Opportunistic TLS (Transport Layer Security) refers to extensions in plain text communication protocols, which offer a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication. Several protocols use a command named "STARTTLS" for this purpose. It is a form of opportunistic encryption and is primarily intended as a countermeasure to passive monitoring.
The STARTTLS command for IMAP and POP3 is defined in RFC 2595, for SMTP in RFC 3207, for XMPP in RFC 6120 and for NNTP in RFC 4642. For IRC, the IRCv3 Working Group defined a STARTTLS extension, though it was later deprecated.[1] FTP uses the command "AUTH TLS" defined in RFC 4217 and LDAP defines a protocol extension OID in RFC 2830. HTTP uses an upgrade header.