IMS-Specific SIP Header Fields

IMS (IP Multimedia Subsystem) is an architectural framework for delivering IP multimedia services. It was initially designed by the 3rd Generation Partnership Project (3GPP) as part of the evolution of mobile telecommunications networks towards 3G and beyond, but its use has expanded to any network that delivers IP multimedia to end-users. IMS enables the convergence of data, voice, video, and mobile network technology over an IP-based infrastructure, making it possible to provide a wide range of telecommunications services across both wireless and fixed networking environments. 

IMS facilitates the integration and delivery of voice, video, messaging, data, and web-based services, enabling rich multimedia experiences for users. Uses SIP for session management, enabling the establishment, modification, and termination of sessions with one or more participants.

In the context of IP Multimedia Subsystem (IMS) communications, certain SIP header fields are specifically designed to support the unique features and requirements of IMS architectures. These headers facilitate enhanced session management, authentication, service delivery, and interoperability within IMS networks. This part focuses on IMS-specific SIP header fields, distinguishing them from the general SIP headers covered in previous sections.

P-Asserted-Identity

Used by network entities to convey the identity of the user as verified by the network, crucial for services that rely on accurate user identification.
Format example: P-Asserted-Identity: <sip:alice@example.com>

P-Preferred-Identity

Allows a user to specify a preferred identity out of multiple possible identities, enabling choice in how they are identified in communications.
Format example: P-Preferred-Identity: <sip:bob@example.com>

P-Charging-Function-Addresses

Communicates the addresses of charging functions within the network, facilitating accurate and efficient charging for IMS services.
Format example: P-Charging-Function-Addresses: ccf=<sip:ccf1.example.com>;ecf=<sip:ecf1.example.com>

P-Charging-Vector

Carries charging-related identifiers and information, enabling correlation of charging records across different IMS network components.
Format example: P-Charging-Vector: icid-value="icid123456789";orig-ioi="originating.example.com"

P-Visited-Network-ID

Indicates the network that the user is currently visiting, essential for applying appropriate policies and charging in roaming scenarios.
Format example: P-Visited-Network-ID: "visitednetwork.example.com"

P-Access-Network-Info

Provides details about the user's access network, supporting optimized session handling based on network characteristics.
Format example: P-Access-Network-Info: IEEE-802.11a; utran-cell-id-3gpp=123456789abc

P-Profile-Key 

The "P-Profile-Key" header is used in SIP to associate a specific profile with the communication session. Profiles may define service settings or configurations that affect how the session is handled, such as feature sets or policy information. This header enables the dynamic application of these profiles to SIP sessions, enhancing the customization and efficiency of service delivery.
Format Example: P-Profile-Key: "profile1"

P-Charging-Function-Addresses 

The "P-Charging-Function-Addresses" header in SIP is designed to convey information about the charging architecture, specifically the addresses of the charging functions (such as the Charging Collection Function (CCF) and the Event Charging Function (ECF)) involved in the call. This header supports billing and charging mechanisms by providing the necessary information for capturing call details for charging purposes.
Format Example: P-Charging-Function-Addresses: ccf="sip:ccf.example.com";ecf="sip:ecf.example.com"

P-Answer-State 

The "P-Answer-State" header is used in the Session Initiation Protocol (SIP) within the context of IP Multimedia Subsystem (IMS) services, particularly to convey the status of a user's answering service. This header is pivotal in scenarios where the network needs to understand or manage the answering behavior of a call recipient, such as in call completion services where the called party’s availability or response to a call is monitored and acted upon. The P-Answer-State header allows the communicating entities to share the status of call answering mechanisms, facilitating enhanced control and decision-making processes in telecommunications systems, especially those conforming to 3GPP standards.
Format Example: P-Answer-State: active;reason=no-answer;retry-after=60

P-Served-User

Specifies the user being served in scenarios where services are invoked on behalf of another user, ensuring the correct application of user-specific services and policies.
Format example: P-Served-User: <sip:carol@example.com>;sescase=orig;regstate=unreg