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CoAP or Constrained Application Protocol for IoT/M2M is being defined by the IETF is on top of UDP. One of the reasons is UDP's simplicity thereby making it easy to implement on microcontrollers. I guess there should be more love for UDP coming.

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There's currently several proposed RFCs for adding TCP to CoAP. The next IETF meeting should (hopefully) standardize one. One is a pretty naive implementation that adds TCP semantics as an almost wrapper, and another removes the application level ACKs because of the transport level ACK associated with TCP, and several other "well TCP takes care of this for us" sorts of changes. ARM's Sensinode library has the first RFC I mentioned currently implemented, so it's being used in the wild.

Once that standardization happens, the OMA LWM2M spec is going to need some revisions, because it's very much written with "CoAP over UDP" in mind. This will ultimately be very good, because it will allow the possibility of other protocols like MQTT, HTTP, etc.

There are several alternative transports being proposed (including the 'unusual' CoAP over websockets), but I wonder if they can be implemented with the original constraints of 8bit micros as servers. Maybe its possible, but not sure if there is a need and hence the new TCP proposals.

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