Traversal Using Relays around NAT (TURN) is a protocol that allows for an element behind a network address translator (NAT) or firewall to receive incoming data over TCP or UDP connections. It is most useful for elements behind symmetric NATs or firewalls that wish to be on the receiving end of a connection to a single peer. TURN does not allow for users to run servers on well known ports if they are behind a NAT; it supports the connection of a user behind a NAT to only a single peer. In that regard, its role is to provide the same security functions provided by symmetric NATs and firewalls, but to turn the tables so that the element on the inside can be on the receiving end, rather than the sending end, of a connection that is requested by the client.
TURN is specified by RFC 5766. An update to TURN for IPv6 is specified in RFC 6156.
Read more about Traversal Using Relays Around NAT: Introduction, Protocol Overview
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