Overview of PCP
This section provides the definition of the Port Control Protocol (PCP) and describes its purpose.
Definition
PCP provides a mechanism to control how incoming packets are forwarded by upstream devices, such as NAT444 devices and firewalls, and a mechanism to reduce application keepalive traffic. PCP allows applications to create mappings from an external IP address, protocol, and port to an internal IP address, protocol, and port. These mappings are required for successful inbound communications destined to machines located behind a NAT or a firewall.
Objective
Currently, the PCP function supported by the FW mainly applies to a NAT444 scenario. In the NAT444 scenario, customer premises equipment (CPE) acts as a PCP client, a Carrier Grade NAT (CGN) (FW) acts as a PCP server, and a PCP connection is established between the CPE and CGN. A private network user obtains a public IP address and port number from the CGN through the CPE and then uses the public IP address to access the Internet. An Internet user can communicate with a private network user through the CGN that has a PCP mapping table. In this way, P2P services, such as file sharing, voice communication, and video transfer can properly run in the NAT444 scenario.
