Abstract:
A method and apparatus for transmitting data wherein it is possible to transmit data on the basis of hardware addresses from a source network device to a destination network device. The source network device assigns the data to be transmitted a fictitious hardware address which is used to identify a transmission destination without matching the latter&#39;s real hardware address prescribed by the manufacturer. The fictitious hardware address is transferred to an inventive address conversion apparatus in which the fictitious hardware address has been assigned an address information item identifying the destination network device. The address information item associated with the fictitious hardware address is then assigned to the data, which are forwarded to the destination network device using this address information item.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for transmitting data from a source network device to a destination network device. The present invention also relates to an address conversion apparatus for implementing the method. In this context, network devices can be understood to be network nodes in a communication and/or data network, router devices, network modules, networked personal computers and other communication and data processing devices which can be coupled to a network. 
     In order to switch data quickly within a network, the data are frequently transmitted in the form of data packets provided with a hardware address as destination address. Hardware addresses, which are also referred to as MAC addresses (medium access control), are addresses which are used in the data link layer (layer 2) of the OSI reference model and with which network devices (usually network modules) are provided by the manufacturer. The hardware address for a network device is stored permanently, i.e., unalterably, in the network device and is unique throughout the world. 
     Switching data packets on the basis of hardware addresses is typical, particularly in local area networks (“LANs”). A data packet which is provided with a hardware address for a destination network device and is sent to a LAN is recognized by the destination network device in question as being addressed to it from the hardware address and is, therefore, received for further processing or forwarding. Normally, hardware addresses are used only for addressing network devices within a local area network. If a transmission destination is situated outside the local area network, the data packets are generally transported to a router device for the local area network, which router device transfers the data packets using a network address which is additionally contained in the data packets and identifies the transmission destination. Unlike a hardware address, a network address is resident above the data link layer and can be reversibly assigned to a network device via system administration. The network addresses used are frequently “Internet Protocol addresses”, also referred to as “IP” addresses below. The router device evaluates the network address in the data packets and, on the basis of this, determines an address information item identifying a destination network device which is registered in the router device and is as close as possible to the transmission destination or is possibly identical thereto. The data packets are then transferred to this destination network device using the address information item. If this destination network device does not match the ultimate transmission destination, this destination network device is responsible for transferring the data packets toward the transmission destination. 
     A method allowing data packets provided with an IP address to be transferred transparently via an ATM network (ATM: asynchronous transfer mode) to a transmission destination which is coupled to the ATM network and is identified by the IP address is known by the name IpoA (IP over ATM), for example. In this context, a router device uses the IP address in the data packets to determine the ATM address of an ATM exit network node which is as close as possible to the transmission destination and is used to forward the data packets in the ATM network. Aspects of this method are described in Internet specification RFC 2225, for example. 
     However, the evaluation of IP addresses for alternate routing which these methods require is a relatively complex operation which can generally be implemented only with a high level of circuit complexity. 
     It is an object of the present invention, therefore, to specify a method for transmitting data from a source network device to at least one destination network device which requires less complex address evaluation. It is also an aim to specify an address conversion apparatus for implementing the method. 
     SUMMARY OF THE INVENTION 
     The inventive method conveys data from a source network device to a destination network device using a fictitious hardware address associated with these data as destination address in the source network device. In this context, the switching on the basis of hardware addresses means that no complex evaluation of network addresses, such as IP addresses, is necessary. The fictitious hardware address referred to in this context is a hardware address information item which the source network device uses, on the basis of a transfer protocol used, to identify a transmission destination without matching the latter&#39;s hardware address prescribed by the manufacturer—also referred to below as the real hardware address. To be able, nonetheless, to transfer data associated with a fictitious hardware address to the intended destination network device, an inventive address conversion apparatus is provided. In the latter, fictitious hardware addresses are associated with a respective address information item which identifies a respective destination network device and can be used to convey the data to the respective destination network device. 
     The direct association between fictitious hardware addresses and address information items identifying destination network devices in the address conversion apparatus allows an address information item to be determined very easily; e.g., by accessing an association table, using a fictitious hardware address. By contrast, it would be much more difficult to determine such an address information item using an IP address instead of the fictitious hardware address. Since an IP address refers to a transmission destination at the end of a transfer chain and, in principle, any device provided with an IP address could be addressed throughout the world, there is no point in assigning IP addresses to destination network devices directly. By contrast with this, the number of fictitious hardware addresses to be managed by an inventive address conversion apparatus is limited by the fact that fictitious hardware addresses are provided only for the purpose of addressing transmission destinations registered in the source network device, so that fictitious hardware addresses can be directly assigned to address information items for destination network devices with little memory requirement. 
     A fundamental advantage of the present invention is that data can also be conveyed, on the basis of hardware addresses, to destination network devices situated outside the local area network of the source network device. To this end, the source network device needs to assign such a destination network device a fictitious hardware address, which the address conversion apparatus in turn assigns an address information item which can be used to convey the data to the destination network device even outside the local area network of the source network device. This significantly extends the range of application for hardware-address based addressing of network devices. 
     Another fundamental advantage of the present invention can be seen in that changing a real hardware address for a network device, e.g., when replacing a network module, at most requires little reconfiguration effort. This is essentially a consequence of using fictitious hardware addresses. Since these do not need to match any real hardware address for a destination network device, fictitious hardware addresses can also be retained when real hardware addresses are changed. By contrast with this, in the case of the current prior art, any change to a real hardware address for a network device requires that the hardware address tables be updated for all network devices which are meant to be able to address the altered network device using its hardware address. 
     The address information item associated with a fictitious hardware address in the address conversion apparatus may advantageously be a real hardware address for the destination network device; e.g., the latter&#39;s MAC address prescribed by the manufacturer. Data which are associated with a real hardware address and are transferred to the destination network device are recognized by the latter as being addressed to it and are received for the purpose of further processing or forwarding. If the real hardware address of the destination network device changes, e.g., due to a network module being replaced, it is necessary only to change the association between real and fictitious hardware addresses in the address conversion apparatus in order to be able to address the destination network device using the unaltered fictitious hardware address. 
     Alternatively, the address information item assigned to the data may also be a network address, for example an IP address or an ATM address, for the destination network device. Such a network address also allows addressing of destination network devices which are situated in a different communication network than the source network device. 
     In accordance with one advantageous embodiment of the present invention, one or more fictitious hardware addresses to be used as destination address can be formed by the address conversion apparatus and transmitted to the source network device. In this context, fictitious hardware addresses need to be formed such that they are unique in the local area network including the source network device and the address conversion apparatus and also do not match any real hardware address in this local area network. 
     In accordance with another advantageous embodiment of the present invention, an inquiry from the source network device after a hardware address for a network device identified by an address information item contained in the inquiry, e.g., an IP address, can be answered by the address conversion apparatus on behalf of the network device in question. Such inquiries may be made, for example, under the “ARP protocol” (address resolution protocol). For this purpose, the address conversion apparatus transmits a fictitious hardware address associated with this address information item in the address conversion apparatus to the inquiring source network device. This is a simple way of entering fictitious hardware addresses into a hardware address table, used for hardware addressing, associated with the source network device. The fictitious hardware addresses entered are then used for all subsequent data transmissions. 
     In accordance with one advantageous embodiment of the present invention, the data to be transferred can be transmitted from the source network device to the destination network device via the address conversion apparatus. As such, the address information item to be used to forward the data to the destination network device can be assigned to the data actually in the address conversion apparatus. In addition, the data can be converted in the address conversion apparatus in line with a transfer protocol used by the destination network device. As such the source network device requires no functionality going beyond the framework of conventional hardware-address based data transmission in order to perform data transmission in accordance with the present invention. The source network device need merely be connected to an inventive address conversion apparatus which can address the destination network device. This allows, by way of example, conventional router devices designed for connecting local area networks directly to be connected to an external communication network, and hence to be coupled to one another, via a respective inventive address conversion apparatus. 
     Additional features and advantages of the present invention are described in and will be apparent from, the following Detailed Description of the Invention and the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic illustration of three local area networks coupled via an ATM network. 
         FIG. 2  shows a schematic illustration of two address conversion devices. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic illustration of three local area networks LAN 1 , LAN 2  and LAN 3  coupled via an ATM network ATM. The local area networks LAN 1 , LAN 2  and LAN 3  each include a number of personal computers PC, PCA, PCB coupled locally via Ethernet or FDDI (fiber distributed data interface), for example, and are respectively coupled to the ATM network ATM via a router device R 1 , R 2  or R 3 . Whereas the router device R 1  for the local area network LAN 1  and the router device R 2  for the local area network LAN 2  are respectively connected to the ATM network ATM via an address conversion apparatus A 1  or A 2 , the router device R 3  for the local area network LAN 3  is connected to the ATM network ATM directly. The router devices R 1  and R 2  are conventional router devices which are actually designed for connecting local area networks directly. By contrast, its direct connection to the ATM network ATM means that the router device R 3  needs to have not only conventional router functionality but the capability for protocol conversion between a transfer protocol used in the ATM network ATM and a transfer protocol used in the local area network LAN 3 . The router devices R 1 , R 2  and R 3  and also the address conversion apparatuses A 1  and A 2  are considered to be special embodiments of network devices within the context of the exemplary embodiment. 
     The router devices R 1  and R 2  have been assigned a respective MAC address MAC 1  and MAC 2  as real hardware address by the manufacturer and have been assigned a respective IP address IP 1  or IP 2  by system administration, in the order specified. The router device R 3  has an associated IP address IP 3  and, in addition, an ATM address ATM 3  as network address. The address conversion apparatuses A 1  and A 2  also have respectively associated ATM addresses ATM 1  or ATM 2  as network address. In addition, the personal computer PCB has the associated IP address IPB. The router devices R 1 , R 2  and R 3 , the address conversion apparatuses A 1  and A 2  and also the personal computer PCB are respectively uniquely identified by the respectively associated addresses MAC 1 , MAC 2 , IP 1 , IP 2 , IP 3 , IPB, ATM 1 , ATM 2  and ATM 3 . 
       FIG. 2  shows a schematic illustration of the address conversion apparatuses A 1  and A 2  via which the router devices R 1  and R 2  are coupled to the ATM network ATM. The address conversion apparatuses A 1  and A 2  each contain, as functional components, a central controller ZS including a protocol conversion apparatus PU, an address assignment device AZ, an address determination device AE and an address checking device AP, and also an association table memory ZT which can be accessed by the central controller ZS. In both address conversion apparatuses A 1  and A 2 , the protocol conversion apparatus PU and the address assignment device AZ, and the address determination device AE and the address checking device AP, are respectively produced in a joint module. 
     In this exemplary embodiment, the association table memories ZT in the address conversion apparatuses A 1  and A 2  each contain at least three entries, in accordance with the number of local area networks LAN 1 , LAN 2  and LAN 3  coupled. The association table memory ZT in the address conversion apparatus A 1  contains a first entry with the IP address IP 1 , the MAC address MAC 1  and the ATM address ATM 1 , a second entry with the IP address IP 2 , a fictitious MAC address FIMAC 2  and the ATM address ATM 2 , and also a third entry with the IP address IP 3 , a fictitious MAC address FIMAC 3  and the ATM address ATM 3 . 
     The association table memory ZT in the address conversion apparatus A 2  correspondingly contains a first entry with the IP address IP 2 , the MAC address MAC 2  and the ATM address ATM 2 , a second entry with the IP address IP 1 , a fictitious MAC address FIMAC 4  and the ATM address ATM 1 , and also a third entry with the IP address IP 3 , a fictitious MAC address FIMAC 5  and the ATM address ATM 3 . In this context, the elements of each entry are respectively stored associated with one another. 
     The fictitious MAC addresses FIMAC 1 , FIMAC 2 , FIMAC 4  and FIMAC 5  each have the format of a hardware address. The fictitious MAC address FIMAC 2  or FIMAC 3  is used by the router device R 1  to address the router device R 2  or R 3 , and the fictitious MAC address FIMAC 4  or FIMAC 5  is used by the router device R 2  to address the router device R 1  or R 3 . To this end, the fictitious MAC addresses FIMMAC 2 , FIMAC 3  are entered into a routing table (not shown) for the router device R 1 , and the fictitious MAC addresses FIMAC 4 , FIMAC 5  are entered into a routing table (not shown) for the router device R 2 . The fictitious MAC addresses FIMAC 1 , FIMAC 2 , FIMAC 4  and FIMAC 5  are fictitious insofar as they match none of the real hardware addresses MAC 1  and MAC 2 , prescribed by the manufacturer, of the router devices R 1  and R 2  which they address, or they are not based on any real hardware address as in the case of the router device R 3 . Nevertheless, the fictitious MAC addresses are treated as real MAC addresses from the point of view of the router devices R 1  and R 2 . 
     The fictitious MAC addresses FIMAC 2 , FIMAC 3  and FIMAC 4 , FIMAC 5  are entered into the routing tables for the router devices R 1  and R 2  by the address conversion apparatuses A 1  and A 2  under the “ARP” protocol (address resolution protocol). Under this ARP protocol, inquiries from the router device R 1  or R 2  after MAC addresses for network devices are answered by the address conversion apparatus A 1  or A 2  on behalf of these network devices. In the course of such a request, the inquiring router device R 1  or R 2  transfers an IP address to the connected network devices with the order to send the MAC address of the network device identified by the IP address to the router device R 1  or R 2 , if known. By way of example, such an inquiry after the MAC address of the router device R 2  identified by the IP address IP 2  may be transmitted from the router device R 1  to the address conversion apparatus A 1 . In such a case, the address conversion apparatus A 1  first checks whether the IP address transferred with the inquiry, in this case IP 2 , is held in the association table memory ZT. If it is, the inquiry is answered by the address conversion apparatus A 1  by sending the MAC address associated with this IP address in the association table memory ZT, in this case the fictitious MAC address FIMAC 2 , to the inquiring router device R 1 . The fictitious MAC address FIMAC 2  transmitted to the router device R 1  is then entered by the router device R 1  into its routing table as MAC address for the router device R 2 . Inquiries from the router device R 2  are answered by the address conversion apparatus A 2  in a similar manner. 
     The fact that such inquiries are answered on a delegated basis by fictitious MAC addresses FIMAC 2 , FIMAC 3  and FIMAC 4 , FIMAC 5  stored in the address conversion apparatuses A 1  and A 2  allows for the real hardware addresses MAC 1 , MAC 2  no longer to be obtained in a complex manner via the communication network ATM. It also becomes unnecessary for each change to a real hardware address for a router device coupled via the ATM network ATM, e.g., as a result of a network module being replaced, to involve the routing tables for all other router devices being updated. 
     The text below considers data being conveyed on the basis of hardware addresses between the local area networks LAN 1 , LAN 2 , LAN 3  using the example of data transmission from the local area network LAN 1  to the local area network LAN 2 . 
     To transmit data from the personal computer PCA situated in the local area network LAN 1  to the destination personal computer PCB situated in the local area network LAN 2 , the data are transmitted to the router device R 1  in the form of data packets containing the IP address IPB of the destination personal computer PCB. The router device R 1  uses the routing table it contains to ascertain that the destination personal computer PC 2  identified by the IP address IPB can be reached via the router device R 2 . Consequently, the router device R 1  provides the data packets to be transmitted with the fictitious MAC address FIMAC 2  obtained from the address conversion apparatus A 1  as the hardware address for the router device R 2 . The IP address IPB of the destination personal computer PCB is maintained unaltered in the data packets in this context. The data packets are then forwarded from the router device R 1  to the address conversion apparatus A 1 . 
     In the address conversion apparatus A 1 , the address checking device AP first checks whether the fictitious MAC address FIMAC 2  received is held in the association table memory ZT. If it is, the address determination device PE determines the ATM address associated with the fictitious MAC address FIMAC 2  in the association table memory ZT (in this case ATM 2 ), which ATM address identifies that address conversion apparatus A 2  via which the router device R 2  is coupled to the ATM network ATM. The data packets to be transmitted are then converted by the protocol conversion apparatus PU, in line with the transfer protocol used in the ATM network ATM, into ATM cells to which the ATM address ATM 2  is assigned as network address by the address assignment device AZ. The ATM cells are then forwarded to the ATM network ATM and are transferred therefrom to the address conversion apparatus A 2  identified by the ATM address ATM 2 . In this context, the ATM cells may be transferred both via one or more fixed connections (PVC: permanent virtual circuit) and via one or more switched connections (SVC: switched virtual circuit) to be set up as required in the ATM network ATM. 
     The address conversion apparatus A 2  converts the ATM cells into data packets in line with the IP protocol again, the data packets being provided with the real MAC address MAC 2  of the router device R 2 , which is held in the association table memory ZT in this address conversion apparatus A 2 . The data packets are then forwarded to the router device R 2 , which recognizes the data packets as being addressed to it from the real MAC address MAC 2  associated with them. The router device R 2  then forwards the data packets received, after evaluating their IP address IPB, to the destination personal computer PCB identified by the IP address IPB. 
     Data packets are transferred from the local area network LAN 1  to a destination personal computer PC situated in the local area network LAN 3  in a largely similar manner, with the difference that, in this case, the data packets are addressed to the router device R 3  directly using the ATM address ATM 3 . The router device R 3  therefore needs to have both protocol conversion functionality and routing functionality. 
     In the present exemplary embodiment, fictitious MAC addresses are effective only for communication between a router device and the respective address conversion apparatus connected directly thereto. In this context, a router device and the address conversion apparatus connected directly thereto form a separate local area network. Hence, the fictitious MAC addresses FIMAC 2 , FIMAC 3  of the address conversion apparatus A 1  can be used independently of the fictitious MAC addresses FIMAC 4 , FIMAC 5  of the address conversion apparatus A 2 . In particular, the fictitious MAC addresses FIMAC 2  and FIMAC 3  used by the address conversion apparatus A 1  can match the fictitious MAC addresses FIMAC 4  and FIMAC 5  used by the address conversion apparatus A 2 . Fictitious MAC addresses can therefore be formed in an address conversion apparatus or assigned by system administration independently of the fictitious MAC addresses of another address conversion apparatus. 
     To allow a router device R 1  or R 2  to address network devices uniquely using fictitious MAC addresses, these MAC addresses need to be unique within the local area network formed by the router device and the address conversion apparatus connected directly thereto. In particular, a fictitious MAC address must not match any real or other fictitious MAC address in this local area network. This may be ensured, by way of example, by allocating real MAC addresses of old network devices no longer in use as fictitious MAC addresses. Alternatively, fictitious MAC addresses may be taken from a contingent reserved only for this purpose. Since fictitious MAC addresses can be repeated in different local area networks, this requires the provision of only one contingent within the scope of an appropriate maximum size for a local area network. The measures cited ensure that no conflict arises between a fictitious MAC address and a real MAC address even when real MAC addresses are changed within the local area network; e.g., due to a network module being replaced. 
     Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the invention as set forth in the hereafter appended claims.