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 only end-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. Disadvantages Slow in spreading the data. Long latency. Duplicate packets at certain points. List of overlay network protocols Overlay network protocols based on TCP/IP include: http / https Distributed hash tables (DHTs) based on the Chord JXTA XMPP: the routing of messages based on an endpoint Jabber ID (Example: nodeId_or_userId@domainId\resourceId) instead of by an IP Address Many peer-to-peer protocols including Gnutella, Gnutella2, Freenet, I2P and Tor. PUCC Solipsis: a France Télécom system for massively shared virtual world Overlay network protocols based on UDP/IP include: Distributed hash tables (DHTs) based on Kademlia algorithm, such as KAD, etc. Real Time Media Flow Protocol – Adobe Flash See also Darknet Mesh network Net Peercasting Virtual Private Network References External links List of overlay network implementations, July 2003 Resilient Overlay Networks Overcast: reliable multicasting with an overlay network OverQoS: An overlay based architecture for enhancing Internet QoS RFC 3170 Multicast over TCP/IP HOWTO End System Multicast Overlay networks Anonymity networks Network architecture Computer networking