為什麼我的web瀏覽器有時無法顯示剩餘的下載時間？

有時，瀏覽器（或其他應用程式）上忠實的下載進度表只是舉手之勞，放棄顯示剩餘的下載時間。為什麼它有時會鎖定預計的下載時間，有時卻無法同時報告？...

有時，瀏覽器（或其他應用程式）上忠實的下載進度表只是舉手之勞，放棄顯示剩餘的下載時間。為什麼它有時會鎖定預計的下載時間，有時卻無法同時報告？

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者讀者Coldblackice想知道為什麼他的瀏覽器不總是會有汙點：

Occasionally, when downloading a file in a web browser, the download progress doesn’t “know” the total size of the file, or how far along in the download it is — it just shows the speed at which it’s downloading, with a total as “Unknown”.

Why wouldn’t the browser know the final size of some files? Where does it get this information in the first place?

到底在哪裡？

答案

超級使用者貢獻者Gronostaj提供了以下見解：

To request documents from web servers, browsers use the HTTP protocol. You may know that name from your address bar (it may be hidden now, but when you click the address bar, copy the URL and paste it in some text editor, you’ll see http:// at the beginning). It’s a simple text-based protocol and it works like this:

First, your browser connects to the website’s server and sends a URL of the document it wants to download (web pages are documents, too) and some details about the browser itself (User-Agent etc). For example, to load the main page on the SuperUser site, http://superuser.com/, my browser sends a request that looks like this:

GET / HTTP/1.1 Host: superuser.com Connection: keep-alive Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) Accept-Encoding: gzip,deflate,sdch Accept-Language: pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4 Cookie: [removed for security] DNT: 1 If-Modified-Since: Tue, 09 Jul 2013 07:14:17 GMT

The first line specifies which document the server should return. The other lines are called headers; they look like this:

Header name: Header value

These lines send additional information that helps the server decide what to do.

If all is well, the server will respond by sending the requested document. The resp***e starts off with a status message, followed by some headers (with details about the document) and finally, if all is well, the document’s content. This is what the SuperUser server’s reply for my request looks like:

HTTP/1.1 200 OK Cache-Control: public, max-age=60 Content-Type: text/html; charset=utf-8 Expires: Tue, 09 Jul 2013 07:27:20 GMT Last-Modified: Tue, 09 Jul 2013 07:26:20 GMT Vary: * X-Frame-Opti***: SAMEORIGIN Date: Tue, 09 Jul 2013 07:26:19 GMT Content-Length: 139672 <!DOCTYPE html> <html> [...snip...] </html>

After the last line, SuperUser’s server closes the connection.

The first line (HTTP/1.1 200 OK) contains the resp***e code, in this case it’s 200 OK. It means that the server will return a document, as requested. When the server doesn’t manage to do so, the code will be something else: you have probably seen 404 Not Found, and 403 Forbidden is quite common, too. Then the headers follow.

When the browser finds an empty line in the resp***e, it knows that everything past that line is the content of the document it requested. So in this case <!DOCTYPE html> is the first line of the SuperUser’s homepage code. If I was requesting a document to download, it would probably be some gibberish characters, because most document formats are unreadable without prior processing.

Back to headers. The most interesting one for us is the last one, Content-Length. It informs the browser how many bytes of data it should expect after the empty line, so basically it’s the document size expressed in bytes. This header isn’t mandatory and may be omitted by the server. Sometimes the document size can’t be predicted (for example when the document is generated on the fly), sometimes lazy programmers don’t include it (quite common on driver download sites), sometimes websites are created by newbies who don’t know of such a header.

Anyway, whatever the reason is, the header can be missing. In that case the browser doesn’t know how much data the server is going to send, and thus displays the document size as unknown, waiting for the server to close the connection. And that’s the reason for unknown document sizes.

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