UDP packets are dropped when its size is less than 12 byte in a certain PC. how do i figure it out the reason?

i've stuck in a problem that is never heard about before.

i'm making an online game which uses UDP packets in a certain character action. after i developed the udp module, it seems to work fine. though most of our team members have no problem, but a man, who is my boss, told me something is wrong for that module.

i have investigated the problem, and finally i found the fact that... on his PC, if udp packet size is less than 12, the packet is never have been delivered to the other host.

the following is some additional information:

• 1~11 bytes udp packets are dropped, 12 bytes and over 12 bytes packets are OK.
• O/S: Microsoft Windows Vista Business
• NIC: Attansic L1 Gigabit Ethernet 10/100/1000Base-T Controller
• WSASendTo returns TRUE.
• loopback udp packet works fine.

how do you think of this problem? and what do you think... what causes this problem? what should i do for the next step for the cause?

PS. i don't want to padding which makes length of all the packets up to 12 bytes.

Just to get one of the non-obvious answers in: maybe UDP checksum offload is broken on that card, i.e. the packets are sent, but dropped by the receiver?

You can check for this by looking at the received packets using Wireshark.