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

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.开发者_开发问答.. 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.

IF you already checked firewall, antivirus, network firewall, network intrusion. read this

For a UDP packet ethernet_header(14 bytes) + IPv4_header(20 bytes min) + UDP_header (8 bytes) = 42 bytes
Now since its less than the 64 bytes or 60 on linux, network driver will pad the packet with (64-42 = 22 ) zeros to make it 60 bytes before it send out the packet.

that's the minimum length for a UDP packet.
theoretically you can send 0 data bytes packet, but haven't tried it yet.

as for your issue it must be an OS issue . check your network's driver's manual or check with manufacturer. because this isn't suuposed to happen.

REF:http://www.freesoft.org/CIE/Course/Section4/8.htm
REF:http://en.wikipedia.org/wiki/User_Datagram_Protocol

Run Wireshark on his PC AND on the destination PC.

Does the log show the udp packet leaving his machine? Does it show it arriving on the destination PC?

What kind of router hardware or switches are between his PC and the destination? Can you remove them and link the two with a cross over cable? (or replace the destination with a laptop and link that to his PC with a cross over cable?)

Have you removed or at least listed all anti virus and firewall products on his machine and anything that installs a Winsock LSP ?

Do ALL 12 byte or less packets get dropped or just some, can you generate packets with random content and see if it's something in the content, rather than just the size, that's causing the issue.

Assuming your problem is with sending from his PC: First, run a packet sniffer on the problematic PC to see if it arrives at the NIC. If it makes it there, there may be a problem in the NIC or NIC driver.

Next, check for any running firewall software. Try disabling it and see what happens.

If that doesn't work, clear out any Winsock Layered Service Providers with netsh winsock catalog reset.

If that doesn't work, I'm stumped :)

Finally, you're probably going to find other customers with the same problem; you might want to think about that workaround anyway. Try sending a few small-size UDP packets on connect, and if they consistently fail to go through, enable a padding workaround. For hosts where the probe packets make it through, you don't need to pad them out.

Pure conjecture: RTP, which is a very common packet to send on UDP, defines a 12 byte header. I wonder if some layer of network software is assuming that anything smaller is a malformed RTP packet and throwing it away?