METHOD AND APPARATUS FOR TRANSPARENTLY BRIDGING TRAFFIC ACROSS WIDE AREA NETWORKS

Abstract: A technique for logically connecting local area networks (LANs) that may be separated by wide area networks containing routers and other network components. A logical link is formed between two bridge-like devices called tunnelers, such that, once a tunnel has been established between two LANs, other devices on the LANs can communicate as if the tunnel were a bridge. The tunneling mechanism of the invention requires that each LAN or extended LAN have only one active tunneler at any particular time, referred to as the designated tunneler, and each of the tunnelers is configured to have knowledge of the identities of the other tunnelers. A tunnel is established after a successful exchange of messages between two tunnelers, and then traffic may be forwarded through the tunnel in a transparent manner. The tunneling mechanism permits messages to be forwarded between LANs separated by a wide area network containing routers. Moreover, the mechanism permits filtering of traffic, such that only selected types of traffic, or messages for selected destinations or from selected sources, are forwarded through tunnels. The tunneling mechanism inherently precludes the formation of closed communication loops. An alternate embodiment of the invention optimizes the configuration process for particular network topologies.

Description: METHOD AND APPARATUS FOR TRANSPARENTLY BRIDGING TRAFFIC ACROSS WIDE AREA NETWORKS BACKGROUND OF THE INVENTION This invention relates generally to intercon¬ nected networks of computers and related devices and, more particularly, to techniques facilitating communica¬ tion between stations connected to widely separated lo- cal area networks (LANs) . A local area network includes a set of stations or nodes connected to a common commu¬ nication bus or ring. Typically, all elements of a sin¬ gle LAN are located in a single building or group of buildings. Multiple LANs are often connected together by devices called bridges, to form an extended LAN. A bridge is a device that is connected to more than one LAN, "listens" to message traffic on each of its LANs, and forwards the traffic onto selected other LANs. The bridge message forwarding operation is limited by a spanning tree algorithm in which all bridges partici¬ pate. Messages are forwarded only over a loop-free span¬ ning tree, to avoid closed loops and multiple copies of messages. Message forwarding is further controlled by the bridge's "learning" function. Each bridge learns the directional locations of various stations that are the sources of messages it hears, and "remembers" these locations for future use in forwarding message traffic. Multiple LANs and extended LANs may also be interconnected to form wide area networks (WANs) . The mechanism through which extended LANs are interconnect¬ ed is typically through devices such as routers. At a broad conceptual level, routers perform a similar task to bridges in that each router is connected to multiple LANs or extended LANs. However, the router operates at a different protocol layer (known as the network lay- er) , and a router can be used for forwarding traffic only if the source of the traffic cooperates by using a network layer protocol that is understood by the rout¬ er. A bridge, on the other hand, is a packet switch that is "transparent." A station that does not have a network layer protocol, or has a network layer protocol that is not implemented by the router, can use the bridge as a packet switch. Unfortunately, however, bridges have significant drawbacks that preclude their use in many contexts. The most significant drawback of a bridge, as compared with a router, is that bridges use only a sub¬ set of the actual topology of the interconnected net¬ works. In particular, redundant paths cannot be used, because to do so would result in closed loops and un¬ wanted multiplication of message traffic. As already mentioned, to ensure a loop-free topology bridges par¬ ticipate in a spanning tree algorithm, which establish¬ es a loop-free tree structure. If two extended LANs were to be interconnected by a bridge, the separate spanning trees of the two networks would have to be combined, forming one larger extended IAN. This might not be desirable from a network administration stand¬ point. Another important disadvantage of bridges is that they cannot forward traffic through a router, be¬ cause the packets handled by the bridge do not neces¬ sarily have the appropriate network layer information needed to utilize the router. Also, since routers do not participate in the spanning tree algorithm, there is a possibility of forming loops in the topology if bridges were to forward traffic through routers. In summary, neither routers nor bridges pro¬ vide an ideal solution to the problems of forwarding message traffic over wide-area networks. Some communica- tion protocols cannot work through routers, since rout- ers support only certain network layer protocols. But bridges are limited, by the spanning tree algorithm, to a subset of the overall network topology, and cannot be used to forward traffic through routers that separate LANs. Application Serial No. 07/489,910, filed March 7, 1990, entitled "Utilization of Redundant Links in Bridged Networks" (which is owned by the assignee of the present application and identified by docket number PD90-0097) , provides for the establishment of point-to- point links outside the spanning tree, but still does not permit forwarding of traffic through traditional routers. What is needed is a new approach that permits traffic to be forwarded transparently across WANs, us- ing more optimal routes and permitting the traffic to traverse traditional routers. The present invention sat¬ isfies this need. Another way of viewing the problem that this invention solves is that, ideally, there should be some way to address messages directly to particular extended LANs. However, the addressing conventions that have evolved for interconnected networks do not include any convenient means for addressing extended LANs. What is needed, then is some way of forwarding a message more directly to the extended LAN in which the message desti¬ nation is located. The present invention also satisfies this need. _- _- SUMMARY OF THE INVENTION The present invention resides in a method and apparatus for logically interconnecting local area net- works and extended local area networks, even if they are separated by wide area networks having traditional routers. Briefly, the method of the invention includes the steps of providing a number of bridges with the ca- pability to form "tunnels" between LANs that may be widely separated, these special bridges being referred to as tunnelers; then electing a unique designated tun¬ neler for each extended IAN (XLAN) for which tunneling is to be provided. Each XLAN has no more than one desig- nated tunneler, but a single tunneler may be designated by more than one XLAN. The next steps are configuring the tunnelers, by supplying each with information iden¬ tifying other tunnelers with which a tunnel may be es¬ tablished, establishing at least one tunnel between two designated tunnelers by exchanging messages between the. two tunnelers, if the configuration rules state that such a tunnel should exist, and then selectively for¬ warding message traffic through the established tunnel, from one LAN to another. In a preferred embodiment of the invention, the step of electing a unique designated tunneler for each XLAN includes executing a spanning tree algorithm among all of the bridges connected to the XLAN, thereby electing a root bridge. Since not all bridges will ne- cessarily have tunnel capability, some adjustment may be needed to bridge priorities used in the spanning tree algorithm, to ensure that the root bridge does have tunneling capability. In this way, the election of a designated tunneler is conveniently effected by means of the existing spanning tree algorithm. The step of configuring the tunnelers includes supplying to each tunneler the identities of other tun¬ nelers and the identities of ports through which the other tunnelers are connected to XIANs with which the ports are associated. The step of establishing a tunnel includes transmitting a greeting from a designated tun¬ neler to each other tunneler that is a potential tunnel endpoint, receiving a message from one of the potential tunnel endpoints confirming that a tunnel may be estab- lished, and recording the successful establishment of a tunnel. Establishing a tunnel may also include the steps of transmitting a greeting to at least one backup tunneler, after failing to receive a confirming message from the other designated tunneler; then receiving a message from the backup tunneler confirming that a tun¬ nel may be established; and finally recording the suc¬ cessful establishment of the tunnel. The step of selectively forwarding message traffic through the established tunnel includes the steps of receiving a message at a tunneler, and deter¬ mining whether the message destination is known to the tunneler. If the destination is known, the method fur¬ ther includes the steps of determining whether a tunnel is required to reach this particular destination, and, if so, forwarding the message through the tunnel, if the tunnel has been established and the message is of a type authorized for forwarding through the tunnel. If the destination is not known, a subsequent step in the method is determining whether the message was received from a tunnel. If so, the message is for¬ warded through a port defined as the tunnel endpoint. If the message was not received from a tunnel and the message destination is not known, the method further includes forwarding the message over all non-tunnel ports in the spanning tree, except the one through which the message was received, and forwarding the mes¬ sage through all established tunnels, if any, for which this tunneler is a sending endpoint, if the message is of a type authorized for transmission through the tun- nel. Forwarding a message through a tunnel includes the step of appending to the message a destination ad¬ dress comprising the address of a receiving endpoint tunneler of the tunnel, and a port identifier that also defines, in part, the receiving endpoint of the tunnel. Then the message is transmitted through an appropriate port to reach the receiving endpoint of the tunnel. If the message was received from a tunnel, the method fur¬ ther comprises the step of stripping the tunnel receiv- ing endpoint address from the received message, before forwarding it through the tunnel receiving endpoint port. In an alternate embodiment of the invention, the configuring step is optimized for specific network topologies. In this case the step of .configuring in¬ cludes supplying the tunnelers with a subset of informa¬ tion concerning other tunnelers, the subset of informa¬ tion being optimized for a particular network topology. The step of establishing a tunnel then includes one or more of the following steps: (1) For a designated tunneler, sending a greet¬ ing to another designated tunneler with which it has been configured, and receiving a response confirming that a tunnel has been established; (2) for a designated tunneler, sending a greet¬ ing to a nondesignated tunneler with which it has been configured, receiving a refusal response identifying a designated tunneler, redirecting the greeting to the designated tunneler identified in the refusal response, and receiving a response confirming that a tunnel has been established; and (3) for a nondesignated tunneler, periodically transmitting multicast messages identifying the tunnel¬ ers with which it has been configured, and, for a desig¬ nated tunneler, responding to a multicast message from a nondesignated tunneler, by sending a greeting to one of the tunnelers contained in the multicast message, and receiving a response message confirming that a tun¬ nel has been established. The optimized configuration approach reduces the burden of having to manually configure every tunnel¬ er with knowledge of every other tunneler. The cost of this simplification is some added complexity in the es¬ tablishment of tunnels, but the approach is worthwhile for some network topologies. It will be understood that the invention also encompasses equivalent apparatus claims similar in scope to the various forms of the method discussed above. It will be appreciated from the foregoing that the present invention represents a significant advance in the field of communication networks. In particular, the invention provides a novel way of logically inter¬ connecting two LANs, even through they may be separated by wide area networks containing routers. The tunneling protocol of the invention effects this logical intercon¬ nection in a manner that permits user controlled filter¬ ing of the tunnel traffic, and precludes the formation of any closed loops. Traffic filtering may fulfill any desired network administration objectives, such as limiting traffic flow through a tunnel to selected protocols, or limiting flow through a tunnel to traffic having selected destination or source addresses. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. — O ~~ BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified schematic diagram of one example of a network topology having multiple ex- tended local area networks (XLANs) that are connected through a wide area network (WAN) having conventional routers; FIG. 2 is a diagram similar to FIG. 1, but showing a different topology example; FIG. 3 is a diagram similar to FIG. 1, but showing yet another topology example; FIG. 4 is a flowchart depicting the operations performed in establishing tunnels; FIG. 5 is a flowchart depicting the operations performed by a tunneler in processing and forwarding received messages; and FIG. 6 is diagram similar to FIGS. 1-3, but showing another topology, in which a configuration opti¬ mization is employed. DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in the drawings by way of illustra¬ tion, the present invention is concerned with a novel technique for logically interconnecting multiple local area networks (LANs) or extended local area networks (XLANs) , which may be separated by wide area networks (WANs) containing traditional routers. Neither routers nor bridges can perform this function in all situa- tions, since routers do not support all communications protocols, and bridges are limited by the spanning tree algorithm and cannot forward traffic through tradition¬ al routers. In accordance with the invention, each LAN or XLAN has a bridge-like device that is designated as the "tunneler" for that network, and logical connections, called tunnels, are established between pairs of tunnel¬ ers. Each such pair of tunnelers functions in much the same way as bridge, in the sense that traffic is for- warded transparently from one network to another. Each tunneler has a connection to the LAN or XLAN of which it is the designated tunneler, and at least one connec¬ tion to a WAN that separates the LANs or XLANs. FIG. 1 is one example, in simplified form, of a network topology in which the invention may be used. The topology includes two extended LANs 10, 12, indicat¬ ed as XLAN1 and XIAN2. XIAN1 has three LANs 14, 16, 18 connected by bridges B, indicated at 20, 22. XLAN2 12 is a single LAN. The rest of the topology is a WAN 24, referred to as a "WAN cloud." This includes any combina¬ tion of LANs, point-to-point links, bridges, and rout¬ ers. The WAN cloud 24 handles message traffic in accor¬ dance with conventional routing protocols, for instance (but not restricted to) the Open Systems Interconnec- tion (OSI) Reference Model, developed by the Inter- national Standards Organization (ISO) . The problem with which the invention is concerned is how to establish a communication link between XIANl and XLAN2, such that traffic can be forwarded through the WAN cloud 24, as optimally as possible, even if the source or destina¬ tion of the traffic is not operating in accordance with the routing protocols in the W.AN cloud. In this illus¬ tration, XIANl is connected to the W.AN cloud 24 by six "tunnelers" indicated as Tl, T2, T3, T4, T5, and T6, and XLAN2 is connected to the WAN cloud by two tunnel¬ ers T7 and T8. Tunneler Tl is connected to LAN 14, tun¬ nelers T2 and T3 are connected to LAN 16, and tunnelers T4, T5 and T6 are connected to LAN 18. Each tunneler may considered as performing in some ways like a bridge, but having additional tunneling capability, to be described in detail below. The tunneling capability involves a prelimi¬ nary procedure for establishing tunnels, and then a traffic handling procedure whereby traffic may be for- warded through the established tunnels. Establishing the tunnels is effected by two basic steps. First, a procedure is used to ensure that there is only one des¬ ignated tunneler for each LAN or extended LAN under con¬ sideration. Second, the designated tunnelers are manual- ly "configured" by an operator or network manager, to define the tunnels that can be established for each pair of LANs or extended LANs. Although each IAN or extended LAN has only one designated tunneler, the same tunneler may perform the function of designated tunneler for more that one LAN or extended LAN. This point is not clear from FIG. 1 but will become apparent from a later example. There are two alternatives for ensuring that each LAN or XLAN has only one designated tunneler. The first alternative, and probably the easiest to i ple- ment, is to use the same procedure that the spanning tree algorithm uses to elect a root bridge. In the span¬ ning tree algorithm, a single bridge is elected and designated the "root bridge" for each extended LAN, to ensure that interconnected bridges in the extended LAN form a loop-free topology. If the same procedure is used to elect a tunneler, it must be used in such a way as to ensure that the elected bridge has tunneling capa¬ bility. If only some of the bridges participating in the spanning tree algorithm have tunneling capability, some adjustment would need to be made to bridge priori¬ ties used in the spanning tree algorithm, to ensure that the bridge elected as the root bridge also has tun¬ neling capability. Using the spanning tree algorithm to elect a designated tunneler has the advantage that the procedure already exists and is utilized by bridges in the network. Moreover, the spanning tree algorithm oper¬ ates effectively even in periods of heavy traffic con¬ gestion. The alternative to using an existing spanning tree algorithm to designate a tunneler is to implement a special election process among the potential tunnel¬ ers in each extended LAN. This algorithm could be close¬ ly duplicative of the spanning tree algorithm, wherein each tunneler transmits a previously assigned priority, and a collective election process eliminates all but one of the tunnelers. This has some disadvantages asso¬ ciated with the implementation of a special election procedure, principally additional cost and complexity. Moreover, there is no guarantee that such a procedure would work effectively in heavy traffic conditions. Since multicast messages loo): like ordinary data packets, and compete with other data packets for access to the network, election messages may be discarded under some conditions. The other preliminary task is to configure, by network management, the tunnelers with the definitions of tunnels that should be built. For example, in FIG. 1, if the designated tunneler for XIANl is Tl and the designated tunneler for XLAN2 is T7, the tunnel between XIANl and XLAN2 would be defined by the network layer address of each end-point of the tunnel and by the port through which the end-point is connected to the appro¬ priate XLAN. If Tl is connected to XIANl by its #1 port, and T7 is connected to XLAN2 by its #2 port, the complete definition of the tunnel is (Tl,l), (T7,7). Tunneler Tl is configured with a corresponding tunnel definition as seen from the Tl end, i.e. (1,T7,2) . Tun¬ neler T7 is configured with a tunnel definition as seen from the T7 end, i.e. (2,T1,1). Configuration optionally involves two other types of information. First, alternate tunnel defini¬ tions may be configured into the tunnelers. For exam¬ ple, Tl may be configured to use T8 as an alternate tun- nel end-point, in the event that T7 is not available for some reason. The second type of additional configu¬ ration data includes a definition of the types of mes¬ sages that will be forwarded over each tunnel. The tun¬ nel may be used for only selected protocols, or for all types of traffic, or for traffic involving only select¬ ed destinations or sources, all at the option of the network manager. After election of a designated tunneler for each LAN or extended LAN, and configuration of the des- ignated tunnelers, tunnels are established by an ex¬ change of messages between tunnelers that have been de¬ fined as tunnel end-points. For example, Tl, having been configured to establish a tunnel to T7, will send a greeting message through the WAN cloud 24, addressed to T7. In essence, the greeting indicates that Tl wishes to establish a tunnel with T7. T7 sends a reply message, either assenting to the establishment of a tun¬ nel, or indicating that it (T7) is not the designated tunneler for the requested port, or indicating that it (T7) has not been configured for this tunnel. Another possibility is that T7 cannot be reached for some rea¬ son, and no reply is received by Tl. This exchange may take place in the opposite direction, depending on timing considerations. Or there may be a symmetrical exchange, with both tunnelers sending messages to their opposite numbers in the defined tunnel. After a tunnel is established, such as the tun¬ nel T1-T7, it functions in a similar fashion to a bridge connecting the two XLANs. When tunneler Tl "hears" a message on XLAN1, Tl forwards the message through the tunnel to tunneler T7, after first append¬ ing the network layer address for T7. On receiving such a message, tunneler T7 strips off the network layer ad¬ dress and forwards the message onto X1AN2. Traffic may also be transmitted through the tunnel in the opposite direction by the same mechanism. The tunnelers have a bridge-like capability of learning the locations of stations from which messages have come. Each message has a source address, in the form of a data link layer address. When tunneler Tl, for example, receives traffic through its #1 port, this traffic can be identified as having a source address in the direction of XIANl. A subsequent message having a destination address that corresponds with a source address previously noted as emanating from XLANl, need not be forwarded through the tunnel to T7. However, when the direction of a destination address is not known to the tunneler, the message is forwarded through the established tunnel to XLAN2. FIG. 2 depicts another example of a topology, having a WAN cloud 24, two tunnelers Tl and T2, and a total of eight extended LANs, indicated as XLANl through XLAN8. XLANl through XLAN5 are connected to tunneler Tl through its ports #1 through #5, and X1AN6 through XIAN8 are connected to tunneler T2 through its ports #1 through #3. Tunneler Tl is connected through its additional port #6 to the WAN cloud 24 and tunneler T2 is connected to the WAN cloud through its additional port #4. Tl is the designated tunneler for extended LANs XLANl through XLAN5, and T2 is the designated tun¬ neler for extended LANs XIAN6 through XLAN8. Tunnelers Tl and T2 may be configured to establish as many as fif¬ teen tunnels between various pairs of XLANs connected to different tunnelers. For example, the tunnels from XLANl will be configured as: (1,T2,1), for tunneling from XLANl to XLAN6, (1,T2,2), for tunneling from XLANl to XLAN7, and (1,T2,3), for tunneling from XLANl to XLAN8. Four additional sets of three tunnels can be configured with XLAN2 through XLAN5 as the starting points. Addi¬ tional tunnels may be established between pairs of XLANs connected to the same tunneler, such as XLAN4 and XLAN5. There are ten possible additional tunnels using only Tl for both tunnel endpoints, and three possible additional tunnels using T2 for both tunnel endpoints. Although XLAN4 and XLAN5, for example, can be connected using Tl as a conventional bridge, a tunnel may be a more desirable solution because it provides a conveni¬ ent mechanism for filtering traffic between the two XLANs. Moreover, if the two XLANs are conventionally bridged by Tl, their spanning trees will necessarily have to be merged and this may not be desirable. For purposes of illustration, the WAN cloud 24 in FIGS. 1 and 2 has been shown as being separate from the other extended LANs in the topology. In general, however, the principles of the invention discussed above also apply if the cloud is considered to encom¬ pass the extended LANs between which the tunnels are established. The requirement that there be only one desig¬ nated tunneler for each extended LAN leads to a desir¬ able, and not immediately obvious property of networks connected by tunnels. When tunnels are established in accordance with the invention, there is no possibility of closed loops being formed by any interconnection of the tunnels. This property can be understood from the further example of FIG. 3, which shows a topology in¬ cluding four extended LANs, XLANl through XLAN4, each of which has a designated tunneler Tl through T4, re- spectively. The established tunnels include T1-T2, T2-T3, T3-T4 and T4-T1, which form the sides of square, together with T1-T3 and T2-T4, which form the diagonals of the square. At first sight, it might appear that a message could be circulated around the sides of the square in a closed loop, and that unwanted duplicate messages would be generated. Suppose a message from XIANl is to be forward¬ ed by tunneler Tl, and that the destination is unknown to Tl. Tl will forward the message over its three estab- lished tunnels. T1-T2, T1-T3 and T1-T4. If, for exam¬ ple, the message destination is on XLAN3, it will be forwarded there via tunnel T1-T3. Tunneler T2 will also receive a copy of the message, through tunnel T1-T2, but will not forward the message through a second tun- nel, such as T2-T3. There is only one established tunnel between Tl and T3, and that is the "diagonal" tunnel T1-T3. Each tunneler is conditioned to operate such that traffic received through one tunnel should not be forwarded through another. Since there is only one designated tunneler for each extended LAN, this rule is simple to put into effect. The procedure performed by each tunneler in establishing tunnels is shown in simplified flowchart form in FIG. 4. The tunneler first determines if it is the uniquely designated tunneler for a particular ex¬ tended LAN (XLAN), as indicated in block 30, and then determines whether this tunneler has been configured as a tunneler to another XLAN, as indicated in block 32. If the answer to either of these questions is negative, the tunneler shifts its attention to other XLANs to which it may be connected (block 34) and repeats the questions of blocks 30 and 32. If the tunneler is a des¬ ignated tunneler for a particular XLAN and has been con¬ figured as a tunneler to some other XLAN, the next step is to perform a communications "handshake" with the tun¬ neler that has been configured as the other endpoint of the tunnel. The handshake, indicated in block 36 may take any of several forms, but two possible results of the handshake are that the establishment of tunnel is confirmed, as indicated by line 38 from block 36, or that the establishment of the tunnel is refused for some reason, as indicated by line 40. In the latter case, there may be subsequent communication with an al¬ ternate tunnel endpoint, if one has been specified by configuration, as indicated in block 42. In any event, after establishment of a tunnel has been confirmed or refused, the tunneler repeats this entire process for other XLANs to which it may connected, as indicated in block 34. The operations performed by a tunneler in for¬ warding messages are summarized in FIG. 5. Like a bridge, a tunneler has multiple ports that are connect¬ ed to separate LANs or XLANs. In addition to its physi¬ cal ports, a tunneler may be considered to have a "tun- neler port" for each established tunnel for which the tunneler serves as an end-point. When a message is re¬ ceived at a tunneler, as indicated at 50, the source address of the message is noted, in conjunction with the port from which the message was received, and a database of addresses and their port locations is devel¬ oped. This learning function, indicated in block 52, includes learning the locations of addresses in terms of their corresponding "tunnel ports" of the tunneler. If the destination of the received message is known from prior message activity, as determined in block 54, the identity of the port through which the message should be forwarded is already known, and the message may be forwarded, as indicated in block 56. The destination may, for example, be located on a IAN that is directly connected to the tunneler, in which case the message can be forwarded through the appropriate non-tunnel port. If the known destination may be reached through an established tunnel, and if the mes¬ sage is of a type that has been authorized for forward- ing through a tunnel, then the message is forwarded through the established tunnel. This requires that a network layer address be appended to the message, giving the address of the receiving end-point of the tunnel. Then the message is forwarded through the tunnel. If the destination of the received message is not known, the tunneler next determines whether the mes¬ sage was received from a tunnel, as indicated in block 58. If so, the message is simply forwarded through the appropriate port that has been designated as the tunnel receiving end-point, as indicated in block 60. The mes¬ sage, having been received from a tunnel, is not for¬ warded through any other tunnels, and the possibility of closed loops is avoided. If the received message was not received from a tunnel, and its destination is un- — lo — known, it should be forwarded through all non-tunnel ports permitted by the spanning tree, and through all tunnel ports, i.e. through all tunnels for which the tunneler is functioning as an end-point, as indicated in block 62. In forwarding through the tunnel ports, the tunneler must first check that the tunnels have been established and that the message is of a type au¬ thorized for forwarding through tunnels. The foregoing description is not intended to preclude the possibility of a tunnel being established between two extended LANs connected to the same tunnel¬ er, i.e. where one tunneler is used as both endpoints of a tunnel, but using different ports, of course . Al¬ though this type of interconnection could be handled by the conventional bridge function, the tunnel mechanism provides the additional capability of filtering traffic in some desired way, such that only certain categories will be allowed to pass through the tunnel. Also, as noted earlier, using the tunneling mechanism avoids having to merge the spanning trees of the two bridged networks. The tunneling mechanism described above works to advantage in any of a variety of network configura¬ tions. In particular, the use of tunnels in accordance with the invention allows traffic to be bridged from one extended IAN to another, even when an interposed WAN includes routers. Considered from another perspec¬ tive, the invention provides a means for addressing ex¬ tended LANs. Conventional communications protocols pro- vide no means for addressing LANs and extended LANs, but the invention achieves this objective since a desig¬ nated tunneler and an associated port are unique to a LAN or extended LAN. Thus the tunneler-port identifica¬ tion also identifies the LAN. Moreover, tunneling is achieved by a mechanism that precludes the formation of undesirable closed loops, allows grouping of tunnelers to designate alternates, and can make use of an exist¬ ing spanning tree algorithm to elect a designated tun¬ neler for each extended IAN. Tunnel configuration, in accordance with the invention as described thus far, requires each poten¬ tial tunneler to be configured with a knowledge of all of the other potential tunnelers. For example, in the topology of FIG. 1, each of the six tunnelers connected to XLANl has to be configured with knowledge of the two tunnelers connected to XIAN2 and each of the two tunnel¬ ers connected to XLAN2 has to be configured with know¬ ledge of all six of the tunnelers connected to XLANl. In total there will be 2 x 6 pieces of configuration information for the FIG. 1 topology. In an alternate embodiment of the invention, the amount of configura¬ tion information can be reduced, or optimized, based on knowledge of the specific network topology in which the invention is being used. An example of a topology in which such an optimization scheme could be used is shown in FIG. 6. FIG. 6 shows four extended LANs, designated XLANl, XLAN2, XLAN3 and XLAN4. XLANl is connected to a tunneler Tl, which is connected to another tunneler T2 through a point-to-point link LI. The tunneler T2 is connected by another port to XIAN4. Similarly, XLAN2 is connected to XLAN4 through a path comprising tunneler T4, link L2, and tunneler T3. Finally, XLAN3 is connect¬ ed to XLAN4 through a path comprising tunneler T5, link L3, and tunneler T6. The links LI, L2, L3, may be con¬ sidered a special case of the WAN shown in FIG. 1. In the configuration of FIG. 6, it is further assumed that there is a desire to build tunnels from XLAN4 to each of the other extended LANs, XLANl, XLAN2 and XLAN3. In accordance with this embodiment of the invention, every tunneler does not have to be config¬ ured with information concerning every other possible tunneler. For example, in the network configuration of FIG. 6 it would only be necessary to configure a tunnel between Tl and T2 (for XLAN1-XLAN4) , another tunnel between T3 and T4 (for X1AN2-XLAN4) , and another tunnel between T5 and T6 (for XLAN3-XLAN4) . There is no point in configuring Tl with knowledge of T3 and T6, because the latter two tunnelers cannot be reached from Tl if T2 is inoperative. Of course, this arrangement using three tunnelers connected with XLAN4 is inconsistent with the notion that there is only one tunneler for each XLAN. In this optimization approach of the inven- tion, there are three possible ways that a tunneler would know whether to build a tunnel. The first is the manual configuration technique as described for the ba¬ sic form of the invention. Thus, if T2 were the desig¬ nated tunneler for XLAN4, the tunnel from Tl to T2 would be established as a result of this basic manual configuration technique. The second technique is by mul¬ ticast advertisement, and the third is by redirection, both of which need further explanation. A multicast message is one directed to ul- tiple destinations. In the second technique for estab¬ lishing tunnels, if a tunneler is not elected the designated tunneler for a particular XIAN, it periodi¬ cally multicasts the set of tunnels with which it has been manually configured. Thus, T3, which is not the designated tunneler, would advertise by multicast messages that it had been configured to form a tunnel with T4. The designated tunneler T2, upon hearing of the advertised T3-T4 tunnel from T3 's multicast, establishes a second tunnel, between T2 and T4. This would certainly not be an optimally routed tunnel. A message from XLAN4 bound for XLAN2 would proceed to tunneler T2 , which would forward the message to the other tunnel endpoint, T4, but through XLAN4 and T3. Similarly, tunneler T6 would advertise a tunnel from T6 to T5, with which it had been configured, and the desi¬ gnated tunneler would receive T6's multicast advertise¬ ment and establish a tunnel from T2 to T5. Tunnel establishment by redirection occurs when a tunneler attempts communication with a tunneler that has not been elected the designated tunneler. The return message from the non-designated tunneler in¬ cludes the identity of the designated tunneler, so the tunneler attempting communication is made aware which tunneler, and which of its ports on that XLAN, will accept the tunnel request. A simple protocol is pro¬ vided, to require the designated tunneler to advertise its identity, including a port number, to others on the same XLAN. For example, tunneler T5 has been configured to establish a tunnel with T6, but T6 is not the desig- nated tunneler. A response message from T6 redirects T5 to the designated tunneler T2. T6 has knowledge of the designated tunneler T2 from "advertising" messages it has previously received from T2. Similarly, tunneler T3 redirects T4 to the designated tunneler T2. Communica- tion in a direction toward XLAN4 is also non-optimal For example, a message from XLAN3 over tunnel T5-T2 is constrained to first visit the designated tunneler T2, which is reached through XLAN4, before returning to XLAN4 for delivery to its final destination. The advantage of optimization by this approach is that the storage and time requirements for manual configuration are greatly reduced. The trade-offs for this improvement are that message routing is likely to be further removed from optimal, and the approach is not universally applicable to all network configura¬ tions. It will be appreciated from the foregoing that the present invention represents a significant advan¬ tage in the field of interconnected local area networks (LANs) . In particular, the invention permits LANs to be bridged together through an established communications tunnel even when they are separated by network compo¬ nents that include routers. Each tunnel established in accordance with the invention provides for user-speci¬ fied filtering, wherein traffic flowing through the tunnel may be limited, at the user's option, to speci¬ fic categories or protocols, or may be limited in other ways, such as by source or destination. As described, the tunnel mechanism of the invention inherently pre¬ cludes closed loops in the interconnected network. The basic tunneling approach provides for grouping of tunnelers to define alternatives in the event of an inoperative tunneler. In the optimization approach described, manual tunneling configuration is greatly reduced for some network configurations. It will also be appreciated that, although a basic embodiment and a specifically optimized version of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. According- ly, the invention is not to be limited except as by the appended claims.

Claims: CLAIMS We claim: 1. A method for logically connecting local area networks (LANs) interconnected by network compo¬ nents that include bridges and may include routers, the method comprising the steps of: providing selected bridges with the capability to form tunnels between LANs that may be widely separ¬ ated, such bridges being referred to as tunnelers; electing a unique designated tunneler for each extended IAN (XLAN) for which tunneling is to be provid- ed, wherein each XLAN has no more than one designated tunneler, but a single tunneler may be designated by more than one XLAN; configuring the tunnelers by supplying each with information identifying other tunnelers with which a tunnel may be established; establishing at least one tunnel between two designated tunnelers, by exchanging messages between the two tunnelers; and selectively forwarding message traffic through the established tunnel, from one IAN to another. 2. A method as defined in claim 1, wherein the step of electing a unique designated tunneler for each XLAN includes: executing a spanning tree algorithm among all of the bridges connected to the XLAN, and thereby elect¬ ing a root bridge; and ensuring that the root bridge has tunneling capability. _-2,4,- 3. A method as defined in claim 1, wherein the step of configuring the tunnelers includes: supplying to each tunneler the identities of other tunnelers and the identities of ports through which the other tunnelers are connected to XLANs with which they are associated. 4. A method as defined in claim 3, wherein the step of establishing at least one tunnel includes: transmitting a greeting from a designated tun¬ neler to each other tunneler that is a potential tunnel endpoint; receiving a message from one of the potential tunnel endpoints confirming that a tunnel may be estab¬ lished; and recording that a tunnel has been established. 5. A method as defined in claim 1, wherein the step of configuring the tunnelers includes: supplying to each tunneler the identities of other tunnelers, the identities of ports through which the other tunnelers are connected to XLANs with which they are associated, and an indication of tunneler pri¬ orities, to facilitate establishment of tunnels between tunnel endpoints. 6. A method as defined in claim 5, wherein the step of establishing at least one tunnel includes: transmitting a greeting from a designated tun¬ neler to another designated tunneler that is a poten¬ tial tunnel endpoint; receiving a message from the other designated tunneler confirming that a tunnel may be established; and recording that a tunnel has been established. 7. A method as defined in claim 5, wherein the step of establishing at least one tunnel includes: transmitting a greeting from a designated tun¬ neler to another designated tunneler that is a poten- tial tunnel endpoint; transmitting, after failing to receive a con¬ firming message from the other designated tunneler, a greeting to at least one backup tunneler; receiving a message from the backup tunneler confirming that a tunnel may be established; and recording that a tunnel has been established. 8. A method as defined in claim 1, wherein the step of selectively forwarding message traffic through the established tunnel includes: receiving a message at a tunneler; determining whether the message destination is known to the tunneler; if the destination is known, performing the steps of determining whether the destination re- quires forwarding through a tunnel, and if so, forwarding the message through the tunnel, if the tunnel has been established and the message is of a type authorized for for¬ warding through the tunnel; if the destination is not known, determining whether the message was received from a tunnel; if the message was received from a tunnel, for¬ warding the message through a port defined as the tun¬ nel endpoint; if the message was not received from a tunnel and the message destination is not known, performing the steps of forwarding the message over all permissi¬ ble non-tunnel ports, and forwarding the message through all estab¬ lished tunnels, if any, for which this tunnel¬ er is a sending endpoint, if the message of a type authorized for tunnel transmission. 9. A method as defined in claim 8, wherein the step of forwarding a message through a tunnel includes: appending to the message a destination address that includes the address of a receiving endpoint tun- neler of the tunnel, and also includes a port identifi¬ er that defines, in part, the receiving endpoint of the tunnel; and transmitting the message through an appropri¬ ate port to reach the receiving endpoint of the tunnel. 10. A method as defined in claim 9, wherein: if the message was received from a tunnel the method further comprises the step of stripping, from the received message, the tunnel receiving endpoint ad- dress, before forwarding the message through the tunnel receiving endpoint port. 11. A method as defined in claim 8, wherein the step of selectively forwarding messages through a tunnel includes the further step of: updating, if necessary, a database of destina- tions, by means of address information obtained from each message source. 12. A method as defined in claim 1, wherein: the step of configuring includes supplying the tunnelers with a subset of information concerning other tunnelers, the subset of information being optimized for a particular network topology; and the step of establishing a tunnel includes the steps of (1) for a designated tunneler, sending a greeting to another designated tunneler with which it has been configured, and receiving a response confirming that a tunnel has been es¬ tablished, (2) for a designated tunneler, sending a greeting to a nondesignated tunneler with which it has been configured, receiving a re¬ fusal response identifying a designated tunnel¬ er, redirecting the greeting to the designated tunneler identified in the refusal response, and receiving a response confirming that a tun- nel has been established, and (3) for a nondesignated tunneler, periodi¬ cally transmitting multicast messages identify¬ ing the tunnelers with which it has been con¬ figured, and, for a designated tunneler, re- sponding to a multicast message from a nondes¬ ignated tunneler, by sending a greeting to one of the tunnelers contained in the multicast message, and receiving a response message con¬ firming that a tunnel has been established. 13. A method for logically connecting local area networks (LANs) interconnected by network compo¬ nents that include bridges and may include routers, the method comprising the steps of: providing some of the bridges with the capabil¬ ity to form tunnels between LANs that may be widely sep¬ arated, such bridges being referred to as tunnelers; electing a unique designated tunneler for each extended LAN (XLAN) for which tunneling is to be provid- ed, wherein each XLAN has no more than one designated tunneler, but a single tunneler may be designated by more than one XLAN; configuring the tunnelers by supplying each tunneler with the identities of other tunnelers and the identities of ports through which the other tunnelers are connected to XLANs with which they are associated; establishing at least one tunnel between two designated tunnelers, by transmitting a greeting from a designated tunneler to each other tunneler that is a potential tunnel endpoint, receiving a message from one of the potential tunnel endpoints confirming that a tun¬ nel may be established, and recording that a tunnel has been established; and selectively forwarding message traffic through the established tunnel, from one LAN to another. 14. A method as defined in claim 13, wherein the step of selectively forwarding message traffic through the established tunnel includes: receiving a message at a tunneler; determining whether the message destination is known to the tunneler; if the destination is known, performing the steps of determining whether the destination re- quires forwarding through a tunnel, and if so, forwarding the message through the tunnel, if the tunnel has been established and the message is of a type authorized for for¬ warding through the tunnel; if the destination is not known, determining whether the message was received from a tunnel; if the message was received from a tunnel, for¬ warding the message through a port defined as the tun¬ nel endpoint; if the message was not received from a tunnel and the message destination is not known, performing the steps of forwarding the message over all non-tun¬ nel ports except one through which the message was received, forwarding the message through all estab¬ lished tunnels, if any, for which this tunnel¬ er is a sending endpoint, if the message of a type authorized for tunnel transmission. 15. A method as defined in claim 14, wherein the step of forwarding a message through a tunnel in¬ cludes: appending to the message a destination address that includes the address of a receiving endpoint tun¬ neler of the tunnel, and also includes a port identifi¬ er that defines, in part, the receiving endpoint of the tunnel; and transmitting the message through an appropri- ate port to reach the receiving endpoint of the tunnel. 16. A method as defined in claim 15, wherein: if the message was received from a tunnel the method further comprises the step of stripping, from the received message, the tunnel receiving endpoint ad- dress, before forwarding the message through the tunnel receiving endpoint port. 17. Apparatus for logically connecting local area networks (LANs) interconnected by network compo¬ nents that include bridges and may include routers, the apparatus comprising: a plurality of bridges with the capability to form tunnels between LANs that may be widely separated, such bridges being referred to as tunnelers; means contained collectively within the tunnel¬ ers, for electing a unique designated tunneler for each extended IAN (XLAN) for which tunneling is to be provid¬ ed, wherein each XLAN has no more than one designated tunneler, but a single tunneler may be designated by more than one XLAN; means for configuring the tunnelers by supply- ing each with information identifying other tunnelers with which a tunnel may be established; means contained collectively in at least two tunnelers, for establishing at least one tunnel between two of the tunnelers, by exchanging messages between the two tunnelers; and means contained within each tunneler, for se¬ lectively forwarding message traffic through the estab¬ lished tunnel, from one IAN to another. 18. Apparatus as defined in claim 17, wherein the means for electing a unique designated tunneler for each XLAN includes: means for executing a spanning tree algorithm among all of the bridges connected to the XLAN, and thereby electing a root bridge; and means for ensuring that the root bridge has tunneling capability. 19. Apparatus as defined in claim 17, wherein the means for configuring the tunnelers includes: means for supplying to each tunneler the iden¬ tities of other tunnelers and the identities of ports through which the other tunnelers are connected to XLANs with which they are associated. 20. Apparatus as defined in claim 19, wherein the means for establishing at least one tunnel in¬ cludes: means for transmitting a greeting from a desig- nated tunneler to each other tunneler that is a poten¬ tial tunnel endpoint; means for receiving a message from one of the potential tunnel endpoints confirming that a tunnel may be established; and means for recording that a tunnel has been es¬ tablished. 21. Apparatus as defined in claim 17, wherein the means for configuring the tunnelers includes: means for supplying to each tunneler the iden¬ tities of other tunnelers, the identities of ports through which the other tunnelers are connected to XLANs with which they are associated, and an indication of tunneler priorities, to facilitate establishment of tunnels between tunnel endpoints. 22. Apparatus as defined in claim 20, wherein the means for establishing at least one tunnel in¬ cludes: means for transmitting a greeting from a desig- nated tunneler to another designated tunneler that is a potential tunnel endpoint; means for receiving a message from the other designated tunneler confirming that a tunnel may be es¬ tablished; and means for recording that a tunnel has been es¬ tablished. 23. Apparatus as defined in claim 20, wherein the means for establishing at least one tunnel in¬ cludes: means for transmitting a greeting from a desig- nated tunneler to another designated tunneler that is a potential tunnel endpoint; means for transmitting, after failing to re¬ ceive a confirming message from the other designated tunneler, a greeting to at least one backup tunneler; means for receiving a message from the backup tunneler confirming that a tunnel may be established; and means for recording that a tunnel has been es¬ tablished. 24. Apparatus as defined in claim 17, wherein the means for selectively forwarding message traffic through the established tunnel includes: means for receiving a message at a tunneler; means for determining whether the message des¬ tination is known to the tunneler; means operative if the destination is known, for determining whether the destination re- quires forwarding through a tunnel, and if so, forwarding the message through the tunnel, if the tunnel has been established and the message is of a type authorized for for¬ warding through the tunnel; means operative if the destination is not known, for determining whether the message was received from a tunnel; means operative if the message was received from a tunnel, for forwarding the message through a port defined as the tunnel endpoint; means operative if the message was not re¬ ceived from a tunnel and the message destination is not known, for forwarding the message over all permissi- ble non-tunnel ports, and forwarding the message through all estab¬ lished tunnels, if any, for which this tunnel¬ er is a sending endpoint, if the message of a type authorized for tunnel transmission. 25. Apparatus as defined in claim 24, wherein the means for forwarding a message through a tunnel in¬ cludes: means for appending to the message a destina- tion address that includes the address of a receiving endpoint tunneler of the tunnel, and also includes a port identifier that defines, in part, the receiving endpoint of the tunnel; and means for transmitting the message through an appropriate port to reach the receiving endpoint of the tunnel. 26. Apparatus as defined in claim 25, and fur¬ ther comprising : means operative if the message was received from a tunnel, for stripping from the received message the tunnel receiving endpoint address, before forward¬ ing the message through the tunnel receiving endpoint port. 27. Apparatus as defined in claim 17, wherein: the means for configuring includes means for supplying the tunnelers with a subset of information concerning other tunnelers, the subset of information being optimized for a particular network topology; and the means for establishing a tunnel includes (1) for a designated tunneler, means for sending a greeting to another designated tun¬ neler with which it has been configured, and receiving a response confirming that a tunnel has been established, (2) for a designated tunneler, means for sending a greeting to a nondesignated tunneler with which it has been configured, means for receiving a refusal response identifying a des¬ ignated tunneler, means for redirecting the greeting to the designated tunneler identified in the refusal response, and receiving a re¬ sponse confirming that a tunnel has been estab- lished, and (3) for a nondesignated tunneler, means for periodically transmitting multicast messag¬ es identifying the tunnelers with which it has been configured, and, for a designated tunnel- er, means for responding to a multicast mes¬ sage from a nondesignated tunneler, by sending a greeting to one of the tunnelers contained in the multicast message, and receiving a re¬ sponse message confirming that a tunnel has been established. 28. A method for logically connecting local area network substantially as described with reference to the accompanying drawings. 29. Apparatus for logically connecting local are networks substantially as described with reference to the accompanying drawings.

Inventors: HARPER JOHN, HAWE WILLIAM, PERLMAN RADIA J, HARPER, JOHN, HAWE, WILLIAM, PERLMAN, RADIA, J.
Applicants: DIGITAL EQUIPMENT CORP, HARPER JOHN, DIGITAL EQUIPMENT CORPORATION, HARPER, JOHN
Application year: 1992
Main classification: G06F15
IPC codes: H04L12