It's true that in general, TCP/IP is relatively error-free, but that assumes you have a solid connection.

If your connection is unreliable, then all bets are off, and the chance of errors increases exponentially.

I would consider even 1% packet loss completely unacceptable, and more than that definitely means your connection is unreliable. If you are seeing that kind of loss, you need to contact your service provider and have them check for problems.

Even bittorrent transfers would have problems with such an unreliable connection.

> -----Original Message-----
> From: curl-users [mailto:curl-users-bounces_at_cool.haxx.se] On Behalf Of
> Daniel Stenberg
> Sent: Wednesday, September 02, 2015 17:39 PM
> To: the curl tool
> Subject: Re: Download of files using curl is not reliable
>
> On Wed, 2 Sep 2015, Freek de Kruijf wrote:
>
> > When downloading rpm files from an openSUSE software repository via a
> > Wi-Fi connection I often get the message that checksum does not match.
> > There are packages lost on the Wi-Fi connection. A ping of 100 shows a
> > few percent loss.
>
> That sounds terrible. A "few" precent is a lot and really hampers your
> connections - like you cannot get very fast transfer rates with that. It is not a
> normal level for normal home conditions I would say.
>
> Maybe it is an indication that your network is shaky to begin with?
>
> > However using shasum to calculate the check sum, I get an answer that
> > does not match the check sum it should have. Repeating the 2 actions
> > often gives a different check sum, especially on large files. But also
> > short files are not always transferred with a matching check sum.
>
> It is a grave indication that things are wrong. But I expect that you get the
> same sort of problems when downloading using other TCP clients as well?
> curl doesn't do anything magic with its connections, it is just a plain old TCP
> connection where it talks HTTP over.
>
> > I assume TCP is used as the transfer protocol, so I expect this
> > transfer to be error free. Is there an explanation for this behavior
> > or is my assumption wrong?
>
> First, TCP is not really 100% error free, it is just unlikely to get errors.
> You can damage the packets and still get a fine checksum as its only 16bit.
> But there's usually more layers of checksums in networks, so actual broken
> content to go through is extremely rare.
>
> How exactly do you download these files, using the curl tool directly? If so,
> you could try alternative tools to see what results they produce.
>
> I would suspect they give roughly the same problems. If that's true, then I
> would suspect something in your surroundings. Network equipment,
> network card, network driver, file system, harddrive...
>
> When you get checksum errors or get differences in downloaded content
> from two identical files, have you tried to compare them to see what the
> damaged areas look like?
>
> --
>
> / daniel.haxx.se
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Received on 2015-09-03