ツナでもわかるmruby [4回目：バイナリデータフォーマットの実装] ref: http://qiita.com/hiroeorz@github/items/27e0866f493e0e7a65c5

mirb

def fixed_header(params)
  message_type   = params[:message_type] || 1
  dup            = params[:dup] || 0
  qos            = params[:qos] || 0
  retain         = params[:retain] || 0
  remaining_size = params[:remaining_size] || 0

  data_1 = (message_type << 4) | (dup << 3) | (qos << 1) | retain
  data_2 = remaining_size
  data_1.chr + data_2.chr
end

fixed_header(message_type:1, dup:0, qos:1, retain:1, remaining_size:20) #=> "\023\024"