code: def start_kex(self):
          
          if self.transport.server_mode:
              self.transport._expect_packet(MSG_KEXGSS_GROUPREQ)
              return
          # request a bit range: we accept (min_bits) to (max_bits), but prefer
          # (preferred_bits).  according to the spec, we shouldn't pull the
          # minimum up above 1024.
          self.gss_host = self.transport.gss_host
          m = Message()
          m.add_byte(c_MSG_KEXGSS_GROUPREQ)
          m.add_int(self.min_bits)
          m.add_int(self.preferred_bits)
          m.add_int(self.max_bits)
          self.transport._send_message(m)
          self.transport._expect_packet(MSG_KEXGSS_GROUP)
docs: Start the GSS-API / SSPI Authenticated Diffie-Hellman Group Exchange