[android help] Fragmented UDP packets not received by application

Fragmented UDP packets not received by application

Very strange behavior...

I have 3 machines:

 -----------     ------------     -----------
 | A (x86) |-----| B (x86)  |-----| C (arm) |
 | sender  |     | receiver |     | sender  |
 -----------     ------------     -----------

• A and B are Linux (Ubuntu 12.04) machines, kernel 3.2;


• C is an android (ICS) machine, kernel 3.0.8;


• All are connected via RJ45 cables;


• Connections are OK, network is set up correctly;

Issue is: when machine C (ARM-android) sends a UDP packet which payload size is over 1472 bytes (maximum payload before packet gets fragmented), server application on machine B is never able to receive it, ... regarding that:

• Source/Dest IP addresses are correct: I can receive all the datagrams I send if I set the payload size less or equal to 1472;


• On machine B (receiver), if I dump network traffic with Wireshark, I can see each fragment, and then re-assembled message => from Wireshark point of view, it's all good!


• Comparing each fragment header as well as re-assembled message with what I can dump when the same message is sent from machine A (which is always received OK), everything seems perfect (only differences are IP addresses, and checksum, since UDP header checksum takes in account IP address fields).


• There is no MTU issue, packets are fragmented as expected.


• There is no router/switch between the machines


• ifconfig shows neither packets drop, nor overflows, nor any other classical error!


• ... this is so weird!!

I've spent some time on Internet, but never found any topic like this one. Each time people has troubles with UDP, either their MTU discovery was not correct, or they did some mishandling in the testing procedure, or they could not dump message on receiver host, ... this is not the case here!!

For sure, I know issue is on sender end (machine C), but maybe is could be easier to enable some logs (at kernel level?) on receiver end to understand why UDP datagram disappears!? Any advice? Are there specific files I could check in /proc/sys/net, or kernel options I should enable?

Thanks a lot.

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