SSL was invented in the mid 90s and has developed ever since. SSL version 2 was the first widespread version used on the Internet but that was deemed insecure already a long time ago. SSL version 3 took over from there, and it too has been deemed not safe enough for use.
TLS version 1.0 was the first "standard". RFC 2246 was published 1999. TLS 1.1 came out in 2006, further improving security, followed by TLS 1.2 in 2008. TLS 1.2 came to be the gold standard for TLS for a decade.
TLS 1.3 (RFC 8446) was finalized and published as a standard by the IETF in August 2018. This is the most secure and fastest TLS version as of date. It is however so new that a lot of software, tools and libraries do not yet support it.
curl is designed to use a "safe version" of SSL/TLS by default. It means that it will not negotiate SSLv2 or SSLv3 unless specifically told to, and in fact several TLS libraries no longer provide support for those protocols so in many cases curl is not even able to speak those protocol versions unless you make a serious effort.
| Option | Use |
|---|---|
| --sslv2 | SSL version 2 |
| --sslv3 | SSL version 3 |
| --tlsv1 | TLS >= version 1.0 |
| --tlsv1.0 | TLS >= version 1.0 |
| --tlsv1.1 | TLS >= version 1.1 |
| --tlsv1.2 | TLS >= version 1.2 |
| --tlsv1.3 | TLS >= version 1.3 |