mms IP comms Host with CPE with NAT/Firewall UPnP IGD PCP client and control UPnP IGD PCP server point Figure 6.5 Three deployment scenarios for PCP. 6.4.2 Protocol Features The core features of the PCP are the following: Reduction of keep-alive traffic: By allowing control of the lifetime of existing or new NAT mapping, PCP allows clients to influence the frequency at which they need to send keep-alive packets. Port forwarding control: The PCP allows applications to create NAT port mapping rules from an internal IP address, protocol, and port to an external IP address, protocol, and port. Mixes between address families are possible. Support for transport layer protocols: The transport layer protocols that utilize 16-bit port numbers are supported. These include TCP, User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP) and Datagram Congestion Control Protocol (DCCP). The transport layer protocols not using ports are only supported with firewall and prefix translation scenarios, but not with NATs. These include, for example, Internet Control Message Protocol (ICMP), ICMPv6, and Encapsulating Security Payload (ESP). While the PCP is not able to control all routers and boxes on the path from the client to the destination, it is usually enough to be able to control the access network where the client is. If the destination is a service in the public Internet, the NATs and firewalls significance are on the client's access network. In the case of peer-to-peer communica- tions, it is enough for both peers to control NATs and firewalls in their respective access networks, and then signal the public-facing IP addresses and ports to the peer using a rendezvous protocol of a specific application, such as Session Initiation Protocol (SIP). The PCP does not care which rendezvous protocol is used. Future of IPv6 in 3GPP Networks 6.4.3 PCP Server Discovery The PCP client can learn the IP address or the fully qualified domain name of a PCP server by manual configuration, provisioning, or with help of Dynamic