How are browsers resuming file downloads? Suppose I'm downloading a large file, I pause it and then resume? What is the underlying HTTP calls to continue the download?
I'm aware of Range Requests, but I've seen cases where the download resumes even when the server doesn't support range requests.
Sai Kishore Komanduri
Engineering an eGovernance Product | Hashnode Alumnus | I love pixel art
If you run the following command:
curl -I http://mirror.pramati.com/ubuntureleases/18.04.1/ubuntu-18.04.1-desktop-amd64.iso...you will get the following output:
HTTP/1.1 200 OK Date: Wed, 10 Oct 2018 13:40:09 GMT Server: Apache Last-Modified: Wed, 25 Jul 2018 03:22:14 GMT ETag: "746dc000-571ca628f7985" Accept-Ranges: bytes Content-Length: 1953349632 Cache-Control: max-age=1209600, public Expires: Wed, 24 Oct 2018 13:40:09 GMT Content-Type: application/x-iso9660-imageSee the
Accept-Rangesheader? That means the server is willing to accept range requests.Browsers have the ability to ensure resumes, using the same concept only. Chrome maintains a temporary
.crdownloadfile; with that information chrome can make a request for a range starting from the non-downloaded byte; on a pause & resume.Sometimes even though servers support range requests (another apt name for this is byte serving), they do not send any
Accept-Rangesheader. The absence of this header is not an accurate indicator of the truth about the server's ability for byte range serving.Some clients assume that the server doesn't support byte serving in either or both of the two cases:
Accept-Rangesheader is absentAccept-Rangesheader is equal to 'none'In summary, if the server truly doesn't support byte serving, then resumes of downloads on any clients, from that server, are really not possible!