On Sat, Aug 28, 2010 at 04:35:04AM +0200, Carlos E. R. wrote:
On 2010-08-27 09:58, Basil Chupin wrote:
On 27/08/2010 00:03, Carlos E. R. wrote:
Incredible....We are in this situation after some close to 30 years of having error-correcting protocols.....Unbelievable :-( .
I use a Firefox Extension called DownThemAll! which has never given me a bad CD or DVD - and it resumes from where I lost the connection (which never happens now :-) .) I also find that the downloader in Firefox now also resumes from where the connection was lost and it also produces a "clean" file - but this latter downloader has not been under any "real strain" since it was upgraded by Mozilla.
Notice that this doesn't guarantee a correct download, the http or ftp protocols don't allow that. You need something else for verification of the download, and then retry. This can be automated, but you need a verification method.
DownThemAll! verifies the download (provided that a metalink contains hashes). However, it doesn't verify chunks of the file - only the end result. Even though this doesn't guarantee that you get a correct result, it guarantess that you don't get an incorrect one. (HTTP/FTP themselves have no provision for this, that's true. That's why there are metalinks. Okay, HTTP does have a special header that has recently been updated to support modern hashes (RFC5843), but it's not used yet. And for FTP people are working on a standard. But it's not ready or even used anywhere. Metalinks are ready, actively used and standardized in RFC 5854.)
For example, if the download manager knows the expected crc, it can insist downloading chunks until the result is good. But blindly, because it can not know what section of the file has the error.
Yes. The download manager aria2c (which has been mentioned before) uses all the hashes and verifies chunks of the file (which it is in transit) and re-downloads pieces from other mirrors if an error happens. Chunked hashes are all available with openSUSE downloads. Hopefully, the number of download clients that actually use them will increase in the future. Peter