How to receive fragmented UDP packets?

[dacoast]

I am having trouble receiving fragmented UDP packets on a MOD 5272, rel 22_rc1

My application works as expected if the UDP packets are < 1500 bytes.
Now, however, I am attempting to receive a UDP message of size about 1650 bytes. Wireshark
shows that the UDP packet is fragmented into 2 pieces.

I attempt to get the UDP packet using:

UDPPacket upkt(&fifo, TICKS_PER_SECOND);

if (upkt.Validate())
{
.... use the UDP message...
}
else
iprintf("udp packet timed out...\n\r");

I never get a valid UDP message, appears to timeout...


Do I need to do something specific to receive a fragmented UDP message?

-Doug

[lgitlitz]

You need enable fragmented UDP RX by uncommenting the following line in C:\Nburn\include\predef.h:
/* #define UDP_FRAGMENTS ( 4 ) */
You will then need to recompile all of the system files for this to take effect. This option can decrease the UDP RX performance which is why it is not enabled by default.

-Larry

[thomastaranowski]

How does the stack handle dropped, missing, or interleaved fragments?

A hypothetical situation:
I have 4 senders on a network, each sending fragmented UDP packets, and a single netburner receiver.

If all 4 senders happen to send the first fragment at the same moment, the netburner stack would receive 4 fragments, but not be able to complete a payload. Would it take the first fragment and discard the rest until it gets the last fragment, or would it take queue up all 4 fragments, but eventually timeout when none are able to complete because the queue is full, or ???.

Thanks
Thomas Taranowski
tom@baringforge.com
Netburner Consultant

[lgitlitz]

Paul created the UDP fragment code and he will not be in till Monday. Here is what I think to be true about fragmented UDP.
The number of fragmented buffers are defined by the UDP_FRAGMENTS you enable in predef.h, default value of 4. You do not want to increase this value too much since each of these buffers will allocate 64K of RAM, the max fragmented datagram size.
For every fragmented packet received the NetBurner must determine where the data should be located in the full datagram and then copy the data there. A non-fragmented UDP receive will just pass a pointer to the packet which is much more efficient.
If any of the fragments are corrupt (bad checksum) or lost then the all successfully received data in that fragmented datagram will be thrown away. This will also keep the buffer allocated until the TTL for that datagram has expired.
I suggest that you do not use fragmented UDP unless absolutely necessary. The overhead required can reduce performance significantly, especially on a noisy network.

[dacoast]

Thanks for the tips, unfortunately, I have to support a server that occasionally sends out a
database as one large multicast UDP datagram, which gets fragmented into multiple UDP
packets (just 2 at the present time).

I uncommented the #define UDP_FRAGMENTS ( 4 ) in predef.h and rebuilt the system files,
but I still cannot seem to receive the fragmented UDP datagram.

Checking the counters using the monitor program, i see an increase in "frames_rx_fragments" but "rx_udp" does not increase and
UDPPacket upkt( &fifo, TICKS_PER_SECOND) never produces a valid UDP packet.

Has anyone actually ever successfully used a MOD5272 to receive fragmented UDP datagrams?

-Doug

[lgitlitz]

I just tested it and it all seems to work. Here was my test...

I loaded the following example on the NetBurner:
C:\Nburn\examples\udp\UDPSendReceive
I then ran the NetBurner UDP terminal PC app:
C:\Nburn\pcbin\UDPTerminal.exe

Send a few small UDP packets back and forth to make sure the NetBurner talks to the PC with normal packets. Then I pasted about 5K worth of text in the UDP terminal send buffer. When I sent it nothing appears on the NetBurner since UDP_FRAGMENTS is not yet enabled. I then enable this and click rebuild all system files in Eclipse. Then do a make clean on the example and load it to the NetBurner. Now I am able to receive the fragmented UDP packets.