path: root/scripts/automation/trex_control_plane/client_utils/packet_builder.py

#!/router/bin/python

import external_packages
import dpkt
import socket
import binascii
import copy
import random
import string
import struct
import re
from abc import ABCMeta, abstractmethod
from collections import namedtuple


class CTRexPktBuilder(object):
    """
    This class defines the TRex API of building a packet using dpkt package.
    Using this class the user can also define how TRex will handle the packet by specifying the VM setting.
    """
    def __init__(self, max_pkt_size=dpkt.ethernet.ETH_LEN_MAX):
        """
        Instantiate a CTRexPktBuilder object

        :parameters:
             None

        """
        super(CTRexPktBuilder, self).__init__()
        self._packet = None
        self._pkt_by_hdr = {}
        self._pkt_top_layer = None
        self._max_pkt_size = max_pkt_size
        self.payload_gen = CTRexPktBuilder.CTRexPayloadGen(self._packet, self._max_pkt_size)
        self.vm = CTRexPktBuilder.CTRexVM()
        self.metadata = ""

    def add_pkt_layer(self, layer_name, pkt_layer):
        """
        This method adds additional header to the already existing packet

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l2", "l4_tcp", etc.

            pkt_layer : dpkt.Packet obj
                a dpkt object, generally from higher layer, that will be added on top of existing layer.

        :raises:
            + :exc:`ValueError`, in case the desired layer_name already exists.

        """
        assert isinstance(pkt_layer, dpkt.Packet)
        if layer_name in self._pkt_by_hdr:
            raise ValueError("Given layer name '{0}' already exists.".format(layer_name))
        else:
            dup_pkt = copy.copy(pkt_layer)  # using copy of layer to avoid cyclic packets that may lead to infinite loop
            if not self._pkt_top_layer:     # this is the first header added
                self._packet = dup_pkt
            else:
                self._pkt_top_layer.data = dup_pkt
            self._pkt_top_layer = dup_pkt
            self._pkt_by_hdr[layer_name] = dup_pkt
            return

    def set_ip_layer_addr(self, layer_name, attr, ip_addr, ip_type="ipv4"):
        """
        This method sets the IP address fields of an IP header (source or destination, for both IPv4 and IPv6)
        using a human readable addressing representation.

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l3_ip", etc.

            attr: str
                a string representation of the sub-field to be set:

                + "src" for source
                + "dst" for destination

            ip_addr: str
                a string representation of the IP address to be set.
                Example: "10.0.0.1" for IPv4, or "5001::DB8:1:3333:1:1" for IPv6

            ip_type : str
                a string representation of the IP version to be set:

                + "ipv4" for IPv4
                + "ipv6" for IPv6

                Default: **ipv4**

        :raises:
            + :exc:`ValueError`, in case the desired layer_name is not an IP layer
            + :exc:`KeyError`, in case the desired layer_name does not exists.

        """
        try:
            layer = self._pkt_by_hdr[layer_name.lower()]
            if not (isinstance(layer, dpkt.ip.IP) or isinstance(layer, dpkt.ip6.IP6)):
                raise ValueError("The specified layer '{0}' is not of IPv4/IPv6 type.".format(layer_name))
            else:
                decoded_ip = CTRexPktBuilder._decode_ip_addr(ip_addr, ip_type)
                setattr(layer, attr, decoded_ip)
        except KeyError:
            raise KeyError("Specified layer '{0}' doesn't exist on packet.".format(layer_name))

    def set_ipv6_layer_addr(self, layer_name, attr, ip_addr):
        """
        This method sets the IPv6 address fields of an IP header (source or destination)

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l3_ip", etc.

            attr: str
                a string representation of the sub-field to be set:

                + "src" for source
                + "dst" for destination

            ip_addr: str
                a string representation of the IP address to be set.
                Example: "5001::DB8:1:3333:1:1"

        :raises:
            + :exc:`ValueError`, in case the desired layer_name is not an IPv6 layer
            + :exc:`KeyError`, in case the desired layer_name does not exists.

        """
        self.set_ip_layer_addr(layer_name, attr, ip_addr, ip_type="ipv6")

    def set_eth_layer_addr(self, layer_name, attr, mac_addr):
        """
        This method sets the ethernet address fields of an Ethernet header (source or destination)
        using a human readable addressing representation.

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l2", etc.

            attr: str
                a string representation of the sub-field to be set:
                + "src" for source
                + "dst" for destination

            mac_addr: str
                a string representation of the MAC address to be set.
                Example: "00:de:34:ef:2e:f4".

        :raises:
            + :exc:`ValueError`, in case the desired layer_name is not an Ethernet layer
            + :exc:`KeyError`, in case the desired layer_name does not exists.

        """
        try:
            layer = self._pkt_by_hdr[layer_name.lower()]
            if not isinstance(layer, dpkt.ethernet.Ethernet):
                raise ValueError("The specified layer '{0}' is not of Ethernet type.".format(layer_name))
            else:
                decoded_mac = CTRexPktBuilder._decode_mac_addr(mac_addr)
                setattr(layer, attr, decoded_mac)
        except KeyError:
            raise KeyError("Specified layer '{0}' doesn't exist on packet.".format(layer_name))

    def set_layer_attr(self, layer_name, attr, val, toggle_bit=False):
        """
        This method enables the user to change a value of a previously defined packet layer.
        This method isn't to be used to set the data attribute of a packet with payload.
        Use :func:`packet_builder.CTRexPktBuilder.set_payload` instead.

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l2", "l4_tcp", etc.

            attr : str
                a string representing the attribute to be changed on desired layer

            val :
                value of attribute.

            toggle_bit : bool
                Indicating if trying to set a specific bit of a field, such as "do not fragment" bit of IP layer.

                Default: **False**

        :raises:
            + :exc:`KeyError`, in case of missing layer (the desired layer isn't part of packet)
            + :exc:`ValueError`, in case invalid attribute to the specified layer.

        """
        try:
            layer = self._pkt_by_hdr[layer_name.lower()]
            if attr == 'data' and not isinstance(val, dpkt.Packet):
                # Don't allow setting 'data' attribute
                raise ValueError("Set a data attribute with object that is not dpkt.Packet is not allowed using "
                                 "set_layer_attr method.\nUse set_payload method instead.")
            if hasattr(layer, attr):
                if toggle_bit:
                    setattr(layer, attr, val | getattr(layer, attr, 0))
                else:
                    setattr(layer, attr, val)
                    if attr == 'data':
                        # re-evaluate packet from the start, possible broken link between layers
                        self._reevaluate_packet(layer_name.lower())
            else:
                raise ValueError("Given attr name '{0}' doesn't exists on specified layer ({1}).".format(layer_name,
                                                                                                         attr))
        except KeyError:
            raise KeyError("Specified layer '{0}' doesn't exist on packet.".format(layer_name))

    def set_layer_bit_attr(self, layer_name, attr, val):
        """
        This method enables the user to set the value of a field smaller that 1 Byte in size.
        This method isn't used to set full-sized fields value (>= 1 byte).
        Use :func:`packet_builder.CTRexPktBuilder.set_layer_attr` instead.

        :parameters:
            layer_name: str
                a string representing the name of the layer.
                Example: "l2", "l4_tcp", etc.

            attr : str
                a string representing the attribute to be set on desired layer

            val : int
                value of attribute.
                This value will be set "ontop" of the existing value using bitwise "OR" operation.

                .. tip:: It is very useful to use dpkt constants to define the values of these fields.

        :raises:
            + :exc:`KeyError`, in case of missing layer (the desired layer isn't part of packet)
            + :exc:`ValueError`, in case invalid attribute to the specified layer.

        """
        return self.set_layer_attr(layer_name, attr, val, True)

    def set_pkt_payload(self, payload):
        """
        This method sets a payload to the topmost layer of the generated packet.
        This method isn't to be used to set another networking layer to the packet.
        Use :func:`packet_builder.CTRexPktBuilder.set_layer_attr` instead.


        :parameters:
            payload:
                a payload to be added to the packet at the topmost layer.
                this object cannot be of type dpkt.Packet.

        :raises:
            + :exc:`AttributeError`, in case no underlying header to host the payload.

        """
        assert isinstance(payload, str)
        try:
            self._pkt_top_layer.data = payload
        except AttributeError:
            raise AttributeError("The so far built packet doesn't contain an option for payload attachment.\n"
                                 "Make sure to set appropriate underlying header before adding payload")

    def load_packet(self, packet):
        """
        This method enables the user to change a value of a previously defined packet layer.

        :parameters:
            packet: dpkt.Packet obj
                a dpkt object that represents a packet.


        :raises:
            + :exc:`CTRexPktBuilder.IPAddressError`, in case invalid ip type option specified.

        """
        assert isinstance(packet, dpkt.Packet)
        self._packet = copy.copy(packet)

        self._pkt_by_hdr.clear()
        self._pkt_top_layer = self._packet
        # analyze packet to layers
        tmp_layer = self._packet
        while True:
            if isinstance(tmp_layer, dpkt.Packet):
                layer_name = self._gen_layer_name(type(tmp_layer).__name__)
                self._pkt_by_hdr[layer_name] = tmp_layer
                self._pkt_top_layer = tmp_layer
                try:
                    # check existence of upper layer
                    tmp_layer = tmp_layer.data
                except AttributeError:
                    # this is the most upper header
                    self._pkt_by_hdr['pkt_final_payload'] = tmp_layer.data
                    break
            else:
                self._pkt_by_hdr['pkt_final_payload'] = tmp_layer
                break
        return

    def load_packet_from_pcap(self, pcap_path):
        """
        This method loads a pcap file into a parsed packet builder object.

        :parameters:
            pcap_path: str
                a path to a pcap file, containing a SINGLE packet.

        :raises:
            + :exc:`IOError`, in case provided path doesn't exists.

        """
        with open(pcap_path, 'r') as f:
            pcap = dpkt.pcap.Reader(f)
            first_packet = True
            for _, buf in pcap:
                # this is an iterator, can't evaluate the number of files in advance
                if first_packet:
                    self.load_packet(dpkt.ethernet.Ethernet(buf))
                else:
                    raise ValueError("Provided pcap file contains more than single packet.")
        # arrive here ONLY if pcap contained SINGLE packet
        return

    def get_packet(self, get_ptr=False):
        """
        This method provides access to the built packet, as an instance or as a pointer to packet itself.

        :parameters:
            get_ptr : bool
                indicate whether to get a reference to packet or a copy.
                Use only in advanced modes
                if set to true, metadata for packet is cleared, and any further modification is not guaranteed.

                default value : False

        :return:
            + the current packet built by CTRexPktBuilder object.
            + None if packet is empty

        """
        if get_ptr:
            self._pkt_by_hdr = {}
            self._pkt_top_layer = None
            return self._packet
        else:
            return copy.copy(self._packet)

    def get_layer(self, layer_name):
        """
        This method provides access to a specific layer of the packet, as a **copy of the layer instance**.

        :parameters:
            layer_name : str
                the name given to desired layer

        :return:
            + a copy of the desired layer of the current packet if exists.
            + None if no such layer

        """
        layer = self._pkt_by_hdr.get(layer_name)
        return copy.copy(layer) if layer else None

    # VM access methods
    def set_vm_ip_range(self, ip_layer_name, ip_field,
                        ip_init, ip_start, ip_end, add_value,
                        operation, is_big_endian=False, val_size=4,
                        ip_type="ipv4", add_checksum_inst=True):
        if ip_field not in ["src", "dst"]:
            raise ValueError("set_vm_ip_range only available for source ('src') or destination ('dst') ip addresses")
        # set differences between IPv4 and IPv6
        if ip_type == "ipv4":
            ip_class = dpkt.ip.IP
            ip_addr_size = val_size if val_size <= 4 else 4
        elif ip_type == "ipv6":
            ip_class = dpkt.ip6.IP6
            ip_addr_size = val_size if val_size <= 8 else 4
        else:
            raise CTRexPktBuilder.IPAddressError()

        self._verify_layer_prop(ip_layer_name, ip_class)
        trim_size = ip_addr_size*2
        init_val = int(binascii.hexlify(CTRexPktBuilder._decode_ip_addr(ip_init, ip_type))[-trim_size:], 16)
        start_val = int(binascii.hexlify(CTRexPktBuilder._decode_ip_addr(ip_start, ip_type))[-trim_size:], 16)
        end_val = int(binascii.hexlify(CTRexPktBuilder._decode_ip_addr(ip_end, ip_type))[-trim_size:], 16)
        # All validations are done, start adding VM instructions
        flow_var_name = "{layer}__{field}".format(layer=ip_layer_name, field=ip_field)
        hdr_offset, field_abs_offset = self._calc_offset(ip_layer_name, ip_field, ip_addr_size)
        self.vm.add_flow_man_inst(flow_var_name, size=ip_addr_size, operation=operation,
                                  init_value=init_val,
                                  min_value=start_val,
                                  max_value=end_val)
        self.vm.add_write_flow_inst(flow_var_name, field_abs_offset)
        self.vm.set_vm_off_inst_field(flow_var_name, "add_value", add_value)
        self.vm.set_vm_off_inst_field(flow_var_name, "is_big_endian", is_big_endian)
        if ip_type == "ipv4" and add_checksum_inst:
            self.vm.add_fix_checksum_inst(self._pkt_by_hdr.get(ip_layer_name), hdr_offset)

    def set_vm_eth_range(self, eth_layer_name, eth_field,
                         mac_init, mac_start, mac_end, add_value,
                         operation, val_size=4, is_big_endian=False):
        if eth_field not in ["src", "dst"]:
            raise ValueError("set_vm_eth_range only available for source ('src') or destination ('dst') eth addresses")
        self._verify_layer_prop(eth_layer_name, dpkt.ethernet.Ethernet)
        eth_addr_size = val_size if val_size <= 4 else 4
        trim_size = eth_addr_size*2
        init_val = int(binascii.hexlify(CTRexPktBuilder._decode_mac_addr(mac_init))[-trim_size:], 16)
        start_val = int(binascii.hexlify(CTRexPktBuilder._decode_mac_addr(mac_start))[-trim_size:], 16)
        end_val = int(binascii.hexlify(CTRexPktBuilder._decode_mac_addr(mac_end))[-trim_size:], 16)
        # All validations are done, start adding VM instructions
        flow_var_name = "{layer}__{field}".format(layer=eth_layer_name, field=eth_field)
        hdr_offset, field_abs_offset = self._calc_offset(eth_layer_name, eth_field, eth_addr_size)
        self.vm.add_flow_man_inst(flow_var_name, size=8, operation=operation,
                                  init_value=init_val,
                                  min_value=start_val,
                                  max_value=end_val)
        self.vm.add_write_flow_inst(flow_var_name, field_abs_offset)
        self.vm.set_vm_off_inst_field(flow_var_name, "add_value", add_value)
        self.vm.set_vm_off_inst_field(flow_var_name, "is_big_endian", is_big_endian)

    def set_vm_custom_range(self, layer_name, hdr_field,
                            init_val, start_val, end_val, add_val, val_size,
                            operation, is_big_endian=False, range_name="",
                            add_checksum_inst=True):
        # verify input validity for init/start/end values
        for val in [init_val, start_val, end_val]:
            if not isinstance(val, int):
                raise ValueError("init/start/end values are expected integers, but received type '{0}'".
                                 format(type(val)))
        self._verify_layer_prop(layer_name=layer_name, field_name=hdr_field)
        if not range_name:
            range_name = "{layer}__{field}".format(layer=layer_name, field=hdr_field)
        trim_size = val_size*2
        hdr_offset, field_abs_offset = self._calc_offset(layer_name, hdr_field, val_size)
        self.vm.add_flow_man_inst(range_name, size=val_size, operation=operation,
                                  init_value=init_val,
                                  min_value=start_val,
                                  max_value=end_val)
        self.vm.add_write_flow_inst(range_name, field_abs_offset)
        self.vm.set_vm_off_inst_field(range_name, "add_value", add_val)
        self.vm.set_vm_off_inst_field(range_name, "is_big_endian", is_big_endian)
        if isinstance(self._pkt_by_hdr.get(layer_name), dpkt.ip.IP) and add_checksum_inst:
            self.vm.add_fix_checksum_inst(self._pkt_by_hdr.get(layer_name), hdr_offset)

    def get_vm_data(self):
        return self.vm.dump()

    def dump_pkt(self):
        """
        Dumps the packet as a decimal array of bytes (each item x gets value between 0-255)

        :parameters:
            None

        :return:
            + packet representation as array of bytes

        :raises:
            + :exc:`CTRexPktBuilder.EmptyPacketError`, in case packet is empty.

        """
        if self._packet is None:
            raise CTRexPktBuilder.EmptyPacketError()
        pkt_in_hex = binascii.hexlify(str(self._packet))
        return {"binary": [int(pkt_in_hex[i:i+2], 16)
                           for i in range(0, len(pkt_in_hex), 2)],
                "meta": self.metadata}
        # return [pkt_in_hex[i:i+2] for i in range(0, len(pkt_in_hex), 2)]

    def dump_pkt_to_pcap(self, file_path, ts=None):
        """
        Dumps the packet as a decimal array of bytes (each item x gets value between 0-255)

        :parameters:
            file_path : str
                a path (including filename) to which to write to pcap file to.

            ts : int
                a timestamp to attach to the packet when dumped to pcap file.
                if ts in None, then time.time() is used to set the timestamp.

                Default: **None**

        :return:
            None

        :raises:
            + :exc:`CTRexPktBuilder.EmptyPacketError`, in case packet is empty.

        """
        if self._packet is None:
            raise CTRexPktBuilder.EmptyPacketError()
        try:
            with open(file_path, 'wb') as f:
                pcap_wr = dpkt.pcap.Writer(f)
                pcap_wr.writepkt(self._packet, ts)
                return
        except IOError:
            raise IOError(2, "The provided path could not be accessed")

    def export_pkt(self, file_path, link_pcap=False, pcap_name=None, pcap_ts=None):
        pass

    # ----- internal methods ----- #
    def _reevaluate_packet(self, layer_name):
        cur_layer = self._packet
        known_layers = set(self._pkt_by_hdr.keys())
        found_layers = set()
        while True:
            pointing_layer_name = self._find_pointing_layer(known_layers, cur_layer)
            found_layers.add(pointing_layer_name)
            if self._pkt_by_hdr[layer_name] is cur_layer:
                self._pkt_top_layer = cur_layer
                disconnected_layers = known_layers.difference(found_layers)
                # remove disconnected layers
                for layer in disconnected_layers:
                    self._pkt_by_hdr.pop(layer)
                break
            else:
                cur_layer = cur_layer.data

    def _gen_layer_name(self, layer_class_name):
        assert isinstance(layer_class_name, str)
        layer_name = layer_class_name.lower()
        idx = 1
        while True:
            tmp_name = "{name}_{id}".format(name=layer_name, id=idx)
            if tmp_name not in self._pkt_by_hdr:
                return tmp_name
            else:
                idx += 1

    def _find_pointing_layer(self, known_layers, layer_obj):
        assert isinstance(known_layers, set)
        for layer in known_layers:
            if self._pkt_by_hdr[layer] is layer_obj:
                return layer

    def _calc_offset(self, layer_name, hdr_field, hdr_field_size):
        pkt_header = self._pkt_by_hdr.get(layer_name)
        hdr_offset = len(self._packet) - len(pkt_header)
        inner_hdr_offsets = []
        for field in pkt_header.__hdr__:
            if field[0] == hdr_field:
                field_size = struct.calcsize(field[1])
                if field_size == hdr_field_size:
                    break
                elif field_size < hdr_field_size:
                    raise CTRexPktBuilder.PacketLayerError(layer_name,
                                                           "The specified field '{0}' size is smaller than given range"
                                                           " size ('{1}')".format(hdr_field, hdr_field_size))
                else:
                    inner_hdr_offsets.append(field_size - hdr_field_size)
                    break
            else:
                inner_hdr_offsets.append(struct.calcsize(field[1]))
        return hdr_offset, hdr_offset + sum(inner_hdr_offsets)

    def _verify_layer_prop(self, layer_name, layer_type=None, field_name=None):
        if layer_name not in self._pkt_by_hdr:
            raise CTRexPktBuilder.PacketLayerError(layer_name)
        pkt_layer = self._pkt_by_hdr.get(layer_name)
        if layer_type:
            # check for layer type
            if not isinstance(pkt_layer, layer_type):
                raise CTRexPktBuilder.PacketLayerTypeError(layer_name, type(pkt_layer), layer_type)
        if field_name and not hasattr(pkt_layer, field_name):
            # check if field exists on certain header
            raise CTRexPktBuilder.PacketLayerError(layer_name, "The specified field '{0}' does not exists on "
                                                               "given packet layer ('{1}')".format(field_name,
                                                                                                   layer_name))
        return

    @staticmethod
    def _decode_mac_addr(mac_addr):
        """
        Static method to test for MAC address validity.

        :parameters:
            mac_addr : str
                a string representing an MAC address, separated by ':' or '-'.

                examples: '00:de:34:ef:2e:f4', '00-de-34-ef-2e-f4

        :return:
            + an hex-string representation of the MAC address.
              for example, ip 00:de:34:ef:2e:f4 will return '\x00\xdeU\xef.\xf4'

        :raises:
            + :exc:`CTRexPktBuilder.MACAddressError`, in case invalid ip type option specified.

        """
        tmp_mac = mac_addr.lower().replace('-', ':')
        if re.match("[0-9a-f]{2}([-:])[0-9a-f]{2}(\\1[0-9a-f]{2}){4}$", tmp_mac):
            return binascii.unhexlify(tmp_mac.replace(':', ''))
            # another option for both Python 2 and 3: 
            # codecs.decode(tmp_mac.replace(':', ''), 'hex')
        else:
            raise CTRexPktBuilder.MACAddressError()

    @staticmethod
    def _decode_ip_addr(ip_addr, ip_type):
        """
        Static method to test for IPv4/IPv6 address validity.

        :parameters:
            ip_addr : str
                a string representing an IP address (IPv4/IPv6)

            ip_type : str
                The type of IP to be checked.
                Valid types: "ipv4", "ipv6".

        :return:
            + an hex-string representation of the ip address.
              for example, ip 1.2.3.4 will return '\x01\x02\x03\x04'

        :raises:
            + :exc:`CTRexPktBuilder.IPAddressError`, in case invalid ip type option specified.

        """
        if ip_type == "ipv4":
            try:
                return socket.inet_pton(socket.AF_INET, ip_addr)
            except AttributeError:  # no inet_pton here, sorry
                # try:
                return socket.inet_aton(ip_addr)
                # except socket.error:
                #     return False
                # return ip_addr.count('.') == 3
            except socket.error:  # not a valid address
                raise CTRexPktBuilder.IPAddressError()
        elif ip_type == "ipv6":
            try:
                return socket.inet_pton(socket.AF_INET6, ip_addr)
            except socket.error:  # not a valid address
                raise CTRexPktBuilder.IPAddressError()
        else:
            raise CTRexPktBuilder.IPAddressError()

    # ------ private classes ------ #
    class CTRexPayloadGen(object):

        def __init__(self, packet_ref, max_pkt_size):
            self._pkt_ref = packet_ref
            self._max_pkt_size = max_pkt_size

        def gen_random_str(self):
            gen_length = self._calc_gen_length()
            # return a string of size gen_length bytes, to pad the packet to its max_size
            return ''.join(random.SystemRandom().choice(string.ascii_letters + string.digits)
                           for _ in range(gen_length))

        def gen_repeat_ptrn(self, ptrn_to_repeat):
            gen_length = self._calc_gen_length()
            if isinstance(ptrn_to_repeat, str):
                # generate repeated string
                return (ptrn_to_repeat * (gen_length/len(ptrn_to_repeat) + 1))[:gen_length]
            elif isinstance(ptrn_to_repeat, int):
                ptrn = binascii.unhexlify(hex(ptrn_to_repeat)[2:])
                return (ptrn * (gen_length/len(ptrn) + 1))[:gen_length]
            elif isinstance(ptrn_to_repeat, tuple):
                if not all((isinstance(x, int) and (x < 255) and (x >= 0))
                           for x in ptrn_to_repeat):
                    raise ValueError("All numbers in tuple must be in range 0 <= number <= 255 ")
                # generate repeated sequence
                to_pack = (ptrn_to_repeat * (gen_length/len(ptrn_to_repeat) + 1))[:gen_length]
                return struct.pack('B'*gen_length, *to_pack)
            else:
                raise ValueError("Given ptrn_to_repeat argument type ({0}) is illegal.".
                                 format(type(ptrn_to_repeat)))

        def _calc_gen_length(self):
            return self._max_pkt_size - len(self._pkt_ref)

    class CTRexVM(object):
        """
        This class defines the TRex VM which represents how TRex will regenerate packets.
        The packets will be regenerated based on the built packet containing this class.
        """
        InstStore = namedtuple('InstStore', ['type', 'inst'])

        def __init__(self):
            """
            Instantiate a CTRexVM object

            :parameters:
                None
            """
            super(CTRexPktBuilder.CTRexVM, self).__init__()
            self.vm_variables = {}
            self._inst_by_offset = {}   # this data structure holds only offset-related instructions, ordered in tuples
            self._off_inst_by_name = {}

        def set_vm_var_field(self, var_name, field_name, val, offset_inst=False):
            """
            Set VM variable field. Only existing variables are allowed to be changed.

            :parameters:
                var_name : str
                    a string representing the name of the VM variable to be changed.
                field_name : str
                    a string representing the field name of the VM variable to be changed.
                val :
                    a value to be applied to field_name field of the var_name VM variable.

            :raises:
                + :exc:`KeyError`, in case invalid var_name has been specified.
                + :exc:`CTRexPktBuilder.VMVarFieldTypeError`, in case mismatch between `val` and allowed type.
                + :exc:`CTRexPktBuilder.VMVarValueError`, in case val isn't one of allowed options of field_name.

            """
            if offset_inst:
                return self._off_inst_by_name[var_name].inst.set_field(field_name, val)
            else:
                return self.vm_variables[var_name].set_field(field_name, val)

        def set_vm_off_inst_field(self, var_name, field_name, val):
            return self.set_vm_var_field(var_name, field_name, val, True)

        def add_flow_man_inst(self, name, **kwargs):
            """
            Adds a new flow manipulation object to the VM instance.

            :parameters:
                name : str
                    name of the manipulation, must be distinct.
                    Example: 'source_ip_change'

                **kwargs** : dict
                    optional, set flow_man fields on initialization (key = field_name, val = field_val).
                    Must be used with legit fields, see :func:`CTRexPktBuilder.CTRexVM.CTRexVMVariable.set_field`.

            :return:
                None

            :raises:
                + :exc:`CTRexPktBuilder.VMVarNameExistsError`, in case of desired flow_man name already taken.
                + Exceptions from :func:`CTRexPktBuilder.CTRexVM.CTRexVMVariable.set_field` method.
                  Will rise when VM variables were misconfiguration.
            """
            if name not in self.vm_variables:
                self.vm_variables[name] = self.CTRexVMFlowVariable(name)
                # try configuring VM instruction attributes
                for (field, value) in kwargs.items():
                    self.vm_variables[name].set_field(field, value)
            else:
                raise CTRexPktBuilder.VMVarNameExistsError(name)

        def add_fix_checksum_inst(self, linked_ipv4_obj, offset_to_obj=14, name=None):
            # check if specified linked_ipv4_obj is indeed an ipv4 object
            if not (isinstance(linked_ipv4_obj, dpkt.ip.IP)):
                raise ValueError("The provided layer object is not of IPv4.")
            if not name:
                name = "checksum_{off}".format(off=offset_to_obj)   # name will override previous checksum inst, OK
            new_checksum_inst = self.CTRexVMChecksumInst(name, offset_to_obj)
            # store the checksum inst in the end of the IP header (20 Bytes long)
            inst = self.InstStore('checksum', new_checksum_inst)
            self._inst_by_offset[offset_to_obj + 20] = inst
            self._off_inst_by_name[name] = inst

        def add_write_flow_inst(self, name, pkt_offset, **kwargs):
            if name not in self.vm_variables:
                raise KeyError("Trying to add write_flow_var instruction to a not-exists VM flow variable ('{0}')".
                               format(name))
            else:
                new_write_inst = self.CTRexVMWrtFlowVarInst(name, pkt_offset)
                # try configuring VM instruction attributes
                for (field, value) in kwargs.items():
                    new_write_inst.set_field(field, value)
                # add the instruction to the date-structure
                inst = self.InstStore('write', new_write_inst)
                self._inst_by_offset[pkt_offset] = inst
                self._off_inst_by_name[name] = inst

        def load_flow_man(self, flow_obj):
            """
            Loads an outer VM variable (instruction) into current VM.
            The outer VM variable must contain different name than existing VM variables currently registered on VM.

            :parameters:
                flow_obj : CTRexVMVariable
                    a CTRexVMVariable to be loaded into VM variable sets.

            :return:
                list holds variables data of VM

            """
            assert isinstance(flow_obj, CTRexPktBuilder.CTRexVM.CTRexVMFlowVariable)
            if flow_obj.name not in self.vm_variables.keys():
                self.vm_variables[flow_obj.name] = flow_obj
            else:
                raise CTRexPktBuilder.VMVarNameExistsError(flow_obj.name)

        def dump(self):
            """
            dumps a VM variables (instructions) into a list data structure.

            :parameters:
                None

            :return:
                list holds variables data of VM

            """
            # at first, dump all CTRexVMFlowVariable instructions
            ret_val = [var.dump()
                       for key, var in self.vm_variables.items()]
            # then, dump all the CTRexVMWrtFlowVarInst and CTRexVMChecksumInst instructions
            ret_val += [self._inst_by_offset.get(key).inst.dump()
                        for key in sorted(self._inst_by_offset)]
            return ret_val

        class CVMAbstractInstruction(object):
            __metaclass__ = ABCMeta

            def __init__(self, name):
                """
                Instantiate a CTRexVMVariable object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CVMAbstractInstruction, self).__init__()
                self.name = name

            def set_field(self, field_name, val):
                if not hasattr(self, field_name):
                    raise CTRexPktBuilder.VMFieldNameError(field_name)
                setattr(self, field_name, val)

            @abstractmethod
            def dump(self):
                pass

        class CTRexVMFlowVariable(CVMAbstractInstruction):
            """
            This class defines a single VM variable to be used as part of CTRexVar object.
            """
            VALID_SIZE = [1, 2, 4, 8]   # size in Bytes
            VALID_OPERATION = ["inc", "dec", "random"]

            def __init__(self, name):
                """
                Instantiate a CTRexVMVariable object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CTRexVMFlowVariable, self).__init__(name)
                # self.name = name
                self.size = 4
                self.big_endian = True
                self.operation = "inc"
                # self.split_by_core = False
                self.init_value = 1
                self.min_value = self.init_value
                self.max_value = self.init_value

            def set_field(self, field_name, val):
                """
                Set VM variable field. Only existing variables are allowed to be changed.

                :parameters:
                    field_name : str
                        a string representing the field name of the VM variable to be changed.
                    val :
                        a value to be applied to field_name field of the var_name VM variable.

                :return:
                    None

                :raises:
                    + :exc:`CTRexPktBuilder.VMVarNameError`, in case of illegal field name.
                    + :exc:`CTRexPktBuilder.VMVarFieldTypeError`, in case mismatch between `val` and allowed type.
                    + :exc:`CTRexPktBuilder.VMVarValueError`, in case val isn't one of allowed options of field_name.

                """
                if not hasattr(self, field_name):
                    raise CTRexPktBuilder.VMFieldNameError(field_name)
                elif field_name == "size":
                    if type(val) != int:
                        raise CTRexPktBuilder.VMFieldTypeError("size", int)
                    elif val not in self.VALID_SIZE:
                        raise CTRexPktBuilder.VMFieldValueError("size", self.VALID_SIZE)
                elif field_name in ["init_value", "min_value", "max_value"]:
                    if type(val) != int:
                        raise CTRexPktBuilder.VMFieldTypeError(field_name, int)
                elif field_name == "operation":
                    if type(val) != str:
                        raise CTRexPktBuilder.VMFieldTypeError("operation", str)
                    elif val not in self.VALID_OPERATION:
                        raise CTRexPktBuilder.VMFieldValueError("operation", self.VALID_OPERATION)
                # elif field_name == "split_by_core":
                #     val = bool(val)
                # update field value on success
                setattr(self, field_name, val)

            def dump(self):
                """
                dumps a variable fields in a dictionary data structure.

                :parameters:
                    None

                :return:
                    dictionary holds variable data of VM variable

                """
                return {"type": "flow_var",  # VM variable dump always refers to manipulate instruction.
                        "name": self.name,
                        "size": self.size,
                        "op": self.operation,
                        # "split_by_core": self.split_by_core,
                        "init_value": str(self.init_value),
                        "min_value": str(self.min_value),
                        "max_value": str(self.max_value)}

        class CTRexVMChecksumInst(CVMAbstractInstruction):

            def __init__(self, name, offset):
                """
                Instantiate a CTRexVMChecksumInst object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CTRexVMChecksumInst, self).__init__(name)
                self.pkt_offset = offset

            def dump(self):
                return {"type": "fix_checksum_ipv4",
                        "pkt_offset": int(self.pkt_offset)}

        class CTRexVMWrtFlowVarInst(CVMAbstractInstruction):

            def __init__(self, name, pkt_offset):
                """
                Instantiate a CTRexVMWrtFlowVarInst object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CTRexVMWrtFlowVarInst, self).__init__(name)
                self.pkt_offset = int(pkt_offset)
                self.add_value = 0
                self.is_big_endian = False

            def set_field(self, field_name, val):
                if not hasattr(self, field_name):
                    raise CTRexPktBuilder.VMFieldNameError(field_name)
                elif field_name == 'pkt_offset':
                    raise ValueError("pkt_offset value cannot be changed")
                cur_attr_type = type(getattr(self, field_name))
                if cur_attr_type == type(val):
                    setattr(self, field_name, val)
                else:
                    CTRexPktBuilder.VMFieldTypeError(field_name, cur_attr_type)

            def dump(self):
                return {"type": "write_flow_var",
                        "name": self.name,
                        "pkt_offset": self.pkt_offset,
                        "add_value": int(self.add_value),
                        "is_big_endian": bool(self.is_big_endian)
                        }

        class CTRexVMChecksumInst(CVMAbstractInstruction):

            def __init__(self, name, offset):
                """
                Instantiate a CTRexVMChecksumInst object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CTRexVMChecksumInst, self).__init__(name)
                self.pkt_offset = offset

            def dump(self):
                return {"type": "fix_checksum_ipv4",
                        "pkt_offset": int(self.pkt_offset)}

        class CTRexVMWrtFlowVarInst(CVMAbstractInstruction):

            def __init__(self, name, pkt_offset):
                """
                Instantiate a CTRexVMWrtFlowVarInst object

                :parameters:
                    name : str
                        a string representing the name of the VM variable.
                """
                super(CTRexPktBuilder.CTRexVM.CTRexVMWrtFlowVarInst, self).__init__(name)
                self.pkt_offset = int(pkt_offset)
                self.add_value = 0
                self.is_big_endian = False

            def set_field(self, field_name, val):
                if not hasattr(self, field_name):
                    raise CTRexPktBuilder.VMFieldNameError(field_name)
                elif field_name == 'pkt_offset':
                    raise ValueError("pkt_offset value cannot be changed")
                cur_attr_type = type(getattr(self, field_name))
                if cur_attr_type == type(val):
                    setattr(self, field_name, val)
                else:
                    CTRexPktBuilder.VMFieldTypeError(field_name, cur_attr_type)

            def dump(self):
                return {"type": "write_flow_var",
                        "name": self.name,
                        "pkt_offset": self.pkt_offset,
                        "add_value": int(self.add_value),
                        "is_big_endian": bool(self.is_big_endian)
                        }

    class CPacketBuildException(Exception):
        """
        This is the general Packet Building error exception class.
        """
        def __init__(self, code, message):
            self.code = code
            self.message = message

        def __str__(self):
            return self.__repr__()

        def __repr__(self):
            return u"[errcode:%r] %r" % (self.code, self.message)

    class EmptyPacketError(CPacketBuildException):
        """
        This exception is used to indicate an error caused by operation performed on an empty packet.
        """
        def __init__(self, message=''):
            self._default_message = 'Illegal operation on empty packet.'
            self.message = message or self._default_message
            super(CTRexPktBuilder.EmptyPacketError, self).__init__(-10, self.message)

    class IPAddressError(CPacketBuildException):
        """
        This exception is used to indicate an error on the IP addressing part of the packet.
        """
        def __init__(self, message=''):
            self._default_message = 'Illegal type or value of IP address has been provided.'
            self.message = message or self._default_message
            super(CTRexPktBuilder.IPAddressError, self).__init__(-11, self.message)

    class MACAddressError(CPacketBuildException):
        """
        This exception is used to indicate an error on the MAC addressing part of the packet.
        """
        def __init__(self, message=''):
            self._default_message = 'Illegal MAC address has been provided.'
            self.message = message or self._default_message
            super(CTRexPktBuilder.MACAddressError, self).__init__(-12, self.message)

    class PacketLayerError(CPacketBuildException):
        """
        This exception is used to indicate an error caused by operation performed on an non-exists layer of the packet.
        """
        def __init__(self, name, message=''):
            self._default_message = "The given packet layer name ({0}) does not exists.".format(name)
            self.message = message or self._default_message
            super(CTRexPktBuilder.PacketLayerError, self).__init__(-13, self.message)

    class PacketLayerTypeError(CPacketBuildException):
        """
        This exception is used to indicate an error caused by operation performed on an non-exists layer of the packet.
        """
        def __init__(self, name, layer_type, ok_type, message=''):
            self._default_message = "The type of packet layer {layer_name} is of type {layer_type}, " \
                                    "and not of the expected {allowed_type}.".format(layer_name=name,
                                                                                     layer_type=layer_type,
                                                                                     allowed_type=ok_type.__name__)
            self.message = message or self._default_message
            super(CTRexPktBuilder.PacketLayerTypeError, self).__init__(-13, self.message)

    class VMVarNameExistsError(CPacketBuildException):
        """
        This exception is used to indicate a duplicate usage of VM variable.
        """
        def __init__(self, name, message=''):
            self._default_message = 'The given VM name ({0}) already exists as part of the stream.'.format(name)
            self.message = message or self._default_message
            super(CTRexPktBuilder.VMVarNameExistsError, self).__init__(-21, self.message)

    class VMFieldNameError(CPacketBuildException):
        """
        This exception is used to indicate that an undefined VM var field name has been accessed.
        """
        def __init__(self, name, message=''):
            self._default_message = "The given VM field name ({0}) is not defined and isn't legal.".format(name)
            self.message = message or self._default_message
            super(CTRexPktBuilder.VMFieldNameError, self).__init__(-22, self.message)

    class VMFieldTypeError(CPacketBuildException):
        """
        This exception is used to indicate an illegal value has type has been given to VM variable field.
        """
        def __init__(self, name, ok_type, message=''):
            self._default_message = "The desired value of field {field_name} is of type {field_type}, " \
                                    "and not of the allowed {allowed_type}.".format(field_name=name,
                                                                                    field_type=type(name).__name__,
                                                                                    allowed_type=ok_type.__name__)
            self.message = message or self._default_message
            super(CTRexPktBuilder.VMFieldTypeError, self).__init__(-31, self.message)

    class VMFieldValueError(CPacketBuildException):
        """
        This exception is used to indicate an error an illegal value has been assigned to VM variable field.
        """
        def __init__(self, name, ok_opts, message=''):
            self._default_message = "The desired value of field {field_name} is illegal.\n" \
                                    "The only allowed options are: {allowed_opts}.".format(field_name=name,
                                                                                           allowed_opts=ok_opts)
            self.message = message or self._default_message
            super(CTRexPktBuilder.VMFieldValueError, self).__init__(-32, self.message)


if __name__ == "__main__":
    pass