STUN Simple traversal of UDP over NAT

STUN, which stands for Simple Traversal of UDP over NAT, is a protocol used in computer networks to facilitate communication between devices located behind Network Address Translators (NATs). NATs are commonly used in residential and corporate networks to enable multiple devices to share a single public IP address. STUN helps devices behind NATs to determine their public IP address and establish communication with devices in different networks.

Here is a detailed explanation of Simple Traversal of UDP over NAT (STUN):

1. Network Address Translation (NAT): NAT is a mechanism used to translate private IP addresses used within a local network into a single public IP address that is visible on the internet. NATs help conserve IP address space and provide a level of security by hiding the internal IP addresses from external networks. However, NATs pose challenges for establishing direct communication between devices located behind different NATs.
2. Problem of NAT Traversal: NATs introduce complications for peer-to-peer communication between devices behind different NATs. Traditional methods of establishing connections, such as IP-based communication, are hindered because devices behind NATs cannot directly reach each other due to the translation and mapping of IP addresses performed by the NAT.
3. STUN Protocol: STUN is a protocol that enables devices behind NATs to discover their public IP address and determine the type of NAT they are located behind. It is a lightweight and straightforward protocol designed to assist with NAT traversal for UDP-based communications.
4. STUN Operation: When a device behind a NAT needs to establish communication with devices in different networks, it sends a STUN request to a STUN server. The STUN server is located in a public network and is reachable by both the requesting device and the target device. The STUN server responds with a STUN response that contains the public IP address and port number as observed by the STUN server.
5. NAT Mapping and Filtering: STUN helps determine the type of NAT a device is located behind. NATs can be categorized into different types based on their behavior, such as full-cone NAT, restricted-cone NAT, port-restricted cone NAT, and symmetric NAT. The information provided by the STUN server helps devices understand the behavior of their own NAT, including the type of mapping and filtering performed by the NAT.
6. ICE and NAT Traversal: STUN is commonly used in conjunction with other protocols like ICE (Interactive Connectivity Establishment) to facilitate NAT traversal for real-time communication applications, such as Voice over IP (VoIP) or video conferencing. ICE utilizes STUN to gather connectivity information and establish the most efficient communication path between devices behind NATs.
7. TURN and NAT Traversal: In cases where direct communication between devices is not possible due to restrictive NATs or firewalls, a technique called TURN (Traversal Using Relays around NAT) can be employed. TURN uses a relay server as an intermediary to relay traffic between devices that cannot establish a direct connection. STUN is often used to assist in discovering the public IP address of the relay server.
8. STUN and IPv6: While STUN was initially designed for IPv4 networks, it can also be used in IPv6 networks for similar purposes. STUN extensions and adaptations exist to support IPv6 address discovery and NAT traversal in IPv6 environments.

In summary, Simple Traversal of UDP over NAT (STUN) is a protocol used for NAT traversal, allowing devices located behind NATs to determine their public IP address and establish communication with devices in different networks. STUN helps address the challenges posed by NATs in peer-to-peer communication scenarios. It is commonly used in conjunction with other protocols like ICE and TURN to facilitate efficient and direct communication between devices located behind NATs.