This is most commonly used for interactive media such as Voice over Internet Protocol (VoIP), peer-to-peer communications, video, and instant messaging.
In such applications, communicating through a central server would be slow and expensive, but direct communication between client applications on the Internet is very tricky due to network address translators (NATs), firewalls, and other network barriers.
VoIP, peer-to-peer, and many other applications require address information of communicating peers within the data streams of the connection, rather than only in the Internet Protocol packet headers.
For example, the Session Initiation Protocol (SIP) communicates the IP address of network clients for registration with a location service, so that telephone calls may be routed to registered clients.
Traversal Using Relays around NAT (TURN) places a third-party server to relay messages between two clients when direct media traffic between peers is not allowed by a firewall.