Overlay network by Ms.Kajal Sharma (Lecturer, CS/IT)

With the rapid advancement of Internet and computing technology, much more aggregate information and computing resources are available, overlay networks provides better utilization of growing Internet information and resources. Overlay network brings more flexibility to add higher layer functionalities for today's Internet to support Quality of Service (QoS). Overlay network is virtual communication structure that is logically laid over a physical network. Nodes in overlay network can be thought of as being connected by virtual or logical link, each of which corresponds to a path, perhaps through many physical links, in the underlying network. An overlay network has also been proposed as a way to improve Internet routing such as through quality of service guarantees to achieve higher-quality streaming media. Overlay networking capability can be used to enhance features like overlay multicast at application service level.

Figure 1
Figure 1 demonstrates an overlay network. An overlay network has a virtual topology, which exists by the agreement of the overlay nodes. Packets are transmitted only along the virtual links between the overlay nodes using the underlying unicast mechanism provided by IP. In contrast to the Internet, in which routers are a shared resource that cannot be specialized for a particular purpose, the members of an overlay network may provide specialized services specific to the application at hand. An Overlay Network is a set of tunnels formed among network edges to support a common packet processing function other than the ones supported in the conventional network. The primary advantage of the overlay network architecture is that it does not require universal network support to be useful. This enables faster deployment of desired network functions and adds flexibility to the service infrastructure, as it allows the co-existence of multiple overlay networks each supporting a different set of service functions. An Overlay Multicast Network is one type of overlay network that provides multicast services to end users on top of the general Internet unicast infrastructure.
Overlay networks allow both network developers and application users to easily design and implement their own communication environment and protocols on top of the Internet, such as data routing and file sharing management. Data routing in overlay networks can be very flexible, quickly detecting and avoiding network congestions by adaptively selecting paths. The high connectivity of increasingly more end-nodes to join overlay networks enables effective sharing of a huge amount of information and resources available on the Internet. QoS (Quality of Service) is one of the most important unsolved research issues in this area.
Overlay networks such as the Chord, Pastry and Content Addressable Network (CAN), create a virtual topology on top of the physical topology. The overlay multicasting approach assumes no multicasting support in the network layer, and constructs a multicast delivery tree in the application layer. The overlay multicast tree consists of IP tunnels connecting edge routers in the network. IP Multicast is the traditional method for group communication over the Internet. Overlay Multicast, a substitute technology for IP Multicast has become a good topic for researchers.