The TRIP Problem Statement

When an End User on an IP network wants to complete a call to a phone number on the circuit switched network, the call must be routed through a telephony gateway. Selection of such a gateway is a non-trivial process because of the following reasons:

1. Number of candidate gateways can be really large.
2. The gateway owners incur cost when completing calls towards the circuit switched networks. Hence, the users are charged for these services. This may restrict access to the user who have an established relationship with the owner.
3. In many cases gateway owner is not the provider of IP telephony services to users. In such cases the provider policies and its relationships with various Gateway owners determine what gateways to choose.
4. Even the end user policy on what features to use or what provider to use can be taken into consideration.
5. The gateway capacity varies. Load balancing is therefore required for better performance.
6. The gateway capacity varies. Load balancing is therefore required for better performance.
7. Protocols & Feature compatibilities: signalling or media protocols that are being used by the calling party must be available at the gateway. This too determines what gateways can be used.

Hence determining available gateways for completing calls requires directory which cannot be global, but is decided on the basis of inter-provider relationships and other factors. For seamless functioning of this service, therefore, a protocol is required to exchange the gateway routing information. TRIP is one such protocol.