Please note that prior art only means relevant information satisfying the priority constraint. The patent application's first claim is quite long and to invalidate this application with a single prior art document (e.g. the RFC), every aspect of the claim must be described in the document.
The IETF RFC6748 draft definitely is relevant to the claimed invention. And the applicant actually mentioned this RFC in their IDS filing to the USPTO, they actually acknowledged it is prior art. Yet, they believe the RFC draft doesn't restrict the novelty of their invention.
For the record, this invention was first filed in the US, then a PCT application was filed, with a search report drafted by the EPO. The EPO found more prior art, including a US patent and a US patent application, but only in category "A" (not particularly relevant).