nameservice servers. History PCP was standardized in 2013 as a successor to the NAT Port Mapping Protocol (NAT-PMP), sharing similar protocol concepts and packet formats with it. As one of the design differences, NAT-PMP is pretty much limited to the deployment on consumer-grade devices, while PCP is designed to also support carrier-grade equipment. Since 2005, NAT-PMP has been implemented in various Apple products. PCP relates to the Internet Gateway Device Protocol (IGDP), which was standardized in 2001 as part of the Universal Plug and Play (UPnP) specification. While the IGDP is complex and tailored toward manual configuration, PCP is designed for simplicity and automated use within software applications. The NAT-PMP specification contains a list of the problems with IGDP that prompted the creation of NAT-PMP, and subsequently, its successor PCP. Security Excluding the attackers capable of altering network packets exchanged while an explicit PCP mapping is created (packets that contain negotiation required for establishing an explicit mapping, which is exchanged between hosts and PCP-enabled NAT devices or firewalls), PCP is considered to be secure as long as created explicit mappings do not exceed the domain of implicit mappings. In other words, implicit mappings are created as a result of the way NAT devices and firewalls are handling regular outbound client connections, meaning that PCP is safe as long as no new mapping possibilities are introduced through the explicit mapping mechanism. From the security standpoint, an important PCP feature is the mapping request option. When used, this option signifies that the IP address specified additionally as part of the mapping request should be used as the internal address for the created explicit mapping, rather than following the default behavior of using source IP address of the actual mapping request packet for that purpose. Such mapping requests can end up with a PCP-enabled NAT device or firewall granting explicit mapping privileges higher than allowed by implicit mappings due to unknown rules imposed elsewhere for the specified IP address, allowing that way an attacker to steal some traffic, or to conduct a denial-of-service (DoS) attack. Additionally, explicit PCP security mechanisms are available as extensions to the PCP protocol, providing authentication and access control mechanisms by using an authenticated and integrity-protected in-band signalling channel, which relies on Extensible Authentication Protocol (EAP) to perform the authentication between devices involved in a PCP negotiation session. Such PCP-enabled NAT devices or firewalls may still accept unauthenticated mapping