My problem was in the IP header checksum.
The NIC card is set for 'IPv4 Checksum Offload - Rx & Tx enabled'. The UDP packets that I was receiving from the PC (LabVIEW) had 0000 for the checksum. Wireshark indicated that the IP checksum was 'disabled'. Header checksum: 0x0000 [validation disabled]
Of course I assumed this was OK for sending packets back to the PC. And Wireshark reported the same information [validation disabled].
When I added the correct checksum to my IP header for packets sent from the DSC, LabVIEW received the packet!
NOTE: I am successfully using 0000 for the UDP checksum, but to do that I have to disable the NIC card's 'UDP Checksum Offload (IPv4).
