Instruction:
Here you have a function in Python 3, explain what it does
def create_extension(namespace, nicira_header, nx_action, nx_stats_request, nx_stats_reply,
                     msg_subtype, action_subtype, stats_subtype):
    '''
    /* This command enables or disables an Open vSwitch extension that allows a
     * controller to specify the OpenFlow table to which a flow should be added,
     * instead of having the switch decide which table is most appropriate as
     * required by OpenFlow 1.0.  Because NXM was designed as an extension to
     * OpenFlow 1.0, the extension applies equally to ofp10_flow_mod and
     * nx_flow_mod.  By default, the extension is disabled.
     *
     * When this feature is enabled, Open vSwitch treats struct ofp10_flow_mod's
     * and struct nx_flow_mod's 16-bit 'command' member as two separate fields.
     * The upper 8 bits are used as the table ID, the lower 8 bits specify the
     * command as usual.  A table ID of 0xff is treated like a wildcarded table ID.
     *
     * The specific treatment of the table ID depends on the type of flow mod:
     *
     *    - OFPFC_ADD: Given a specific table ID, the flow is always placed in that
     *      table.  If an identical flow already exists in that table only, then it
     *      is replaced.  If the flow cannot be placed in the specified table,
     *      either because the table is full or because the table cannot support
     *      flows of the given type, the switch replies with an OFPFMFC_TABLE_FULL
     *      error.  (A controller can distinguish these cases by comparing the
     *      current and maximum number of entries reported in ofp_table_stats.)
     *
     *      If the table ID is wildcarded, the switch picks an appropriate table
     *      itself.  If an identical flow already exist in the selected flow table,
     *      then it is replaced.  The choice of table might depend on the flows
     *      that are already in the switch; for example, if one table fills up then
     *      the switch might fall back to another one.
     *
     *    - OFPFC_MODIFY, OFPFC_DELETE: Given a specific table ID, only flows
     *      within that table are matched and modified or deleted.  If the table ID
     *      is wildcarded, flows within any table may be matched and modified or
     *      deleted.
     *
     *    - OFPFC_MODIFY_STRICT, OFPFC_DELETE_STRICT: Given a specific table ID,
     *      only a flow within that table may be matched and modified or deleted.
     *      If the table ID is wildcarded and exactly one flow within any table
     *      matches, then it is modified or deleted; if flows in more than one
     *      table match, then none is modified or deleted.
     */
    '''
    with _warnings.catch_warnings():
        _warnings.filterwarnings('ignore', '^padding', StructDefWarning)
        nx_flow_mod_table_id = nstruct(
            (uint8, 'set'),                 # /* Nonzero to enable, zero to disable. */
            (uint8[7],),
            name = 'nx_flow_mod_table_id',
            base = nicira_header,
            criteria = lambda x: getattr(x, msg_subtype) == NXT_FLOW_MOD_TABLE_ID,
            classifyby = (NXT_FLOW_MOD_TABLE_ID,),
            init = packvalue(NXT_FLOW_MOD_TABLE_ID, msg_subtype)
        )
        namespace['nx_flow_mod_table_id'] = nx_flow_mod_table_id

        '''
        /* NXT_SET_PACKET_IN_FORMAT request. */
        '''
        nx_set_packet_in_format = nstruct(
            (uint32, 'format'),           # /* One of NXPIF_*. */
            name = 'nx_set_packet_in_format',
            base = nicira_header,
            criteria = lambda x: getattr(x, msg_subtype) == NXT_SET_PACKET_IN_FORMAT,
            classifyby = (NXT_SET_PACKET_IN_FORMAT,),
            init = packvalue(NXT_SET_PACKET_IN_FORMAT, msg_subtype)
        )
        namespace['nx_set_packet_in_format'] = nx_set_packet_in_format
        '''
        /* NXT_PACKET_IN (analogous to OFPT_PACKET_IN).
         *
         * NXT_PACKET_IN is similar to the OpenFlow 1.2 OFPT_PACKET_IN.  The
         * differences are:
         *
         *     - NXT_PACKET_IN includes the cookie of the rule that triggered the
         *       message.  (OpenFlow 1.3 OFPT_PACKET_IN also includes the cookie.)
         *
         *     - The metadata fields use NXM (instead of OXM) field numbers.
         *
         * Open vSwitch 1.9.0 and later omits metadata fields that are zero (as allowed
         * by OpenFlow 1.2).  Earlier versions included all implemented metadata
         * fields.
         *
         * Open vSwitch does not include non-metadata in the nx_match, because by
         * definition that information can be found in the packet itself.  The format
         * and the standards allow this, however, so controllers should be prepared to
         * tolerate future changes.
         *
         * The NXM format is convenient for reporting metadata values, but it is
         * important not to interpret the format as matching against a flow, because it
         * does not.  Nothing is being matched; arbitrary metadata masks would not be
         * meaningful.
         *
         * Whereas in most cases a controller can expect to only get back NXM fields
         * that it set up itself (e.g. flow dumps will ordinarily report only NXM
         * fields from flows that the controller added), NXT_PACKET_IN messages might
         * contain fields that the controller does not understand, because the switch
         * might support fields (new registers, new protocols, etc.) that the
         * controller does not.  The controller must prepared to tolerate these.
         *
         * The 'cookie' field has no meaning when 'reason' is OFPR_NO_MATCH.  In this
         * case it should be UINT64_MAX. */
         '''

        if 'ofp_oxm' in namespace:
            nx_match = namespace['ofp_oxm']
            namespace['nx_match'] = nx_match
            nx_match_mask = namespace['ofp_oxm_mask']
            namespace['nx_match_mask'] = nx_match_mask
            nx_match_nomask = namespace['ofp_oxm_nomask']
            namespace['nx_match_nomask'] = nx_match_nomask
            create_nxm = namespace['create_oxm']
            namespace['create_nxm'] = create_nxm

            nx_match_nomask_ext = nstruct(
                base = nx_match_nomask,
                criteria = lambda x: NXM_VENDOR(x.header) <= 1,
                extend = {'header': nxm_header},
                name = 'nx_match_nomask_ext'
            )
            namespace['nx_match_nomask_ext'] = nx_match_nomask_ext
            nx_match_mask_ext = nstruct(
                base = nx_match_mask,
                criteria = lambda x: NXM_VENDOR(x.header) <= 1,
                extend = {'header': nxm_header},
                name = 'nx_match_mask_ext'
            )
            namespace['nx_match_mask_ext'] = nx_match_mask_ext
        else:
            nx_match = nstruct(
                (nxm_header, 'header'),
                name = 'nx_match',
                padding = 1,
                size = lambda x: NXM_LENGTH(x.header) + 4
            )
            namespace['nx_match'] = nx_match
            nx_match_nomask = nstruct(
                (raw, 'value'),
                base = nx_match,
                criteria = lambda x: not NXM_HASMASK(x.header),
                init = packvalue(NXM_OF_IN_PORT, 'header'),
                name = 'nx_match_nomask'
            )
            namespace['nx_match_nomask'] = nx_match_nomask
            _nxm_mask_value = nstruct(
                (raw, 'value'),
                name = 'nxm_mask_value',
                size = lambda x: NXM_LENGTH(x.header) // 2,
                padding = 1
            )
            nx_match_mask = nstruct(
                (_nxm_mask_value,),
                (raw, 'mask'),
                base = nx_match,
                criteria = lambda x: NXM_HASMASK(x.header),
                init = packvalue(NXM_OF_ETH_SRC_W, 'header'),
                name = 'nx_match_mask',
            )
            namespace['nx_match_mask'] = nx_match_mask
            def create_nxm(header, value = None, mask = None):
                if NXM_HASMASK(header):
                    nxm = nx_match_mask.new()
                    size = NXM_LENGTH(header) // 2
                else:
                    nxm = nx_match_nomask.new()
                    size = NXM_LENGTH(header)
                nxm.header = header
                nxm.value = common.create_binary(value, size)
                if NXM_HASMASK(header):
                    nxm.mask = common.create_binary(mask, size)
                nxm._pack()
                nxm._autosubclass()
                return nxm
            namespace['create_nxm'] = create_nxm
            nx_match_nomask_ext = nx_match_nomask
            nx_match_mask_ext = nx_match_mask
            namespace['nx_match_nomask_ext'] = nx_match_nomask_ext
            namespace['nx_match_mask_ext'] = nx_match_mask_ext
        from namedstruct.namedstruct import rawtype as _rawtype
        import socket as _socket
        if 'ip4_addr_bytes' in namespace:
            ip4_addr_bytes = namespace['ip4_addr_bytes']
        else:
            ip4_addr_bytes = prim('4s', 'ip4_addr_bytes')
            ip4_addr_bytes.formatter = lambda x: _socket.inet_ntoa(x)
            namespace['ip4_addr_bytes'] = ip4_addr_bytes

        nxm_mask_ipv4 = nstruct(name = 'nxm_mask_ipv4',
                                    base = nx_match_mask_ext,
                                    criteria = lambda x: x.header in (NXM_OF_IP_SRC_W, NXM_OF_IP_DST_W, NXM_OF_ARP_SPA_W, NXM_OF_ARP_TPA_W, NXM_NX_TUN_IPV4_SRC_W, NXM_NX_TUN_IPV4_DST_W),
                                    init = packvalue(NXM_OF_IP_SRC_W, 'header'),
                                    extend = {'value' : ip4_addr_bytes, 'mask' : ip4_addr_bytes}
                                    )
        namespace['nxm_mask_ipv4'] = nxm_mask_ipv4
        nxm_nomask_ipv4 = nstruct(name = 'nxm_nomask_ipv4',
                                    base = nx_match_nomask_ext,
                                    criteria = lambda x: x.header in (NXM_OF_IP_SRC, NXM_OF_IP_DST, NXM_OF_ARP_SPA, NXM_OF_ARP_TPA, NXM_NX_TUN_IPV4_SRC, NXM_NX_TUN_IPV4_DST),
                                    init = packvalue(NXM_OF_IP_SRC, 'header'),
                                    extend = {'value' : ip4_addr_bytes}
                                    )
        namespace['nxm_nomask_ipv4'] = nxm_nomask_ipv4
        if 'mac_addr_bytes' in namespace:
            mac_addr_bytes = namespace['mac_addr_bytes']
        else:
            mac_addr_bytes = _rawtype()
            mac_addr_bytes.formatter = lambda x: ':'.join('%02X' % (c,) for c in bytearray(x))
            namespace['mac_addr_bytes'] = mac_addr_bytes

        nxm_mask_eth = nstruct(name = 'nxm_mask_eth',
                                   base = nx_match_mask_ext,
                                   criteria = lambda x: x.header in (NXM_OF_ETH_SRC_W, NXM_OF_ETH_DST_W),
                                    init = packvalue(NXM_OF_ETH_SRC_W, 'header'),
                                   extend = {'value' : mac_addr_bytes, 'mask' : mac_addr_bytes})
        namespace['nxm_mask_eth'] = nxm_mask_eth

        nxm_nomask_eth = nstruct(name = 'nxm_nomask_eth',
                                   base = nx_match_nomask_ext,
                                   criteria = lambda x: x.header in (NXM_OF_ETH_SRC, NXM_OF_ETH_DST, NXM_NX_ND_SLL, NXM_NX_ND_TLL, NXM_NX_ARP_SHA, NXM_NX_ARP_THA),
                                    init = packvalue(NXM_OF_ETH_SRC, 'header'),
                                   extend = {'value' : mac_addr_bytes})
        namespace['nxm_nomask_eth'] = nxm_nomask_eth

        ofp_port_no = namespace['ofp_port_no']

        nx_port_no = enum('nx_port_no', None, uint16,
                           **dict((k, v & 0xffff) for k,v in ofp_port_no.getDict().items())
                           )
        nxm_port_no_raw = _rawtype()
        nxm_port_no_raw.formatter = lambda x: nx_port_no.formatter(nx_port_no.parse(x)[0])
        namespace['nx_port_no'] = nx_port_no
        namespace['nxm_port_no_raw'] = nxm_port_no_raw
        nxm_nomask_port = nstruct(name = 'nxm_nomask_port',
                                        base = nx_match_nomask_ext,
                                        criteria = lambda x: x.header == NXM_OF_IN_PORT,
                                        init = packvalue(NXM_OF_IN_PORT, 'header'),
                                        extend = {'value': nxm_port_no_raw}
                                        )
        namespace['nxm_nomask_port'] = nxm_nomask_port
        if 'ethtype_raw' in namespace:
            ethtype_raw = namespace['ethtype_raw']
        else:
            ethtype_raw = _rawtype()
            ethtype_raw.formatter = lambda x: ethertype.formatter(ethertype.parse(x)[0])
            namespace['ethtype_raw'] = ethtype_raw

        nxm_nomask_ethertype = nstruct(name = 'nxm_nomask_ethertype',
                                           base = nx_match_nomask_ext,
                                           criteria = lambda x: x.header == NXM_OF_ETH_TYPE,
                                           init = packvalue(NXM_OF_ETH_TYPE, 'header'),
                                           extend = {'value': ethtype_raw})
        namespace['nxm_nomask_ethertype'] = nxm_nomask_ethertype
        if 'arpop_raw' in namespace:
            arpop_raw = namespace['arpop_raw']
        else:
            arpop_raw = _rawtype()
            arpop_raw.formatter = lambda x: arp_op_code.formatter(arp_op_code.parse(x)[0])
            namespace['arpop_raw'] = arpop_raw

        nxm_nomask_arpopcode = nstruct(name = 'nxm_nomask_arpopcode',
                                           base = nx_match_nomask_ext,
                                           criteria = lambda x: x.header == NXM_OF_ARP_OP,
                                           init = packvalue(NXM_OF_ARP_OP, 'header'),
                                           extend = {'value': arpop_raw})
        namespace['nxm_nomask_arpopcode'] = nxm_nomask_arpopcode

        if 'ip_protocol_raw' in namespace:
            ip_protocol_raw = namespace['ip_protocol_raw']
        else:
            ip_protocol_raw = _rawtype()
            ip_protocol_raw.formatter = lambda x: ip_protocol.formatter(ip_protocol.parse(x)[0])
            namespace['ip_protocol_raw'] = ip_protocol_raw

        nxm_nomask_ip_protocol = nstruct(name = 'nxm_nomask_ip_protocol',
                                             base = nx_match_nomask_ext,
                                             criteria = lambda x: x.header == NXM_OF_IP_PROTO,
                                             init = packvalue(NXM_OF_IP_PROTO, 'header'),
                                             extend = {'value': ip_protocol_raw})
        namespace['nxm_nomask_ip_protocol'] = nxm_nomask_ip_protocol
        if 'ip6_addr_bytes' in namespace:
            nxm_nomask_ipv6 = nstruct(name = 'nxm_nomask_ipv6',
                                          base = nx_match_nomask_ext,
                                          criteria = lambda x: x.header in (NXM_NX_IPV6_SRC, NXM_NX_IPV6_DST, NXM_NX_ND_TARGET),
                                          init = packvalue(NXM_NX_IPV6_SRC, 'header'),
                                          extend = {'value': ip6_addr_bytes})
            namespace['nxm_nomask_ipv6'] = nxm_nomask_ipv6
            nxm_mask_ipv6 = nstruct(name = 'nxm_mask_ipv6',
                                          base = nx_match_mask_ext,
                                          criteria = lambda x: x.header in (NXM_NX_IPV6_SRC_W, NXM_NX_IPV6_DST_W),
                                          init = packvalue(NXM_NX_IPV6_SRC_W, 'header'),
                                          extend = {'value': ip6_addr_bytes, 'mask': ip6_addr_bytes})
            namespace['nxm_mask_ipv6'] = nxm_mask_ipv6

        nx_ip_frag_raw = _rawtype()
        nx_ip_frag_raw.formatter = lambda x: nx_ip_frag.formatter(nx_ip_frag.parse(x)[0])
        nxm_nomask_ipfrag = nstruct(name = 'nxm_nomask_ipfrag',
                                    base = nx_match_nomask_ext,
                                    criteria = lambda x: x.header == NXM_NX_IP_FRAG,
                                    init = packvalue(NXM_NX_IP_FRAG, 'header'),
                                    extend = {'value': nx_ip_frag_raw})
        namespace['nxm_nomask_ipfrag'] = nxm_nomask_ipfrag
        nxm_mask_ipfrag = nstruct(name = 'nxm_mask_ipfrag',
                                    base = nx_match_mask_ext,
                                    criteria = lambda x: x.header == NXM_NX_IP_FRAG_W,
                                    init = packvalue(NXM_NX_IP_FRAG_W, 'header'),
                                    extend = {'value': nx_ip_frag_raw, 'mask': nx_ip_frag_raw})
        namespace['nxm_mask_ipfrag'] = nxm_mask_ipfrag

        nx_matches = nstruct(
                (nx_match[0], 'matches'),
                name = 'nx_matches',
                size = sizefromlen(65536, 'match_len'),
                prepack = packrealsize('match_len'),
                padding = 8
        )

        namespace['nx_matches'] = nx_matches

        nx_packet_in = nstruct(
            (uint32, 'buffer_id'),     #  /* ID assigned by datapath. */
            (uint16, 'total_len'),     #  /* Full length of frame. */
            (uint8, 'reason'),         #  /* Reason packet is sent (one of OFPR_*). */
            (uint8, 'table_id'),       #  /* ID of the table that was looked up. */
            (uint64, 'cookie'),        #  /* Cookie of the rule that was looked up. */
            (uint16, 'match_len'),     # /* Size of nx_match. */
            (uint8[6],),               # /* Align to 64-bits. */
            (nx_matches,),
            (uint8[2],),
            (raw, 'data'),
            name = 'nx_packet_in',
            base = nicira_header,
            classifyby = (NXT_PACKET_IN,),
            criteria = lambda x: getattr(x, msg_subtype) == NXT_PACKET_IN,
            init = packvalue(NXT_PACKET_IN, msg_subtype)
        )
        namespace['nx_packet_in'] = nx_packet_in
        '''
        /* Configures the "role" of the sending controller.  The default role is:
         *
         *    - Other (NX_ROLE_OTHER), which allows the controller access to all
         *      OpenFlow features.
         *
         * The other possible roles are a related pair:
         *
         *    - Master (NX_ROLE_MASTER) is equivalent to Other, except that there may
         *      be at most one Master controller at a time: when a controller
         *      configures itself as Master, any existing Master is demoted to the
         *      Slave role.
         *
         *    - Slave (NX_ROLE_SLAVE) allows the controller read-only access to
         *      OpenFlow features.  In particular attempts to modify the flow table
         *      will be rejected with an OFPBRC_EPERM error.
         *
         *      Slave controllers do not receive OFPT_PACKET_IN or OFPT_FLOW_REMOVED
         *      messages, but they do receive OFPT_PORT_STATUS messages.
         */
         '''
        nx_role_request = nstruct(
            (nx_role, 'role'),       # /* One of NX_ROLE_*. */
            name = 'nx_role_request',
            base = nicira_header,
            classifyby = (NXT_ROLE_REQUEST, NXT_ROLE_REPLY),
            criteria = lambda x: getattr(x, msg_subtype) == NXT_ROLE_REQUEST or getattr(x, msg_subtype) == NXT_ROLE_REPLY,
            init = packvalue(NXT_ROLE_REQUEST, msg_subtype)
        )
        namespace['nx_role_request'] = nx_role_request
        '''
        /* NXT_SET_ASYNC_CONFIG.
         *
         * Sent by a controller, this message configures the asynchronous messages that
         * the controller wants to receive.  Element 0 in each array specifies messages
         * of interest when the controller has an "other" or "master" role; element 1,
         * when the controller has a "slave" role.
         *
         * Each array element is a bitmask in which a 0-bit disables receiving a
         * particular message and a 1-bit enables receiving it.  Each bit controls the
         * message whose 'reason' corresponds to the bit index.  For example, the bit
         * with value 1<<2 == 4 in port_status_mask[1] determines whether the
         * controller will receive OFPT_PORT_STATUS messages with reason OFPPR_MODIFY
         * (value 2) when the controller has a "slave" role.
         *
         * As a side effect, for service controllers, this message changes the
         * miss_send_len from default of zero to OFP_DEFAULT_MISS_SEND_LEN (128).
         */
         '''
        ofp_packet_in_reason = namespace['ofp_packet_in_reason']
        if 'ofp_packet_in_reason_bitwise' in namespace:
            ofp_packet_in_reason_bitwise = namespace['ofp_packet_in_reason_bitwise']
        else:
            ofp_packet_in_reason_bitwise = enum('ofp_packet_in_reason_bitwise', None, uint32,
                                            **dict((k, 1<<v) for k,v in ofp_packet_in_reason.getDict().items()))
            namespace['ofp_packet_in_reason_bitwise'] = ofp_packet_in_reason_bitwise

        ofp_port_reason = namespace['ofp_port_reason']
        if 'ofp_port_reason_bitwise' in namespace:
            ofp_port_reason_bitwise = namespace['ofp_port_reason_bitwise']
        else:
            ofp_port_reason_bitwise = enum('ofp_port_reason_bitwise', None, uint32,
                                            **dict((k, 1<<v) for k,v in ofp_port_reason.getDict().items()))
            namespace['ofp_port_reason_bitwise'] = ofp_port_reason_bitwise

        ofp_flow_removed_reason = namespace['ofp_flow_removed_reason']
        if 'ofp_flow_removed_reason_bitwise' in namespace:
            ofp_flow_removed_reason_bitwise = namespace['ofp_flow_removed_reason_bitwise']
        else:
            ofp_flow_removed_reason_bitwise = enum('ofp_flow_removed_reason_bitwise', None, uint32,
                                            **dict((k, 1<<v) for k,v in ofp_flow_removed_reason.getDict().items()))
            namespace['ofp_flow_removed_reason_bitwise'] = ofp_flow_removed_reason_bitwise

        nx_async_config = nstruct(
            (ofp_packet_in_reason_bitwise[2], 'packet_in_mask'),  #     /* Bitmasks of OFPR_* values. */
            (ofp_port_reason_bitwise[2], 'port_status_mask'),     #     /* Bitmasks of OFPRR_* values. */
            (ofp_flow_removed_reason_bitwise[2], 'flow_removed_mask'), #/* Bitmasks of OFPPR_* values. */
            name = 'nx_async_config',
            base = nicira_header,
            classifyby = (NXT_SET_ASYNC_CONFIG,),
            criteria = lambda x: getattr(x, msg_subtype) == NXT_SET_ASYNC_CONFIG,
            init = packvalue(NXT_SET_ASYNC_CONFIG, msg_subtype)
        )
        namespace['nx_async_config'] = nx_async_config
        '''
        /* Nicira vendor flow actions. */
        '''
        '''
        /* Action structures for NXAST_RESUBMIT and NXAST_RESUBMIT_TABLE.
         *
         * These actions search one of the switch's flow tables:
         *
         *    - For NXAST_RESUBMIT_TABLE only, if the 'table' member is not 255, then
         *      it specifies the table to search.
         *
         *    - Otherwise (for NXAST_RESUBMIT_TABLE with a 'table' of 255, or for
         *      NXAST_RESUBMIT regardless of 'table'), it searches the current flow
         *      table, that is, the OpenFlow flow table that contains the flow from
         *      which this action was obtained.  If this action did not come from a
         *      flow table (e.g. it came from an OFPT_PACKET_OUT message), then table 0
         *      is the current table.
         *
         * The flow table lookup uses a flow that may be slightly modified from the
         * original lookup:
         *
         *    - For NXAST_RESUBMIT, the 'in_port' member of struct nx_action_resubmit
         *      is used as the flow's in_port.
         *
         *    - For NXAST_RESUBMIT_TABLE, if the 'in_port' member is not OFPP_IN_PORT,
         *      then its value is used as the flow's in_port.  Otherwise, the original
         *      in_port is used.
         *
         *    - If actions that modify the flow (e.g. OFPAT_SET_VLAN_VID) precede the
         *      resubmit action, then the flow is updated with the new values.
         *
         * Following the lookup, the original in_port is restored.
         *
         * If the modified flow matched in the flow table, then the corresponding
         * actions are executed.  Afterward, actions following the resubmit in the
         * original set of actions, if any, are executed; any changes made to the
         * packet (e.g. changes to VLAN) by secondary actions persist when those
         * actions are executed, although the original in_port is restored.
         *
         * Resubmit actions may be used any number of times within a set of actions.
         *
         * Resubmit actions may nest to an implementation-defined depth.  Beyond this
         * implementation-defined depth, further resubmit actions are simply ignored.
         *
         * NXAST_RESUBMIT ignores 'table' and 'pad'.  NXAST_RESUBMIT_TABLE requires
         * 'pad' to be all-bits-zero.
         *
         * Open vSwitch 1.0.1 and earlier did not support recursion.  Open vSwitch
         * before 1.2.90 did not support NXAST_RESUBMIT_TABLE.
         */
        '''
        nx_action_resubmit = nstruct(
            (nx_port_no, 'in_port'),        # /* New in_port for checking flow table. */
            (uint8, 'table'),               # /* NXAST_RESUBMIT_TABLE: table to use. */
            (uint8[3],),
            base = nx_action,
            criteria = lambda x: getattr(x, action_subtype) == NXAST_RESUBMIT_TABLE or getattr(x, action_subtype) == NXAST_RESUBMIT,
            classifyby = (NXAST_RESUBMIT_TABLE, NXAST_RESUBMIT),
            name = 'nx_action_resubmit',
            init = packvalue(NXAST_RESUBMIT_TABLE, action_subtype)
        )
        namespace['nx_action_resubmit'] = nx_action_resubmit
        '''
        /* Action structure for NXAST_SET_TUNNEL.
         *
         * Sets the encapsulating tunnel ID to a 32-bit value.  The most-significant 32
         * bits of the tunnel ID are set to 0. */
        '''
        nx_action_set_tunnel = nstruct(
            (uint8[2],),
            (uint32, 'tun_id'),         #         /* Tunnel ID. */
            name = 'nx_action_set_tunnel',
            base = nx_action,
            classifyby = (NXAST_SET_TUNNEL,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_TUNNEL,
            init = packvalue(NXAST_SET_TUNNEL, action_subtype)
        )
        namespace['nx_action_set_tunnel'] = nx_action_set_tunnel
        '''
        /* Action structure for NXAST_SET_TUNNEL64.
         *
         * Sets the encapsulating tunnel ID to a 64-bit value. */
        '''
        nx_action_set_tunnel64 = nstruct(
            (uint8[6],),
            (uint64, 'tun_id'),          #       /* Tunnel ID. */
            name = 'nx_action_set_tunnel64',
            base = nx_action,
            classifyby = (NXAST_SET_TUNNEL64,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_TUNNEL64,
            init = packvalue(NXAST_SET_TUNNEL64, action_subtype)
        )
        namespace['nx_action_set_tunnel64'] = nx_action_set_tunnel64
        '''
        /* Action structure for NXAST_SET_QUEUE.
         *
         * Set the queue that should be used when packets are output.  This is similar
         * to the OpenFlow OFPAT_ENQUEUE action, but does not take the output port as
         * an argument.  This allows the queue to be defined before the port is
         * known. */
        '''
        nx_action_set_queue = nstruct(
            (uint8[2],),
            (uint32, 'queue_id'),             #    /* Where to enqueue packets. */
            name = 'nx_action_set_queue',
            base = nx_action,
            classifyby = (NXAST_SET_QUEUE,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_QUEUE,
            init = packvalue(NXAST_SET_QUEUE, action_subtype)
        )
        namespace['nx_action_set_queue'] = nx_action_set_queue

        '''
        /* Action structure for NXAST_POP_QUEUE.
         *
         * Restores the queue to the value it was before any NXAST_SET_QUEUE actions
         * were used.  Only the original queue can be restored this way; no stack is
         * maintained. */
        '''
        nx_action_pop_queue = nstruct(
            (uint8[6],),
            name = 'nx_action_pop_queue',
            base = nx_action,
            classifyby = (NXAST_POP_QUEUE,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_POP_QUEUE,
            init = packvalue(NXAST_POP_QUEUE, action_subtype)
        )
        namespace['nx_action_pop_queue'] = nx_action_pop_queue

        '''
        /* Action structure for NXAST_REG_MOVE.
         *
         * Copies src[src_ofs:src_ofs+n_bits] to dst[dst_ofs:dst_ofs+n_bits], where
         * a[b:c] denotes the bits within 'a' numbered 'b' through 'c' (not including
         * bit 'c').  Bit numbering starts at 0 for the least-significant bit, 1 for
         * the next most significant bit, and so on.
         *
         * 'src' and 'dst' are nxm_header values with nxm_hasmask=0.  (It doesn't make
         * sense to use nxm_hasmask=1 because the action does not do any kind of
         * matching; it uses the actual value of a field.)
         *
         * The following nxm_header values are potentially acceptable as 'src':
         *
         *   - NXM_OF_IN_PORT
         *   - NXM_OF_ETH_DST
         *   - NXM_OF_ETH_SRC
         *   - NXM_OF_ETH_TYPE
         *   - NXM_OF_VLAN_TCI
         *   - NXM_OF_IP_TOS
         *   - NXM_OF_IP_PROTO
         *   - NXM_OF_IP_SRC
         *   - NXM_OF_IP_DST
         *   - NXM_OF_TCP_SRC
         *   - NXM_OF_TCP_DST
         *   - NXM_OF_UDP_SRC
         *   - NXM_OF_UDP_DST
         *   - NXM_OF_ICMP_TYPE
         *   - NXM_OF_ICMP_CODE
         *   - NXM_OF_ARP_OP
         *   - NXM_OF_ARP_SPA
         *   - NXM_OF_ARP_TPA
         *   - NXM_NX_TUN_ID
         *   - NXM_NX_ARP_SHA
         *   - NXM_NX_ARP_THA
         *   - NXM_NX_ICMPV6_TYPE
         *   - NXM_NX_ICMPV6_CODE
         *   - NXM_NX_ND_SLL
         *   - NXM_NX_ND_TLL
         *   - NXM_NX_REG(idx) for idx in the switch's accepted range.
         *   - NXM_NX_PKT_MARK
         *   - NXM_NX_TUN_IPV4_SRC
         *   - NXM_NX_TUN_IPV4_DST
         *
         * The following nxm_header values are potentially acceptable as 'dst':
         *
         *   - NXM_OF_ETH_DST
         *   - NXM_OF_ETH_SRC
         *   - NXM_OF_IP_TOS
         *   - NXM_OF_IP_SRC
         *   - NXM_OF_IP_DST
         *   - NXM_OF_TCP_SRC
         *   - NXM_OF_TCP_DST
         *   - NXM_OF_UDP_SRC
         *   - NXM_OF_UDP_DST
         *   - NXM_NX_ARP_SHA
         *   - NXM_NX_ARP_THA
         *   - NXM_OF_ARP_OP
         *   - NXM_OF_ARP_SPA
         *   - NXM_OF_ARP_TPA
         *     Modifying any of the above fields changes the corresponding packet
         *     header.
         *
         *   - NXM_OF_IN_PORT
         *
         *   - NXM_NX_REG(idx) for idx in the switch's accepted range.
         *
         *   - NXM_NX_PKT_MARK
         *
         *   - NXM_OF_VLAN_TCI.  Modifying this field's value has side effects on the
         *     packet's 802.1Q header.  Setting a value with CFI=0 removes the 802.1Q
         *     header (if any), ignoring the other bits.  Setting a value with CFI=1
         *     adds or modifies the 802.1Q header appropriately, setting the TCI field
         *     to the field's new value (with the CFI bit masked out).
         *
         *   - NXM_NX_TUN_ID, NXM_NX_TUN_IPV4_SRC, NXM_NX_TUN_IPV4_DST.  Modifying
         *     any of these values modifies the corresponding tunnel header field used
         *     for the packet's next tunnel encapsulation, if allowed by the
         *     configuration of the output tunnel port.
         *
         * A given nxm_header value may be used as 'src' or 'dst' only on a flow whose
         * nx_match satisfies its prerequisites.  For example, NXM_OF_IP_TOS may be
         * used only if the flow's nx_match includes an nxm_entry that specifies
         * nxm_type=NXM_OF_ETH_TYPE, nxm_hasmask=0, and nxm_value=0x0800.
         *
         * The switch will reject actions for which src_ofs+n_bits is greater than the
         * width of 'src' or dst_ofs+n_bits is greater than the width of 'dst' with
         * error type OFPET_BAD_ACTION, code OFPBAC_BAD_ARGUMENT.
         *
         * This action behaves properly when 'src' overlaps with 'dst', that is, it
         * behaves as if 'src' were copied out to a temporary buffer, then the
         * temporary buffer copied to 'dst'.
         */
         '''
        nx_action_reg_move = nstruct(
            (uint16, 'n_bits'),         #       /* Number of bits. */
            (uint16, 'src_ofs'),        #       /* Starting bit offset in source. */
            (uint16, 'dst_ofs'),        #       /* Starting bit offset in destination. */
            (nxm_header, 'src'),        #       /* Source register. */
            (nxm_header, 'dst'),        #       /* Destination register. */
            name = 'nx_action_reg_move',
            base = nx_action,
            classifyby = (NXAST_REG_MOVE,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_REG_MOVE,
            init = packvalue(NXAST_REG_MOVE, action_subtype),
            formatter = _createdesc(lambda x:'move:%s[%d..%d]->%s[%d..%d]' % (x['src'], x['src_ofs'], x['src_ofs'] + x['n_bits'] - 1, x['dst'], x['dst_ofs'], x['dst_ofs'] + x['n_bits'] - 1))
        )
        namespace['nx_action_reg_move'] = nx_action_reg_move
        '''
        /* Action structure for NXAST_REG_LOAD.
         *
         * Copies value[0:n_bits] to dst[ofs:ofs+n_bits], where a[b:c] denotes the bits
         * within 'a' numbered 'b' through 'c' (not including bit 'c').  Bit numbering
         * starts at 0 for the least-significant bit, 1 for the next most significant
         * bit, and so on.
         *
         * 'dst' is an nxm_header with nxm_hasmask=0.  See the documentation for
         * NXAST_REG_MOVE, above, for the permitted fields and for the side effects of
         * loading them.
         *
         * The 'ofs' and 'n_bits' fields are combined into a single 'ofs_nbits' field
         * to avoid enlarging the structure by another 8 bytes.  To allow 'n_bits' to
         * take a value between 1 and 64 (inclusive) while taking up only 6 bits, it is
         * also stored as one less than its true value:
         *
         *  15                           6 5                0
         * +------------------------------+------------------+
         * |              ofs             |    n_bits - 1    |
         * +------------------------------+------------------+
         *
         * The switch will reject actions for which ofs+n_bits is greater than the
         * width of 'dst', or in which any bits in 'value' with value 2**n_bits or
         * greater are set to 1, with error type OFPET_BAD_ACTION, code
         * OFPBAC_BAD_ARGUMENT.
         */
        '''
        nx_action_reg_load = nstruct(
            (uint16, 'ofs_nbits'),           #  /* (ofs << 6) | (n_bits - 1). */
            (nxm_header, 'dst'),             #  /* Destination register. */
            (uint64, 'value'),               #  /* Immediate value. */
            name = 'nx_action_reg_load',
            base = nx_action,
            classifyby = (NXAST_REG_LOAD,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_REG_LOAD,
            init = packvalue(NXAST_REG_LOAD, action_subtype),
            formatter = _createdesc(lambda x: 'load:0x%x->%s[%d..%d]' % (x['value'], x['dst'], x['ofs_nbits'] >> 6, (x['ofs_nbits'] >> 6) + (x['ofs_nbits'] & 0x3f)))
        )
        namespace['nx_action_reg_load'] = nx_action_reg_load

        '''
        /* Action structure for NXAST_STACK_PUSH and NXAST_STACK_POP.
         *
         * Pushes (or pops) field[offset: offset + n_bits] to (or from)
         * top of the stack.
         */
        '''
        nx_action_stack = nstruct(
            (uint16, 'offset'),          #      /* Bit offset into the field. */
            (nxm_header, 'field'),       #      /* The field used for push or pop. */
            (uint16, 'n_bits'),          #      /* (n_bits + 1) bits of the field. */
            (uint8[6],),                 #      /* Reserved, must be zero. */
            name = 'nx_action_stack',
            base = nx_action,
            classifyby = (NXAST_STACK_PUSH, NXAST_STACK_POP),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_STACK_PUSH or getattr(x, action_subtype) == NXAST_STACK_POP,
            init = packvalue(NXAST_STACK_PUSH, action_subtype),
            formatter = _createdesc(lambda x: '%s:%s[%d..%d]' % ('push' if x[action_subtype] == 'NXAST_STACK_PUSH' else 'pop', x['field'], x['offset'], (x['offset'] + x['n_bits'] - 1)))
        )
        namespace['nx_action_stack'] = nx_action_stack

        '''
        /* Action structure for NXAST_NOTE.
         *
         * This action has no effect.  It is variable length.  The switch does not
         * attempt to interpret the user-defined 'note' data in any way.  A controller
         * can use this action to attach arbitrary metadata to a flow.
         *
         * This action might go away in the future.
         */
        '''
        nx_action_note = nstruct(
            (varchr, 'note'),
            name = 'nx_action_note',
            base = nx_action,
            classifyby = (NXAST_NOTE,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_NOTE,
            init = packvalue(NXAST_NOTE, action_subtype)
        )
        namespace['nx_action_note'] = nx_action_note

        '''
        /* Action structure for NXAST_MULTIPATH.
         *
         * This action performs the following steps in sequence:
         *
         *    1. Hashes the fields designated by 'fields', one of NX_HASH_FIELDS_*.
         *       Refer to the definition of "enum nx_mp_fields" for details.
         *
         *       The 'basis' value is used as a universal hash parameter, that is,
         *       different values of 'basis' yield different hash functions.  The
         *       particular universal hash function used is implementation-defined.
         *
         *       The hashed fields' values are drawn from the current state of the
         *       flow, including all modifications that have been made by actions up to
         *       this point.
         *
         *    2. Applies the multipath link choice algorithm specified by 'algorithm',
         *       one of NX_MP_ALG_*.  Refer to the definition of "enum nx_mp_algorithm"
         *       for details.
         *
         *       The output of the algorithm is 'link', an unsigned integer less than
         *       or equal to 'max_link'.
         *
         *       Some algorithms use 'arg' as an additional argument.
         *
         *    3. Stores 'link' in dst[ofs:ofs+n_bits].  The format and semantics of
         *       'dst' and 'ofs_nbits' are similar to those for the NXAST_REG_LOAD
         *       action.
         *
         * The switch will reject actions that have an unknown 'fields', or an unknown
         * 'algorithm', or in which ofs+n_bits is greater than the width of 'dst', or
         * in which 'max_link' is greater than or equal to 2**n_bits, with error type
         * OFPET_BAD_ACTION, code OFPBAC_BAD_ARGUMENT.
         */
        '''
        nx_action_multipath = nstruct(

            #/* What fields to hash and how. */
            (nx_hash_fields, 'fields'),       #     /* One of NX_HASH_FIELDS_*. */
            (uint16, 'basis'),                #     /* Universal hash parameter. */
            (uint16,),

            #/* Multipath link choice algorithm to apply to hash value. */
            (nx_mp_algorithm, 'algorithm'),   #     /* One of NX_MP_ALG_*. */
            (uint16, 'max_link'),             #     /* Number of output links, minus 1. */
            (uint32, 'arg'),                  #     /* Algorithm-specific argument. */
            (uint16,),

            # /* Where to store the result. */
            (uint16, 'ofs_nbits'),            #     /* (ofs << 6) | (n_bits - 1). */
            (nxm_header, 'dst'),              #     /* Destination. */
            name = 'nx_action_multipath',
            base = nx_action,
            classifyby = (NXAST_MULTIPATH,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_MULTIPATH,
            init = packvalue(NXAST_MULTIPATH, action_subtype),
            formatter = _createdesc(lambda x: 'multipath(%s,%d,%s,%d,%d,%s[%d..%d])' % (x['fields'], x['basis'], x['algorithm'],x['max_link'] + 1, x['arg'], x['dst'], x['ofs_nbits'] >> 6, (x['ofs_nbits'] >> 6) + (x['ofs_nbits'] & 0x3f)))
        )
        namespace['nx_action_multipath'] = nx_action_multipath

        '''
        /* Action structure for NXAST_LEARN.
         *
         * This action adds or modifies a flow in an OpenFlow table, similar to
         * OFPT_FLOW_MOD with OFPFC_MODIFY_STRICT as 'command'.  The new flow has the
         * specified idle timeout, hard timeout, priority, cookie, and flags.  The new
         * flow's match criteria and actions are built by applying each of the series
         * of flow_mod_spec elements included as part of the action.
         *
         * A flow_mod_spec starts with a 16-bit header.  A header that is all-bits-0 is
         * a no-op used for padding the action as a whole to a multiple of 8 bytes in
         * length.  Otherwise, the flow_mod_spec can be thought of as copying 'n_bits'
         * bits from a source to a destination.  In this case, the header contains
         * multiple fields:
         *
         *  15  14  13 12  11 10                              0
         * +------+---+------+---------------------------------+
         * |   0  |src|  dst |             n_bits              |
         * +------+---+------+---------------------------------+
         *
         * The meaning and format of a flow_mod_spec depends on 'src' and 'dst'.  The
         * following table summarizes the meaning of each possible combination.
         * Details follow the table:
         *
         *   src dst  meaning
         *   --- ---  ----------------------------------------------------------
         *    0   0   Add match criteria based on value in a field.
         *    1   0   Add match criteria based on an immediate value.
         *    0   1   Add NXAST_REG_LOAD action to copy field into a different field.
         *    1   1   Add NXAST_REG_LOAD action to load immediate value into a field.
         *    0   2   Add OFPAT_OUTPUT action to output to port from specified field.
         *   All other combinations are undefined and not allowed.
         *
         * The flow_mod_spec header is followed by a source specification and a
         * destination specification.  The format and meaning of the source
         * specification depends on 'src':
         *
         *   - If 'src' is 0, the source bits are taken from a field in the flow to
         *     which this action is attached.  (This should be a wildcarded field.  If
         *     its value is fully specified then the source bits being copied have
         *     constant values.)
         *
         *     The source specification is an ovs_be32 'field' and an ovs_be16 'ofs'.
         *     'field' is an nxm_header with nxm_hasmask=0, and 'ofs' the starting bit
         *     offset within that field.  The source bits are field[ofs:ofs+n_bits-1].
         *     'field' and 'ofs' are subject to the same restrictions as the source
         *     field in NXAST_REG_MOVE.
         *
         *   - If 'src' is 1, the source bits are a constant value.  The source
         *     specification is (n_bits+15)/16*2 bytes long.  Taking those bytes as a
         *     number in network order, the source bits are the 'n_bits'
         *     least-significant bits.  The switch will report an error if other bits
         *     in the constant are nonzero.
         *
         * The flow_mod_spec destination specification, for 'dst' of 0 or 1, is an
         * ovs_be32 'field' and an ovs_be16 'ofs'.  'field' is an nxm_header with
         * nxm_hasmask=0 and 'ofs' is a starting bit offset within that field.  The
         * meaning of the flow_mod_spec depends on 'dst':
         *
         *   - If 'dst' is 0, the flow_mod_spec specifies match criteria for the new
         *     flow.  The new flow matches only if bits field[ofs:ofs+n_bits-1] in a
         *     packet equal the source bits.  'field' may be any nxm_header with
         *     nxm_hasmask=0 that is allowed in NXT_FLOW_MOD.
         *
         *     Order is significant.  Earlier flow_mod_specs must satisfy any
         *     prerequisites for matching fields specified later, by copying constant
         *     values into prerequisite fields.
         *
         *     The switch will reject flow_mod_specs that do not satisfy NXM masking
         *     restrictions.
         *
         *   - If 'dst' is 1, the flow_mod_spec specifies an NXAST_REG_LOAD action for
         *     the new flow.  The new flow copies the source bits into
         *     field[ofs:ofs+n_bits-1].  Actions are executed in the same order as the
         *     flow_mod_specs.
         *
         *     A single NXAST_REG_LOAD action writes no more than 64 bits, so n_bits
         *     greater than 64 yields multiple NXAST_REG_LOAD actions.
         *
         * The flow_mod_spec destination spec for 'dst' of 2 (when 'src' is 0) is
         * empty.  It has the following meaning:
         *
         *   - The flow_mod_spec specifies an OFPAT_OUTPUT action for the new flow.
         *     The new flow outputs to the OpenFlow port specified by the source field.
         *     Of the special output ports with value OFPP_MAX or larger, OFPP_IN_PORT,
         *     OFPP_FLOOD, OFPP_LOCAL, and OFPP_ALL are supported.  Other special ports
         *     may not be used.
         *
         * Resource Management
         * -------------------
         *
         * A switch has a finite amount of flow table space available for learning.
         * When this space is exhausted, no new learning table entries will be learned
         * until some existing flow table entries expire.  The controller should be
         * prepared to handle this by flooding (which can be implemented as a
         * low-priority flow).
         *
         * If a learned flow matches a single TCP stream with a relatively long
         * timeout, one may make the best of resource constraints by setting
         * 'fin_idle_timeout' or 'fin_hard_timeout' (both measured in seconds), or
         * both, to shorter timeouts.  When either of these is specified as a nonzero
         * value, OVS adds a NXAST_FIN_TIMEOUT action, with the specified timeouts, to
         * the learned flow.
         *
         * Examples
         * --------
         *
         * The following examples give a prose description of the flow_mod_specs along
         * with informal notation for how those would be represented and a hex dump of
         * the bytes that would be required.
         *
         * These examples could work with various nx_action_learn parameters.  Typical
         * values would be idle_timeout=OFP_FLOW_PERMANENT, hard_timeout=60,
         * priority=OFP_DEFAULT_PRIORITY, flags=0, table_id=10.
         *
         * 1. Learn input port based on the source MAC, with lookup into
         *    NXM_NX_REG1[16:31] by resubmit to in_port=99:
         *
         *    Match on in_port=99:
         *       ovs_be16(src=1, dst=0, n_bits=16),               20 10
         *       ovs_be16(99),                                    00 63
         *       ovs_be32(NXM_OF_IN_PORT), ovs_be16(0)            00 00 00 02 00 00
         *
         *    Match Ethernet destination on Ethernet source from packet:
         *       ovs_be16(src=0, dst=0, n_bits=48),               00 30
         *       ovs_be32(NXM_OF_ETH_SRC), ovs_be16(0)            00 00 04 06 00 00
         *       ovs_be32(NXM_OF_ETH_DST), ovs_be16(0)            00 00 02 06 00 00
         *
         *    Set NXM_NX_REG1[16:31] to the packet's input port:
         *       ovs_be16(src=0, dst=1, n_bits=16),               08 10
         *       ovs_be32(NXM_OF_IN_PORT), ovs_be16(0)            00 00 00 02 00 00
         *       ovs_be32(NXM_NX_REG1), ovs_be16(16)              00 01 02 04 00 10
         *
         *    Given a packet that arrived on port A with Ethernet source address B,
         *    this would set up the flow "in_port=99, dl_dst=B,
         *    actions=load:A->NXM_NX_REG1[16..31]".
         *
         *    In syntax accepted by ovs-ofctl, this action is: learn(in_port=99,
         *    NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],
         *    load:NXM_OF_IN_PORT[]->NXM_NX_REG1[16..31])
         *
         * 2. Output to input port based on the source MAC and VLAN VID, with lookup
         *    into NXM_NX_REG1[16:31]:
         *
         *    Match on same VLAN ID as packet:
         *       ovs_be16(src=0, dst=0, n_bits=12),               00 0c
         *       ovs_be32(NXM_OF_VLAN_TCI), ovs_be16(0)           00 00 08 02 00 00
         *       ovs_be32(NXM_OF_VLAN_TCI), ovs_be16(0)           00 00 08 02 00 00
         *
         *    Match Ethernet destination on Ethernet source from packet:
         *       ovs_be16(src=0, dst=0, n_bits=48),               00 30
         *       ovs_be32(NXM_OF_ETH_SRC), ovs_be16(0)            00 00 04 06 00 00
         *       ovs_be32(NXM_OF_ETH_DST), ovs_be16(0)            00 00 02 06 00 00
         *
         *    Output to the packet's input port:
         *       ovs_be16(src=0, dst=2, n_bits=16),               10 10
         *       ovs_be32(NXM_OF_IN_PORT), ovs_be16(0)            00 00 00 02 00 00
         *
         *    Given a packet that arrived on port A with Ethernet source address B in
         *    VLAN C, this would set up the flow "dl_dst=B, vlan_vid=C,
         *    actions=output:A".
         *
         *    In syntax accepted by ovs-ofctl, this action is:
         *    learn(NXM_OF_VLAN_TCI[0..11], NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],
         *    output:NXM_OF_IN_PORT[])
         *
         * 3. Here's a recipe for a very simple-minded MAC learning switch.  It uses a
         *    10-second MAC expiration time to make it easier to see what's going on
         *
         *      ovs-vsctl del-controller br0
         *      ovs-ofctl del-flows br0
         *      ovs-ofctl add-flow br0 "table=0 actions=learn(table=1, \
                  hard_timeout=10, NXM_OF_VLAN_TCI[0..11],             \
                  NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],                   \
                  output:NXM_OF_IN_PORT[]), resubmit(,1)"
         *      ovs-ofctl add-flow br0 "table=1 priority=0 actions=flood"
         *
         *    You can then dump the MAC learning table with:
         *
         *      ovs-ofctl dump-flows br0 table=1
         *
         * Usage Advice
         * ------------
         *
         * For best performance, segregate learned flows into a table that is not used
         * for any other flows except possibly for a lowest-priority "catch-all" flow
         * (a flow with no match criteria).  If different learning actions specify
         * different match criteria, use different tables for the learned flows.
         *
         * The meaning of 'hard_timeout' and 'idle_timeout' can be counterintuitive.
         * These timeouts apply to the flow that is added, which means that a flow with
         * an idle timeout will expire when no traffic has been sent *to* the learned
         * address.  This is not usually the intent in MAC learning; instead, we want
         * the MAC learn entry to expire when no traffic has been sent *from* the
         * learned address.  Use a hard timeout for that.
         */
         '''
        def _nx_flow_mod_spec_formatter(x):
            if NX_FLOWMODSPEC_SRC(x['header']):
                srcdesc = '0x' + ''.join('%02x' % (c,) for c in bytearray(x['value']))
            else:
                srcdesc = '%s[%d..%d]' % (x['src'], x['src_ofs'], x['src_ofs'] + NX_FLOWMODSPEC_NBITS(x['header']) - 1)
            dstv = NX_FLOWMODSPEC_DST(x['header'])
            if dstv != NX_LEARN_DST_OUTPUT:
                dstdesc = '%s[%d..%d]' % (x['dst'], x['dst_ofs'], x['dst_ofs'] + NX_FLOWMODSPEC_NBITS(x['header']) - 1)
            if dstv == NX_LEARN_DST_MATCH:
                x['_desc'] = '%s=%s' % (dstdesc, srcdesc)
            elif dstv == NX_LEARN_DST_LOAD:
                x['_desc'] = 'load:%s->%s' % (srcdesc, dstdesc)
            elif NX_FLOWMODSPEC_SRC(x['header']):
                x['_desc'] = 'output:%s' % nxm_port_no_raw.formatter(common.create_binary(x['value'], 2))
            else:
                x['_desc'] = 'output:%s' % (srcdesc,)
            x['header'] = nx_flow_mod_spec_header.formatter(x['header'])
            return x

        nx_flow_mod_spec = nstruct(
            (uint16, 'header'),
            (_nx_flow_mod_spec_src,),
            (_nx_flow_mod_spec_dst,),
            name = 'nx_flow_mod_spec',
            padding = 1,
            formatter = _nx_flow_mod_spec_formatter,
            lastextra = False
            # if x.header == 0, size is 14, the padding should not be so large so it will not be successfully parsed
        )
        namespace['nx_flow_mod_spec'] = nx_flow_mod_spec
        def create_nxfms_matchfield(src, dst, src_ofs = 0, dst_ofs = 0, n_bits = None):
            if n_bits is None:
                n_bits = min(NXM_LENGTH(dst) * 8 - dst_ofs, NXM_LENGTH(src) * 8 - src_ofs)
            if n_bits <= 0:
                raise ValueError('Cannot create flow mod spec with 0 bits')
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_FIELD, NX_LEARN_DST_MATCH, n_bits) + _create_field(src, src_ofs) + _create_field(dst, dst_ofs))[0]
        namespace['create_nxfms_matchfield'] = create_nxfms_matchfield
        def create_nxfms_matchvalue(dst, value, dst_ofs, n_bits = None):
            if n_bits is None:
                n_bits = NXM_LENGTH(dst) * 8 - dst_ofs
            if n_bits <= 0:
                raise ValueError('Cannot create flow mod spec with 0 bits')
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_IMMEDIATE, NX_LEARN_DST_MATCH, n_bits) + common.create_binary(value, (n_bits + 15) // 16 * 2) + _create_field(dst, dst_ofs))[0]
        namespace['create_nxfms_matchvalue'] = create_nxfms_matchvalue
        def create_nxfms_loadfield(src, dst, src_ofs = 0, dst_ofs = 0, n_bits = None):
            if n_bits is None:
                n_bits = min(NXM_LENGTH(dst) * 8 - dst_ofs, NXM_LENGTH(src) * 8 - src_ofs)
            if n_bits <= 0:
                raise ValueError('Cannot create flow mod spec with 0 bits')
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_FIELD, NX_LEARN_DST_LOAD, n_bits) + _create_field(src, src_ofs) + _create_field(dst, dst_ofs))[0]
        namespace['create_nxfms_loadfield'] = create_nxfms_loadfield
        def create_nxfms_loadvalue(dst, value, dst_ofs, n_bits = None):
            if n_bits is None:
                n_bits = NXM_LENGTH(dst) * 8 - dst_ofs
            if n_bits <= 0:
                raise ValueError('Cannot create flow mod spec with 0 bits')
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_IMMEDIATE, NX_LEARN_DST_LOAD, n_bits) + common.create_binary(value, (n_bits + 15) // 16 * 2) + _create_field(dst, dst_ofs))[0]
        namespace['create_nxfms_loadvalue'] = create_nxfms_loadvalue
        def create_nxfms_outputfield(src, src_ofs = 0, n_bits = None):
            if n_bits is None:
                n_bits = NXM_LENGTH(src) * 8 - src_ofs
            if n_bits <= 0:
                raise ValueError('Cannot create flow mod spec with 0 bits')
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_FIELD, NX_LEARN_DST_OUTPUT, n_bits) + _create_field(src, src_ofs))[0]
        namespace['create_nxfms_outputfield'] = create_nxfms_outputfield
        def create_nxfms_outputvalue(dst, value):
            return nx_flow_mod_spec.parse(_create_header(NX_LEARN_SRC_IMMEDIATE, NX_LEARN_DST_OUTPUT, 16) + common.create_binary(value, 2))[0]
        namespace['create_nxfms_outputvalue'] = create_nxfms_outputvalue

        ofp_flow_mod_flags = namespace['ofp_flow_mod_flags']

        nx_action_learn = nstruct(
            (uint16, 'idle_timeout'),    #  /* Idle time before discarding (seconds). */
            (uint16, 'hard_timeout'),    #  /* Max time before discarding (seconds). */
            (uint16, 'priority'),        #  /* Priority level of flow entry. */
            (uint64, 'cookie'),          #  /* Cookie for new flow. */
            (ofp_flow_mod_flags, 'flags'),  #          /* Either 0 or OFPFF_SEND_FLOW_REM. */
            (uint8, 'table_id'),         #  /* Table to insert flow entry. */
            (uint8,),                    #  /* Must be zero. */
            (uint16, 'fin_idle_timeout'),#  /* Idle timeout after FIN, if nonzero. */
            (uint16, 'fin_hard_timeout'),#  /* Hard timeout after FIN, if nonzero. */
            (nx_flow_mod_spec[0], 'specs'),
            base = nx_action,
            name = 'nx_action_learn',
            classifyby = (NXAST_LEARN,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_LEARN,
            init = packvalue(NXAST_LEARN, action_subtype),
        )
        namespace['nx_action_learn'] = nx_action_learn

        '''
        /* Action structure for NXAST_FIN_TIMEOUT.
         *
         * This action changes the idle timeout or hard timeout, or both, of this
         * OpenFlow rule when the rule matches a TCP packet with the FIN or RST flag.
         * When such a packet is observed, the action reduces the rule's idle timeout
         * to 'fin_idle_timeout' and its hard timeout to 'fin_hard_timeout'.  This
         * action has no effect on an existing timeout that is already shorter than the
         * one that the action specifies.  A 'fin_idle_timeout' or 'fin_hard_timeout'
         * of zero has no effect on the respective timeout.
         *
         * 'fin_idle_timeout' and 'fin_hard_timeout' are measured in seconds.
         * 'fin_hard_timeout' specifies time since the flow's creation, not since the
         * receipt of the FIN or RST.
         *
         * This is useful for quickly discarding learned TCP flows that otherwise will
         * take a long time to expire.
         *
         * This action is intended for use with an OpenFlow rule that matches only a
         * single TCP flow.  If the rule matches multiple TCP flows (e.g. it wildcards
         * all TCP traffic, or all TCP traffic to a particular port), then any FIN or
         * RST in any of those flows will cause the entire OpenFlow rule to expire
         * early, which is not normally desirable.
         */
         '''
        nx_action_fin_timeout = nstruct(
            (uint16, 'fin_idle_timeout'),   #  /* New idle timeout, if nonzero. */
            (uint16, 'fin_hard_timeout'),   #  /* New hard timeout, if nonzero. */
            (uint16,),
            base = nx_action,
            name = 'nx_action_fin_timeout',
            criteria = lambda x: getattr(x, action_subtype) == NXAST_FIN_TIMEOUT,
            classifyby = (NXAST_FIN_TIMEOUT,),
            init = packvalue(NXAST_FIN_TIMEOUT, action_subtype)
        )
        namespace['nx_action_fin_timeout'] = nx_action_fin_timeout
        '''
        /* Action structure for NXAST_BUNDLE and NXAST_BUNDLE_LOAD.
         *
         * The bundle actions choose a slave from a supplied list of options.
         * NXAST_BUNDLE outputs to its selection.  NXAST_BUNDLE_LOAD writes its
         * selection to a register.
         *
         * The list of possible slaves follows the nx_action_bundle structure. The size
         * of each slave is governed by its type as indicated by the 'slave_type'
         * parameter. The list of slaves should be padded at its end with zeros to make
         * the total length of the action a multiple of 8.
         *
         * Switches infer from the 'slave_type' parameter the size of each slave.  All
         * implementations must support the NXM_OF_IN_PORT 'slave_type' which indicates
         * that the slaves are OpenFlow port numbers with NXM_LENGTH(NXM_OF_IN_PORT) ==
         * 2 byte width.  Switches should reject actions which indicate unknown or
         * unsupported slave types.
         *
         * Switches use a strategy dictated by the 'algorithm' parameter to choose a
         * slave.  If the switch does not support the specified 'algorithm' parameter,
         * it should reject the action.
         *
         * Several algorithms take into account liveness when selecting slaves.  The
         * liveness of a slave is implementation defined (with one exception), but will
         * generally take into account things like its carrier status and the results
         * of any link monitoring protocols which happen to be running on it.  In order
         * to give controllers a place-holder value, the OFPP_NONE port is always
         * considered live.
         *
         * Some slave selection strategies require the use of a hash function, in which
         * case the 'fields' and 'basis' parameters should be populated.  The 'fields'
         * parameter (one of NX_HASH_FIELDS_*) designates which parts of the flow to
         * hash.  Refer to the definition of "enum nx_hash_fields" for details.  The
         * 'basis' parameter is used as a universal hash parameter.  Different values
         * of 'basis' yield different hash results.
         *
         * The 'zero' parameter at the end of the action structure is reserved for
         * future use.  Switches are required to reject actions which have nonzero
         * bytes in the 'zero' field.
         *
         * NXAST_BUNDLE actions should have 'ofs_nbits' and 'dst' zeroed.  Switches
         * should reject actions which have nonzero bytes in either of these fields.
         *
         * NXAST_BUNDLE_LOAD stores the OpenFlow port number of the selected slave in
         * dst[ofs:ofs+n_bits].  The format and semantics of 'dst' and 'ofs_nbits' are
         * similar to those for the NXAST_REG_LOAD action. */
        '''
        nx_action_bundle = nstruct(
    #        /* Slave choice algorithm to apply to hash value. */
            (nx_bd_algorithm, 'algorithm'),  #         /* One of NX_BD_ALG_*. */

    #        /* What fields to hash and how. */
            (nx_hash_fields, 'fields'),     #         /* One of NX_HASH_FIELDS_*. */
            (uint16, 'basis'),              #         /* Universal hash parameter. */

            (nxm_header, 'slave_type'),     #         /* NXM_OF_IN_PORT. */
            (uint16, 'n_slaves'),           #         /* Number of slaves. */

            (uint16, 'ofs_nbits'),          #         /* (ofs << 6) | (n_bits - 1). */
            (nxm_header, 'dst'),            #         /* Destination. */

            (uint8[4],),                    #         /* Reserved. Must be zero. */
            name = 'nx_action_bundle',
            base = nx_action,
            criteria = lambda x: getattr(x, action_subtype) == NXAST_BUNDLE or getattr(x, action_subtype) == NXAST_BUNDLE_LOAD,
            classifyby = (NXAST_BUNDLE, NXAST_BUNDLE_LOAD),
            init = packvalue(NXAST_BUNDLE, action_subtype)
        )
        namespace['nx_action_bundle'] = nx_action_bundle

        def _nx_slave_ports_prepack(x):
            x.n_slaves = len(x.bundles)
        _nx_slave_ports = nstruct(
            (nx_port_no[0], 'bundles'),
            name = '_nx_slave_ports',
            size = lambda x: x.n_slaves * 2,
            prepack = _nx_slave_ports_prepack,
            padding = 1
        )

        nx_action_bundle_port = nstruct(
            (_nx_slave_ports,),
            base = nx_action_bundle,
            name = 'nx_action_bundle_port',
            criteria = lambda x: x.slave_type == NXM_OF_IN_PORT,
            init = packvalue(NXM_OF_IN_PORT, 'slave_type'),
            lastextra = False,
            formatter = _createdesc(lambda x: 'bundle_load(%s,%d,%s,%s,%s[%d..%d],slaves:%r)' % \
                (x['fields'], x['basis'], x['algorithm'], x['slave_type'], x['dst'], x['ofs_nbits'] >> 6, (x['ofs_nbits'] >> 6) + (x['ofs_nbits'] & 0x3f), x['bundles']) \
                if x[action_subtype] == 'NXAST_BUNDLE_LOAD' else 'bundle(%s,%d,%s,%s,slaves:%r)' % (x['fields'], x['basis'], x['algorithm'], x['slave_type'], x['bundles']))
        )
        namespace['nx_action_bundle_port'] = nx_action_bundle_port

        def _nx_slave_others_prepack(x):
            x.n_slaves = len(x.bundlesraw) // NXM_LENGTH(x.slave_type)

        _nx_slave_others = nstruct(
            (raw, 'bundlesraw'),
            name = '_nx_slave_others',
            size = lambda x: x.n_slaves * NXM_LENGTH(x.slave_type),
            prepack = _nx_slave_others_prepack,
            padding = 1
        )

        nx_action_bundle_others = nstruct(
            (_nx_slave_others,),
            base = nx_action_bundle,
            name = 'nx_action_bundle_others',
            criteria = lambda x: x.slave_type != NXM_OF_IN_PORT,
            lastextra = False,
            init = packvalue(NXM_OF_ETH_DST, 'slave_type'),
            formatter = _createdesc(lambda x: 'bundle_load(%s,%d,%s,%s,%s[%d..%d],slaves:%r)' % \
                (x['fields'], x['basis'], x['algorithm'], x['slave_type'], x['dst'], x['ofs_nbits'] >> 6, (x['ofs_nbits'] >> 6) + (x['ofs_nbits'] & 0x3f), x['bundleraw']) \
                if x[action_subtype] == 'NXAST_BUNDLE_LOAD' else 'bundle(%s,%d,%s,%s,slaves:%r)' % (x['fields'], x['basis'], x['algorithm'], x['slave_type'], x['bundleraw']))
        )
        namespace['nx_action_bundle_others'] = nx_action_bundle_others
        '''
        /* Action structure for NXAST_DEC_TTL_CNT_IDS.
         *
         * If the packet is not IPv4 or IPv6, does nothing.  For IPv4 or IPv6, if the
         * TTL or hop limit is at least 2, decrements it by 1.  Otherwise, if TTL or
         * hop limit is 0 or 1, sends a packet-in to the controllers with each of the
         * 'n_controllers' controller IDs specified in 'cnt_ids'.
         *
         * (This differs from NXAST_DEC_TTL in that for NXAST_DEC_TTL the packet-in is
         * sent only to controllers with id 0.)
         */
         '''

        def _nx_action_cnt_ids_ids_prepack(x):
            x.n_controllers = len(x.cnt_ids)
        _nx_action_cnt_ids_ids = nstruct(
            (uint16[0], 'cnt_ids'),
            name = '_nx_action_cnt_ids_ids',
            size = lambda x: 2 * x.n_controllers,
            prepack = _nx_action_cnt_ids_ids_prepack
        )

        nx_action_cnt_ids = nstruct(
            (uint16, 'n_controllers'),    # /* Number of controllers. */
            (uint8[4],),                  # /* Must be zero. */
            (_nx_action_cnt_ids_ids,),
            base = nx_action,
            classifyby = (NXAST_DEC_TTL_CNT_IDS,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_DEC_TTL_CNT_IDS,
            init = packvalue(NXAST_DEC_TTL_CNT_IDS, action_subtype),
            lastextra = False,
            name = 'nx_action_cnt_ids'
        )
        namespace['nx_action_cnt_ids'] = nx_action_cnt_ids

        '''
        /* Action structure for NXAST_OUTPUT_REG.
         *
         * Outputs to the OpenFlow port number written to src[ofs:ofs+nbits].
         *
         * The format and semantics of 'src' and 'ofs_nbits' are similar to those for
         * the NXAST_REG_LOAD action.
         *
         * The acceptable nxm_header values for 'src' are the same as the acceptable
         * nxm_header values for the 'src' field of NXAST_REG_MOVE.
         *
         * The 'max_len' field indicates the number of bytes to send when the chosen
         * port is OFPP_CONTROLLER.  Its semantics are equivalent to the 'max_len'
         * field of OFPAT_OUTPUT.
         *
         * The 'zero' field is required to be zeroed for forward compatibility. */
        '''
        nx_action_output_reg = nstruct(
            (uint16, 'ofs_nbits'),      # /* (ofs << 6) | (n_bits - 1). */
            (nxm_header, 'src'),        # /* Source. */
            (uint16, 'max_len'),        # /* Max length to send to controller. */
            (uint8[6],),                # /* Reserved, must be zero. */
            base = nx_action,
            classifyby = (NXAST_OUTPUT_REG,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_OUTPUT_REG,
            init = packvalue(NXAST_OUTPUT_REG, action_subtype),
            name = 'nx_action_output_reg',
            formatter = _createdesc(lambda x: 'output:%s[%d..%d]' % (x['src'], x['ofs_nbits'] >> 6, (x['ofs_nbits'] >> 6) + (x['ofs_nbits'] & 0x3f)))
        )
        namespace['nx_action_output_reg'] = nx_action_output_reg

        '''
        /* NXAST_EXIT
         *
         * Discontinues action processing.
         *
         * The NXAST_EXIT action causes the switch to immediately halt processing
         * actions for the flow.  Any actions which have already been processed are
         * executed by the switch.  However, any further actions, including those which
         * may be in different tables, or different levels of the NXAST_RESUBMIT
         * hierarchy, will be ignored.
         *
         * Uses the nx_action_header structure. */

        /* ## --------------------- ## */
        /* ## Requests and replies. ## */
        /* ## --------------------- ## */
        '''

        '''
        /* NXT_SET_FLOW_FORMAT request. */
        '''
        nx_set_flow_format = nstruct(
            (nx_flow_format, 'format'),         #            /* One of NXFF_*. */
            name = 'nx_set_flow_format',
            base = nicira_header,
            criteria = lambda x: getattr(x, msg_subtype) == NXT_SET_FLOW_FORMAT,
            classifyby = (NXT_SET_FLOW_FORMAT,),
            init = packvalue(NXT_SET_FLOW_FORMAT, msg_subtype)
        )
        namespace['nx_set_flow_format'] = nx_set_flow_format

        '''
        /* NXT_FLOW_MOD (analogous to OFPT_FLOW_MOD).
         *
         * It is possible to limit flow deletions and modifications to certain
         * cookies by using the NXM_NX_COOKIE(_W) matches.  The "cookie" field
         * is used only to add or modify flow cookies.
         */
         '''
        ofp_flow_mod_command = namespace['ofp_flow_mod_command']
        nx_flow_mod = nstruct(
            (uint64, 'cookie'),                  #     /* Opaque controller-issued identifier. */
            (ofp_flow_mod_command, 'command'),   #     /* OFPFC_* + possibly a table ID (see comment
    #                                       * on struct nx_flow_mod_table_id). */
            (uint16, 'idle_timeout'),            #     /* Idle time before discarding (seconds). */
            (uint16, 'hard_timeout'),            #     /* Max time before discarding (seconds). */
            (uint16, 'priority'),                #     /* Priority level of flow entry. */
            (uint32, 'buffer_id'),               #     /* Buffered packet to apply to (or -1).
    #                                         Not meaningful for OFPFC_DELETE*. */
            (nx_port_no, 'out_port'),            #     /* For OFPFC_DELETE* commands, require
    #                                         matching entries to include this as an
    #                                         output port.  A value of OFPP_NONE
    #                                         indicates no restriction. */
            (ofp_flow_mod_flags, 'flags'),       #     /* One of OFPFF_*. */
            (uint16, 'match_len'),               #     /* Size of nx_match. */
            (uint8[6],),                         #     /* Align to 64-bits. */
            (nx_matches,),
            base = nicira_header,
            criteria = lambda x: getattr(x, msg_subtype) == NXT_FLOW_MOD,
            classifyby = (NXT_FLOW_MOD,),
            init = packvalue(NXT_FLOW_MOD, msg_subtype),
            name = 'nx_flow_mod'
        )
        namespace['nx_flow_mod'] = nx_flow_mod

        '''
        /* NXT_FLOW_REMOVED (analogous to OFPT_FLOW_REMOVED).
         *
         * 'table_id' is present only in Open vSwitch 1.11 and later.  In earlier
         * versions of Open vSwitch, this is a padding byte that is always zeroed.
         * Therefore, a 'table_id' value of 0 indicates that the table ID is not known,
         * and other values may be interpreted as one more than the flow's former table
         * ID. */
         '''
        nx_flow_removed = nstruct(
            (uint64, 'cookie'),             # /* Opaque controller-issued identifier. */
            (uint16, 'priority'),           # /* Priority level of flow entry. */
            (ofp_flow_removed_reason, 'reason'),    #    /* One of OFPRR_*. */
            (uint8, 'table_id'),                #    /* Flow's former table ID, plus one. */
            (uint32, 'duration_sec'),           #    /* Time flow was alive in seconds. */
            (uint32, 'duration_nsec'),          #    /* Time flow was alive in nanoseconds beyond
    #                                     duration_sec. */
            (uint16, 'idle_timeout'),           #    /* Idle timeout from original flow mod. */
            (uint16, 'match_len'),              #    /* Size of nx_match. */
            (uint64, 'packet_count'),
            (uint64, 'byte_count'),
            (nx_matches,),
            base = nicira_header,
            criteria = lambda x: getattr(x, msg_subtype) == NXT_FLOW_REMOVED,
            classifyby = (NXT_FLOW_REMOVED,),
            init = packvalue(NXT_FLOW_REMOVED, msg_subtype),
            name = 'nx_flow_removed'
        )
        namespace['nx_flow_removed'] = nx_flow_removed

        '''
        /* Nicira vendor stats request of type NXST_FLOW (analogous to OFPST_FLOW
         * request).
         *
         * It is possible to limit matches to certain cookies by using the
         * NXM_NX_COOKIE and NXM_NX_COOKIE_W matches.
         */
         '''
        nx_flow_stats_request = nstruct(
            (nx_port_no, 'out_port'),       #/* Require matching entries to include this
    #                                     as an output port.  A value of OFPP_NONE
    #                                     indicates no restriction. */
            (uint16, 'match_len'),          #       /* Length of nx_match. */
            (uint8, 'table_id'),            #       /* ID of table to read (from ofp_table_stats)
    #                                     or 0xff for all tables. */
            (uint8[3],),                    #       /* Align to 64 bits. */
            (nx_matches,),
            base = nx_stats_request,
            criteria = lambda x: getattr(x, stats_subtype) == NXST_FLOW,
            classifyby = (NXST_FLOW,),
            init = packvalue(NXST_FLOW, stats_subtype),
            name = 'nx_flow_stats_request'
        )
        namespace['nx_flow_stats_request'] = nx_flow_stats_request
        '''
        /* Body for Nicira vendor stats reply of type NXST_FLOW (analogous to
         * OFPST_FLOW reply).
         *
         * The values of 'idle_age' and 'hard_age' are only meaningful when talking to
         * a switch that implements the NXT_FLOW_AGE extension.  Zero means that the
         * true value is unknown, perhaps because hardware does not track the value.
         * (Zero is also the value that one should ordinarily expect to see talking to
         * a switch that does not implement NXT_FLOW_AGE, since those switches zero the
         * padding bytes that these fields replaced.)  A nonzero value X represents X-1
         * seconds.  A value of 65535 represents 65534 or more seconds.
         *
         * 'idle_age' is the number of seconds that the flow has been idle, that is,
         * the number of seconds since a packet passed through the flow.  'hard_age' is
         * the number of seconds since the flow was last modified (e.g. OFPFC_MODIFY or
         * OFPFC_MODIFY_STRICT).  (The 'duration_*' fields are the elapsed time since
         * the flow was added, regardless of subsequent modifications.)
         *
         * For a flow with an idle or hard timeout, 'idle_age' or 'hard_age',
         * respectively, will ordinarily be smaller than the timeout, but flow
         * expiration times are only approximate and so one must be prepared to
         * tolerate expirations that occur somewhat early or late.
         */
         '''
        ofp_action = namespace['ofp_action']

        nx_flow_stats = nstruct(
            (uint16, 'length'),         # /* Length of this entry. */
            (uint8, 'table_id'),        # /* ID of table flow came from. */
            (uint8,),
            (uint32, 'duration_sec'),   # /* Time flow has been alive in seconds. */
            (uint32, 'duration_nsec'),  # /* Time flow has been alive in nanoseconds
    #                                     beyond duration_sec. */
            (uint16, 'priority'),       # /* Priority of the entry. */
            (uint16, 'idle_timeout'),   # /* Number of seconds idle before expiration. */
            (uint16, 'hard_timeout'),   # /* Number of seconds before expiration. */
            (uint16, 'match_len'),      # /* Length of nx_match. */
            (uint16, 'idle_age'),       # /* Seconds since last packet, plus one. */
            (uint16, 'hard_age'),       # /* Seconds since last modification, plus one. */
            (uint64, 'cookie'),         # /* Opaque controller-issued identifier. */
            (uint64, 'packet_count'),   # /* Number of packets, UINT64_MAX if unknown. */
            (uint64, 'byte_count'),     # /* Number of bytes, UINT64_MAX if unknown. */
            #=======================================================================
            # /* Followed by:
            #  *   - Exactly match_len (possibly 0) bytes containing the nx_match, then
            #  *   - Exactly (match_len + 7)/8*8 - match_len (between 0 and 7) bytes of
            #  *     all-zero bytes, then
            #  *   - Actions to fill out the remainder 'length' bytes (always a multiple
            #  *     of 8).
            #  */
            #=======================================================================
            (nx_matches,),
            (ofp_action[0], 'actions'),
            name = 'nx_flow_stats',
            size = sizefromlen(65536, 'length'),
            prepack = packsize('length')
        )
        namespace['nx_flow_stats'] = nx_flow_stats

        nx_flow_stats_reply = nstruct(
            (nx_flow_stats[0], 'stats'),
            base = nx_stats_reply,
            classifyby = (NXST_FLOW,),
            criteria = lambda x: getattr(x, stats_subtype) == NXST_FLOW,
            init = packvalue(NXST_FLOW, stats_subtype),
            name = 'nx_flow_stats_reply'
        )
        namespace['nx_flow_stats_reply'] = nx_flow_stats_reply

        '''
        /* Nicira vendor stats request of type NXST_AGGREGATE (analogous to
         * OFPST_AGGREGATE request).
         *
         * The reply format is identical to the reply format for OFPST_AGGREGATE,
         * except for the header. */
        '''
        nx_aggregate_stats_request = nstruct(
            (nx_port_no, 'out_port'),           #       /* Require matching entries to include this
    #                                     as an output port.  A value of OFPP_NONE
    #                                     indicates no restriction. */
            (uint16, 'match_len'),              #       /* Length of nx_match. */
            (uint8, 'table_id'),                #       /* ID of table to read (from ofp_table_stats)
    #                                     or 0xff for all tables. */
            (uint8[3],),                        #       /* Align to 64 bits. */
            #=======================================================================
            # /* Followed by:
            #  *   - Exactly match_len (possibly 0) bytes containing the nx_match, then
            #  *   - Exactly (match_len + 7)/8*8 - match_len (between 0 and 7) bytes of
            #  *     all-zero bytes, which must also exactly fill out the length of the
            #  *     message.
            #  */
            #=======================================================================
            (nx_matches,),
            base = nx_stats_request,
            name = 'nx_aggregate_stats_request',
            criteria = lambda x: getattr(x, stats_subtype) == NXST_AGGREGATE,
            classifyby = (NXST_AGGREGATE,),
            init = packvalue(NXST_AGGREGATE, stats_subtype),
            lastextra = False
        )
        namespace['nx_aggregate_stats_request'] = nx_aggregate_stats_request

        nx_aggregate_stats_reply = nstruct(
            (uint64, 'packet_count'),           # /* Number of packets in flows. */
            (uint64, 'byte_count'),             # /* Number of bytes in flows. */
            (uint32, 'flow_count'),             # /* Number of flows. */
            (uint8[4],),
            base = nx_stats_reply,
            name = 'nx_aggregate_stats_reply',
            criteria = lambda x: getattr(x, stats_subtype) == NXST_AGGREGATE,
            classifyby = (NXST_AGGREGATE,),
            init = packvalue(NXST_AGGREGATE, stats_subtype)
        )
        namespace['nx_aggregate_stats_reply'] = nx_aggregate_stats_reply

        '''
        /* NXT_SET_CONTROLLER_ID.
         *
         * Each OpenFlow controller connection has a 16-bit identifier that is
         * initially 0.  This message changes the connection's ID to 'id'.
         *
         * Controller connection IDs need not be unique.
         *
         * The NXAST_CONTROLLER action is the only current user of controller
         * connection IDs. */
         '''
        nx_controller_id = nstruct(
            (uint8[6],),                    # /* Must be zero. */
            (uint16, 'controller_id'),      # /* New controller connection ID. */
            base = nicira_header,
            name = 'nx_controller_id',
            criteria = lambda x: getattr(x, msg_subtype) == NXT_SET_CONTROLLER_ID,
            init = packvalue(NXT_SET_CONTROLLER_ID, msg_subtype),
            classifyby = (NXT_SET_CONTROLLER_ID,)
        )
        namespace['nx_controller_id'] = nx_controller_id

        '''
        /* Action structure for NXAST_CONTROLLER.
         *
         * This generalizes using OFPAT_OUTPUT to send a packet to OFPP_CONTROLLER.  In
         * addition to the 'max_len' that OFPAT_OUTPUT supports, it also allows
         * specifying:
         *
         *    - 'reason': The reason code to use in the ofp_packet_in or nx_packet_in.
         *
         *    - 'controller_id': The ID of the controller connection to which the
         *      ofp_packet_in should be sent.  The ofp_packet_in or nx_packet_in is
         *      sent only to controllers that have the specified controller connection
         *      ID.  See "struct nx_controller_id" for more information. */
         '''
        nx_action_controller = nstruct(
            (uint16, 'max_len'),                    # /* Maximum length to send to controller. */
            (uint16, 'controller_id'),              # /* Controller ID to send packet-in. */
            (ofp_packet_in_reason, 'reason'),       # /* enum ofp_packet_in_reason (OFPR_*). */
            (uint8,),
            base = nx_action,
            name = 'nx_action_controller',
            criteria = lambda x: getattr(x, action_subtype) == NXAST_CONTROLLER,
            classifyby = (NXAST_CONTROLLER,),
            init = packvalue(NXAST_CONTROLLER, action_subtype)
        )
        namespace['nx_action_controller'] = nx_action_controller
        '''
        /* Flow Table Monitoring
         * =====================
         *
         * NXST_FLOW_MONITOR allows a controller to keep track of changes to OpenFlow
         * flow table(s) or subsets of them, with the following workflow:
         *
         * 1. The controller sends an NXST_FLOW_MONITOR request to begin monitoring
         *    flows.  The 'id' in the request must be unique among all monitors that
         *    the controller has started and not yet canceled on this OpenFlow
         *    connection.
         *
         * 2. The switch responds with an NXST_FLOW_MONITOR reply.  If the request's
         *    'flags' included NXFMF_INITIAL, the reply includes all the flows that
         *    matched the request at the time of the request (with event NXFME_ADDED).
         *    If 'flags' did not include NXFMF_INITIAL, the reply is empty.
         *
         *    The reply uses the xid of the request (as do all replies to OpenFlow
         *    requests).
         *
         * 3. Whenever a change to a flow table entry matches some outstanding monitor
         *    request's criteria and flags, the switch sends a notification to the
         *    controller as an additional NXST_FLOW_MONITOR reply with xid 0.
         *
         *    When multiple outstanding monitors match a single change, only a single
         *    notification is sent.  This merged notification includes the information
         *    requested in any of the individual monitors.  That is, if any of the
         *    matching monitors requests actions (NXFMF_ACTIONS), the notification
         *    includes actions, and if any of the monitors request full changes for the
         *    controller's own changes (NXFMF_OWN), the controller's own changes will
         *    be included in full.
         *
         * 4. The controller may cancel a monitor with NXT_FLOW_MONITOR_CANCEL.  No
         *    further notifications will be sent on the basis of the canceled monitor
         *    afterward.
         *
         *
         * Buffer Management
         * =================
         *
         * OpenFlow messages for flow monitor notifications can overflow the buffer
         * space available to the switch, either temporarily (e.g. due to network
         * conditions slowing OpenFlow traffic) or more permanently (e.g. the sustained
         * rate of flow table change exceeds the network bandwidth between switch and
         * controller).
         *
         * When Open vSwitch's notification buffer space reaches a limiting threshold,
         * OVS reacts as follows:
         *
         * 1. OVS sends an NXT_FLOW_MONITOR_PAUSED message to the controller, following
         *    all the already queued notifications.  After it receives this message,
         *    the controller knows that its view of the flow table, as represented by
         *    flow monitor notifications, is incomplete.
         *
         * 2. As long as the notification buffer is not empty:
         *
         *        - NXMFE_ADD and NXFME_MODIFIED notifications will not be sent.
         *
         *        - NXFME_DELETED notifications will still be sent, but only for flows
         *          that existed before OVS sent NXT_FLOW_MONITOR_PAUSED.
         *
         *        - NXFME_ABBREV notifications will not be sent.  They are treated as
         *          the expanded version (and therefore only the NXFME_DELETED
         *          components, if any, are sent).
         *
         * 3. When the notification buffer empties, OVS sends NXFME_ADD notifications
         *    for flows added since the buffer reached its limit and NXFME_MODIFIED
         *    notifications for flows that existed before the limit was reached and
         *    changed after the limit was reached.
         *
         * 4. OVS sends an NXT_FLOW_MONITOR_RESUMED message to the controller.  After
         *    it receives this message, the controller knows that its view of the flow
         *    table, as represented by flow monitor notifications, is again complete.
         *
         * This allows the maximum buffer space requirement for notifications to be
         * bounded by the limit plus the maximum number of supported flows.
         *
         *
         * "Flow Removed" messages
         * =======================
         *
         * The flow monitor mechanism is independent of OFPT_FLOW_REMOVED and
         * NXT_FLOW_REMOVED.  Flow monitor updates for deletion are sent if
         * NXFMF_DELETE is set on a monitor, regardless of whether the
         * OFPFF_SEND_FLOW_REM flag was set when the flow was added. */

        /* NXST_FLOW_MONITOR request.
         *
         * The NXST_FLOW_MONITOR request's body consists of an array of zero or more
         * instances of this structure.  The request arranges to monitor the flows
         * that match the specified criteria, which are interpreted in the same way as
         * for NXST_FLOW.
         *
         * 'id' identifies a particular monitor for the purpose of allowing it to be
         * canceled later with NXT_FLOW_MONITOR_CANCEL.  'id' must be unique among
         * existing monitors that have not already been canceled.
         *
         * The reply includes the initial flow matches for monitors that have the
         * NXFMF_INITIAL flag set.  No single flow will be included in the reply more
         * than once, even if more than one requested monitor matches that flow.  The
         * reply will be empty if none of the monitors has NXFMF_INITIAL set or if none
         * of the monitors initially matches any flows.
         *
         * For NXFMF_ADD, an event will be reported if 'out_port' matches against the
         * actions of the flow being added or, for a flow that is replacing an existing
         * flow, if 'out_port' matches against the actions of the flow being replaced.
         * For NXFMF_DELETE, 'out_port' matches against the actions of a flow being
         * deleted.  For NXFMF_MODIFY, an event will be reported if 'out_port' matches
         * either the old or the new actions. */
         '''
        ofp_table = namespace['ofp_table']
        nx_flow_monitor_request = nstruct(
            (uint32, 'id'),                     # /* Controller-assigned ID for this monitor. */
            (nx_flow_monitor_flags, 'flags'),   # /* NXFMF_*. */
            (nx_port_no, 'out_port'),           # /* Required output port, if not OFPP_NONE. */
            (uint16, 'match_len'),              # /* Length of nx_match. */
            (ofp_table, 'table_id'),            # /* One table's ID or 0xff for all tables. */
            (uint8[5],),                        # /* Align to 64 bits (must be zero). */
            (nx_matches,),
            name = 'nx_flow_monitor_request',
            base = nx_stats_request,
            criteria = lambda x: getattr(x, stats_subtype) == NXST_FLOW_MONITOR,
            init = packvalue(NXST_FLOW_MONITOR, stats_subtype),
            classifyby = (NXST_FLOW_MONITOR,)
        )
        namespace['nx_flow_monitor_request'] = nx_flow_monitor_request
        '''
        /* NXST_FLOW_MONITOR reply header.
         *
         * The body of an NXST_FLOW_MONITOR reply is an array of variable-length
         * structures, each of which begins with this header.  The 'length' member may
         * be used to traverse the array, and the 'event' member may be used to
         * determine the particular structure.
         *
         * Every instance is a multiple of 8 bytes long. */
        '''
        nx_flow_update = nstruct(
            (uint16, 'length'),            #/* Length of this entry. */
            (nx_flow_update_event, 'event'),        #             /* One of NXFME_*. */
            name = 'nx_flow_update',
            size = sizefromlen(65536, 'length'),
            prepack = packsize('length')
        )
        namespace['nx_flow_update'] = nx_flow_update

        '''
        /* NXST_FLOW_MONITOR reply for NXFME_ADDED, NXFME_DELETED, and
         * NXFME_MODIFIED. */
         '''
        nx_flow_update_full = nstruct(
            (ofp_flow_removed_reason, 'reason'),        #            /* OFPRR_* for NXFME_DELETED, else zero. */
            (uint16, 'priority'),                       #          /* Priority of the entry. */
            (uint16, 'idle_timeout'),                   #      /* Number of seconds idle before expiration. */
            (uint16, 'hard_timeout'),                   #      /* Number of seconds before expiration. */
            (uint16, 'match_len'),                      #         /* Length of nx_match. */
            (uint8, 'table_id'),                        #           /* ID of flow's table. */
            (uint8,),                                   #                /* Reserved, currently zeroed. */
            (uint64, 'cookie'),                         #            /* Opaque controller-issued identifier. */
            #=======================================================================
            # /* Followed by:
            #  *   - Exactly match_len (possibly 0) bytes containing the nx_match, then
            #  *   - Exactly (match_len + 7)/8*8 - match_len (between 0 and 7) bytes of
            #  *     all-zero bytes, then
            #  *   - Actions to fill out the remainder 'length' bytes (always a multiple
            #  *     of 8).  If NXFMF_ACTIONS was not specified, or 'event' is
            #  *     NXFME_DELETED, no actions are included.
            #  */
            #=======================================================================
            (nx_matches,),
            (ofp_action[0], 'actions'),
            name = 'nx_flow_update_full',
            base = nx_flow_update,
            criteria = lambda x: x.event in (NXFME_ADDED, NXFME_DELETED, NXFME_MODIFIED),
            init = packvalue(NXFME_ADDED, 'event')
        )
        namespace['nx_flow_update_full'] = nx_flow_update_full
        '''
        /* NXST_FLOW_MONITOR reply for NXFME_ABBREV.
         *
         * When the controller does not specify NXFMF_OWN in a monitor request, any
         * flow tables changes due to the controller's own requests (on the same
         * OpenFlow channel) will be abbreviated, when possible, to this form, which
         * simply specifies the 'xid' of the OpenFlow request (e.g. an OFPT_FLOW_MOD or
         * NXT_FLOW_MOD) that caused the change.
         *
         * Some changes cannot be abbreviated and will be sent in full:
         *
         *   - Changes that only partially succeed.  This can happen if, for example,
         *     a flow_mod with type OFPFC_MODIFY affects multiple flows, but only some
         *     of those modifications succeed (e.g. due to hardware limitations).
         *
         *     This cannot occur with the current implementation of the Open vSwitch
         *     software datapath.  It could happen with other datapath implementations.
         *
         *   - Changes that race with conflicting changes made by other controllers or
         *     other flow_mods (not separated by barriers) by the same controller.
         *
         *     This cannot occur with the current Open vSwitch implementation
         *     (regardless of datapath) because Open vSwitch internally serializes
         *     potentially conflicting changes.
         *
         * A flow_mod that does not change the flow table will not trigger any
         * notification, even an abbreviated one.  For example, a "modify" or "delete"
         * flow_mod that does not match any flows will not trigger a notification.
         * Whether an "add" or "modify" that specifies all the same parameters that a
         * flow already has triggers a notification is unspecified and subject to
         * change in future versions of Open vSwitch.
         *
         * OVS will always send the notifications for a given flow table change before
         * the reply to a OFPT_BARRIER_REQUEST request that follows the flow table
         * change.  Thus, if the controller does not receive an abbreviated (or
         * unabbreviated) notification for a flow_mod before the next
         * OFPT_BARRIER_REPLY, it will never receive one. */
         '''
        nx_flow_update_abbrev = nstruct(
            (uint32, 'xid'),                    # /* Controller-specified xid from flow_mod. */
            name = 'nx_flow_update_abbrev',
            base = nx_flow_update,
            criteria = lambda x: x.event == NXFME_ABBREV,
            init = packvalue(NXFME_ABBREV, 'event')
        )
        namespace['nx_flow_update_abbrev'] = nx_flow_update_abbrev

        nx_flow_monitor_reply = nstruct(
            (nx_flow_update[0], 'stats'),
            base = nx_stats_reply,
            classifyby = (NXST_FLOW_MONITOR,),
            name = 'nx_flow_monitor_reply',
            criteria = lambda x: getattr(x, stats_subtype) == NXST_FLOW_MONITOR,
            init = packvalue(NXST_FLOW_MONITOR, stats_subtype)
        )
        namespace['nx_flow_monitor_reply'] = nx_flow_monitor_reply
        '''
        /* NXT_FLOW_MONITOR_CANCEL.
         *
         * Used by a controller to cancel an outstanding monitor. */
        '''
        nx_flow_monitor_cancel = nstruct(
            (uint32, 'id'),                     # /* 'id' from nx_flow_monitor_request. */
            name = 'nx_flow_monitor_cancel',
            base = nicira_header,
            classifyby = (NXT_FLOW_MONITOR_CANCEL,),
            criteria = lambda x: getattr(x, msg_subtype) == NXT_FLOW_MONITOR_CANCEL,
            init = packvalue(NXT_FLOW_MONITOR_CANCEL, msg_subtype)
        )
        namespace['nx_flow_monitor_cancel'] = nx_flow_monitor_cancel

        '''
        /* Action structure for NXAST_WRITE_METADATA.
         *
         * Modifies the 'mask' bits of the metadata value. */
        '''
        nx_action_write_metadata = nstruct(
            (uint8[6],),                        # /* Must be zero. */
            (uint64, 'metadata'),               # /* Metadata register. */
            (uint64, 'mask'),                   # /* Metadata mask. */
            base = nx_action,
            classifyby = (NXAST_WRITE_METADATA,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_WRITE_METADATA,
            init = packvalue(NXAST_WRITE_METADATA, action_subtype),
            name = 'nx_action_write_metadata'
        )
        namespace['nx_action_write_metadata'] = nx_action_write_metadata

        '''
        /* Action structure for NXAST_PUSH_MPLS. */
        '''
        nx_action_push_mpls = nstruct(
            (ethertype, 'ethertype'),           # /* Ethertype */
            (uint8[4],),
            base = nx_action,
            classifyby = (NXAST_PUSH_MPLS,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_PUSH_MPLS,
            init = packvalue(NXAST_PUSH_MPLS, action_subtype),
            name = 'nx_action_push_mpls'
        )
        namespace['nx_action_push_mpls'] = nx_action_push_mpls

        '''
        /* Action structure for NXAST_POP_MPLS. */
        '''
        nx_action_pop_mpls = nstruct(
            (ethertype, 'ethertype'),           # /* Ethertype */
            (uint8[4],),
            base = nx_action,
            classifyby = (NXAST_POP_MPLS,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_POP_MPLS,
            init = packvalue(NXAST_POP_MPLS, action_subtype),
            name = 'nx_action_pop_mpls'
        )
        namespace['nx_action_pop_mpls'] = nx_action_pop_mpls

        '''
        /* Action structure for NXAST_SET_MPLS_LABEL. */
        '''
        nx_action_mpls_label = nstruct(
            (uint8[2],),                    # /* Must be zero. */
            (uint32, 'label'),              # /* LABEL */
            base = nx_action,
            classifyby = (NXAST_SET_MPLS_LABEL,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_MPLS_LABEL,
            init = packvalue(NXAST_SET_MPLS_LABEL, action_subtype),
            name = 'nx_action_mpls_label'
        )
        namespace['nx_action_mpls_label'] = nx_action_mpls_label

        '''
        /* Action structure for NXAST_SET_MPLS_TC. */
        '''
        nx_action_mpls_tc = nstruct(
            (uint8, 'tc'),                      # /* TC */
            (uint8[5],),
            base = nx_action,
            classifyby = (NXAST_SET_MPLS_TC,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_MPLS_TC,
            init = packvalue(NXAST_SET_MPLS_TC, action_subtype),
            name = 'nx_action_mpls_tc'
        )
        namespace['nx_action_mpls_tc'] = nx_action_mpls_tc

        '''
        /* Action structure for NXAST_SET_MPLS_TTL. */
        '''
        nx_action_mpls_ttl = nstruct(
            (uint8,  'ttl'),                    # /* TTL */
            (uint8[5],),
            base = nx_action,
            classifyby = (NXAST_SET_MPLS_TTL,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SET_MPLS_TTL,
            init = packvalue(NXAST_SET_MPLS_TTL, action_subtype),
            name = 'nx_action_mpls_ttl'
        )
        namespace['nx_action_mpls_ttl'] = nx_action_mpls_ttl

        '''
        /* Action structure for NXAST_SAMPLE.
         *
         * Samples matching packets with the given probability and sends them
         * each to the set of collectors identified with the given ID.  The
         * probability is expressed as a number of packets to be sampled out
         * of USHRT_MAX packets, and must be >0.
         *
         * When sending packet samples to IPFIX collectors, the IPFIX flow
         * record sent for each sampled packet is associated with the given
         * observation domain ID and observation point ID.  Each IPFIX flow
         * record contain the sampled packet's headers when executing this
         * rule.  If a sampled packet's headers are modified by previous
         * actions in the flow, those modified headers are sent. */
        '''
        nx_action_sample = nstruct(
            (uint16, 'probability'),            #           /* Fraction of packets to sample. */
            (uint32, 'collector_set_id'),       #      /* ID of collector set in OVSDB. */
            (uint32, 'obs_domain_id'),          #         /* ID of sampling observation domain. */
            (uint32, 'obs_point_id'),           #          /* ID of sampling observation point. */
            base = nx_action,
            classifyby = (NXAST_SAMPLE,),
            criteria = lambda x: getattr(x, action_subtype) == NXAST_SAMPLE,
            init = packvalue(NXAST_SAMPLE, action_subtype),
            name = 'nx_action_sample'
        )
        namespace['nx_action_sample'] = nx_action_sample

Response: This function creates an Open vSwitch extension.