DLT reference architecture

@techreport{itu-dlt-3.1,
   apice = {ItuFgdltD31v2019},
   author = {FG DLT},
   institution = {International Telecommunication Union},
   month = aug,
   number = {FG DLT D3.1},
   title = {DLT reference architecture},
   type = {ITU Technical Specification},
   url = {https://www.itu.int/en/ITU-T/focusgroups/dlt/Documents/d31.pdf},
   year = 2019
}