Routing on Overlay Networks: Developments and Challenges

Overlay networks are networks operating on the inter-domain level, where the edge hosts learn of each other and, based on knowledge of underlying network performance, they form loosely coupled neighboring relationships. These relationships can be used to induce a specific graph, where nodes are representing hosts and edges are representing neighboring relationships. Graph abstraction and the associated graph theory can be further used to formulate routing algorithms on overlay networks. The main advantage of overlay networks is that they offer the possibility to augment the IP routing as well as the QoS functionality offered by the Internet. One can state that, generally, every Peer-to-Peer (P2P) network has an overlay network at the core, which is mostly based on TCP or HTTP connections. Because of the abstraction offered by the TCP/IP protocol stack at the application layer, the overlay and the physical network can be completely separated from each other as the overlay connections do not reflect the physical connections. Overlay networks allow designers to develop own routing and packet management algorithms atop of the Internet. Overlay networks can therefore be used to deploy new protocols and functionality atop of IP routers without the need to upgrade the routers. New services can be easily developed, with their own routing algorithms and policies. Actually, there are two general classes of overlay networks, i.e., routing overlays and storage and lookup overlays. Routing overlays operate on the inter-domain IP level and are used to enhance the Border Gateway Protocol (BGP) routing and to provide new functionality or improved service, e.g., as reported in [2]. However, the overlay nodes operate, with respect to each other, as if they are belonging to the same domain on the overlay level. QoS guarantees can be provided as well. On the other hand, storage and lookup overlays have the focus on techniques to use the power of large, distributed collections of machines, e.g., Chord [15]. These overlays are actually used to support a number of projects on large distributed systems.