Bell Communications Research, Inc. v. Vitalink Communications Corp., 94-1516

Citation34 USPQ2d 1816,55 F.3d 615
Decision Date23 May 1995
Docket NumberNo. 94-1516,94-1516
PartiesBELL COMMUNICATIONS RESEARCH, INC., Plaintiff-Appellant, v. VITALINK COMMUNICATIONS CORPORATION, Defendant-Appellee.
CourtUnited States Courts of Appeals. United States Court of Appeals for the Federal Circuit

James S. Renard, Bickel & Brewer, Dallas, TX, argued for plaintiff-appellant. With him on the brief were William A. Brewer, III and Eric G. Calhoun.

Richard J. Anderson, Fish & Richardson, Minneapolis, MN, argued for defendant-appellee. With him on the brief was Wayne E. Willenberg.

Before MICHEL, RADER, and BRYSON, Circuit Judges.

MICHEL, Circuit Judge.

Bell Communications Research, Inc. ("Bellcore") appeals from the August 16, 1994 decision of the United States District Court for the District of New Jersey, No. 92-4104, granting summary judgment in favor of Vitalink Communications Corporation ("Vitalink") on the latter's counterclaim for a declaration of noninfringement and dismissing Bellcore's infringement suit against Vitalink. Because the trial court partially misconstrued the scope of the asserted claim of U.S. Patent No. 4,706,080 ('080), and consequently erred in granting summary judgment of noninfringement, we vacate and remand.

BACKGROUND
A. The Technology

Local Area Networks ("LANs") consist of a number of devices, such as computers or telephones, attached to a shared communications medium. The communications medium permits the devices to transmit bundles of data, or "packets," back and forth to one another. Such packets, which contain fields of information that function as source device and destination device addresses, are broadcast through the communications medium; only the device that recognizes its own address as the destination address receives the broadcast.

The quality of a LAN's performance degrades in proportion to both the number of devices in the network and the speed at which each device processes information. One can, however, recover some of this lost performance by creating networks of multiple LANs rather than simply adding devices to a single, larger LAN. These multiple LANs are connected by means of "bridges," each of which are themselves composed of two paired "gateways." The gateways, each of which have memory capacity, maintain running lists of the source addresses of the packets they have forwarded, allowing the bridges they form gradually to "learn" how to broadcast packets selectively in order to reduce the network's overall load.

The existence of multiple pathways between a given pair of devices in two different LANs creates the potential for loops and thus thwarts the learning function of memory-capable gateways--a packet that cycles through a complete loop causes its source address to appear on both sides of the relevant gateway pairs, eliminating the advantage of equipping gateways to keep lists of source addresses from the packets they have broadcast. For purposes of describing and solving this looping problem, one can depict a While the use of one spanning tree deals with multiple path and looping problems, two problems remain: spanning tree backup paths remain inactive unless bridge failures require that they be used, and the spanning tree's root may become a performance bottleneck for the system. The use of multiple spanning trees thus represents an improvement over the use of only one spanning tree. But one cannot implement a system for the use of multiple spanning trees without some means of differentiating among the spanning trees, such that a packet is always forwarded over some tree. A system can achieve the required differentiation in one of a number of ways: for example, the system could randomly assign different device addresses to different trees, or, alternatively, the source device could specify a tree by means of a tree identifier when it originates the packet.

group of interconnected networks as a graph, with lines and vertices used to represent connecting bridges and connected LANs, respectively. In this graphical notation, a "tree" is a graph in which a sequence of one or more lines connects two vertices, while a "spanning tree" is a graph in which all the vertices are connected. Such a spanning tree can be superimposed on the complete graph of the bridged networks and used to determine a set of loop-free spanning tree paths among the vertices. This process of determination can be accomplished either by some oversight mechanism or automatically by the bridges.

B. The Patent in Suit

Bellcore's '080 patent, entitled "Interconnection of Broadcast Networks," discloses a method for interconnecting networks, such as LANs, that uses multiple concurrent spanning trees for packet delivery while preserving loop-free paths. According to the summary of the invention contained in the specification,

[e]ach spanning tree is uniquely identified. Each message packet that traverses the overall system is assigned to a specific spanning tree so the packet travels between nodes [i.e., device networks] along edges [i.e., bridges] contained in the specified spanning tree. Each gateway, with an expanded store-and-forward protocol [in its memory], parses the packet to determine the assigned spanning tree and forwards the message accordingly. In one embodiment of the present invention, the device originating the packet specifies the spanning tree identifier and conveys it either explicitly or implicitly in the packet.

Col. 2, 11. 14-25. As the more detailed description explains,

To implement the improvement in the gateway protocol arrangement in accordance with one aspect of the present invention, a set of spanning trees is selected for the cyclic graph according to predetermined guidelines. Each spanning tree is assigned a unique identifier or number and each message traversing the system is assigned to a unique spanning tree via its identifier. Any gateway receiving this message determines the tree number and then routes the message over the specified spanning tree and drops all packets of other spanning trees. Typically, the device originating the message specifies the spanning tree number, either explicitly or implicitly. For instance, with the explicit approach, a "tree number" field could be added to the packet specifications, say as an extra bit in the header of the packet. With the implicit approach, a spanning tree number could be generated from fields normally occurring in the packet such as the source and destination addresses. An appropriate example function might be

spanning tree number = (source 'exclusive or' destination) modulo N,

where N is the number of spanning trees in the network. This has the benefit that all traffic between a pair of hosts will travel on only one spanning tree, thus minimizing the occupied drop lists across the system.

Col. 5, ll. 11-35. Bellcore's method also preserves the "transparency" of the interconnections among the LANs, according to which the existence of gateways does not require modifications to the networked devices or the packets they broadcast. Col. 1, ll. 29-35.

Claim 6 of the '080 patent, the only claim asserted by Bellcore, reads as follows:

6. A method for transmitting a packet over a system comprising a plurality of networks interconnected by gateways, said packet originated by a source device connected to one of said networks and destined for a destination device connected to one of said networks, said packet including a source address and a destination address, and said method comprising the steps of

defining an undirected graph representative of the system wherein said networks comprise graph nodes and said gateway[s] comprise graph paths,

defining a spanning tree on said graph such that every pair of said nodes is connected by only one of said paths and selecting a plurality of spanning trees for said graph according to pre-determined system guidelines,

configuring each gateway with source address lists in correspondence to the number of trees having said each gateway comprising one of said paths, wherein said lists reduce to a common list whenever said selection of spanning trees results in identical ones of said lists for said each gateway,

assigning, by said source device, one of said trees to broadcast said packet and associating with said packet an identifier indicative of said one of said trees,

broadcasting said packet by said source device through the system on said one of said trees, and

for each gateway receiving said packet,

(i) determining for each said packet said source address, said destination address and said packet identifier,

(ii) if said receiving gateway does not process packets having said identifier, inhibiting forwarding of said packet; otherwise, inserting said source address in the corresponding one of said lists associated with said identifier, and

(iii) inhibiting forwarding of said packet if said destination address is in said corresponding list; otherwise, forwarding said packet by said receiving gateway.

Col. 10, ll. 18-57.

C. The Accused Products

Vitalink manufactures and markets communications products, including bridges and bridge-routers used in the networking of LANs. Specifically, Vitalink sells a series of products including a Distributed Load Sharing ("DLS") feature, which is itself the subject of a Vitalink patent. These products are alleged by Bellcore to use the method claimed in the '080 patent.

Both Bellcore and Vitalink agree that, in a Vitalink product using the DLS feature, a message packet contains both a source address and a destination address but no separate packet identifier. In other words, DLS uses only the implicit approach to spanning tree identification in the packet. The parties also agree that the tree along which a message packet travels in a DLS system may change mid-course in response to such phenomena as link failures and the opening of additional links or, when the system is in a steady state, remain the same throughout its transmission from the source device to the destination device.

D...

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