Summary

Discovery is the process of locating peers and resources in a P2P network. Discovery is based on three messaging techniques: broadcast, selective broadcast, and adaptive broadcast.

Broadcast, multicast, and replicated unicast all represent viable discovery transport services when used appropriately. Flooding broadcast techniques are only applicable for small work groups. A combination of constrained multicast and targeted unicast is gaining in popularity. It provides the necessary balance between connectivity and bandwidth consumption.

The TTL parameter is a technique used to limit the broadcast horizon in P2P networks. TTL is also used in the network layer to control packet routing in general.

Heartbeat polling in large P2P networks should be minimized, if not eliminated. The frequency of broadcast requests should be monitored. Controls and rules placed on bandwidth consumption can help to mitigate network problems, resource allocation, and security breaches.

Discovery protocols are maturing and becoming more complex. Metadata is providing an important element to extending protocol definition and enabling a more robust network.

Special-purpose discovery roles, such as relays and rendezvous points, provide a number of attractive alternatives for scaling a P2P network.