get_hdrs_len computes the length of the hdrs field

        To do this computation, the length of the padlen field, reserved,
        stream_id and the actual padding is subtracted to the string that was
        provided to the pre_dissect fun of the pkt parameter.
        @return int: the length of the hdrs field
        @raise AssertionError

def get_hdrs_len(self):
        # type: () -> int
        """ get_hdrs_len computes the length of the hdrs field

        To do this computation, the length of the padlen field, reserved,
        stream_id and the actual padding is subtracted to the string that was
        provided to the pre_dissect fun of the pkt parameter.
        @return int: the length of the hdrs field
        @raise AssertionError
        """
        fld, padding_len = self.getfield_and_val('padlen')
        padding_len_len = fld.i2len(self, padding_len)
        bit_len = self.get_field('reserved').size
        bit_len += self.get_field('stream_id').size

        ret = int(self.s_len -
                  padding_len_len -
                  padding_len -
                  (bit_len / 8)
                  )
        assert(ret >= 0)
        return ret