pptp on freebsd

[vladone]

Hi!
I have an gateway freebsd that make nat with ipnat and traffic shaping
with ipfw + dummynet. I want to require for my users authentication to
access the internet via my gateway. I want to use pptp because is
supported by most version of windows operating systems.
My questions is about traffic flow. How is traffic flow using pptp?
This implement an virtual interface, or how?
How i can filter traffic when use pptp?

For example, now i can traffic shaping very easy with this rule in
ipfw:

ipfw pipe 1 config bw 128kbits/s mask dts-ip 0xffffff
ipfw add 100 pipe 1 ip from any to any out via $private_interface

If someone have experience with this, please!