为什么我的web浏览器有时无法显示剩余的下载时间？

有时，浏览器（或其他应用程序）上忠实的下载进度表只是举手之劳，放弃显示剩余的下载时间。为什么它有时会锁定预计的下载时间，有时却无法同时报告？...

为什么我的web浏览器有时无法显示剩余的下载时间？

有时，浏览器（或其他应用程序）上忠实的下载进度表只是举手之劳，放弃显示剩余的下载时间。为什么它有时会锁定预计的下载时间，有时却无法同时报告？

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

问题

超级用户读者Coldblackice想知道为什么他的浏览器不总是会有污点：

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?

到底在哪里？

答案

超级用户贡献者Gronostaj提供了以下见解：

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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