Traversal Using Relays around NAT
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Traversal Using Relays around NAT (TURN) is a protocol that allows for a client behind a network address translator (NAT) or firewall to receive incoming data over TCP or UDP connections. It is most useful for clients 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, as in video telephony, for example. 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 client on the inside can be on the receiving end, rather than the sending end, of a connection that is requested by the client.
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.
Session Traversal Utilities for 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 through 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.
Protocol Overview 
The process begins when a Client Computer ("Client") wants to contact a Peer Computer ("Peer") for a data transaction, but cannot do so due to both Client and Peer being behind respective NATs. If STUN is not an option because one of the NATs is a symmetric NAT (a type of NAT known to be non-STUN compatible), TURN must be used.
First, the Client contacts a TURN server with an "Allocate" request. The Allocate request asks the TURN server to allocate some of its resources for the Client so that he/she may contact Peer. If allocation is possible, the server will allocate an address for Client to use as a relay, and send the Client a "Allocation Successful" response, which contains a "allocated relayed transport address" located at the TURN server.
Second, the Client sends in a CreatePermissions request to the TURN server to create a permissions check system for Peer/Server communications. In other words, when Peer is finally contacted and sends information back to the TURN server to be relayed to Client, the TURN server uses the permissions to verify that the Peer to TURN server communication is valid.
After permissions have been created, Client has two choices of sending the actual data, (1) he/she can use the Send mechanism, or (2) he/she can reserve a channel using the ChannelBind request. The Send mechanism is more straight forward, but contains a larger header, 36 bytes, that can substantially increase the bandwidth in a TURN relayed conversation. In contrast, the channel method is lighter: the header is only 4 bytes, but it requires a channel being reserved which needs to be periodically refreshed, among other considerations.
Using either method, Send or channel binding, the TURN server will receive the data from Client and relay it to Peer using UDP datagrams, which contain as their Source Address the "Allocated Relayed Transport Address". The Peer receives the data and responds, again using a UDP datagram as the transport protocol, sending the UDP datagram to the relay address at the TURN server.
The TURN server receives the Peer UDP datagram, checks permissions and if they are valid, sends it back to Client.
This process gets around even symmetric NATs because both the Client and Peer can at least talk to TURN server, which has allocated a relay IP address for communication.
While TURN is more robust than STUN in that it can traverse more types of NATs, it is prudent to note that a TURN communication relays the entire communication through the server requiring far more bandwidth than STUN protocol (which typically only resolves the public facing IP address then relays the information to Client and Peer for them to use in direct communication). For this reason, the ICE protocol mandates STUN usage as a first resort, and TURN usage when dealing with symmetric NATs, or other situations where STUN cannot be used.
See also 
- Traversal Using Relays around NAT (TURN): RFC5766
- Traversal Using Relays around NAT (TURN) Extension for IPv6: RFC5766
- Yahoo! - Director of Engineering explaining STUN and TURN (Video)
- Restund OpenSource Modular STUN/TURN Server (BSD License)
- Numb is a free STUN/TURN server.
- TurnServer - OpenSource TURN server.
- reTurn - opensource STUN/TURN server and client library (C++)
- TURN Server - High-Performance Open Source TURN/STUN server (BSD license) and client library (C)
- AnyFirewall - STUN, TURN & ICE library.
- Libnice - STUN, TURN & ICE library used in Pidgin.
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