Protocol Overview
STUN is a lightweight client-server protocol requiring only simple query and response. The client side is implemented in the user's communications application, such as a voice over Internet Protocol (VoIP) phone or instant messaging client.
The base protocol operates essentially as follows. The client, often operating inside a private network, sends a binding request to a STUN server on the public Internet. The STUN server sends a success response that contains the IP address and port as observed from its perspective. The result is usually XOR mapped to avoid translation of packet contents.
STUN usually operates on a User Datagram Protocol (UDP) messaging transport. Since UDP does not provide reliable transport guarantees, reliability is achieved by application-controlled retransmissions of the STUN requests. STUN servers do not implement any reliability mechanism for their responses. When reliability is mandatory, the Transmission Control Protocol (TCP) may be used, but induces extra networking overhead. In security-sensitive applications, STUN may be transported and encrypted by Transport Layer Security (TLS).
An application may automatically determine a suitable STUN server for communications with a particular peer by querying the Domain Name System (DNS) for the stun (for UDP) or stuns (for TCP/TLS) server record (SRV) resource record, e.g., _stun._udp.example.com. The standard listening port number for a STUN server is 3478 for UDP and TCP, and 5349 for TLS. Alternatively, TLS may also be run on the TCP port if the server implementation can de-multiplex TLS and STUN packets. In case no STUN server is found using DNS lookups, the standard recommends that the destination domain name should be queried for address records (A or AAAA) which would be used with the default port numbers.
In addition to using protocol encryption via TLS, STUN also has built-in authentication and message-integrity mechanisms via specialized STUN packet types.
When a client has discovered its external address, it can use this as a candidate for communicating with peers by sharing the external NAT address rather than the private address (which is, by definition, not reachable from peers on the public network).
If both peers are located in different private networks behind a NAT, the peers must coordinate to determine the best communication path between them. Some NAT behavior may restrict peer connectivity even when the public binding is known. The Interactive Connectivity Establishment (ICE) protocol provides a structured mechanism to determine the optimal communication path between two peers. Session Initiation Protocol (SIP) extensions are defined to enable the use of ICE when setting up a call between two hosts.
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