ated to address family selections, depending on network conditions, is a typical IP-version- aware application protocol requirement. 2. FTP needs means of communicating IPv6 related endpoint information. In practice this means replacement of the FTP command PASV with Enhanced PASV - EPSV and PORT with Enhanced PORT - EPRT commands. Generally speaking, this means that whatever signaling a protocol is using in relation to IP addresses, the protocol must support the existence of both address families. 3. The EPRT command enhances the PORT command with additional fields for commu- nicating which address family is used (IPv4 or IPv6) and an IP address in address family specific format. The command can also fail in new ways: the remote peer can fail with an error indicating unknown or unsupported address family. Such errors usu- ally trigger fallback to another address family, if available. This kind of approach for passing protocol family and address in specific format is typical for protocols involving IP addresses, although often lists of both IPv4 and IPv6 addresses are provided. 4. The EPSV command enhances the PASV command only with placeholders for address family and IP address. Interestingly enough, the FTP passive mode listens only to one address family - the one that the control channel is using. However, in many protocols listening for incoming connections needs to be conducted on both IPv4 and IPv6, as the remote peers may have only either available. 3.10.4 Internet Routing Introduction of IPv6 into the Internet required changes to the existing IPv4 routing infrastructure. All routers had to, and have to, be modified to support IPv6 related Deploying IPv6 in 3GPP Networks forwarding functions. Additionally, routing protocols had to be enhanced with IPv6 capabilities. It is also worth noting that the mere existence of both IPv4 and IPv6 routes increases quite significantly resource (mostly memory) requirements for routers' routing table management, but that is not a problem we are focusing on in this book. B