In the context of IMS (IP Multimedia Subsystem) and SIP (Session Initiation Protocol), User IDs are crucial identifiers that facilitate user authentication, session management, and routing of multimedia services. These identifiers are integral to the functioning of IMS and SIP, as they ensure that the correct user is associated with the correct services, and that signaling messages are appropriately routed.

Types of User ID

Understanding the different types of User IDs in the IP Multimedia Subsystem (IMS) and Session Initiation Protocol (SIP) is essential for grasping how modern communication networks manage user interactions. These User IDs are specialized identifiers that serve various roles, from ensuring secure user authentication to accurately routing calls, messages, and multimedia services. Each type of User ID—whether public, private, temporary, or session-specific—plays a unique role in the seamless operation of communication systems. These identifiers enable the network to not only recognize and differentiate users but also to protect their privacy and maintain the integrity of their communication sessions. By categorizing User IDs into different types, IMS and SIP can efficiently manage the complexities of multiple user identities, dynamic session requirements, and the diverse service needs of users, ensuring a personalized and secure communication experience for all.

Private User Identity (IMPI)

The Private User Identity, also known as the IMS Private Identity (IMPI), is a unique identifier assigned to each user within the IMS network. It is not intended to be exposed to external entities and is primarily used for internal purposes such as authentication and authorization.

• Format: Typically, the IMPI is in the form of an International Mobile Subscriber Identity (IMSI) for mobile networks or another operator-defined identifier. It usually follows the format of user@realm, where the realm is the domain of the operator (e.g., 123456789012345@ims.operator.com).
• Usage: The IMPI is used during the registration process with the IMS network. It is included in SIP REGISTER messages and is key to authenticating the user and associating them with their subscription profile in the Home Subscriber Server (HSS).

Public User Identity (IMPU)

The Public User Identity, or IMS Public Identity (IMPU), is the user’s addressable identity within the IMS network. It is what other users and services interact with and is used for routing calls, messages, and other communications.

• Format: The IMPU is typically in the form of a SIP URI (e.g., sip:username@domain.com) or a TEL URI (e.g., tel:+1234567890). It represents the user in a way that is both human-readable and routable within the SIP infrastructure.
• Usage: The IMPU is used to identify the user in SIP signaling, such as in INVITE, MESSAGE, and REGISTER requests. Multiple IMPUs can be associated with a single IMPI, allowing users to have multiple addresses (e.g., different phone numbers or email addresses) linked to the same subscription.

P-Preferred-Identity

The P-Preferred-Identity is a header field in SIP that indicates the user’s preferred identity for outgoing requests. This identity is used by the network when setting the user’s identity in outgoing messages, such as the From header in an INVITE.

• Format: The P-Preferred-Identity can be in the form of a SIP URI or a TEL URI, depending on the user’s preferred way of being identified.
• Usage: This identity is used in cases where the user has multiple public identities (IMPUs) and wants to ensure that a specific one is used when initiating a session. It allows users to control which of their identities is presented to the recipients of their communications.

P-Asserted-Identity

The P-Asserted-Identity is a header field used by the network to assert the identity of the user who is sending a SIP request. It is added by the network after authenticating the user and is trusted by downstream network elements.

• Format: The P-Asserted-Identity is usually in the form of a SIP URI or TEL URI and is often the same as or derived from the IMPU.
• Usage: This identity is used for call routing, billing, and presenting the caller ID to the recipient. It ensures that the identity presented in SIP signaling is verified by the network, preventing identity spoofing.

Contact URI

Definition: The Contact URI is a SIP URI that specifies the address where the user can be reached for the duration of a session. It is included in SIP REGISTER and INVITE messages.

• Format: The Contact URI typically includes the device’s IP address and port number, along with the user’s identity (e.g., sip:username@device-ip:port).
• Usage: The Contact URI is critical for routing responses to SIP requests back to the correct device. It ensures that incoming communications reach the user at their current location, which may vary depending on mobility or network changes.

URI and Address Assignment

Base Specification : 24.229 4.2 URI and Address Assignment

Case 1 > When ISIM is available (24.229 5.1.1.1A Parameters contained in the ISIM)

: UE Extract User ID and Domain Name information from IMPI, IMPU,DOMAIN parameters (See TS 31.103 for ISIM Parameters)

Case 2 > When ISIM is NOT available and USIM is available (24.229 5.1.1.1A Parameters contained in the ISIM)

: UE has to provide all the information required for registration (private user identity, temporary public user identity, home network domain name).

Case 3 > When No ISIM, No USIM, ony IMC(IMS Credential) is available (24.229 5.1.1.1B.1 Parameters provisioned in the IMC)

: UE has to provide all the information required for registration(private user identity,a public user identity,a home network domain name) in the form of IMC.

Case 4 > When No ISIM, No USIM, No IMC(IMS Credential) is available (5.1.1.1B.2 Parameters when UE does not contain ISIM, USIM or IMC)

: UE shall generate a temporary public user identity, a private user identity and a home network domain name to address the SIP REGISTER request based on 3GPP TS 23.003: "Numbering, addressing and identification"

Requirement from 3GPP 23.228

3GPP 23.228 describes about this topic in E.3 Address and identity management concepts

If the UICC does not contain an ISIM application, The Private User Identity shall be derived from the USIM's IMSI

• The format of the Private User Identity derived from the IMSI is specified in TS 23.003
• A Temporary Public User Identity shall be derived from the USIM's IMSI, and shall be used in SIP registration procedures. The format of the Temporary Public User Identity is specified in TS 23.003

Which one is used ? IMPI or IMPU ? or MSISDN

NOTE : When there are multiple items stored in IMPU list, which one should be used for the steps shown above ? - It usually depends on the requirement from Service Provider.

For example : A service provider may set the requirement as follows.

• i) If a provisioned SIM is used, use the first item in IMPU list
• ii) If a non-provisioned SIM is used, use the second item in IMPU list

NOTE :This is only a general rule..  there may be variations of UE ID application for each steps depending on the requirement from each service provider. For example, Some service provider put MSISDN ID at higher priority and require UE to use this ID for registration.

Requirement from IR 92

IR 92 2.2.1 SIP Registration Procedures specifies as follows.

All IMS public user identities provided in the implicit registration set used for VoLTE by the IMS core network must be alias user identities and must include a Tel URI. The following public user identity must be assigned to the implicit registration set used for VoLTE and it must be used by the UE when registering for VoLTE:

• a) When ISIM is used, the public user identity in the first (or only) record in the Elementary File in the ISIM
• b) The temporary public user identity derived from the IMSI.

No other implicit registration set must be used for VoLTE.