Session Traversal Utilities for NAT (STUN)
Session Traversal Utilities for NAT (STUN) is a protocol that serves as a tool for other protocols in dealing with NAT traversal. It can be used by an endpoint to determine the IP address and port allocated to it by a NAT. It can also be used to check connectivity between two endpoints and as a keep-alive protocol to maintain NAT bindings. STUN works with many existing NATs and does not require any special behavior from them.
STUN is not a NAT traversal solution by itself. Rather, it is a tool to be used in the context of a NAT traversal solution.
This document obsoletes RFC 5389.