Android
Session’s Android client has two options for notifications: background polling (slow mode), and Firebase Cloud Messaging (fast mode).
If you choose the slow mode, the Session application runs in the background and periodically polls its swarm (see What is a swarm) for new messages. If a new message is found, it is presented to you as a local notification on your device.
If you choose the fast mode, Session will use Google’s FCM push notification service to deliver push notifications to your device. This requires that your device's IP address and unique push notification token are exposed to a Google-operated push notification server. Additionally, you will expose your Account ID and unique push notification token to an OPTF-operated push notification server, for the purpose of providing the actual notifications to the Google FCM server.
These exposures are fairly minimal, Google will likely already know your device’s IP address through telemetry data or other applications on your device using push notifications. Registration of your Session ID and unique push notification token to the OPTF push notification server is necessary for the detection and signalling of new messages and is low impact as registration occurs using onion requests meaning your Account ID and push notification token are never tied to any real-world identifier (such as your IP address).
When using fast mode neither Google nor the OPTF can see the contents of your messages, who you’re talking to, or exactly when messages are sent or received.
iOS
Session’s iOS client has two options for notifications: background polling (slow mode), and Apple Push Notification Service (APNs) (fast mode).
If you choose the slow mode, the Session application runs in the background and periodically polls its swarm (see What is a swarm) for new messages. If a new message is found, it is presented to you as a notification on your device.
If you choose the fast mode, Session will use the APNs push notification service to deliver push notifications to your device. This requires your device's IP address and unique push notification token to be exposed to an Apple-operated push notification server. Additionally, you will expose your Account ID and unique push notification token to an OPTF-operated push notification server, for the purpose of providing notifications to the APNs server.
These exposures are fairly minimal because Apple will likely already know your device’s IP address through telemetry data or other applications on your device using push notifications. Registration of your Account ID and unique push notification token to the OPTF push notification server is necessary for detection and signalling of new messages and is low impact as registration occurs using onion requests meaning your Account ID and push notification token is never tied to any real-world identifier (such as your IP address).
When using fast mode neither Apple nor the OPTF can see the contents of your messages, who you’re talking to, or exactly when messages are sent or received.
To learn more about notifications, check out this video: