Session Control in the IMS
We saw in Section 4.1 how SIP is used in a public Internet environment. We have also explored the core SIP functionality and a few important extensions that SIP User Agents may support. Each implementation of SIP is free to implement the options or SIP extensions that the particular application requires.
3GPP is one of those particular applications of SIP where SIP is used in a wireless environment. In the case of 3GPP, SIP is used over an underlying packet network that defines a number of constraints. The result of the evaluation of SIP in wireless environments led to the definition of a set of requirements that accommodates SIP in 3GPP networks. The implementation of solutions to these wireless requirements led 3GPP to mandate the use of a number of options and extensions to SIP and other protocols. We can consider 3GPP’s function as creating a profile of utilization of SIP and other protocols in the IP Multimedia Subsystem. The 3GPP SIP profile utilization for IMS is specified in 3GPP TS 24.229 . We call it a profile because there are no differences with respect to the usage of SIP on the public Internet. However, 3GPP has mandated the implementation of a number of extensions and options in both the IMS network nodes and IMS terminals. This section focuses on describing how SIP is used in the IMS as well as highlighting differences in the utilization of SIP with respect to the public Internet.
When 3GPP began the work on session control ...