Concept: Session Resumption

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When and how to reconnect a session.

Description

Session resumption happens, when a new connection for an existing session is established.

Reconnect Types

We differentiate between two types of session resumption: A wakeup and a full reconnect:

• Full Reconnect: During a full reconnect, the chunk cache SHALL NOT be used. All initial messages MUST be sent again and the connection is treated as if the session were newly created.
• Wakeup: When doing a wakeup, the initial messages SHALL NOT be sent again. The chunk cache MUST be transferred from the previous connection to the new connection (both identified with a unique connection ID) See sequence number for more details.

Reconnect Triggers

Session resumption SHALL be triggered in one of the following cases:

Triggered by App

• A session is currently active, but disconnected, and
• an outgoing message to the client is enqueued.

As an example, when a new incoming Threema message from a contact arrives, that will result in a new update/messages message being enqueued for the client. If at that point in time a session is active but not connected, session resumption will be triggered.

Triggered by Push

• A wakeup push message arrives.

Triggered Manually

• The user manually starts the session from the Threema app.