As any other Internet protocol, the HTTP protocol has kept evolving over the years and now there are clients and servers distributed over the world and over time that speak different versions with varying levels of success. So in order to get libcurl to work with the URLs you pass in libcurl offers ways for you to specify which HTTP version that request and transfer should use. libcurl is designed in a way so that it tries to use the most common, the most sensible if you want, default values first but sometimes that is not enough and then you may need to instruct libcurl what to do.