay networks because their nodes form networks over existing network connections.[2][citation needed]
The Internet was originally built as an overlay upon the telephone network, while today (through the advent ofVoIP), the telephone network is increasingly turning into an overlay network built on top of the Internet.[citation needed]
Overlay networks have a certain set of attributes, including separation of logical addressing,security andquality of service. Other optional attributes includeresiliency,encryption andbandwidth control.
Guaranteeing bandwidth through marking traffic has multiple solutions, includingIntServ andDiffServ. IntServ requires per-flow tracking and consequently causes scaling issues in routing platforms. It has not been widely deployed. DiffServ has been widely deployed in many operators as a method to differentiate traffic types. DiffServ itself provides no guarantee of throughput; it does allow the network operator to decide which traffic is higher priority, and hence will be forwarded first in congestion situations.
Overlay networks implement a much finer granularity of quality of service, allowing enterprise users to decide on an application and user or site basis which traffic should be prioritized.
Manytelcos 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. TheseVPNs 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 is that the telecom operator does not need to manage addressing or other enterprise-specific network attributes.
Within data centers, it was more common to useVXLAN, however due to its complexity and the need to stitchlayer-2 VXLAN-based overlay networks tolayer-3 IP/BGP networks, it has become more common to use BGP within data centers to provide layer-2 connectivity betweenvirtual machines orKubernetes clusters.
Enterprise private networks were first overlaid ontelecommunication networks such asFrame Relay andAsynchronous Transfer Mode (ATM)packet switching infrastructures but migration from these (now legacy) infrastructures to IP-basedMPLS networks andvirtual 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, SD-WAN overlay networks offer additional flexibility.
The Internet is the basis for more overlaid networks that can be constructed in order to permitrouting of messages to destinations not specified by anIP address. For example,distributed hash tables can be used to route messages to a node having a specificlogical address, whose IP address is not known in advance.[needs context]
Overlay networks can be incrementally deployed at end-user sites or on hosts running the overlay protocol software, without cooperation fromInternet service providers. The overlay has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes a message traverses before reaching its destination.
The objective of resilience intelecommunications networks is to enable automated recovery during failure events in order to maintain a wantedservice level oravailability. As telecommunications networks are built in a layered fashion, resilience can be used in the physical, optical, IP or session to 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 all the layers below them. The advantage of overlays is that they are more flexible and programmable than traditional network infrastructure, which can outweigh the disadvantages of additional latency, complexity and bandwidth overheads.
Resilient Overlay Networks (RON) are architectures that allow distributed Internet applications to detect and recover from disconnection or interference. Current wide-area routing protocols that take at least several minutes to recover from are improved upon with this application-layer overlay. The RON nodes monitor the Internet paths among themselves and will determine whether or not to reroute packets directly over the Internet or over other RON nodes, thus optimizing application-specific metrics.[3]
The Resilient Overlay Network has a relatively simple conceptual design. RON nodes are deployed at various locations on the Internet. These nodes form an application-layer overlay that cooperates in routing packets. Each of the RON nodes monitors the quality of the Internet paths between each other and uses this information to accurately and automatically select paths from each packet, thus reducing the amount of time required to recover from poorquality of service.[3]
Overlay multicast is also known asEnd System orPeer-to-Peer Multicast.[4] High bandwidth multi-source multicast among widely distributed nodes is a critical capability for a wide range of applications, including audio and video conferencing, multi-party games and content distribution. Throughout the last decade, a number of research projects have explored the use of multicast as an efficient and scalable mechanism to support such group communication applications. Multicast decouples the size of the receiver set from the amount of state kept at any single node and potentially avoids redundant communication in the network.
The limited deployment of IP Multicast, a best-effort network layer multicast protocol, has led to considerable interest in alternate approaches that are implemented at the application layer, using onlyend-systems. In an overlay or end-system multicast approach, participating peers organize themselves into an overlay topology for data delivery. Each edge in this topology corresponds to a unicast path between two end-systems or peers in the underlying internet. All multicast-related functionality is implemented at the peers instead of at routers, and the goal of the multicast protocol is to construct and maintain an efficient overlay for data transmission.
Overlay network protocols based onTCP/IP include:
Overlay network protocols based on UDP/IP include:
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