 peer's IP address and port. Both request messages include a suggested external IP address, which is useful, for example, when a client is restoring an old, perhaps lost, state to the NAT or firewall. This is also a useful property with applications such as active mode File Transfer Protocol (FTP) that require different transport layer flows, within a single application layer session, to use the same IP addresses. If a request is for a protocol that does not have 16-bit port numbers, the request is considered to apply to all messages of that protocol. Deploying IPv6 in 3GPP Networks The protocol itself allows mapping for all protocols and all ports, in which case the NAT/firewall would essentially have 1:1 mapping between external and internal IP addresses. PCP Responses All PCP responses include the response type a result indicating success or failure reasons, the server's epoch time value, optional information elements, and then a 32-bit lifetime indicating the lifetime of a mapping or in case of error, how long the response is likely to remain the same. The responses are defined for the two specified requests: MAP: The PCP server returns to the client, in the case when mapping was allowed, the same nonce, protocol, the internal port, and assigned external IP address and port, which may or may not be the same as the client requested. PEER: The PCP server returns to the client, in the case of success, information about the external address and port assigned for the client, and also the remote peer's port and IP address. The epoch lifetime value provides hints for PCP clients about the state of the PCP server; if client notices surprisingly low epoch value, it is an indication of server reboot and hence could trigger the client to refresh its mappings. 6.4.5 Cascaded NATs It can happen that the network has cascaded NATs. The PCP client can detect the presence of multiple NATs by sending a PCP request to the PCP server that the client is configured with, and observing if the PCP response shows a different IP addre