At the level at which you are asking questions, I'm not sure you'll hear this answer, but, here goes...

Besides all of that, you'll also need to tell the server to send only the data that you don't have. Otherwise, it'd be simpler just to move your file out of the way and start over.

See "Re: Re: Re: download part of remote files" for a snippet of code that uses the Range: header to ask for the last 128 bytes. You'll need to do something similar to ask for the bytes that you don't have. The response may or may not be a "partial content" response: if it is, you can append it to your existing data; otherwise, you'll need to replace your existing data. Oh, and you'll have to verify that the modification date of the response is the same as your original response. I hope you saved the original modification date somewhere.

What you ask is do-able, but not easy. And not at the level you are asking questions. Good luck.

-- Randal L. Schwartz, Perl hacker