Class H2PaddedDataFrame

scapy/contrib/http2.py:1444–1489

H2DataFrame implements RFC7540 par6.1 This packet is the Data Frame to use when there is padding.

class H2PaddedDataFrame(H2DataFrame):
    """ H2DataFrame implements RFC7540 par6.1
    This packet is the Data Frame to use when there is padding.
    """
    __slots__ = ['s_len']

    name = 'HTTP/2 Padded Data Frame'
    fields_desc = [
        fields.FieldLenField('padlen', None, length_of='padding', fmt="B"),
        fields.StrLenField('data', '',
                           length_from=lambda pkt: pkt.get_data_len()
                           ),
        fields.StrLenField('padding', '',
                           length_from=lambda pkt: pkt.getfieldval('padlen')
                           )
    ]

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

        To do this computation, the length of the padlen field and the actual
        padding is subtracted to the string that was provided to the pre_dissect  # noqa: E501
        fun of the pkt parameter
        :return: int; length of the data part of the HTTP/2 frame packet provided as parameter  # noqa: E501
        :raises: AssertionError
        """
        padding_len = self.getfieldval('padlen')
        fld, fval = self.getfield_and_val('padlen')
        padding_len_len = fld.i2len(self, fval)

        ret = self.s_len - padding_len_len - padding_len
        assert ret >= 0
        return ret

    def pre_dissect(self, s):
        # type: (str) -> str
        """pre_dissect is filling the s_len property of this instance. This
        property is later used during the getfield call of the "data" field when  # noqa: E501
        trying to evaluate the length of the StrLenField! This "trick" works
        because the underlayer packet (H2Frame) is assumed to override the
        "extract_padding" method and to only provide to this packet the data
        necessary for this packet. Tricky, tricky, will break some day probably!  # noqa: E501
        """
        self.s_len = len(s)
        return s


#                                             HTTP/2 Header Frame Packets                                              #  # noqa: E501