From what I understand, what multiplexing means is that the client just need to create one TCP connection with the server and it can send multiple requests at the same time without having to wait for the response of one request before continuing another. So if I send 3 request at the same time, there are also 3 response streams.
And for server push, the client sends one request to the server, the server then guesses that the client needs other resources (also called promises) other than the one it requested, so it sends push promise streams hinting the client with the URL of the additional resources. The client may choose to request those additional resources or not.
My questions are:
- For any response sent from the server to the client, does it have to be a request initiated first? I mean, it I created a connection to the server, I did not send any request. Could I be getting responses from server via server push? In multiplexing, I get same number of responses for same number of requests. In server push, I can get multiple responses for one request. So does there always have to be a request first?
- In server push, when a promise stream is sent by the server to the client containing the URL of the additional resources, does that mean the server will only push the additional resources only when the client accepts the promises?
[the server] sends push promise streams hinting the client with the URL of the additional resources. The client may choose to request those additional resources or not.
This is not correct. When the server sends to the client a PUSH_PROMISE, the server will then send the resource content associated to that pushed resource.
The only thing that the client can do is to reset the pushed stream via a RST_STREAM frame, but it may well be that the whole pushed resource is already in-flight so resetting the pushed stream has no effect: the client will receive the pushed resource bytes and may discard them if not interested.
To answer your specific questions:
Yes, a response from the server is always client-initiated. If a client does not send any request to the server, the server cannot push to the client. Even in the case of server push, the client always initiated a stream by making a request, and server pushes are always associated to that "parent" request.
The PUSH_PROMISE frame is an indication from the server to the client of what resource the server is about to push. The client does not "accept" pushes, the server forces them to the client. The only thing the client can do is to reset the stream associated with the pushed resource; as I said, the server may have already pushed the whole resource by the time it receives the RST_STREAM frame from the client.