Solaris Version 9 (SunOS 5.9) and earlier have the xntpd version 3 protocol see RFC 1305.., The most current protocol is version 4 - only RFC 2030 exists now for it. This is not complete.
Read your man page - it tells you the default protocol for xntpd (NTP)
I am not aware of specific problems. Go to the ntp website and search in reports there.
ntp.org: Home of the Network Time Protocol
---------- Post updated at 14:36 ---------- Previous update was at 14:35 ----------
Tony -
do you have a link for the denial of service attack warning?