How is a browser able to resume downloads?

Question

Do downloads use HTTP? How can they resume downloads after they have been suspended for several minutes? Can they request a certain part of the file?

Answer 1

Downloads are done over either HTTP or FTP.

For a single, small file, FTP is slightly faster (though you'll barely notice a differece). For downloading large files, HTTP is faster due to automatic compression. For multiple files, HTTP is always faster due to reusing existing connections and pipelining.

Parts of a file can indeed be requested independent of the whole file, and this is actually how downloads work. This is a process known as 'Chunked Encoding'. A browser requests individual parts of a file, downloads them independently, and assembles them in the correct order once all parts have been downloaded:

In chunked transfer encoding, the data stream is divided into a series of non-overlapping "chunks". The chunks are sent out and received independently of one another. No knowledge of the data stream outside the currently-being-processed chunk is necessary for both the sender and the receiver at any given time.

And according to FTP vs HTTP:

During a "chunked encoding" transfer, the sending party sends a stream of [size-of-data][data] blocks over the wire until there is no more data to send and then it sends a zero-size chunk to signal the end of it.

This is combined with a process called 'Byte Serving' to allow for resuming of downloads:

Byte serving begins when an HTTP server advertises its willingness to serve partial requests using the Accept-Ranges response header. A client then requests a specific part of a file from the server using the Range request header. If the range is valid, the server sends it to the client with a 206 Partial Content status code and a Content-Range header listing the range sent.