The destination device could be another subscriber device, or it may be an application
server providing a service to the subscriber (such as a Web server supporting a Webinar
session). The next session looks at how the INVITE is routed followed by a discussion
regarding the establishment of a dialog, and session management.
Routing the Request The 3GPP has specified the use of loose routing in the IMS (vs.
strict routing) in accordance with RFC 3261. However, actual implementation by operators
may use strict routing as an alternative. There are many instances outlined in
the various 3GPP documents that describe strict routing, so one can assume that while
loose routing is supported, the recommendation is for strict routing.
There are many advantages to strict routing, as there are to loose routing. In loose
routing the network makes routing decisions based on traffic in the network and the
status of the various network elements. Traditional data networks may choose loose
routing simply because this is how they route traffic today.
Strict routing, on the contrary, is more controlled and can cause unbalanced traffic
in the network. However, traditional voice service providers use this method today and
are likely to use strict routing in their IMS implementations for security purposes. The
procedures discussed here are per the 3GPP specifications for IMS.
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