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?
2 Answers
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.
Do downloads use HTTP?
Yes. Especially since major browsers had deprecated FTP.
How can they resume downloads after they have been suspended for several minutes?
Not all downloads can resume after this long. If the (TCP or SSL/TLS) connection had been closed, another one has to be initiated to resume the download. (If it's HTTP/3 over QUIC, then it's another story.)
Can they request a certain part of the file?
Yes. This can be done with Range Requests. But it require server-side support (especially when the requested resource is provided by a dynamic script).
That other answer mentioning chunked transfer had mistaken it for the underlaying mechanism of TCP. Chunked transfer is not designed for the purpose of resuming partial downloads. It's designed for delimiting message boundary when the Content-Length header is not present, and when the communicating parties wish to reuse the connection. It is also used when the protocol version is HTTP/1.1 and there's a trailer fields section (which is similar to header fields section, but comes after the message body). HTTP/2 and HTTP/3 have their own way to convey trailers.
Even if multiple non-overlapping "chunks" of the resource is requested, it's encapsulated in a multipart/* message.
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