How does the client know its user_id and the user_id of the interlocutor? How does the initial setup happen?

The client usually receives their user_id after successful authentication on the server. Upon first connection, the client sends login data (username/password, token, etc.), the server verifies them and returns a unique user_id, which the client stores locally (for example, in memory or in localStorage for web).

The user_id of the interlocutor is learned by the client through the application interface — for example, from the contact list, chat history, or by invitation. Initially, the server provides the client with a list of available users or a specific user_id for communication.

Initialization occurs as follows:

1. The client authenticates and receives their user_id.
2. The client requests a list of users or a specific user_id of the interlocutor.
3. After selecting the interlocutor, the client begins exchanging messages using these identifiers.

Thus, the user_id is a unique identifier assigned and controlled by the server, which the client stores and uses to identify itself and its interlocutors.