IPIP-tunnel with 1500 MTU

Who is setting the DF flag and why? If an end host is doing it,

All windows machines do this by default.

check to see if P-MTUd is set on the end host and turn it off.
Several OSs try to do this by default (Solaris, HP-UX, etc).
>From my past experience with P-MTUd (dates back a few years),
almost no vendor understood how to implement it.

Even cisco recognises the problem and offer no solution :confused: