Certain protocols such as FTP and SIP can carry addressing information such as source and destination IP addresses and ports. When these protocols pass through NAT , this addressing will not be changed, since NAT typically works at the Transport and Network Layers. Therefore, the receiving entity will be supplied with addressing that is not relevant, such as private IP addressing or the wrong version of IP addressing. NAT traversal describes the process behind overcoming this issue, ensuring that addressing information delivered to the intended recipient is valid. There are several techniques which can be used for NAT traversal, such as ICE (Interactive Connectivity Establishment – a combination of STUN and TURN ), Session Border Controllers and UPnP (Universal Plug and Play).