IMS single registration

Android 12 introduces support for a single registration model for providing MMTEL and RCS features. This model allows devices to have all IMS features managed through a single IMS registration provided by the device's ImsService and to comply with requirements introduced by some telecom carriers. Compared with a dual registration model, where multiple IMS registrations are managed on one device, single registration reduces traffic on the carrier's network and increases reliability.

Android 12 supports this single registration model through an architecture with a set of APIs that allow the AOSP telephony stack to manage both MMTEL features provided by ImsService and RCS features provided by the user-selected RCS messaging app. To support IMS single registration, device manufacturers and SoC vendors must implement these APIs to enable RCS features in the user-selected RCS messaging app.

Figure 1 illustrates the device's IMS stack when using the IMS single registration model. All IMS apps use the device's default ImsService for MMTEL and RCS features over a single IMS registration. This includes provisioning, SIP message forwarding, and RCS user capability exchange.

Figure 1. Single registration model architecture

Android 11 and lower only supports a dual registration model for providing MMTEL and RCS features, where MMTEL is provided by the device's ImsService and RCS features are implemented over the top and manage their own IMS stack and connection to the carrier's network independently.

Figure 2 illustrates the architecture for the dual registration model. In this model, each app is responsible for connecting to the carrier's network and establishing an IMS registration for MMTEL and RCS features. The device's ImsService implements MMTEL, uses the device's IMS data connection to the carrier network, and operates independently of other RCS apps.

Figure 2. Dual registration model architecture

IMS single registration APIs

Devices that range on carriers that require IMS single registration must support the IMS single registration APIs and define the Android feature PackageManager#FEATURE_TELEPHONY_IMS_SINGLE_REGISTRATION. Figure 3 shows the APIs that support IMS single registration.

Figure 3. High-level API surfaces that support IMS single registration

Android devices that support IMS single registration as part of the AOSP telephony stack are required to support all of the AOSP APIs described in the following table.

API Surface Area	RCS app APIs	Vendor IMS APIs	Description
RCS provisioning	ProvisioningManager	ImsConfigImplBase	Allows an OEM or carrier to provide an app to update the RCS provisioning status if the carrier uses a proprietary carrier entitlement mechanism. The ImsService must also support the standard AutoConfigurationServer (ACS) for provisioning for carriers that don't use a proprietary mechanism.
SIP message forwarding	SipDelegateManager	SipTransportImplBase	Allows an RCS app to first associate specific RCS feature tags with the device ImsService, and then send and receive SIP messages and IMS registration updates associated with those RCS feature tags.
Dedicated bearer notifications	ConnectivityManager	DataCallResponse	Allows an app to listen to QoS notifications on a socket that's associated with a specific local port.
GBA authentication	bootstrapAuthenticationRequest	GbaService	Allows an RCS app to authenticate with the network and access keys used for RCS features such as file transfer.
RCS user capability exchange	ImsRcsManager	RcsCapabilityExchangeImplBase	Provides AOSP the ability to send its MMTEL and RCS capabilities to the vendor ImsService so that they can be published under one entity to the network for RCS user capability exchange. Also allows other apps interested in the RCS capabilities of one or more contacts to query the network for the contacts RCS capabilities.

Security and permissions

Android 12 introduces the following permissions to ensure secure access to the carrier's network and user's data:

• android.permission.PERFORM_IMS_SINGLE_REGISTRATION
• android.permission.ACCESS_RCS_USER_CAPABILITY_EXCHANGE

The android.permission.PERFORM_IMS_SINGLE_REGISTRATION permission must be defined by the messaging app with the RCS features. For this permission to be granted, the following must be true:

• The app must be installed as a privileged app, meaning that it's preinstalled on the device and is allowed to access privileged permissions
• The app must be set as the user's default SMS role using RoleManager

If both of these conditions aren't met, the app is denied access to the android.permission.PERFORM_IMS_SINGLE_REGISTRATION permission. This means that third party apps aren't allowed to access RCS single registration APIs as they require carrier certification on the device.

The android.permission.ACCESS_RCS_USER_CAPABILITY_EXCHANGE permission when granted to an app that also has the READ_CONTACTS permission allows the app to request the RCS capabilities of phone numbers using RcsUceAdapter. For this permission to be granted, the following must be true:

• The app must be installed as a privileged app, meaning that it's preinstalled on the device and is allowed to access privileged permissions.

• The app must be defined as one of the following RoleManager roles:

  • Default messaging app: Set by the user.
  • Default dialer app: Set by the user.
  • Default contacts app: A role introduced in Android 12 that allows the OEM to define a package name through the device overlay value config_systemContacts, which must correspond to the device's contacts app. That app is then given the contacts role.

To access the IMS APN using ConnectivityManager to set up and manage data traffic, apps must also request the android.permission.CONNECTIVITY_USE_RESTRICTED_NETWORKS permission.

Examples and source

Android provides an app in AOSP that implements a test messaging app with basic RCS messaging support for testing and development purposes. You can find the app at testapps/TestRcsApp. When the app is installed on a device, it can be set as the user's default messaging app and will have the permissions required to access the IMS single registration APIs.

Android also provides a sample implementation of ImsService for RCS. The source code is at /testapps/ImsTestService.

Implementation

For more implementation details, download IMS Single Registration in Android.

Validation

To validate your implementation of IMS single registration, do the following:

• Ensure the CtsTelephonyTestCases CTS test suite passes.
• Install and run the TestRcsApp to run basic single registration test cases during integration.
• Pass carrier certification for IMS single registration test cases.