SIP or other protocols, and we expect that individual members of the SIP community will work with 3GPP to achieve a better understanding of these mechanisms. Some of the requirements in this document may not be addressed at all by the IETF, although we believe that the act of documenting and discussing them is in itself helpful in achieving a better all-around understanding of the task at hand.
2. Conventions
This document does not specify any protocol of any kind. Therefore, the usage of the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document, as described in RFC 2119 [1], does not apply.
3. Overview of the 3GPP IMS
This section gives the reader an overview of the 3GPP IM CN Subsystem (IMS). It is not intended to be comprehensive, but it provides enough information to understand the basis of the 3GPP IMS. Readers are encouraged to find a more detailed description in the 3GPP Technical Specifications 23.060 [27], 23.228 [28], and 24.228 [29].
For a particular cellular device, the 3GPP IMS network is further decomposed in a home network and a visited network.
An IMS subscriber belongs to his or her home network. Services are triggered and may be executed in the home network. One or more SIP servers are deployed in the SIP home network to support the IP Multimedia Subsystem. Among those SIP servers, there is a SIP serving proxy, which is also acting as a SIP registrar. Authentication/Authorization servers may be part of the home network as well. Users are authenticated in the home network.
A SIP outbound proxy is provided to support the User Agent (UA). The SIP outbound proxy is typically located in the visited network, although it may be located in the home network as well. The SIP outbound proxy maintains security associations between itself and the terminals, and interworks with the resource management in the packet
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