Patent Publication Number: US-7583603-B2

Title: Scalable and fault-tolerant link state routing protocol for packet-switched networks

Description:
CROSS-REFERENCE TO RELATED DOCUMENTS 
   The present application is a continuation application of patent application Ser. No. 10/219,565 entitled “Scalable and Fault-Tolerant Link State Routing Protocol for Packet-Switched Networks,” filed on Aug. 14, 2002 and issued as U.S. Pat. No. 6,762,999 on Jul. 13, 2004, which is a continuation of application Ser. No. 09/727,907, filed Nov. 30, 2000 (U.S. Pat. No. 6,529,481, issued Mar. 4, 2003), and which are incorporated in their entirety by reference. 

   FIELD OF THE INVENTION 
   The present invention is in the area of packet routers for use with wide-area packet networks, such as the Internet, and pertains more particularly to Link State Routing Protocol(LSRP) for such routers and networks. 
   BACKGROUND OF THE INVENTION 
   At the time of the present patent application demand for increased data capacity and efficiency in Internet traffic continues to increase dramatically as more individuals and businesses increase their use of the Internet. The ever-increasing demand also drives development of equipment for the Internet such as data packet routers. A number of enterprises are developing routers that are capable of faster and higher capacity handling of data packets. 
   The Internet, operating globally, comprises components from a wide variety of companies and organizations. It is, of course, necessary that such equipment conform to certain hardware and connection standards and operate by certain data transfer standards and protocols. These standards are all well known to the skilled artisan. 
   As new and better equipment for routing data traffic in the Internet continues to be developed, researchers developing such equipment are including fault tolerance, diagnostic capabilities, and redundancy into the equipment, links between instances of routing equipment, and in routing protocols. 
   One of the important developments in Internet technology as of the time of filing of the present patent application is in development of faster, and scalable routers. The present inventors, for example, are involved in development of what are known in the art as Terabit routers, capable of much higher packet transfer rates than currently available in the art. These routers are scalable to higher and higher overall data transfer rates, by allowing upwards of seven thousand interfaces to network traffic. 
   As the number of interfaces to network traffic grows, the importance of scalability and fault-tolerance grows apace. What is clearly needed is a method for making link-state routing protocols both scalable and fault tolerant. 
   SUMMARY OF THE INVENTION 
   In a preferred embodiment a distributed Link-State Routing Protocol (LSRP) system in a packet switch having external communication ports is provided, comprising a first card executing an update process of the LSRP, making database updates, a second card executing Decision (Shortest Path First (SPF)) process of the LSRP, and a communication fabric coupling the cards with each other and the external ports. 
   In some embodiments the LSRP is one of an Intermediate System to Intermediate System (IS-IS) LSRP, and in others an Open Shortest Path First (OSPF) LSRP. Also in some embodiments there may be a third card communicating through the fabric with the first and second cards and the ports, wherein the first card updates both the second and third cards, maintaining synchronized copies of the Link State, Adjacency, and Circuit databases. 
   In some embodiments there is also a location service (LS) controller operating external to the LSRP-related processes on the cards, wherein the LS controller configures and monitors the LSRP system. The LS controller may configure one of the three cards to execute the LSRP Update process, another to execute the LSRP Decision process, and the third to act as a backup, and may reassign roles according to monitored condition of the three cards. 
   In another aspect of the invention a distributed Link-State Routing Protocol (LSRP) system in a packet switch having external communication ports is provided, comprising a set of two or more cards in communication with one another and with the external communication ports, and a location service (LS) controller separate from but in communication with the cards. In this system the LS controller monitors and configures the cards, assigning LSRP functions and backup functions to individual cards. 
   In some embodiments of the system the LS controller reassigns functions to cards in the system in response to failures of cards in the system, providing fault tolerance. 
   In still another aspect of the invention a method for distributing a Link-State Routing Protocol in a packet switch having more than a single card is provided, comprising the steps of (a) configuring a first card to execute update process of the protocol; (b) configuring a second card to execute Decision process(Shortest-Path First (SPF)) of the protocol; and (c) executing the configured functions on the cards, and synchronizing copies of a the Adjacency, Circuit, and Link State databases on the cards. 
   In some embodiments of the method the LSRP is one of an Intermediate System to Intermediate System (IS-IS) LSRP or an Open Shortest Path First LSRP. There may further be a location service (LS) controller operating external to the card executing LSRP process, wherein the LS controller configures and monitors the LSRP system. 
   In some embodiments of the method there is at least a third card, wherein the LS controller configures one of the three to execute the LSRP Update process, another to execute the LSRP Decision(SPF) process, and the third to act as a backup, and reassigns roles according to monitored condition of the three cards. Two or more cards may be configured to execute the Update functions of the protocol. 
   In various embodiments of the invention described in enabling detail below, for the first time a scalable and fault tolerant Link State Routing Protocol is provided 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is a diagram of a switch operating a Link State Routing Protocol in the prior art. 
       FIG. 2  is a table showing possible states of a single control card running an IS-IS protocol. 
       FIG. 3  is a block diagram of two control cards sharing functions of an IS-IS protocol according to an embodiment of the present invention. 
       FIG. 4  is a block diagram of three control cards sharing functions of an IS-IS protocol in an embodiment of the invention, with one of the control cards serving a backup role. 
       FIG. 5  is a block diagram of four control cards sharing functions of IS-IS protocol, with one card acting in a backup role. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As briefly described in the Background section above, the present inventors are engaged in development of new and improved packet switches (routers). In the present case, routers are under development having massive scalability and upwards of seven thousand I/O interfaces to other nodes in a packet network. With the development of higher capacity and scalability of capacity the importance of scalability of function and fault tolerance has grown apace. 
   One of the important functions of a packet switch in a network is to keep track of changes in network topology, meaning the existence of neighboring routers and connectivity, and the relative latency of alternate paths through the network to intermediate destinations. On a micro-scale, when a packet arrives at a packet switch, the switch needs to decide, and quickly, based upon destination information received with the packet for example, where to send the packet next. The decision is not a simple process, because, as hinted above, topology is constantly changing. 
   One widely-used and well-known type of routing protocol used for routing control is known generally as Link-State Routing Protocol (LSRP), of which two prime examples are Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) protocol, used in the art in Internet Protocol (IP) and Open Systems Interconnect (OSI) based networks. 
   In the present document and disclosure the IS-IS protocol is used as a prime example, although it is to be understood that the invention is not limited to the IS-IS protocol but may be applied to other LSRPs such as OSPF. 
     FIG. 1  is a simplified diagrammatical representation of a packet switching apparatus  100  in the prior art. Apparatus  100  may be referred to in the art as a packet switch or as a router. Apparatus  100  in this example has a plurality of line cards  103  and at least one control card  105  serving more than one line card. Each line card has a plurality of ports through which data is sent and received, indicated in  FIG. 1  by bi-directional arrows. Control card  105  in this example is configured and enabled to perform a Link State Routing Protocol (LSRP), wherein the protocol maintains adjacencies with neighboring packet switches by means of advertisements, and builds a network topology database, which is also referred to as a Link State Database. In practice the LSRP calculates the shortest path between points in the network connected through ports of the line cards. 
   In current routers the processes of the routing protocol are all performed on one control card. A particular drawback in this prior art system is that the protocol is neither scalable nor fault tolerant. 
   As mentioned above, IS-IS protocol is used exemplary in this specification to illustrate the features and advantages of the present invention. IS-IS protocol is an Interior Gateway Protocol for routing packets between routers belonging to a single domain. 
   Since the size of a domain can be significantly large, especially in the case of large Internet Service Providers (ISPs), fault tolerance and scalability are critical requirements for a robust protocol implementation. In a preferred embodiment of the present invention fault tolerance requires at least two cards running IS-IS with one acting as a backup for the other. In many embodiments there are plural active cards with functions of the protocol distributed among the active cards and at least one backup card ready to take over for any active card that might fail. The backup card in this embodiment has exactly the same configuration as the active card and maintains a copy of dynamic databases synchronized with the state of the databases on the active card. In the event of failure of the active card, this allows the backup to assume the active role without affecting the externally visible behavior of the router. 
   The databases utilized by the protocol are:
         1) Link State Database—This is a description of the network topology.   2) Adjacency Database—This is a description of the neighboring equipment, such as routers, to which the local router is immediately connected.   3) Circuit Database—A description of the network layer circuits which are directly accessible by the local router.   4) Forwarding Database—The output of the decision process, this describes the neighbor to which PDUs addressed to particular destinations should be forwarded.       

   There are generally four processes in a Link State Routing Protocol (LSRP), whether in current art, or as used in embodiments of the present invention. The processes are: 
   1) The Decision Process 
   The Decision process calculates routes to destinations-commonly known as the Shortest Path First (SPF) calculation. As the size of the network grows, the computational requirements of the algorithm grow as the square of the number of nodes (worst case). This makes this process potentially the most CPU-intensive process of the four processes described herein. As the size of a network grows, the Decision Process is allowed to take somewhat longer to complete, so long as the maximum completion time is bounded. (Note: The ISO 10589 standard specifies a maximum compute time of 5 seconds.) 
   The inputs for the decision process are: Link State Database and Adjacency Database. The Outputs are the Forwarding Database. 
   2) The Update Process 
   This process constructs, processes incoming, and propagates Link State Protocol Data Units (PDUs) for database updates. This process also constructs, processes incoming, and transmits hello PDUs, which are data updates that announce the presence of nodes. The process also maintains the state of the adjacency, circuit, and link state databases. The real time demands on the Update Process are more restrictive than that on the Decision Process in that, as the network grows in size, the response times of the Update Process to the receipt of PDUs should remain constant. 
   The inputs for this process are: Received PDUs, which may be Link-State PDUs (LSPs), Sequence Number Packets (SNPs), which may be partial or complete, and Hello PDUs (Hellos). The outputs are Adjacency and Link State Databases. 
   3) The Receive Process 
   This process is responsible for obtaining incoming PDUs from supported subnetworks and for distributing the PDUs to the appropriate processes (forwarding or update). 
   4) The Forwarding Process 
   This process supports relaying PDUs to all potential destinations in the supported network. 
   In some preferred embodiments of the present invention only the Update and Decision processes are performed by software. The receive and forwarding processes may be performed by hardware. 
   Terminology 
   In the present specification the following terminology is used: 
   A card running the IS-IS Update process in the active role has state “Active”. 
   A card running the IS-IS Update process in the backup role has state “Backup”. 
   A card running the IS-IS Decision process in the active role has state “SPF”. 
   There is, in a preferred embodiment, no card running the IS-IS Decision process in the backup role. The potential state combinations for a given single card may therefore be listed as in  FIG. 2 . 
   Preferred embodiments of this invention require that the state of the Link State, Adjacency, and Circuit databases be synchronized between cards with state Active and cards with state Backup and/or SPF. Synchronization of these databases allows a Backup card to assume the role of an Active card in the event of failure. Synchronization of the databases also allows the SPF card to utilize the correct inputs in a distributed environment. 
     FIG. 3  is a simplified block diagram of an IS-IS process in an embodiment of the present invention running on two active cards, these being cards  301  and  303 . In this example the Update process is running on C  301  and the Decision (SPF) process is running on C  303 . Distributing the functions of the IS-IS protocol allows for scalability in a scalable router. 
     FIG. 4  is a simplified block diagram of an IS-IS process in an embodiment of the present invention, running on three cards, these being cards  401 ,  403  and  405 . In this particular embodiment card  401  executes the Update process (ACTIVE), card  403  executes the Decision process (SPF), and card  405  is a redundant card acting as a BACKUP for either or both of cards  401  and  403 . In  FIG. 4  incoming protocol packets (PDUs) are processed by card  401 , which performs updates, card  403  does SPF calculations, and card  401  synchronizes a copy of the databases in card  405 , such that in the event that card  405  has to become active, it does so with an updated and current copy of all topology and path information. 
     FIG. 5  is a block diagram representing IS-IS protocol shared among four cards, these being cards  501 ,  503 ,  505 , and  507 . In this case the Update process is distributed on cards  501  and  503 , and the Decision process is performed by one card ( 507 ). Card  505  acts as a backup for any of the three other cards. 
     FIGS. 3-5  illustrate in general, sharing and scalability of an IS-IS protocol on several cards of a packet switch apparatus in various embodiments of the present invention, and are meant to illustrate a few of many ways such sharing might be done. The skilled artisan will recognize that the distribution of functionality could be different than that shown in the specific examples, and that the communication paths between cards represent interconnectivity of cards through the fabric of a packet switching apparatus. 
   In various embodiments of the present invention the IS-IS protocol can run on one or more cards. If only one card is available in a particular piece of equipment then IS-IS runs in a uniprocessor mode. 
   In preferred embodiments of the invention distributed IS-IS with backup is provided for a scalable Terrabit Network Router (TNR) wherein additional cards and fabric may be added, and routing equipment may therefore be provided to almost any reasonable rate level. Scalability for such equipment requires response time for the novel IS-IS protocol implementation to remain stable under large load conditions. Achieving this scalability requires distribution of IS-IS protocol processing among multiple cards so that additional CPU resources can be applied in real time. 
   In preferred embodiments of the invention a system is provided for monitoring of performance of cards involved in distributed protocol processing and backup, and for switching available cards into and out of active states as required. 
   In preferred embodiments the use or non-use of multiple cards for an IS-IS instance within a TNR does not affect the externally-visible behavior of the router. Also, when multiple cards are being used to run IS-IS, the failure of a single card is operationally transparent to the external behavior of the router (fault tolerance). As the size of the network increases the performance of the protocol does not degrade, and the system is extensible to the use of 2-N cards where “N” is a number larger than 3. 
   In a preferred embodiment of the present invention, when multiple cards are available to execute IS-IS processes, one card performs the Decision Process (SPF state), one (or more) cards perform the Update Process in the active role (ACTIVE state), and one card performs the Update Process in the backup role (Backup state). In the case where only two cards are available, the SPF card also assumes the role of Backup. When multiple cards are performing the Update Process in the Active role, each card is responsible for a disjoint set of circuits. 
   For multiple cards to operate together in embodiments of the invention, it is necessary to synchronize the databases on all cards running IS-IS processes. The approach used in preferred embodiments is referred to as Database State Change Synchronization (DSCS) i.e. the backup database(s) are synchronized with the Active database when state changes occur. Synchronization of the databases is directed by the Active card, if only one is active, or active cards if the processes are distributed over several cards. In preferred embodiments the Active card(s) process incoming PDUs and notify the Backup and/or SPF cards of any resulting state changes in the IS-IS databases (adjacency, link state, and/or circuit) via a reliable inter-process communications path. This path is referred to herein as the Database State Change synchronization Link (DSCSLink). All exchanges over DSCSLink are acknowledged. 
   Failure of the Backup or SPF card to acknowledge the receipt of a DSCSLink message in a timely fashion results in the receiving processor being identified as having failed. The state transitions occur in the remaining IS-IS cards to assume the IS-IS role of the failed card. The Backup and/or SPF cards periodically issue queries as to the health of the Active card. Failure of the Active card to respond to this query in a timely fashion results in the Active card as being identified as having failed. 
   It will be apparent to the skilled artisan that there are many alterations that might be made in embodiments of the inventions described herein without departing from the spirit and scope of the invention. There are many other possible alterations, and the invention should accordingly be granted the scope of the claims that follow.