STARTTLS is an extension to plain text communication protocols, which offers a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication.
TLS is application-neutral; in the words of RFC 5246
- One advantage of TLS is that it is application protocol independent. Higher-level protocols can layer on top of the TLS protocol transparently. The TLS standard, however, does not specify how protocols add security with TLS; the decisions on how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS.
The style used to specify how to use TLS matches the same layer distinction that is also conveniently supported by several library implementations of TLS. E.g., the RFC 3207 SMTP extension illustrates with the following dialog how a client and server can start a secure session:
S: <waits for connection on TCP port 25> C: <opens connection> S: 220 mail.example.org ESMTP service ready C: EHLO client.example.org S: 250-mail.example.org offers a warm hug of welcome S: 250 STARTTLS C: STARTTLS S: 220 Go ahead C: <starts TLS negotiation> C & S: <negotiate a TLS session> C & S: <check result of negotiation> C: EHLO client.example.org . . .
The last EHLO command above is issued over a secure channel. Note that authentication is optional in SMTP, and the omitted server reply may now safely advertise an AUTH PLAIN SMTP extension, which is not present in the plain-text reply.
SSL ports 
Before STARTTLS was well established, a number of TCP ports were defined for SSL-secured versions of well-known servers. These establish secure communications and then present a communication stream identical to the old un-encrypted protocol. These are no longer recommended, since STARTTLS makes more efficient use of scarce port numbers and allows simpler device configuration. Some examples include:
|Protocol||Purpose||Normal port||SSL variant||SSL port|
- Tim Dierks; Eric Rescorla (August 2008). "The Transport Layer Security (TLS) Protocol". RFC Editor. Retrieved 2009-10-08.
- Paul Hoffman (February 2002). "SMTP Service Extension for Secure SMTP over Transport Layer Security". RFC Editor. Retrieved 2009-10-08.
- The last line in the example added for clarity. See e.g. the thread started by Paul Smith (26 January 2009). "STARTTLS & EHLO". ietf-smtp mailing list. Internet Mail Consortium. Retrieved 2009-10-08.
- SMTP TLS Tests and Tools (the "Receiver Test" shows dialogs like the one above)
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