Port control protocol

The Port Control Protocol (PCP) provides a mechanism to control the forwarding of incoming packets by upstream devices such as NAT64, NAT44, and firewall devices, and a mechanism to reduce application keepalive traffic. PCP is designed to be implemented in the context of both carrier-grade NATs (CGNs) and small NATs (for example, residential NATs). PCP allows hosts to operate servers for a long time (for example, a webcam) or a short time (for example, while playing a game or on a phone call) when behind a NAT device, including when behind a CGN operated by their Internet service provider. PCP allows applications to create mappings from an external IP address and port to an internal IP address and port.