Yes, it's a common error, but there is in fact only one correct way: IPsec.
See RFC 2401 or the working group discussion on this topic for more information.
Ian BROWN wrote:
IPsec is spelled "IPsec". Not "IPSEC".
grepping through the messages in my ipsec WG folder, I find:
IPSEC 113 times
IPsec 104 times
IPSec 38 times
I also see that the IETF Working Group description begins:
Rapid advances in communication technology have
accentuated the need for security in the Internet.
The IP Security Protocol Working Group (IPSEC)
will develop mechanisms to protect client
protocols of IP. A security protocol in the
network layer will be developed to provide
cryptographic security services that will flexibly
support combinations of authentication, integrity,
access control, and confidentiality.
--
Will Price, Director of Engineering
PGP Security, Inc.
a division of Network Associates, Inc.