Is it correct to say that a web browser always knows when a web page is completely loaded?

A browser sends a GET request for a static web page to a server. The server sends back HTTP OK response with the HTML page in the HTTP body. Looking at the Content-Length field or looking for the terminating chunk or some other delimiter for some other encoding the browser can know if it has received the web page and subsequently all its embedded objects (images etc.). Is it correct to say that in this case the browser always knows when a web page has completely loaded and that it will see no further network traffic?

Now if the page is dynamic (lets say facebook or gmail), where you might receive notifications or parts of the page gets updated using AJAX or javascript running in the background, here also the browser should know when the page has loaded. What if the server is pushing some updates to the client. Is it possib开发者_StackOverflow社区le in this scenario for the browser to know when it has received the full update?

So, is there any scenario in which a browser doesn't know when it has fully received the data (static or dynamic) it has requested from a web server or push-based updates the server is forwarding to it?

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I can only imagine (for the static case) the one scenario when Content-Length is not set. It's not mandatory to send it for the server.

Potentially, of course, in a page containing scripts, one could also have other scenarios where the script loads bits and pieces one by one with delays (including the AJAX scenario you mentioned). This way the browser would not know in advance either. In such a case it would know "for the moment" that the page has loaded completely, but the next action from the script would invalidate that assertion again.

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You do not need AJAX to get in a situation where not all elements in the page are loaded even after the page itself has been loaded. A little javascript is all that you need (been a while since I last worked with JS, there might be some syntax errors)

<img id="dyn_image" src="/not_clicked.gif">

<input type="button" onclick="javascrit:document.get("dyn_image").src="/clicked.gif">

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There are cases when the server uses some kind of push technology, for example Comets. In this case a request (generally Ajax request) is sent, without receiving any response (obvoiusly no HTTP headers as well), but leaving the TCP connection open. This may take long time, but still may be considered as a sub-case of Ajax calls.

The other case is HTML5's WebSocket technology. In a WebSocket the server side can push data to the client side without explicit request from the client side.

These two can be combined, so the answer to your question is: yes, there can be cases when you cannot predict that the network traffic is over or not. The common (in all cases) is that the client side must leave a channel open to the server.