How does IDM resume a paused HTTP download?

Question

How do programs like Internet Download Manager resume a download from a new URL? I think that it may store the original URL of the page in which the download link found and opens that page to refresh the download URL. Is it so?

EDIT:

What do you mean by "refresh expired URL"?

Currently am downloading a file in idm and now I have paused the download. After, say 3 hours am resuming the same download and the URL has been expired. IDM couldn't continue the download and it takes me to the original webpage where the download link exist saying please wait while refreshing the URL and now I have to click the download link again and IDM catches the new URL. Now it resumes the download from new URL.

Answer 1

An HTTP transaction consists of a request and a reply. The request can specify more than just the name of the file, but also certain characteristics of the reply.

Commonly supported characteristics include language, document type (e.g. a music file vs plain text file), character encoding, and (importantly) the byte range.

For example, if a download is interrupted 500KB into a 1MB download, IDM will specify that it only wants bytes 501KB through 1MB when it resumes the download from a different URL, skipping the already downloaded part.

This also allows IDM (and other tools) to use multiple connections to download a file. For example, three connections would make three independent requests: one for the first third of the file, one for the middle third, and one for the final third. In some cases this can be faster than using only one connection for the whole file.

If the server is unable or unwilling to supply the requested characteristics in its reply, it will return an error reply instead (e.g. Requested range not satisfiable). In which case IDM would have to re-download the whole file over a single connection.

Answer 2

Hmm, I think I now get what you are saying.

I am supposing that you are trying to download a file from download sites such as MediaFire or RapidShare. As those sites implement control measures to avoid unregistered users from depleting their resources, when you pause your download, their servers assume that the connection has been lost, mainly because they haven't received any communication (i.e. download or upload request) from your part.

When you resume your download, although the file's URL doesn't change, the service in practice forces you to redo the previous steps, as it doesn't remember that you tried to download a file before. When you get around and actually download the file, IDM knows that the file you have paused and the one you are now downloading are the same (because the URL is the same). As such, (and this might vary from service to service), IDM requests the server to only send the parts you don't have.

To answer your question, IDM doesn't refresh the URL, as they are the same. It doesn't make sense for it to refresh the URL as you say, because the files might not be the same if the URLs are different.

Answer 3

1. Let IDM re-download the file from new address for few seconds then pause it.
2. Open IDM
3. Right-click and select properties of the file you just start to download
4. Copy Address of the new file
5. Right-click and select properties of the old file
6. Paste in the address field.
7. Click OK
8. Go to IDM and resume to download the old file.

That's it. It should work.