make libcURL timeout only on connection failure - c++

Question

I'm using some auto-update function in my program. In case of connection failure, I would like the program to keep trying up to 15 seconds, and then announce failure. In order to achieve that, I used the following curl_easy_setopt for cURL easy option:
curl_easy_setopt(curl, CURLOPT_TIMEOUT, 15);
But then I discovered that if the download takes more than 15 seconds, the timeout error is announced.
How can I restrict the 15 seconds only to the case of failure? I.e., if there's no connection for 15 seconds?
More information
The full option list I use is the following:
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L); //verify ssl peer
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L); //verify ssl hostname
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, CurlWrite_CallbackFunc_StdString);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &s);
curl_easy_setopt(curl, CURLOPT_NOPROGRESS, FALSE);
curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, this); //pointer to the current class as it's a GUI program
curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, CurlProgress_CallbackFunc_UpdateProgress);
curl_easy_setopt(curl, CURLOPT_TIMEOUT, 20);
and the call to start is done through:
CURLcode res = curl_easy_perform(curl);
If you require more information, please let me know.
Thank you.

Answer 1

Instead of
curl_easy_setopt(curl, CURLOPT_TIMEOUT, 20);
use
curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 15);
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, 1);
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, 15);
The first line is the timeout for the connection phase. After the connection is established the timeout becomes irrelevant, but the two following lines make sure that if the average speed in a 15 second time frame drops below 1 byte per second, then the operation is aborted.
Also worth noting that curl will not try to reestablish any connection if it is dropped, because a TCP connection can still be hold if the physical connection is (temporarly) lost up until one of the sides decides to timeout.