opposite direction, from the PSTN to the IP C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Tier 0 ".xxx" Tier 1 ".3.3.xxx" ".4.4.xxx" "0.0.0.7.7.4.0.4.1.3.3.xxx" Tier 2 > SIP URL for IP phone, address, email, web, email phone Tier 3 FIGURE 21.5 Different ENUM layer. (From Faltstrom, P. 2000. E.164 number and DNS; Network Working Group, RFC 2916. With permission.) network, a gateway must also be selected. The TRIP framework [10] divides the problem into three subproblems: 1. Given a phone number corresponding to a specific host on the IP net- work, determine the IP address of the host. 2. Given a phone number corresponding to a terminal on the PSTN, deter- mine the IP address of a gateway capable of completing calls to that phone. This is called the gateway location problem. 3. Given a phone number corresponding to a user of a terminal on the PSTN, determine the IP address of an IP terminal owned by the same user [10]. As the usage of ToIP grows and the number of gateways increase, the man- agement of gateways and routes between the IP and PSTN becomes more complex. In the situation where the IP address approaches the size of the PSTN address, a large part of the calls will pass through one or even several gateways on their path. The caller must locate a gateway for calls from the IP network to the PSTN, the gateway is able to complete calls to the desired destination. There may be several available gateways, and selecting the most suitable one is not a trivial process. Currently the gateway must be selected by the user or by the signaling servers. TRIP uses the concept of Internet Telephony Administrative Domains (ITADs) and location server (LS) nodes which exchange information with © 2009 by Taylor & Francis Group, LLC Secure Intelligent SIP Services other location servers. The location servers are administered by a single ITAD. TRIP distributes information between ITADs and contains functions for inter- domain synchronization of routing information. The decision about w