Iptables udp forwarding and QoS

Greetings,

I use iptables forward chain to forward udp packets from one network interface throughout another. I also use QoS mechanism (applied to FORWARD chain of mangle table) to limit net traffic. Of course I realize, that if incoming packets are received too fast, some of transmission will be lost – at last that is UDP. Incoming frames are bigger than MTU (1500 bytes).
The problem is, that when frames buffers overflow, fragmented UDP packets (MTU sized) are missed and reordered and in this form are send through outgoing network interface, which can be observed using tcpdump tool. Of course, this packets can not be collected by receiving host. It looks like they are already miss-ordered and lost in UDP buffers. The result is, that from that moment all sent UDP frames are damaged.

My questions are:
1. How can I get information about such situation:
- “netstat -su” shows “0 packet receive errors”.
- “ifconfig” also shows 0 errors on both network interfaces, worse - RX bytes of input interfaces shows all bytes sent by remote client, which cannot be true.
- Content of “/sys/class/net/” directories also shows nothing wrong.
2. Can I change the behavior of kernel packets forwarding – so some frames should be lost, but another should be forwarded correctly?

I use RedHat 6.2 x86_64 with kernel 2.6.32-642

Thanks in advance,