I´m making a work about CoAP extension that allows it to use the Publish/Subscribe Broker model and I´m currently analyzing how is the Transmision Reliabilty gets done here. In CoAP are used CONFIRMABLE and NON-CONFIRMABLE messages, to guarantee that a message arrived to destination. My cuestion is if this extension of CoAP relies on this CoAP messages to guarantee transmision reliability, because i can´t find anything that talks about this. Thank you.
In CoAP are used CONFIRMABLE and NON-CONFIRMABLE messages, to guarantee that a message arrived to destination.
CON messages are retransmitted until either the retransmission counter expires or a ACK/RST is received as answer. That makes it very probable, that the message arrives the destination, but a "guarantee" is something else. "Cut the wire" and no technology can grant, that the message arrives the destination. Receiving a ACK you could rely, that the message arrived, but if you don't receive it, you don't know it.
Retransmission consumes bandwidth, and the benefit from sending "many different" message ("unreliable") vs. "some" messages ("reliable") is application specific. That application specific decision is also true for pubsub. I'm not sure, why pubsub should therefore explicit mention the CON/NON behavior.
Just to mention: Such questions may be asked directly in the IETF Core Mailinglist. There you may reach the authors of the RFC and you have a chance, to get their feedback. You will need to register there.