Overlay network

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An overlay network is a computer network which is built on top of another network. Nodes in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example, many peer-to-peer networks are overlay networks because they run on top of the Internet. Dial-up Internet is an overlay upon the telephone network.

Uses of overlay networks

Overlay networks can be constructed in order to permit routing messages to destinations not specified by an IP address. For example, distributed hash tables can be used to route messages to a node having specific logical address, whose IP address is not known in advance.

Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees to achieve higher-quality streaming media. Previous proposals such as IntServ, DiffServ, and IP Multicast have not seen wide acceptance largely because they require modification of all routers in the network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from ISPs. 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.

For example, Akamai Technologies manages an overlay network which provides reliable, efficient content delivery (a kind of multicast). Academic research includes End System Multicast and Overcast for multicast; RON (Resilient Overlay Network) for resilient routing; and OverQoS for quality of service guarantees, among others.

List of overlay network protocols

• JXTA
• Distributed hash tables (DHTs), such as KAD and other protocols based on the Kademlia algorithm, for example.
• Many peer-to-peer protocols including Gnutella, Gnutella2, Freenet and I2P. (Examples: Limewire, Shareaza, utorrent, etc.)
• Solipsis: a France Télécom system for massively shared virtual world

References

• D. Andersen, H. Balakrishnan, M. Kaashoek, and R. Morris. Resilient Overlay Networks. In Proc. ACM SOSP, Oct. 2001.
• J. Jannotti, D. K. Gifford, K. L. Johnson, M. F. Kaashoek, and J. O'Toole. Overcast: reliable multicasting with an overlay network. In Proc. USENIX OSDI, Oct. 2000.
• L. Subramanian, I. Stoica, H. Balakrishnan, and R. H. Katz. OverQoS: An overlay based architecture for enhancing Internet QoS. In Proc. NSDI, 2004.

External links

• List of overlay network implementations, July 2003