Traversal Using Relay NAT
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Traversal Using Relay NAT (TURN) is a protocol that allows for an element behind a 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 currently an Internet draft.
[edit] Introduction
Network Address Translators (NATs), while providing many benefits, also come with many drawbacks. The most troublesome of those drawbacks is the fact that they break many existing IP applications, and make it difficult to deploy new ones. Guidelines have been developed that describe how to build "NAT friendly" protocols, but many protocols simply cannot be constructed according to those guidelines. Examples of such protocols include multimedia applications and file sharing.
Simple Traversal of UDP Through NAT (STUN) provides one means for an application to traverse a NAT. STUN allows a client to obtain a transport address (an IP address and port) which may be useful for receiving packets from a peer. However, addresses obtained by STUN may not be usable by all peers. Those addresses work depending on the topological conditions of the network. Therefore, STUN by itself cannot provide a complete solution for NAT traversal.
A complete solution requires a means by which a client can obtain a transport address from which it can receive media from any peer which can send packets to the public Internet. This can only be accomplished by relaying data though a server that resides on the public Internet. This specification describes Traversal Using Relay NAT (TURN), a protocol that allows a client to obtain IP addresses and ports from such a relay.
Although TURN will almost always provide connectivity to a client, it comes at high cost to the provider of the TURN server. It is therefore desirable to use TURN as a last resort only, preferring other mechanisms (such as STUN or direct connectivity) when possible. To accomplish that, the Interactive Connectivity Establishment (ICE) methodology can be used to discover the optimal means of connectivity.
