Joe,
This article is good intro to how the STARTTLS command would be used:
http://en.wikipedia.org/wiki/STARTTLS
In short, the client sends the STARTTLS command to the server to
indicate a desire to use TLS encryption for the connection.
STARTTLS is most widely used with SMTP, POP, and IMAP.
The genesis of the STARTTLS command was a realization that the earlier
approaches to adding TLS security to existing TCP protocol-based
services suffered from a systemic flaw: "wrapping" the connection in TLS
and offering the "wrapped" service on a different port in effect
required doubling the number of assigned ports. One port for the
cleartext version, one port for the TLS version.
(This turned out to be less of a problem in practice than anticipated at
the time of the creation of the STARTTLS command, as the growth of
encryption was paralleled by a reduction in ports on which many hosts
connected to the Internet may transmit packets due to ISP level
filtering and the rise of NAT. But that's a discussion for a different
mailing list).
--Lucky
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