I'm having problems with custom K60 hardware that essentially translates ethernet to/from CAN.
Original code was built with MQX4/RTCS3.8 and "works". However when we put it in a more time critical system we saw our box would occasionally not ack a tcp packet. (verified wireshark) When this occurred, recv() would not return any data until the tcp packet was retransmitted 1 second later. We didn't loose any data, but the latency receiving the tcp packet caused problems with the CAN bus which expected a new message within 100msec. We have a switch in the system and originally thought hardware might be the root of the problem, but reading all the MIB registers does not reveal any problems and same exact problem occurs with fiber or copper connections. We switched to MQX5.1/RTCS5.1 and still see the same problem.
We don't control the tcp timeout and so we changed to UDP. With UDP we still see an occasional(two 140msec delays in six hour test) delay in the recv() function. Unfortunately, we have a 100msec requirement to talk on the CAN bus or the box we communicate with will sleep causing a system problem.