Abstract:
A method and system for quickly informing a backup unit that a primary unit has failed. Normally an exception handler is activated when a software failure occurs and network controller chips or the ASIC interface to a signal bus can operate even though there is a software failure. A software failure notification packet is programmed and stored in a location that is not affected by a software system failure. When a software failure occurs, control is shifted to the exception handler. The exception handler sends a pre-established and pre-addressed packet to the network controller card which transmits this packet to the backup unit. Upon receipt of the packet, the backup unit goes into operation. In some alternate embodiments that include multiple line cards in a single unit, the exception handler sends a signal to a backup unit via a signal bus or a data bus.

Description:
FIELD OF THE INVENTION 
     The present invention relates to communication systems and more particularly to communication system that includes a redundant or back-up unit. 
     BACKGROUND OF THE INVENTION 
     Many modern communication networks include routers that inter-connect various paths in the network. Routers generally include tables which provide a map of routes through the network. The technology for routing messages through networks is well know. For example, see books such as “ Designing Routing and Switching Architectures ” Network Architecture and Development Series, by Howard C Berkowitz, Published by Que; 1st edition Nov. 15, 1999, ISBN: 1578700604 or “ OSPF Anatomy of An Internet Routing Protocol ” by John T. Moy, published by Addison-Wesley Pub. Co.; 1st edition, Jan. 15, 1998 ISBN: 0201634724. 
     Reliability is of primary importance in modern day communication systems. Reliability is often increased by the use of stand-by routers which are brought into operation when a primary router fails. 
     When a router becomes inoperable, a new map of the paths through the network must be calculated and propagated to all routers in the network. There are known protocols and techniques for doing this type of re-routing such as the “Link State Routing Protocol” or the “Distance-Vector Routing Protocol”. Using these protocols, routers talk to adjacent routers, informing each other of what network routes are currently active. 
     Many different types of failures can occur in a system. However, one of the most common failures is a software failure. A software failure occurs when, for some reason, the software in a unit stops operating properly. In many systems when a software failure occurs, the system branches to an exception handler routine. The exception handler routine is an independent program thread of execution, that generally performs a number of operations that facilitate handling and post-mortem analysis. For example the exception handler may perform a memory dump so that programmers can determine what caused a software failure. 
     Communication systems that include backup routers, usually include a mechanism to detect software failures in the primary unit. When a software failure is detected by these mechanisms, operation of the backup unit is initiated. In currently available systems, there are a variety of different types of mechanisms for detecting failure and activating backup units. 
     Some systems include a hardware implemented mechanism for detecting software failure and for activating a backup router. A hardware failure detection mechanism may for example include a special signal line that activates a standby unit when a software failure occurs in a primary unit. 
     For systems that do not include a hardware failure detection mechanism, there are several known types of failure detection mechanisms in widespread use. One known type of software failure detection uses a simple time out mechanism. For example, a primary unit can be programmed to periodically send a signal to a standby unit (for example every 1 to 30 seconds). If the standby unit does not receive this signal within a defined period, it concludes that the primary unit has failed and the backup unit goes into operation. This type of failure detection is sometimes called a “heart-beat” method. Another type of known failure detection can be termed “hello-acknowledge”. When a “hello-acknowledge” methodology is used, the backup unit (or a central unit) periodically polls the main unit. If a response is not received in a specified period, the system concludes that the primary unit is not operating. 
     With the known types of software failure detection mechanisms there can be a delay between when the failure occurs and when the backup unit detects the failure. For example with “heart-beat” systems, there is a period of time between heart beats. While this period of time may be quite short (i.e. 1 to 30 seconds) with a communication system, much data can be lost in short period of time. 
     The present invention is directed to a proactive software mechanism for detecting failure and for activating a backup unit. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is directed to method and system for quickly informing a backup unit that a primary unit has failed. The present invention takes advantage of the fact that in computer systems, an exception handler is activated when a software failure occurs. Furthermore the invention takes advantage of the fact that certain components (for example the network controller chip, or the ASIC interface to a signal bus) can operate even though there is a software failure. In one embodiment of the invention, a software failure notification packet is programmed and stored in a location that is not affected by a software system failure. When a software failure occurs, as is normal, control is shifted to the exception handler. With the present invention, an augmented exception handler sends a pre-established and pre-addressed packet to the network controller card which transmits this packet to the backup unit. Upon receipt of the packet, the backup unit goes into operation. In other embodiments, (where the primary unit and the backup unit are in the same physical box) the exception handler sends a signal to the backup unit via a signal bus or a data bus. Thus, with the present invention, a signal is sent to the backup unit as soon as a software failure occurs. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of a first preferred embodiment. 
         FIG. 2  is a block flow diagram showing the operation of the system shown in  FIG. 1 . 
         FIG. 3  is a block diagram of a second embodiment of the invention.  FIG. 4  is a block diagram of a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Several different embodiments of the invention will be described. It should, however, be understood that the invention is equally applicable to various other types of routers in addition to those shown in the embodiments specifically described herein. This invention may be embodied in many different forms and the scope of the invention is not limited to the embodiments specifically described herein. 
     In the first embodiments described herein, the invention is applied in a Cable Modem Termination System (hereinafter referred to as a CMTS). CMTS units are normal parts of cable networks. CMTS units are, for example, often part of the systems used by cable system operators to offer high-speed Internet access to home computers. Such systems generally include “cable modems” at the user premises and a CMTS unit located at the cable head-end. The CMTS unit receives signals sent upstream from the cable modems. The signals received by the CMTS are converted into IP packets and routed to an Internet Service Provider for connection to the Internet. The CMTS also can receive IP packets from the Internet and send signals downstream to the user&#39;s cable modem. In general, cable modems cannot communicate directly with each other; they must communicate by channeling their signals through a CMTS. 
     In order to achieve high reliability, cable systems frequently include back-up CMTS units which become operable if a primary unit fails. A system including a back-up router is shown in  FIG. 1 . The system shown in  FIG. 1  includes a CMTS  10  which provides an interface between coaxial cable  5  and local area network  6 . The system includes a backup CMTS  10 A which can be brought into operation if the CMTS  10  becomes inoperable. 
     The CMTS  10  includes a CPU  11 , an operating system  12 , an exception handler  13  and a network controller card  14 . There are commercially available CMTS units that include s CPU, an operating system, an exception handler and network controller card. Such CMTS units are commercially available for a number of vendors such as Motorola Inc., Cisco Systems Inc., Juniper Networks Inc. etc. The CPU  11 , the operating system  12 , exception handler  13  and network controller card  14  can be the same as those in the commercially available units. 
     Network controller card  14  includes a Direct Memory Access (DMA) ring. This is the same as the DMA ring in commercially available network controller cards. Packets placed in the DMA ring of card  14  are sent out on the network  6 , independent of whether or not the CPU  11  and the operating system  12  are operational. 
     The unit  10 A is identical to the unit  10 . The units  10  and  10 A normally interact through network  6  in order to update routing tables and other aspects of the system. Router  10 A includes programming which can receive a control packet from the local area network  6 , which instruct unit  10 A to begin operating. Controlling the operation of a router in response to control packets is conventional. 
     The software operating system  12  is a very complex set of interacting software programs. Operating systems for routers are well known and in commercial use, hence software operating system  12  is not described in detail herein. Such operating systems generally consist of a number of different planes of software which together control the router and perform the various routing function. In general, an operating system in a router is the primary software that runs on the CPU which is a part of the router hardware platform. The operating system drives, controls, manages and coordinates the hardware and software applications that are needed by a user on that given platform. It also supports execution of software applications and their coordination. It achieves this with the help of device drivers, process scheduler, and various other software libraries. 
     An example of a commercially available operating system for an Internet router is the operating system marketed under the trademark Cisco IOS™. The Cisco IOS program includes an operating system and various other software needed to enable data-networking on various hardware platforms marketed by the Cisco corporation. The IOS data-networking software also has support for various defined layers, such as layer 2 to layer 7, where layer 2 is implemented in drivers, layer 3 in IP, etc, layer 4 in TCP, UDP, RTP, etc. and so on. Thus, the operating system  12  is a very complicated piece of software that has many layers of programming which interact to provide a variety of functions. 
     Since units are commercially available that include CPU  11 , an operating system  12  and a network controller card  14 , no further detailed explanation will be given of the details these units. 
     With respect to the present invention, operating system  12  (which could for example be the Cisco IOS) provides a mechanism to handle various software faults like bad memory accesses, CPU instruction execution errors, etc. These faults generate an exception signal that is sent to the exception handler  13 . That is, as is customary in this type of system, when an error or fault occurs in the operating system  12 , the exception handler routine  13  is activated. The normal purpose of this routine is to dump (i.e. preserve) the data in memory and in various registers, so that the cause of the failure can be diagnosed later. 
     The present invention adds a relatively small amount of code to the exception handler  13 . In  FIG. 1  this additional code is designated  13 A. When exception handler  13  is activated, the code  13 A sends a pre-established packet to network controller card  14 . The packet is placed in the DMA ring of controller card  14 . As previously explained the network controller card  14  is a separate hardware device that remains functional even when a fault occurs in the operating system  12 . Thus, network controller card  14  can transmit the special packet provided by code  13 A even though the operating system  12  has stopped functioning. 
     Exception handler extension  13 A includes a pre-established pre-formatted control packet, stored in a memory location that is not affected by an operating system crash. This pre-established control packet has appropriate addressing and control information stored therein. The addressing is such that this packet is directed to unit  10 A. The control information in this packet instructs unit  10 A to begin operating. When program  13 A is activated it retrieves this pre-established packet and places it directly in the DMA ring of Network controller card  14 . Network Controller card  14  then transmits this packet on network  6  and it is delivered to system  10 A. The control packet (sent by code  13 A via network card  14 ) instructs unit  10 A to begin functioning. 
     It is noted that when the system is crashing, that is, when a software fault or crash occurs, the CPU and/or the software layers can not be assumed to be available to deliver packets to the standby router in the normal manner. However, as is conventional the network controller chip  14  runs independent of the CPU and it is fully functional and available during a software crash. Thus the CPU exception handler extension  13 A directly places the crash notification packet into the DMA ring of the network controller  14 . The network controller card  14  can then reliably deliver the crash notification packet onto the network and thus to the standby router. 
     Systems that send control packets between units are known. Such packets contain an address and a control code which the unit receiving the packet recognizes as a control code and thus takes the action designated by the particular control code. In the embodiment shown the control packet contains the network address of redundant system  10 A and a control code that causes unit  10 A to begin operating. 
     Using the technique described herein, the redundant system  10 A is notified almost immediately that unit  10  has stopped functioning. There is no need to wait until a timeout occurs as is the situation with the software detection systems known in the prior art. 
     It is noted that while in the embodiment shown in  FIG. 1 , the units are interconnected by a local area network. In other units the interconnection could be by a wide area network or WAN such as the commercial Internet network. 
       FIG. 2  is block flow diagram which illustrates the operation of the system shown in  FIG. 1 . Block  201  represents the normal operation of the system. This is where the flow diagram in  FIG. 2  starts. As indicated by blocks  202  and  203 , the fault notification process begins when an error occurs in the operating system  12 , causing a branch to the exception handler  13 . As indicated by block  210 , program  13 A sends a control packet directly to the network controller card  14 . This packet is preset and stored in a particular memory location, such that none of the packet assembly routines in operating system  12  are utilized. The packet is transmitted to system  10 A by network controller card  14 , and the backup system  10 A them begins operating on signals from coaxial cable  5 . 
     An alternate embodiment of the invention is shown in  FIG. 3 . In this alternate embodiment, the router unit  300  includes a number of what are conventionally known as “line cards”. The line cards in unit  300  are designated A, B, and C. One of the cards, (that is, the card designated C) is a backup card that only operates if one of the other cards stops operating for some reason. It is noted that while only three line cards are shown, the system could have many such cards. Systems that have a number of line cards are commercially available. Examples of such commercially available systems are the “ubr7200” and “ubr10000” series of routers marketed by the Cisco corporation. These routers utilize the Cisco&#39;s IOS operating system software. 
     Each of the line cards include a CPU  311 , a software operating system  312 , and exception handler  313  and an ASIC  314  that is connected to a control bus  303  as is conventional. The system  300  also includes a CPU  301  and a box operating system  302  as is conventional. The control signal bus  303  can, for example be the PCI bus of CPU  301 . The system  300  may also include a data bus that interconnects the cards and various other components that are not particularly relevant to the present invention, hence, they are not shown. 
     In the embodiment shown in  FIG. 3  each line card includes an extension  313   x  to the exception handler code  313 . This extension is similar to the extension  13 A shown in  FIG. 1 . 
     During normal operation of the system, line card A handles signals from coaxial cable  307 . It is noted that coaxial cable  307  is also connected to card C, so that if card A become inoperable, the signals on coaxial cable  307  can be handled by card C. Other coaxial cables not shown would be connected to the other line cards. 
     The operation of card A will now be discussed. It should be understood that the other cards operate in a similar manner. If software operating system  312 A is for some reason disabled due to a fault, the exception handler  313 A is activated in a normal manner. When exception handler  313 A is activated programming code  313   x  is activated. Program  313   x  sends a signal to ASIC  314 A. ASIC  314  sends this signal out on bus  303  to card C informing card C that it should begin operating. 
     The notification of card C occurs almost immediately when the fault occurs. There is no waiting for a time out to occur. 
       FIG. 4  shows still another alternate embodiment. In the embodiment shown in  FIG. 4 , the line cards including the CPU  411 , the operating system  412 , the exception handler  413  and the extension to the extension  413   x  are similar to the corresponding components in the previously described embodiments. 
     In the embodiment shown in  FIG. 4 , there is no control bus that connects the line cards. Thus, control signals can not be sent directly between the line cards. The system does however, have a data bus  404  that can transmit packets between the cards. Is the system shown in  FIG. 4 , a control packet is sent from a failing line card to the backup line card, similar to the operation of the first embodiment of the invention, with the exception that the packet transmission is entirely within one box or unit. 
     In this embodiment, the data forwarding plane  404 , which is part of a conventional Network Processing unit (NPU) is used to exchange control traffic. In this type of architecture, traffic that the NPU cannot handle is diverted to the CPU. This fact is leveraged for crash notification. The extension to the exception handler injects a pre-created crash notification packet into the data plane stream. The forwarding plane of the NPU on the standby unit diverts this notification to the standby CPU, thus enabling fault detection and notification without interrupt support or network controller support. 
     The three embodiments of the invention discussed above can be summarized as follows: In the first embodiment, there is an independent standby unit connected to the primary unit by a local area network. When a software crash or a software fault occurs in the primary unit, an extension to the exception handler in the primary unit sends a pre-created crash notification packet to the standby unit. It is noted that since the system is crashing, the CPU and the software layers are not available to deliver the packet to the standby router in the normal manner. However, the network controller chip runs independent of the CPU and it is fully functional and available during a software crash. Thus the CPU exception handler extension directly places the pre-established crash notification packet into the DMA ring of the network controller which then reliably delivers the crash notification packet onto the network and thus to the standby router. 
     In the second embodiment, there is not an independent control network interface that is available to deliver a crash notification packet to the backup unit. In this embodiment, the exception handler extension sends a control signal to the backup unit via a signal bus which interconnects the units. 
     In the third embodiment, traffic that a network processing unit can not handle is diverted to a CPU data bus. In this embodiment, a pre-created crash notification packet is embedded into the data plane stream. The forwarding plane network processing unit diverts this notification to the standby CPU, thus enabling software fault notification without interrupt support or network controller support. 
     In each of the embodiments, the backup unit is notified of a software fault without waiting for any time out or heart beat mechanism to operate. 
     While the invention has been shown with respect to a number of different embodiment, it should be understood that various changes in form and detail can be made without departing from the spirit and scope of the invention. The scope of the invention is only limited by the appended claims.