Chapter 3

UDP

• UDP packets are very simple. They only add an additional 4 headers to the payload. Checksum, length, source port, and destination port. Of which, only length and destination port are required.

NAT

Because UDP does not maintain connection state, NAT devices do not know when a connection is no longer active. NAT Translators expire UDP connections based on a timer. This timer is typically unique across manufacturers of NAT devices.

NAT Traversal

NAT can cause issues for client applications that need to be aware of the public IP of the connection. Some example applications are P2P apps such as VOIP, games, and file sharing. To workaround this issue, protocols such as STUN, TURN, and ICE were created.

• STUN - Session Traversal Utilities for NAT. A protocol that allows the host application to discover the presence of a NAT device on the network, and when present obtain the allocated public IP address and port tuple for the current connection. To do this, the application requires assistance from a well-known, third party STUN server that resides on the network. The IP address of the STUN server can be shared via DNS.
• TURN - Traversal Using Relays around NAT. Runs over UDP, but can switch to TCP when it fails. Requires a well-known public relay to shuttle the data between peers.