How to use FreeBSD/Dummynet to fragment IP packets

[Andrew Wenlang Zhu]

Hello:

I use a freebsd system running Dummynet as a middle box to test network
behavior. The setup likes this:

System A -------------  FreeBSD with Dummynet ------------ System B

Network traffic flows between System A and B, and the traffic is a mix of
TCP, UDP and ICMP.

I need the Dummynet system to fragment IP packet to a specified packet size.
It is OK that all traffic get fragmented, but even it is better if I can
specify a certain percentage of packet to be fragmented. I looked into
Dummynet man page, but cannot find how to do packet fragmentation.

Does any one know how to use Dummynet to fragment packet? Or any other open
source software can achieve this?

Thanks,

Andrew