Is TCP/IP a mandatory for MQTT?

Question

If so, do you know examples of what can go wrong in a non-TCP network?

Learning about MQTT I came across several mentions of the fact that MQTT relies on TCP/IP stack. For example, from mqtt.org:

MQTT for Sensor Networks is aimed at embedded devices on non-TCP/IP networks, whereas MQTT itself explicitly expects a TCP/IP stack.

But if you read the reference documents, you won't finding anything like that. Moreover, there's QoS field that can be used for reliable delivery and whose values other than 0 are essentially useless in TCP/IP networks. Right now I see nothing that would prevent me from establishing an MQTT connection using UNIX pipe, domain or UDP socket rather than TCP socket.

Answer 1

MQTT just needs a delivery that is ordered and reliable, it doesn't have to be TCP. SCTP works just fine, for example, but UDP doesn't because there is no way to guarantee your large PUBLISH packet made up of multiple UDP packets will arrive in order and complete.

With regards TCP reliability, in theory what you are saying is true, but in practice when an application calls write() and receives a successful return, it can't guarantee when the data has actually made it out of the computer to the remote host. All write() (or send()) does is copy the data to the kernel buffers, at which point you have no further control.

The only way to be sure that a message reaches the remote host at the application level is to have the remote host reply.