This Recommendation defines the principles applicable to international public correspondence services provided by IP-based networks interworking with the ITU-defined, circuit-switched, public, international telecommunication networks (for example, the PSTN, ISDN and PLMN). This Recommendation is applicable to those cases where the IP-based network is implemented by a separate service provider (e. g. ROA) from the service provider of the public, circuit-switched international telecommunication network. It does not cover the case where IP technology is integrated within the international telecommunication network of a single service provider.