Wide Area Network (WAN) Interface for a Transmission Control Protocol/Internet Protocol (TCP/IP) in a Local Area Network (LAN)

The invention relates to interfacing a TCP/IP (transmission control protocol/internet protocol) software of a local area network (LAN) to a wide area network (WAN) via a media access control (MAC) manager when the TCP/IP software utilizes a lower-level ARP (address resolution protocol) protocol to adapt physical network addresses to logical IP addresses, and a lower-level IP protocol to transfer useful traffic in IP data packets.

Computers can be interconnected to form various computer networks generally called local area networks (LAN). Ethernet and Token Ring are the most typical types of LAN. The equipment connected to the LAN may be a workstation, a server, a bridge, a router, etc. Computer application programs may use various network protocols to communicate with each other over the LAN. One such network protocol is TCP/IP (Transmission Control Protocol/Internet Protocol), which is used widely in equipments from different manufacturers especially in the UNIX operating system but also in the MS-DOS and OS/2 operating systems. TCP/IP is a protocol family consisting of a number of layered protocols complying with the Open Systems Interconnection (OSI) model specified by the International Standards Organization (ISO). The most important OSI layers are the transmission protocol layer (layer 4) and the network layer (layer 3). When the IP protocol is used in a LAN (Ethernet, Token Ring, FDDI), an ARP (Address Resolution Protocol) is needed to find a physical LAN address (or more correctly an MAC address) corresponding to the requested IP address. Typically, each computer maintains an ARP database from which it can look up the physical destination address for to the logical IP destination address. If the physical destination address cannot be found in the ARP database, the computer transmits a special ARP request message to the LAN so as to request a computer identifying the IP address to transmit the respective physical destination address.

It may also be advantageous that a workstation remote from the LAN (a remote workstation), or another LAN, can be connected to the LAN. Due to the long distance it is thereby necessary to make the interconnection via a so-called wide area network (WAN). The simplest form of the WAN is an asynchronous modem link from the remote workstation to a network element. On the other hand, the WAN may also be established over a transfer network. The ARP protocol is not needed over the WAN as the WAN does not contain LAN addresses for which the respective IP addresses should be found. A remaining problem is that the TCP/IP software designed for the LAN always attempts to use the ARP protocol to find out the correspondence between the addresses. TCP/IP software utilizing an MAC-level interface cannot therefore operate directly over the WAN.

The object of the present invention is to enable a computer using a TCP/IP software designed for a LAN to be interfaced to a WAN, thus enabling a more efficient utilization of WANs than previously.

According to the invention, the MAC manager of the WAN simulates operation complying with the ARP protocol towards the TCP/IP software, and transmits and receives only IP data packets over the WAN in a WAN-specific manner.

The basic idea of the invention is that the WAN MAC manager controlling traffic over the WAN simulates operation LAN workstation complying with the ARP protocol towards the TCP/IP application program. Accordingly, when the TCP/IP application program transmits an ARP request message to the MAC manager so as to request a physical destination address (network address) corresponding to the IP destination address, the MAC manager responds with an appropriate reply message which, however, contains an arbitrary physical address for said IP address. The TCP/IP application assumes to have received a reply and an appropriate address from another computer connected to the LAN and starts to transfer useful data with the other computer in compliance with the IP protocol. It thereby transmits IP data packets provided with its own address and the above-mentioned arbitrary physical address. The MAC manager identifies the IP packets, removes the above-mentioned physical addresses and other possible LAN control data and transmits only the IP data packet over the WAN in a WAN-specific manner. Correspondingly, the IP data packets are extracted from data received over the WAN and provided with the physical address of the TCP/IP application and the above-mentioned arbitrary address, and then transmitted to the TCP/IP application program. The WAN is thus all the time transparent to the TCP/IP application, and the operation of the TCP/IP application is equal to communication over a normal LAN.

The invention is very easy and economical to implement as modifications and extra functions are required only in the MAC manager. The TCP/IP applications and WANs communication can be accomplished in a conventional way.

In the following the invention will be described in more detail by means of illustrating embodiments with reference to the attached drawings, in which

FIG. 1 illustrates schematically a LAN interconnected with another LAN or another remote workstation over a WAN according to the invention;

FIG. 2 illustrates a workstation provided with a MAC manager according to the invention;

FIG. 3 illustrates a router provided with a MAC manager according to the invention;

FIGS. 4A, 4B, 4C and 4D referred collectively as FIG. 4 illustrate the simulation of an ARP protocol; and

FIGS. 5A, 5B, and 5C referred collectively as FIG. 5 illustrate the transfer of useful traffic.

The invention will be described below by way of example with reference to Ethernet LANs, but the invention may also be applied to other types of LANs, such as Token Ring. FIG. 1 shows two separate Ethernet LANs 1 and 2 both having a plurality of associated workstations WS and servers SR (FIG. 1 shows only a few units for the sake of clarity). The workstations WS may be any computers, such as the MIKROMIKKO 4 from ICL Personal Systems Ltd. The servers SR may be e.g. printers, disk drives, routers RO or workstation servers implemented by computers. The workstations and servers are typically supported by the UNIX, OS/2 or DOS operating system.

The routers RO connect together the geographically separate LANs 1 and 2 by means of a WAN. The WAN 3 may be any connection, such as a telephone line, X.25 line, leased line, asynchronous link, SNA network. In addition, an asynchronous remote workstation RAWS is connected to both LANs over the WAN 3.

In the preferred embodiment of the invention, the network software of the remote workstation RAWS is such as shown in FIG. 2. As used herein the network software refers to system software needed to make the workstations and servers communicate with each other. There are two main types of network software: network operating systems and telecommunication software. The network operating system is usually an application program in a server and requires its own counterpart at the workstation. The network operating system is usually on the highest ISO level of hierarchy. Network operating-system softwares include e.g. IBM PC Lan Program, OS/2 Lan Manager, and ICL MS Lan Manager.

Telecommunication software is intended for intra-network communication. All workstations and servers connected to the network have to use the same protocol. The remote workstation RAWS according to the invention uses TCP/IP telecommunication software connected to the network operating-system software via a NETBIOS interface. The TCP protocol is on the fourth level of ISO hierarchy, and the IP protocol is on the third level of ISO hierarchy. The TCP/IP software uses the IP protocol on the lower level of hierarchy (OSI layer 3) to transfer useful traffic (payload) in IP data packets. It also uses the ARP (Address Resolution Protocol) to adapt the physical network addresses to the logical addresses used by the IP protocol. The ARP is described in the standard RFC-826: An Ethernet Address Resolution Protocol. The IP is described in the standard RFC-894: Standard for the Transmission of IP Datagrams over Ethernet Networks. Typically each computer maintains an ARP database from which it can look up an appropriate physical destination address on the basis of the logical IP destination address. If the physical destination address cannot be found in the ARP database, the computer transmits a special ARP request message to the LAN so as to request a computer identifying the IP address to transmit the respective physical destination address.

The next lower OSI layer (layer 2) uses an MAC (Media Access Control) manager software which controls traffic over the serial WAN 3. The TCP/IP software and the MAC software are interconnected via an NDIS interface. The NDIS interface is described in the Microsoft/3Com Network Driver Interface specification. Below the MAC manager there is a physical layer (OSI 1) comprising a physical transfer path and a physical interface to it.

The TCP/IP software assumes that the network utilized on the transfer network level is a LAN (in this specific case Ethernet) while the MAC manager, in fact, operates in a WAN environment. According to the invention, the MAC manager simulates operation of LAN workstation complying with the ARP protocol and the Ethernet frame format in communication towards the TCP/IP software so that the TCP/IP software assumes that it is connected to the Ethernet network. An example of this ARP simulation is shown in FIG. 4, where the MAC manager responds to the ARP request. The TCP/IP software transmits the ARP Request packet shown in FIG. 4A to the MAC manager by the NDIS command TransmitChain. The packet is transmitted in order to find out the physical address corresponding to the requested IP address. As shown in the figure, the ARP Request packet contains the field A of the physical destination address (in this specific case an empty location for the requested physical destination address), the field B of the physical source address (the physical address of the TCP/IP software), the type field C of the ARP message, and the ARP frame D. As shown in FIG. 4B, the ARP frame D contains e.g. a physical source address D1, an IP source address D2, a physical destination address D3 (unknown) and an IP destination address D4. The MAC manager assembles an ARP Reply frame E shown in FIG. 4C from the received ARP Request frame D. An arbitrary 6-byte constant is inserted to the frame E as a physical source address E1; the IP source address E2 will be the IP destination address D4 of the frame D; the physical destination address E3 will be the destination address D1 of the frame D; and the IP destination address E4 will be the IP source address D2 of the frame D. The NDIS command ReceiveLookAhead illustrated in FIG. 4D is then formed by incorporating the ARP type field F, the physical source address G (equal to E1) and the physical destination address H (equal to E4) to the ARP frame E.

After having received the message of FIG. 4D, the TCP/IP software assumes that the ARP request has been successful and starts to transmit IP packets to the MAC manager, which is illustrated in FIG. 5. FIG. 5A shows a packet K transmitted by the TCP/IP software via the NDIS interface, the LAN control data comprising a physical destination address K1 (equal to D1), a physical source address K2 (equal to D3), an IP type K3 and an IP packet containing the header IP-Header and IP data. The MAC manager extracts the LAN control data from the IP packet, as shown in FIG. 5B, and inserts the IP packet in the data field of the SDLC packet, which is preceded by the SDLC-Header when the WAN is an SDLC link (Synchronous Data Link Control) e.g. in an SNA network (Systems Network Architecture), as shown in FIG. 5C. If the WAN 3 is an X.25 link, the IP packet is transmitted in the data field of the X.25 packet. If the WAN 3 is an asynchronous connection, the IP packet is SLIP encoded before being transmitted. SLIP encoding is specified in the standard RFC-1055: Non-Standard for Transmission of IP Datagrams over Serial Lines.

Correspondingly, when SDLC data packets are received from the WAN 3, the MAC manager executes procedures inverse to those shown in FIG. 5: the MAC manager extracts the WAN control data (SDLC-Header) from the IP data packets and provides IP data packets with the LAN control data and then transmits the packets to the TCP/IP software.

FIG. 3 illustrates the implementation of the invention in a server computer operating as a router such as the router RO in FIG. 1. In the embodiment of FIG. 1, the TCP/IP software is connected to the IP router via the NDIS interface. The IP router in turn is connected via another NDIS interface to the WAN MAC manager, the Token Ring LAN MAC manager, and the Ethernet LAN manager. The IP router is able to route normal traffic complying with the ARP and IP protocols between the MAC managers and between the MAC managers and the TCP/IP software. Communication between the TCP/IP software and the WAN MAC manager takes place as described above with reference to FIG. 4.

The figures and the description related to them are only intended to illustrate the present invention. In its details, the method according to the invention may vary within the scope of the attached claims.